Early Proterozoic crustal evolution: Geochemical and Nd-Pb ... et al... · Paolo et al., 1991;...

Pergamon Geochimica et Cosmochimica Acta, Vol. 59, No. 6. pp. I IX- 1177, 1995

Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved

0016-7037/95 $9.50 + .oO

0016-7037(95)00032-l

Early Proterozoic crustal evolution: Geochemical and Nd-Pb isotopic evidence from metasedimentary rocks, southwestern North America

S. M. MCLENNAN, ‘.* S. R. HEMMING,‘.+ S. R. TAYLOR,‘.~ and K. A. ERIKSSON”

‘Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100. USA ‘Research School of Earth Sciences, Australian National University, Canberra. ACT 2601, Australia

‘Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blaksburg. VA 24061. USA

(Received June 24. 1994; accepted in revised form December 29. 1994)

Abstract-Early Proterozoic ( 1X- 1.7 Ga) metasedimentary rocks in northern New Mexico and southern Colorado, USA, can be divided into turbidite successions (commonly volcanogenic) associated with mafic/ felsic metavolcanic successions (e.g., Irving Fm.) and stable shelf quartzite-pelite successions of shallow marine origin (e.g., Hondo Gp.) . Metapelites from the turbidite successions reported here have low K20/ Na20, low Th/U ( <3.0), low to moderate Th/Sc (O.l-0.6), and slight negative Eu-anomalies, although regionally, negative Eu-anomalies in such rocks are common. At the time of sedimentation (ca. 1.7- 1.8 Ga), cNd values were in the range +3 to +7, indistinguishable from associated metavolcanic and plutonic rocks. Similarly, lead isotopic data scatter about a 1.7 Ga reference isochron. Low K ( *3*Th/‘38U) values for the Irving Formation are consistent with derivation from crustal sources similar to the southern Colo- rado/northern New Mexico lead isotope crustal province. These data are further consistent with a volcanic arc related origin. In contrast, stable shelf metapelites have high K20/Na20, variable but commonly high Th/U (2.0-7.0), moderate to high Th/Sc (0.51.4), and substantial negative Eu-anomalies. Although compositions are rather variable, they are typical of post-Archean shales. Neodymium isotopes are surprisingly radiogenic with cNd( 1.7 Ga) in the range -0.2 to +4. Lead isotopic data for the least radiogenic samples also are consistent with a dominantly juvenile source and on a 207Pb/204Pb vs. ‘06Pb/204Pb diagram, data scatter slightly above the 1.7 Ga reference isochron, suggesting minor components of significantly older material. Lead isotopic systematics suggest that a major component of the provenance was derived from the immediately associated metavolcanic-plutonic terranes, consistent with suggestions of a first-cycle origin, but with an Archean component. Isotopic data restrict the Archean component to about lo%, on average, and no more than 25% in any sample. This older crustal component may be derived either by direct erosion of Archean rocks, such as the Wyoming Province, or indirectly through assimilation into Early Proterozoic igneous rocks. Although the stable shelf sedimentary rocks are derived from a provenance with similar ages as the volcanogenic turbidites, the geochemical characteristics of the provenance are significantly different. Accordingly, these data are consistent with especially rapid and widespread crustal growth and evolution in southwestern North America during the period 1.9- 1.7 Ga. Several samples from the Hondo Group and Uncompahgre Formation have REE patterns that are rotated to LREE depletion and perhaps HRFE enrichment. The change in REEs correlate with MO, U, and V abundances and Pb isotopic characteristics suggesting sedimentary processes similar to those operating in black shales affected these REE patterns. REE patterns and Th/U ratios of Early Proterozoic volcanogenic turbidites examined in this and other studies differ on average from turbidites found in Archean greenstone belts. Negative Eu-anomalies are common, HREE-depletion is seen but comparatively rare, and Th/U ratios are commonly below 3.0. Accordingly, these data are consistent with models suggesting that the upper crust had a different composition in the Archean.

EARLY PROTEROZOIC CRUSTAL EVOLUTION

The Early Proterozoic, especially between about 2.0 and 1.7 Ga, is seen increasingly as an important period of growth and evolution of the continental crust (DePaolo, 1981; Patchett and Arndt, 1986; McCulloch, 1987; De- Paolo et al., 1991; Taylor and McLennan, 1995). In North America, a major site of crustal additions at about 1.9 to 1.7 Ga appears in a broad belt running northeast from

southern California and Arizona to the Grenville Province (Hoffman, 1989a). In this paper, we examine the geochemistry and Nd-Pb isotopic characteristics of some metasedimentary rocks deposited between about 1.80 and 1.69 Ga in northern New Mexico and southern Colorado. The chemical composition of sedmentary rocks provides a record of the nature and evolution of the provenance and of the sedimentary history. Accordingly, the major purposes of this study are threefold:

* On leave 1994; Max-Planck-Institut fur Chemie, Abt. Geo- chemie, Postfach 3060, 55020 Mainz, Germany.

+ Present Address: Lamont-Doherty Earth Observatory of Colum- bia University, Palisades, NY 10964, USA.

‘Present Address: Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 2601, Australia.

1)

2)

1153

to further evaluate the nature of Early Proterozoic crustal growth and establish any possible consequences for the chemical evolution of the continental crust in this area (e.g., Taylor and McLennan, 198.5, 1995 ) ; to evaluate the sedimentary history of these metasedimentary rocks, particularly as it relates to the questions of tec-

1154 S. M. McLennan et al.

tonic regime in this area and of Early Proterozoic climate tercalated with a bimodal metavolcanic succession. Condie (e.g., McLennan et al., 1993); and ( 1992) included these rocks in the Dubois Terrane.

3) to examine the utility of combined geochemical and Nd/ Pb isotopic analyses in determining the provenance and sedimentary history of structurally complex sedimentary sequences.

GEOLOGICAL RELATIONSHIPS

Proterozoic rocks, of ca. 1.8 to 1.6 Ga age, are preserved in the southwestern United States as a series of basement uplifts, largely composed of metamorphosed volcano-plutonic sequences. Detailed geological relationships among many of these blocks are commonly unclear or are in dispute. There is a general younging of rocks from northwest (Nevada, Utah, USA) to southeast (Arizona, New Mexico, USA) although, in detail, distinct geochronological boundaries do not neces- sarily exist (Condie, 1986; Karlstrom and Bowring, 1988). Some workers have recognized distinct tectonostratigraphic terranes in this region, largely based on U-Pb zircon geochronology, common-Pb studies of feldspars and whole rocks, and Nd model age studies (e.g., Bennett and DePaolo, 1987; Wooden and Dewitt, 1991; Aleinikoff et al., 1993). These blocks may represent accreted island-arcs or juxtaposed allochthonous terranes or both. In some cases there is dispute over the allochthonous/autochthonous relationships within individual basement blocks (e.g., Grambling et al., 1988, 1989).

The second area selected is the Salida area of south-central Colorado (Bickford and Boardman, 1984; Boardman, 1986; Boardman and Condie, 1986). In this area, deformed and comparatively less deformed bimodal metavolcanic and metasedimentary rocks are exposed and are part of the Cochetopa- Salida Terrane of Condie ( 1992). These rocks have been interpreted as representing a back-arc environment, underlain by sialic crust (Boardman, 1986; Boardman and Condie, 1986). A U-Pb zircon date of a felsic metavolcanic rock within this sequence was reported as 1,728 + 6 Ma (Bickford et al., 1989).

Quartzite-Shale Successions

Sedimentary rocks may be divided broadly into two associations. The first and most abundant are volcanogenic and other turbidites, associated with the volcano-plutonic terranes, generally thought to have been deposited in proximity to island arcs or arc-continent collision zones. The petrography and geochemistry of these rocks have been studied in some detail, especially in probable time-equivalent terranes exposed further west in Arizona (Condie and Nuter, 198 1; Con- die and Martell, 1983; Condie and DeMalas, 1985; Condie et al., 1992). The second variety includes quartzite-shale successions, generally thought to have been deposited in shallow marine settings. The geochemistry of similar rocks preserved in Arizona, have been studied by Condie et al. ( 1992). On a regional scale, turbidite sediments are interpreted to have been deposited both before and after the quartzite-shale association, however, where in proximity, the quartzite-shale association always appears to be younger than the volcanogenic successions.

For this paper, most attention has been placed on metapelitic rocks of the Hondo Group (or Ortega Group) of northern New Mexico, USA (Figs. 1,2) because of the extensive studies of the geology (e.g., Grambling et al., 1988), geochronology (see review in Grambling et al., 1988), structure (e.g., Williams, 1991), metamorphism (Grambling et al., 1989), stratigraphy (Soegaard and Eriksson, 1986; Williams et al., 1986; Bauer and Williams, 1989), and sedimentology (Soe- gaard andEriksson, 1985,1986,1989) of these and associated rocks (e.g., Robertson and Condie, 1989). The stratigraphy of northern New Mexico has been recently redefined by Bauer and Williams ( 1989) and for this paper, we use this termi- nology, with stratigraphic terms used by Soegaard and Eriks- son (1986) in parentheses at first use.

Volcanogenic Successions

Metapelitic rocks from two volcanogenic successions were selected for analysis (Figs. 1, 2). The Irving Formation is exposed within the Needle Mountains in southern Colorado, USA (Barker, 1969; Tewksbury, 1985, 1989; Harris et al., 1987). This unit is generally considered equivalent to the Twilight Gneiss which accumulated and was metamorphosed during the interval of 1,780- 1,750 Ma (see review in Condie, 1992). The Irving Formation is a complex sequence of elastic metasedimentary rocks ( amphibolite grade), commonly showing characteristic turbidite features, and Fe formation in-

The Hondo (Ortega) Group is > 1,000 m thick and consists of a basal quartzite unit, the Ortega Formation (Ortega Quartzite), overlain by three fine-grained units, the Rinco- nada Formation (from the base includes units RI/R2 through R6), Pilar Formation (Pilar Slate), and Piedra Lumbre For- mation. This sequence of sedimentary rocks has been interpreted as being deposited on a stable continental shelf with facies relationships indicating the continent was to the present-day north and northwest (Soegaard and Eriksson, 1985, 1986). Quartz arenites of the Ortega Formation have also been interpreted as first cycle sediments on sedimentological grounds (Soegaard and Eriksson, 1989). The Hondo Group is constrained to have been deposited between about 1,713 Ma, the minimum U-Pb zircon age of detrital zircons that appear to have suffered only a single episode of Pb loss (Al- einikoff et al., 1985, 1993), and about 1,691 Ma, the U-Pb zircon age of cross-cutting plutons (see review in Bauer and Williams, 1989). The veracity of the cross-cutting relationships, in establishing the minimum age, does not appear to be unequivocally accepted and the Hondo Group could be much younger, perhaps as young as 1,650 Ma. The entire sequence was deformed and metamorphosed to near aluminosilicate tri- ple-point conditions between about 1,650 and 1,250 Ma (Grambling et al., 1989).

The Hondo Group rests more or less conformably on an extensive felsic volcanic sequence of variable thickness, the Vadito Group, which appears to be about 1,714 t 5 Ma (Bowring and Condie, 1982). In turn, this unit is in contact with a series of metavolcanic sequences, including the Pecos, Gold Hill, and Moppin Complexes. Whether this relationship

Geochemistry of Early Proterozoic sedimentary rocks 1155

I

S. Colorado / - N. New Mexico

Pb Province

100km

4km

0)

FIG. 1. Location map showing (A) generalized map of northern New Mexico and southern Colorado showing the location of Precambrian basement uplifts (stippled pattern). Also shown are the locations of the Archean Wyoming Structural Province, lead isotope Provinces with the Transition Zone between provinces shown in dotted pattern (Al- einikoff et al.. 1993) and Nd-model age (TDM) boundaries (Bennett and DePaolo, 1987; DePaolo et al., 1991). (B) Generalized geology of the Picuris (north), Truchas (southwest) and Pecos (southeast) Ranges and location of stratigraphic sections in the Hondo Group. (C) Generalized geology of the Needle Mountains area showing sample locations for the Irving Formation and Uncompahgre Formation. (D) Generalized geology and sample locations in the Salida area. Geological maps compiled from Tewsksbury (1985); Soegaard and Eriksson (1986); Boardman (1986).

represents an unconformable contact or a tectonic boundary is an issue in dispute, but one that has fundamental significance for understanding the tectonic development of southwestern North America. These metavolcanic successions (greenstone belts ) vary in age between about 1,765 and 1,720 Ma. The Pecos and Gold Hill Complexes have been studied in greatest detail and have been interpreted as a continental arc and a remnant of a back-arc basin, respectively (Condie and McCrink, 1982; Robertson and Condie, 1989).

The final sequence considered is the Uncompahgre For- mation, also exposed in the Needle Mountains of southern Colorado (Fig. 1, 2) (Tewksbury, 1985, 1989; Harris et al., 1987). The age of this sequence is not well established, but there is growing consensus, based on structural data, that the

Uncompahgre Formation postdates the 1.78- 1.76 Ga Irving Formation and Twilight Gneiss (Harris et al., 1987). Detailed sedimentological work reveals close similarities between the Uncompahgre Formation and Hondo Group (Harris and Er- iksson, 1990), supporting suggestions of stratigraphic correlation (e.g., Tewksbury, 1985, 1989). U-Pb ages of detrital zircons from the two units (Hondo and Uncompahgre) have differing age distributions, suggesting differing provenance, but are not inconsistent with the suggested correlation (Al- einikoff et al., 1993).

ANALYTICAL METHODS

Samples were ground in agate to approximately 200 mesh. Major elements and trace elements. Cr. V. SC, Ni, Co, Cu. Zn, Ba. Sr, Rb,


AGE NORTHERN NEW MEXICO NEEDLE MTNS. SALlDAlGUNNlSON

1660

? ?

1700

i

R5 ~‘~~~~~&{m 1~~~~~~~iona

“ondo $$XUrnb”/R6

\ mconada Fm ~4 ? ?

Vadito Gp Ortega Fm

\ R3 RI/R2

Metavolcanic Sequences Catchetopa 1740

I

I (Pecos (1720 Ma), Gold Hill (1765 Ma), Moppin

1760

I!ZYi$ZmZeiss y

Succession

(21755 Ma) Complexes) Dubois Succession

FIG. 2. Generalized stratigraphic and radiometric age relationships for Early Proterozoic supracrustal rocks in northern New Mexico and southern Colorado that are considered in this paper. See text for sources of data.

and Zr, were analysed by direct current argon plasma spectroscopy (DCP-AES), using analytical techniques adapted from Feigenson and Carr ( 1985) and Klein et al. ( 1991). For elements well above detection limits, analytical uncertainties are typically about ?5- 10% for trace elements, except Co and Rb ( lo- 15%) and better than ?2- 3% for major elements except Si ( 1%). The rare earth elements and trace elements, Cs, Tl, Pb, Th, U, Hf, Nb, Sn, MO, W, Bi, and Y were analysed by spark source mass spectrometry, using techniques adapted from Taylor and Gorton ( 1977). Analytical uncertainty is typically better than about 5% for elements with concentrations above about 0.2 ppm, although it varies somewhat with individual element and analytical conditions. In addition, the coarse grained metamorphic texture of some samples adversely affects precision. For most of the samples analysed for neodymium isotopic composition, Sm, Nd and in many cases other REE abundances were analysed by isotope dilution mass spectrometry (IDMS) and these can be identified in the tables by an additional significant figure. Major and trace element data are reported in Tables l-3, where an asterisk indicates concentrations below detection limit.

The neodymium isotopic compositions were determined using a Finnigan MAT 262 thermal ionization mass spectrometer in the static mode. Samples were dissolved after fusion with lithium metaborate flux and Nd was separated by coprecipitation with iron and aluminum hydroxides followed by standard ion exchange procedures (see Stem and Hanson, 199 1) Data are normalized to ‘46Nd/‘44Nd = 0.7219 and corrected to La Jolla standard ‘43Nd/‘44Nd = 0.511865 (from measured value of 0.5 11820). Run precision was in all cases better than 0.002% and external reproducibility for the La Jolla standard was 0.0049% (2~). Lead isotopes were measured on an NBS 12 inch radius Nier-type mass spectrometer. Rock powders were dissolved overnight in HF-HN03 in a bomb at 220°C. Lead was run using the HIPO,/silica gel method (Cameron et al., 1969). The Pb blank is <300 pg. Average ratios for twelve analyses of NBS Pb 982 (2~) are 208Pb/*06Pb = 0.99750(39); ‘“‘Pb/‘06Pb = 0.46652( 17); and 206Pb/*wPb = 36.637(28). Correction for mass discrimination is taken as 0.125%/a.m.u. Sample uncertainties on *“Pb/*@‘Pb and 2mPb/‘04Pb ratios are approximately 0.10% and on ‘tr8Pb/2~Pb ratios are approximately 0.13%. Neodymium and lead isotopic data are reported in Table 4.

Sample numbers for the Hondo Group refer to both the formation or unit and the stratigraphic section from which they were taken (see Fig. 1). Thus, sample R2-1 is from Rinconada Formation Unit R2 and from the stratigraphic section 1, shown in Fig. 1. Sample PL-I is from the Piedra Lumbre Formation in section 1, and so forth. Un- compahgre Formation samples are taken from the upper half of the unit, in a single stratigraphic section along Lime Creek, with sample numbers increasing up section. Irving Formation samples were each taken from different outcrops west of Emerald Lake, anywhere from 0.1 to > 1 .O km apart. Samples from the Salida area are located in- dividually in Fig. 1.

RESULTS

Major Elements

The analysed volcanogenic metasedimentary rocks are characterized by low Si02 contents ( Si02/A1203 = 2-4), low to intermediate K,0/Na20 (0.3-3) (Fig. 3), and relatively high Fe0 + MgO contents in comparison to metapelitic rocks from the Hondo (Ortega) and Uncompahgre. An exception is sample 34.4 from the Salida area. This sample is characterized by high SiOl and K20/Na,0 (Fig. 3 ). In addition, this sample has a number of other unusual trace element and isotope features, including high REE abundances and a distinctive chondrite-normalized REE pattern, exceptionally low ferromagnesian trace elements and Sr (leading to high Rb/Sr and Ba/ Sr) abundances, and an extremely high zosPb/204Pb ratio. On balance, it is not clear that this is a metasedimentary rock but instead may be an altered felsic metavolcanic. If it is of sedimentary origin then it was likely derived from a very localized source with an uncharacteristic composition. In either case, data from this sample are reported in the various tables but are not discussed further.

Metapelitic rocks from the Hondo Group and Uncompah- gre Formation are indistinguishable in major and trace element compositions, although there is some variability. The major element data may be divided into low Si ( SiOZ/A1203 = 2-5) and high Si (Si02/A1203 > 6) groups (Fig. 3). In addition to having high Si02/A1203, the high Si samples also have the highest Si02 contents in their respective stratigraphic units. For samples from the Hondo Group, high silica samples are represented in each formation but all except one (R6-3 ) come from the northern stratigraphic sections (Fig. 1B). The ratio K20/Na20 is intermediate to very high, mostly in the range 2-20. The abundance of CaO is very low, always less than 1.4% and typically less than 0.5%. This latter observation indicates a dearth of original carbonate minerals in these metasedimentary rocks, in addition to a severe weathering history (see discussion below).

Rare Earth Elements

Chondrite-normalized REE patterns for volcanogenic metasedimentary rocks are displayed in Fig. 4. Samples from the


Table 1. Geochemical data for metasedimentary rocks from the Rinconada Formation.

Rl-1 RZ-1 RZ-2 R2-3 R2-4 R4-1 R4-2 R4-3 R4-4 R4-5 R6-1 R6-2 R6-3 R6-5

Si@ 65.6 TQ 0.57

2% 18.1 4.74

g 0.117 2.26 cao 0.94

81.8 0.44 a.37 4.33 0.013 1.28 0.11 0.44 2.45 0.051 1.33

100.6 5.57

19.0 71

65.4 69.0 66.4 64.1 0.74 0.84 0.79 0.98

19.1 16.3 18.0 22.6

70.3 62.6 65.1 0.77 0.92 0.80

15.8 19.3 19.4 4.35 6.71 4.54 0.016 0.039 0.090 0.53 1 .oa 1.05 0.03 0.22 0.09 0.54 0.46 0.64 3.08 3.78 4.45 0.050 0.206 0.060 3.46 3.81 3.33

98.9 99.1 99.5 5.7 8.22 6.95

29.3 42.0 30.3 79 80 77

64.8 0.82

18.6 6.57 0.102 1.30 0.08 0.35 4.26 0.068 2.01

99.0 12.2 53.1 78

64.8 0.78

18.2 5.93 0.031 1.75 0.13 0.78 4.63 0.065 2.61

99.7 5.94

23.3 74

54.16 45.29 121.8 95.88

52.25 10.24 1.980 8.990

38.06 7.203 1.350 5.890

7.99 5.540

4.65 3.255 4.55 3.318 8.04 9.22 3.33 3.96 1.60 1.44 0.63 0.63

65.1 77.9 60.4 0.97 0.46 0.95

18.4 8.59 19.6 7.45 7.06 9.87 0.095 0.008 0.144 0.46 0.34 2.15 0.06 0.03 0.16 0.36 0.15 0.84 2.75 2.5 1 4.43 0.056 0.041 0.076 4.79 1.80 2.27

100.5 98.9 100.9 7.64 16.7 5.27

51.1 57.3 23.3 84 74 76

6.44 5.34 6.58 3.51 0.028 0.053 0.271 0.085 1.34 1.39 1.80 0.81 0.23 0.11 1.34 0.06

Na20 1.34 W 4.18 p205 0.154 L.O.I. 2.83 Total 100.8 K20iNa20 3.12 Al2Qmdfl 13.5 CLA 69 J-a 47.33 Ce 103.0

Ed 39.87 Sm a.195

El 1.765 8.624

Tb Dy 8.29 Un

0.26 0.56 1.32 0.46 2.54 2.87 2.95 4.61 0.187 0.073 0.138 0.074 3.96 3.06 1.18 2.84

100.2 99.6 100.8 loo.2 9.77 5.13 2.23 10.0

73.5 29.1 13.6 49.1 86 80 71 80 38.8 32.6 66.4 78.1 12.0 7.88 55.0 34.2

8.13 7.25 1.53 1.29 7.04 5.16 1.25 0.79 6.67 4.62 1.25 1.00 3.62 3.02 3.65 2.81 7.18 7.84 3.00 2.83 1.56 1.49 0.62 0.64

44.15 53.5 22.6 35.2 43.41 104.3 125.3 50.4 79.8 90.43

13.6 6.68 a .44 36.53 54.36 32.2 36.7 38.7 7.139 10.68 6.05 9.70 7.570 2.101 2.03 1.36 2.01 1.441 6.427 8.57 5.34 10.6 6.609

1.37 0.91 1.87 5.945 7.72 5.72 11.1 6.781

1.58 3.517 4.76 3.698 4.36 8.07 8.29 3.89 3.15 1.41 1.59 0.95 0.65

27.31 64.22

45.5 26.1 53.4 80.3 58.8 90.5 12.5 7.22 13.9 46.4 28.5 55.1

9.96 5.74 8.59 26.73

5.378 1.015 5.487

1.82 1.08 1.74 9.79 5.94 6.65 1.62 1.05 1.16 9.73 6.69 7.41 1.93 1.33 1.51

5.141

2.881 2.904 6.35 3.20 1.53 0.57

.--

FL 4.32 4.125 bN/ybN 7.74 hN/SmN 3.64 %d%v 1.69 Eu/Eu* 0.64

5.40 3.91 4.62 4.83 3.55 4.48 6.37 4.97 8.05 2.88 2.86 3.91 1.64 1.36 1.20 0.56 0.57 0.70 0.73 0.61 0.62

41 25 33 40 38 40 31 55 32 41 37 34 68 12.1 25 13 18

107

39 42

5.8 18

at 18

66 71 53 74 84 64 11.5 13.0 18.6 40 28 31 12 13 7.6 ; 140

153 73 81 0.89 0.85 0.83 1 .a5 3.00 2.06 3.33 3.11 2.38 3.37 2.51 2.37

99 140

18.7 34 14 4.7

116 0.7 1

69 95 94 78 82 75 127 109 91 96 12.6 16.2 15.7 15.0 14.5 22 62 21 28 31

a 15 9 11

;4 147 5 ;: :; 89 i!; 0.92 0.75 0.86 0.86 0.85

47 71

34 39

38 95 47 105

6.5 16.3 20

7.4 :

1bS 14.0

z? 24

110 0.68 1.69 2.91 3.25

3 13 9; 0.81 0.90 2.35 2.39

0.50 0.93 2.72 2.33 1.92 3.00 3.91 4.71

4.12 3.41 2.05 5.19 3.25 3.10 2.43 2.75 4.13 2.33 2.33 2.82 2.86 1.79 2.17 2.76 3.61 3.12

2.70 2.10 4.02 3.28

43 199

*

6.6 108

*

6.2 1:: 104

16.8 146

0.52 0.17 * 275 431 696

14.6 22.8 31 22 94

136 2;; 168 7.05 1.68 1.55

12.5 6.95 7.40

10.6 180

55;

2.1 69 1: 2%

22: 0.55

732 44; 26

705 251 20.4 12.5 88 32

61 18 36 31 132 213 2:: 1;: 266 :fi 188

1.36 1.58 2.19 1.60 1.70 4.08 4.20 8.48 7.95 6.83 6.08 14.2

11.1 6.88 12.2 9.41 15.9 9.43 4.45 2.23 5 .oo 2.73 3.72 2.32

243 146 214 177 227 170

7.1 24.5 65 22 66

331 145 183 1.06 6.55 3.05 6.91 10.3 11.1

174 188 2.26 3.38 8.01 7.84

16.4 2.56

213 7.0

7.99 2.56

333 11.0 10.7

12.6 10.6 12.2 2.08 2.16 1.73

173 181 217 5.6 6.3 8.1

16 20 22 5.2 5.7 6.7 0.6 0.4 1.4 1.0 1.3 2.0

11.6 5.70

242 8.9

6.92 12.5 2.12 2.32

225 233 7.2 7.1

12 26 5.6 4.85 6.3 7.8 7.4 6.0

17 27 15 26 32 6.2 9.0 8.8 10.0 8.0 1.0 1.3 0.8 0.9 0.60 1.7 2.2 4.2 2.7 2.6 0.24 0.37 * 0.20 0.20

30.1 34.0 22.7 30.7 28.3

21 9.3 1.6 3.7

29 a.3 0.8 1.4 0.15

27.2 2.04 3.92 0.83

4.5 0.8 0.9

3.9 8.0 1.0 0.6 0.5

4.0

3t.4 6.41 2.89 1.36

2.0 1.9 0.32 0.15

31.3 32.8 3.26 5.39 3.77 4.27 1.06 0.77

0.13 0.24 0.11 1.4 0.18 30.3 30.9 28.7 26.8 43.4 3.12 6.06 4.91 7.05 2.49 3.42 3.08 3.08 3.62 4.82 1.38 1.10 0.82 0.66 0.59

3.09 2.44 3.45 4.27 4.06 3.28 2.89 4.61 3.41 4.80 0.55 0.75 0.60 1.06 0.65

Irving Formation are fairly uniform with moderately enriched Rare earth patterns for metapelites from the Hondo LREE (La&Sm, = 2.53.6), fairly flat but slightly concave up Group and Uncompahgre Formation are shown in Figs. 5 HREE patterns (Gd,/yb, = 1.3-1.9) and slightly positive to and 6. With a few exceptions, discussed below, the patterns moderate negative Eu anomalies (Eu/Eu* = 0.76-1.13). Sam- are uniform and similar to typical post-Archean shales such ples from the Salida area are mom variable. Apart from Sample as PAAS or NASC (Taylor and McLennan, 1985), with 34.4, shown above to have other unusual chemical characteris- LRFE enrichment (LaN/SmN = 3.3, s.d. = 0.7), flat HREE tics, REE patterns for two of the samples am within the range of ( GdN/YbN = 1.3, s.d. = 0.3), and significant negative Eu- the Irving Formation. Sample 14.5 is distinctive in having a no- anomalies (Eu/Eu* = 0.64, s.d. = 0.06, excluding R2-4, table negative Eu-anomaly and flat HREE. see below ) .


Table 2. Geochemical data for metasedimentary rocks from the Pilar, Piedra Lumbre and Uncompahgre Formations.

______pil~ Fo~a~on______

PS-1 PS-4 PS-5 --P-,iedra Lumbre Formation------

PL-3 PL-4 PL-5 ------Uncoqahgre Formation------ S3a S3b S4 S5

SQ 17.7 78.0 84.0 Tie;? 0.59 0.41 0.31

s? 9.41 0.57 9.65 1.41 5.10 0.86

!z 0.013 0.69 0.38 0.012 0.59 0.26 0.010 0.67 0.23 NaZo 0.35 0.68 0.12 KqO 2.92 2.21 1 .I5

0.210 0.130 0.107 5.63 4.90 5.04

Total 98.5 98.3 98.2 KzOiNazO 8.34 3.34 14.6 Al&?+/NqO 27.1 14.2 42.5 CIA 71 72 69

63.5 68.5 78.0 68.0 59.3 57.4 67.4 68.5 0.70 0.64 0.49 0.55 0.80 1.10 0.93 0.50

20.2 16.5 8.84 17.9 21.3 24.6 16.3 10.1 4.14 4.92 0.36 3.56 1.46 6.75 5.71 2.51 0.035 0.011 0.010 0.047 0.042 0.040 0.036 0.036 1.70 0.83 0.46 1.01 2.21 1.79 1.81 3.24 0.72 0.09 0.42 0.18 0.05 0.02 0.21 0.18 1.60 0.34 1.57 1.10 0.13 1.12 0.81 0.04 4.43 3.72 2.37 3.70 4.50 3.69 3.40 2.68 0.029 0.029 0.049 0.050 0.064 0.061 0.068 0.259 3.20 3.35 6.40 3.27 4.18 4.90 3.41 10.9

100.3 98.9 99.0 99.4 100.0 101.5 100.1 98.9 2.71 10.9 1.51 3.36 34.6 3.29 4.20 67.0

12.6 48.5 5.63 16.3 164 22.0 20.1 253 70 78 60 75 81 81 76 77

28.5 12.07 17.04 34.2 33.54 20.9 27.01 41.42 54.11 32.3 12.16 47.6 22.24 31.16 82.9 76.47 40.8 53.86 92.54 119.3 69.8 22.36

6.01 10.4 7.40 20.0- 11.12 17.07

3.05 2.558 3.578 0.52 0.566 0.528 1.92 3.120 3.433 0.30 1.83 4.41 3.537 0.38 1.12 3.480 2.278 1.04 3.667 2.261

18.5 2.22 5.09 5.88 2.97 3 .oo 1.50 0.69 1.23 0.66 0.61 0.46 9.0 30 19

:z 29

173 1:: 12.6 8.1 4.7 22 6 24

2: 2; 8: 72 18 49

0.92 0.17 0.11 3.41 28.8 6.63 2.15 2.26 l-.49 1.63

4.7 2.9 2.60 95 79 45

0.95 0.43 509 51s 500

9.9 1.1 --

256: 2:; 3:; 1.58 0.87 0.90 8.48 5.69 10.0 6.76 5.44 2.10 2.81 4.76 7.80

146 121 47 5.2 4.7 1.8

16 12 6.0 E

419

23 4.6 22 2.2

2.1 2.0 0.26 -- 0.20

28.1 25.7 26.1 2.41 1.14 0.27 4.22 2.22 8.11 0.54 0.67 0.45

40.2 7.05 1.37 5.09 0.90 5.62 1.30 4.23 4.52 5.11 3.06 0.91 0.70

32

68 105

14.9

31.24 6.418 1.230 6.040

6.067

3.740 3.580 6.33 3.29 1.37 0.60

31

:4 11.3

18.10 3.944 0.900 4.31 0.81 5.10 1.06 3.21 3.46 4.08 3.34 1.02 0.66

21

29 189

I

23.70 4.691 0.925 4.182

4.564

3.040 3.179 5.74 3.62 1.07 0.64

25

z:: 12.5

14 19 4 15 15 12

:8 4; ; 5;

138 0.65 0.65 :15 0.68 7.50 4.37 4713 5.80 0.93 1.58 1.33 3.00 2.30 2.97 2.99 2.16

2:: 3.1 2.9 31

114 65 188 1.0 1.1 0.70

991 83; 385 735 1E :: 88 9.0 177 22

159 271 303 163 2.20 4.22 0.74 1.06 9.44 31.0 4.38 4.15

15.6 15.9 3.16 8.35 2.55 3.17 5.96 2.16

169 118 123 128 7.1 5.2 4.7 4.4

14 19 12 15 10.7 9.1 4.3 9.5 0.8 1.1 1.5 3.8

:::3 2.7 2.2 3.2 0.47 0.20 0.85

23.8 22.7 26.2 29.1 6.12 4.22 0.63 3.87 2.19 2.11 5.56 3.23 1.05 1.41 0.54 0.67

Sample R2-4 from the Rinconada Formation is distinc- 1989). However, such a process is unlikely to be signifi- tive in having no significant Eu-anomaly. This sample cant in pelitic rocks such as these. This sample has distinc- shows no anomalous trace element characteristics (Table tive isotopic characteristics in having the highest end value 1) except in having the highest Th/U ratio (7.05), and high at 1.7 Ga (+3.3) and having the least radiogenic lead iso- Cu content. However, it does have distinctive major ele- topic compositions among the Rinconada samples. These ment chemistry (Table 1) in having high Ca and Mn con- data are consistent with a provenance more similar (though tent and having the lowest KzO/NazO ratio. Sedimentary not identical) to the Irving Formation but without addi- sorting of feldspar is one mechanism that can account for tional similar samples, it is not possible to further evaluate Eu-enrichments in some first-cycle sands (e.g., McLennan, the origin of this anomalous sample.

33.60 52.39 6.072 9.974 1.177 1.970 4.641 7.438

4.066 6.507

2.410 4.054 2.395 4.152

11.7 8.81 4.29 3.41 1.57 1.45 0.68 0.70

22 35

94 93 107 172

16.0 21.0

28.88 12.07 5.361 2.784 1.210 0.651 4.39 3.372 0.72 4.61 5.150 1.08 3.16 3.938 3.17 4.418 6.89 1.86 3.19 2.75 1.12 0.62 0.76 0.65

31 36.5

836 38

188 14.3 10.5 32 21 32 23

18 16 18 113 ::

:; 2; 71 31

0.88 0.54 1.03 0.20 3.34 7.48 2.91 8.95 1.78 1.44 2.67 3.50 2.59 2.58 2.26 1.16

2:: 11 5.6 8.5

168 130 71 0.95 0.9 0.18 1.1

522 538 488 310 5.0 3.5 80

43 :: 41 20’ 173 182 217 313

5.02 2.07 3.17 3.55 12.1 6.64 11.9 15.5 13.8 15.9 15.7 7.87 2.68 3.17 3.23 8.84

128 153 193 178 3.9 5.2 6.8 6.4

14 19 22 10 IX 9.1 1.1 5.7 0.4 37 4.4

1.5 0.37 z7 El h.266

32.8 2914 28.4 27:8 5.15 4.22 4.86 0.89 3.00 3.40 2.06 1.55 0.86 0.76 1.10 0.75

Geochemistry of Early Proterozoic sedimentary rocks 11.59

Table 3. Geochemical data for metasedimentary rocks from the Irving Formation and Salida area.

37A _____ _ ____. _ .________ hjq Formation ___________._ ______________ _________ _________S&a _____ ____.________ 61 81 117A 130 132 135 14.5 19.5 34.1 34.4

SiO2 Tie Al-&

2.974 5.128 3.42

1.22 2.93 0.48

1.806 1.808 7.30 3.23 1.44 0.93

2.871 1.951 2.715 1.924 9.09 8.29 3.02 3.03 1.85 1.71 0.76 0.80

1.75 0.873 5.73 2.93

.87 0.48 4.95 2.82 1.02 0.58 2.68 1.64 2.50 1.72

0.54 1.50

11.2 3.58

17.7 43.1

5.28 22.1 4.48 1.27 3.59 0.57 3.28 0.64 1.84 1.73

1.86 0.83

4.32 2.70 1.38 0.97

1.89 5.61 2.65 1.26 1.13

6.91 2.49 1.68 0.97

Y 16 33 24 18 14 20 17.5 Cr V SC Ni CO CU zn Cr/v V/T% Ni/Co La/SC CS Rb n Ba Pb Sr WRb RblSr Ba/Sr

170 1:: 42 189 242 188 123 180 149 240 163 211 21 I 25.1 21.2 22.6 26.7 24.8 26.6 31.2 45 26 21 43 54 58 44 27 22 36

171 69 42: : 156 86 115 98 111 78 1n1 1Ml 0.94 0.57 0.28 0.79 i.48 --0.89 --6.58 4.00 5.31 7.10 5.58 3.02 3.64 4.80 1.67 1.18 0.91 1.43 2.16 1.61 1.19 0.78 1.72 1.04 1.55 0.44 0.59 0.57

4.8 3.2 2.2 1.7 1.3 6.7 2.2 108 56 50 80 39 122 71

* * * * * * * 1051 786 525 1280 464 1147 430

10.5 6.3 8.6 6.3 6.2 3.7 7.3 350 490 371 375 542 133 584 314 320 384 386 283 274 27n

- 0.31 0.11 0.13 0.21 0.07 0.92 -.0.12 3.00 1.60 1.42 3.41 0.86 8.62 0.74

3.71 6.37 4.16 6.74 2.89 3.16 2.96 1.62 5.16 1.83 4.67 1.35 1.61 1 .oo

128 227 105 134 104 117 119 3.5 5.6 3.0 5.5 3.0 2.8 2.8 7.6 6.4 5.2 7.6 3.2 ;.6 2.5 2.4 2.0 3.5 ;.2 0.8 1.3 1.0 0.9 1.0 0.9 1.3 1.8 1.0 ;.4 1.4

36.7 40.5 0.8 35.0 1.8 24.4 0.8 34.7 l-.7 41.8 0.9 0.6 42.5

2.29 1.23 2.27 1.44 2.14 1.96 2.96 5.27 5.73 5.67 6.16 3.81 4.91 5.98 0.15 0.30 0.18 0.25 0.12 0.12 0.095

55.1 58.4 62.2 54.8 51.9 55.5 0.85 0.84 0.60 1.15 0.65 0.90

19.7 19.0 15.7 21.7 16.6 18.3 8.49 5.60 8.64 8.80 8.02 8.74 0.097 0.073 0.200 0.032 0.140 0.100 3.08 2.96 2.29 1.36 3.81 4.09 3.55 3.10 3.56 3.30 4.52 3.86 1.55 5.26 4.02 3.47 4.61 2.20 4.09 2.16 2.31 3.72 1.33 4.03 0.214 0.270 0.232 0.233 0.174 0.198

53.7 1.02

18.8 10.1 0.130 2.32 6.72 3.49 2.31 0.227

2.99 1.80 0.93 1.99 1.62 1.94 0.86 99.1 99.5 100.7 100.6 99.4 99.9 99.7

2.64 0.41 0.57 1.07 0.29 1.83 0.66 12.7 3.31 3.91 6.25 3.60 8.32 5.39 67 54 51 59 50 61 57 19.54 36.52 23.6 41.5 11.0 15.7 40.71 82.82 52.2 96.2 26.1 35.9

8.95 3.12 4.66 32.5 11.55 19.7

7.29 2.567 3.73 19.41 3.807 1.070 3.217

38.30 7.615 1.709 6.201

25.30 4.906 1.163 4.07

REE patterns of samples PS-4 (and possibly PS-5) from the Pilar Formation, PL-4 (and possibly PL-5) from the

respective formations. There are several systematic differences in some of the trace element relations (Table 2). These

Piedra Lumbre Formation, and S-5 from the Uncompahgre Formation are anomalous in that they appear to be rotated, to

samples have low Th/U ratios compared to other samples in

LREE-depletion and, in the cases of PS-4 and S-5, to HREE- the same units. It is thought that the low Th/U is primarily related to enriched U abundances (note Th/Sc ratios are sim-

enrichment, relative to associated samples. Although these ilar), however, the high La/Th ratios in some of the affected samples show some major element differences (Table 2) compared to other samples in the same units, the only system-

samples suggests that some Th loss cannot be entirely ruled

atic difference is that all are from the Si-rich group (Fig. 3) out. The samples with rotated REE patterns also have low

and all are the most carbonaceous of the samples from their levels of a number of trace elements, including Cr, SC, Co, Rb, Th, Sn, and possibly Nb. In contrast, V is enriched, lead-

60.2 60.3 60.9 77.8 0.63 0.91 0.96 0.24

20.9 18.1 18.4 10.3 4.47 6.75 8.10 3.60 0.100 0.180 0.120 0.079 3.08 3.14 2.35 3.29 0.70 2.70 2.43 o.oi 2.04 4.01 2.95 0.15 4.77 3.09 3.13 4.15 0.189 0.279 0.185 0.040 3.10 1.06 1.28 1.33

100.2 100.5 100.8 101.0 2.34 0.77 1.06 27.7

10.3 4.51 6.24 68.7 69 56 60 69 27.21 38.6 18.0 47.6 77.13 87.2 40.8 124.4

10.7 4.24 14.9 41.79 44.47 20.5 56.3

9.224 7.496 5.29 12.9 1.485 1.86 1.50 1.70 8.147 5.91 4.04 12.2

1.05 0.64 2.20 8.209 5.69 3.81 12.2

1.08 0.80 2.92 5.181 3.05 2.34 8.95 5.099 2.80 2.28 8.93 3.61 9.32 5.33 3.60 1.86 3.24 2.14 2.32 1.29 1.71 1.44 1.11 0.52 0.85 0.99 0.42

54 31 23 72 20 61 71 4 64 115 142 8 13.4 20.5 22.4 21 37 1:

:: ;: 6 105 25 ; 109 90 109 252

0.31 0.53 0.50 0.50 3.05 3.59 3.84 0.84 1.62 1.60 1.48 2.03 1.88 0.80 lI.9

5.5 7.2 6.6 0.40 142 79 84 88

97; 0.12 * *

1171 1253 1003 8.1 9.0 15.8 4.2

149 250 284 3.9 279 325 309 391

0.95 0.32 0.30 22.6 6.54 4.68 4.41 257.2

6.19 10.8 3.96 9.64 3.76 2.63 1.47 4.43

307 215 176 436 9.4 6.8 4.2

34 25 20 :!: 8.5 4.0 4.6 15.4 0.8 0.2 0.6 1.4

2.2 1.0 4.9

32.7 0.10

31.6 4i.9 2i.8 1.65 4.11 2.69 2.18 4.40 3.57 4.55 4.94 0.64 0.53 0.18 2.47


Table 4. Neodymium and lead isotopic data for early Proterozoic metasedimentary rocks from northern New Mexico and southern Colorado.

SUllpk ‘47Sm/‘44Nd fSm/Nd 143Nd/‘44Nd eNd(O) ENd(1.7) TDM 2”Pbp”Pb 207Pb/20+b zosPb/204Pb

Hondo Grow - Rinconada Formation Rl-1 011243 -0.368 R2-1 0.1217 -0.381 R2-1 (Duplicate) R2-4 0.1182 -0.399 R4-1 0.1188 -0.396 R44 0.1183 -0.399 R4-5 0.1185 -0.397 R6-1 0.1145 -0.418 Hondo Group - Pilar Slate PS-1 PS-4 0.1391 -0.293

0.511856 -15.2 +0.6 0.511792 -16.5 -0.1

0.511929 -13.8 +3.3 0.511833 -15.7 +1.3 0.511748 -17.3 -0.2 0.511899 -14.4 +2.1 0.511773 -16.8 +l.l

0.512135 -9.8 +2.8

;;.S (Duplicate) 0.1268 -0.356 0.512140 -9.7 +5.6 PS-5 (Duplicate) 0.512163 Hondo Group - Piedra Lumbre Formation PL-3 0.1242 -0.368 0.511881 -14.7 +l.l PL-4 0.1318 -0.330 0.512113 -10.2 +4.0 PL-5 0.1199 -0.391 0.511892 -14.5 +2.3 PL5 (Duplicate) Uncompahgre Formation S-3a 0.1093 -0.444 0.511743 -17.4 +1.6 S-3b 0.1151 -0.415 0.511788 -16.5 +1.3 S-3b (Duplicate) s-4 0.1123 -0.429 0.511861 -15.1 s-5 0.1395 -0.29 1 0.512075 -10.9 S-5 (Duplicate) 0.512053 Irving Formation Vok;y9ynic Metasedimentary NM-57A 0.1186 0.511914 -14.1 NM-57A (Duplicate) 0.511919 NM-67 0.1202 -0.389 0.511941 -13.6 NM-67 (Duplicate) 0.511942 NM-81 0.1173 -0.404 0.511931 -13.8 NM-130 0.1344 -0.317 0.512089 -10.7 NM-135 0.1226 -0.377 0.512000 -12.4

1.96 2.00

+3.3 1.85 +1.5 2.08

23.358 16.073 50.199 20.267 15.757 42.072 20.310 15.763 42.828 21.599 15.853 39.892 29.613 16.512 39.398

Rocks +3.0 1.88 18.670 15.536 36.764

+3.1 1.87 21.948 15.899 38.304

+3.6 1.83 19.058 15.647 37.278 i2.9 1.93 18.979 15.570 37.098 +3.8 1.83 19.087 15.668 38.025

Salida Area Volcanogenic Metasedimentary Rocks 14.5 0.1335 -0.321 0.512095 -10.6 +3.3 19.5 0.1019 -0.482 0.511917 -14.0 +6.7 34.4 0.1386 -0.295 0.5 12200 -8.5 +4.2

2.09 2.14

1.85 2.01 2.13 1.90 2.01

1.95

1.68

2.05 1.82 1.94

1.89 20.734 15.780 43.176 1.61 20.227 15.752 40.034 1.82 35.772 17.245 44.867

18.246 15.550 39.427 20.126 15.813 39.895 19.961 15.795 39.374 18.066 15.532 39.411 18.915 15.637 38.423 19.344 15.733 39.797 18.937 15.669 38.686 19.007 15.676 40.545

25.539 16.259 39.024 27.612 16.415 39.179 27.654 16.427 38.727 32.805 16.927 38.395

18.000 15.551 37.782 23.526 16.087 38.211 20.412 15.773 41.183 20.205 15.734 40.273

Nd isotopes normalized to ‘46Nd/‘44Nd = 0.7219, corrected to La Jolla standard ‘43Nd/14“Nd = 0.511865. ENI(T) = {[143Nd/‘44Nd,,,‘(T)1 / [‘43Nd/‘44N~‘~~(T)] 1) * 10‘! and fSm/Nd = (‘47Sm/‘+‘Ndsmpl) / ( ‘47Sm/‘44NdC,j& 1; where “3Nd/‘44NdCmR = 0.512636 and ‘47S”,,‘44N+~u~ = 0.1967 and Is,,, = 6.54 x lo-‘* yr’. TDM = l/&,, * In( 1+[(‘43Nd/‘44Nd smp, - “3Nd/‘44NdDM) / (‘47Sm/‘44Nd,,l 147Sm/144Nd’,M)]); where ‘43Nd/‘44NdDM = 0.51315 and 147Sm/‘44NdDM = 0.217 Pb isotopes corrected for mass discrimination of 0.125%/a.m.u.; analytical uncertainties are approximately 0.1% for 2°7Pb/2MPb and 206Pb/204Pb and 0.13% for *08PbP04Pb.

ing to exceptionally low Cr/V ratios. Molybdenum is also substantially enriched in these samples. The cause of these anomalous characteristics is thought to be related to sedimentary/diagenetic processes and is discussed in greater detail below.

Other Trace Elements

The Irving Formation has variable, but somewhat elevated levels for most ferromagnesian trace elements. For example, Cr averages 147 ppm (s.d. = 70), which compares to the upper crustal average of 35 ppm and shale average of about 100 ppm (Taylor and McLennan, 1985). This is consistent with a mafic component in the provenance. Nickel, however, is not as enriched. Apart from Ba and Sr, levels of incompat- ible trace elements are low, as are the ratios Rb/Sr, Th/Sc, and ThfU.

The Hondo Group and Uncompahgre Formation generally have trace element abundances that are variable but on average close to typical post-Archean shales, with the exception of the samples having unusual REE patterns, described above. For example, Th/Sc averages 0.84 (s.d. = 0.28) which com-

pares to the upper crustal and average shale value of about 1 .O (Taylor and McLennan, 1985). With few exceptions, Rb/ Sr is greater than 1 .O, consistent with an extended weathering history. Although Tb/U is commonly greater than the upper crustal value of 3.8, lower values are also common and these data are surprisingly variable. In general, trace element abundances are more variable than typically seen in Phanerozoic shales (e.g., Nance and Taylor, 1976).

Neodymium and Lead Isotopes

The neodymium isotopic data are presented in Table 4. The Irving Formation and Salida volcanogenic samples have a restricted range of eNd at the time of sedimentation (ca. 1.7 Ga). Excluding sample 19.5 (+6.7) and 34.4 (see above), they are in the range of +2.9 to +3.8. This compares with the range of similarly aged volcanic and plutonic rocks in this area of about +3 to +7 (e.g., Nelson and DePaolo, 1984), suggesting relatively little, if any, older (i.e., Archean) crust was being em&d and incorporated as part of the provenance. The values of ??Nd( 1.7 Ga) in the Hondo Group and Uncompahgre Formation are in the range of -0.2 to +5.6 and thus overlap the range of the


0

Oo 8” 0 0 ??

Shale

1 ’ 0.1

I I J 1 10 100

K,O / Na,O

FIG. 3. Plot of Si02/A1203 vs. K,0/Na20 for Early Proterozoic metasedimentary rocks from New Mexico and Colorado. Note the low silica and low K,0/Na20 characteristics of the volcanogenic sediments of the Irving Formation and Salida area. Also note that samples from the Hondo Group and Uncompahgre Formation fall into low and high silica groups and that these samples have relatively high K,0/Na20. Also plotted is an estimate of average shale (Taylor and McLennan, 1985).

volcanogenic sediments. The lower cNd( 1.7 Ga) for some of the Hondo/Uncompahgre samples indicate a significant but relatively small amount (<25%) of old (Archean) crust in the provenance. For example, average Archean metasedimentary rocks preserved in the Wyoming Province to the north (see Fig. 1) would have cNd < -12 at 1.7 Ga (Frost, 1993).

Lead isotopic compositions show wide variations, with 206Pb/204Pb ranging from about 18.0-32.8 (Table 4), the most radiogenic samples being those with rotated REE patterns. Figure 7 plots 207Pb/204Pb and 20*Pb/204Pb vs. 206Pb/ 2arPb for the Irving and Salida samples and the least radio- genie samples from the Hondo Group and Uncompahgre For- mation. Also shown in Fig. 7 is the Stacey-Kramers (1975) model growth curve and a line corresponding to Pb growth between 1.7 and 0.0 Ga and K (232Th/238U) = 1 and 4 (both from the least radiogenic K-feldspar from 1.7 Ga plutonic rocks taken from the region (Aleinikoff et al., 1993), approximately corresponding to a single stage ,LL (238U/204Pb) = 8.1). For comparison, also shown are the regression line for lead isotope data for igneous rocks from the Early Protero- zoic Mojave Pb Province to the west and a field representing available data for Archean rocks from the Wyoming Province to the north (Wooden and Miller, 1990; Wooden and Dewitt, 1991). Irving and Salida samples generally fall along or slightly above the 1.7 Ga reference line. Irving Formation samples all correspond to K significantly less than 4, whereas the two Salida samples that are plotted have K equal or greater than 4. The least radiogenic Hondo and Uncompahgre samples overlap with the volcanogenic samples but differ in vir- tually all falling above the 1.7 Ga reference line. Most of these samples also scatter about the K = 4 line. The more radiogenic samples are discussed separately below.

Aleinikoff et al. ( 1993) have identified two possible lead isotope provinces in this region on the basis the lead isotopic composition of ca. 1.7 Ga and 1.4 Ga plutonic rocks (see Fig. 1). These provinces may reflect terrane boundaries or merely differing sources for the plutonic rocks. The Northern Colorado

Province is characterized by slightly higher *@‘Pblw4Pb ratio for a given 206Pb/2”‘Pb ratio and K values that are scattered but commonly above 4. In contrast, the Southern Colorado/ Northern New Mexico Province is characterized by lower *“Pbl’04Pb and K values almost exclusively less than 4 and commonly less than 2. This southern province is comparable to the lead isotopic characteristics of the Central Arizona lead isotope crustal province, defined by Wooden and coworkers (Wooden et al., 1988; Wooden and Dewitt, 1991). In southeastern Arizona, an additional high K province is also recognized (Wooden et al., 1988; Wooden and Dewitt, 1991). An additional Early Proterozoic lead isotope province, the Mojave Province (Wooden and Miller, 1990; Wooden and Dewitt, 1991) is recognized in eastern California and likely extends north into Nevada and Utah. This province is characterized by relatively high *“Pb/*04Pb (see Fig. 7), K values generally >4.0 and relatively old Nd-model ages ( TDM = 2.0-2.3) in- dicative of a substantial Archean component (Bennett and DePaolo, 1987).

PROVENANCE AND SEDIMENTARY HISTORY

Provenance

Rare earth element patterns are generally accepted as among the most reliable indicators of sediment provenance (Taylor and

- 57A - 67 - 61

117A 130

- 132

1 ““““““” Lace PrNd SmEuGdTbDyHoEr Yb

FIG. 4. Chondrite-normalized REE diagrams for metasedimentary rocks of the Irving Formation and Salida area. Most samples are characterized by relatively low REE abundances (La, = 30-100) fairly flat HKEE distributions and no to slight negative Eu-anomalies.


Rincunada Fm. - m

- R4-1 - R4-2 - R4-3 - R4-4

f!inconada Fm. - 174

100

10

1

I 1 1 I I I I I I I 1 I I I-

- Rl-1 - WI - R2-2 - m-3

I I I I I I I I I I I I I I

Lace PrNd SmEuGdTbDyHoEr Yb

- PS-1 - PM - Psi-5 1

Lace PrNd SmEuGdTbDyHoEr

FIG. 5. Chondrite-normalized REE diagrams for metasedimentary rocks from the Hondo Group. Most samples are of uniform composition and similar to typical post-Archean shales. One exception is sample R2-4, which lacks an Eu- anomaly. Samples PS-5 from the Pilar Formation is exceptional in appearing to have a rotated REE pattern compared to the other samples of the formation. Samples PS-4 and PL-4 (Piedra Lumbre Formation) may have a similar, but less well developed character.

McLennan, 1985 ). The presence of only slight negative Eu- alkaline) and relatively Wenriched felsic volcanics; plutonic

anomalies for most sedimentary rocks of the volcanogenic Irving rocks are mostly LREE enriched and a wide range in composi- Formation and Salida area (Fig. 4) indicates that intracrustal tion (e.g., Condie, 1980; Condie and Nuter, 1981; Robertson and igneous processes such as partial melting or fractional crystalli- Condie, 1989). Variable mixtures of such a provenance would zation involving plagioclase separation probably did not affect tend to lead to sedimentary REE patterns with variable La/Yb the provenance rocks to a large degree. For the Irving Formation, ratios that correlate with other chemical characteristics sensitive the REE patterns vary in abundances but are rather uniform in to composition. Figure 8 plots Cr/Th vs. La+Jm and the neg- terms of the overall shape. Most of the volcanic sequences in ative correlation, that is expected for variable mixtures of mafic this area are of bimodal character with mafic volcanics ranging and felsic compositions, is observed. Measured Th/U ratios (and from LREEdepleted (tholeiitic ) through LREE-enriched (calc- implied K) are mostly less than 3.0 and are accompanied by fairly


8 .z 2 100 0 6

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11 I 11 11 1 I ” 11 “I ’ Lace PrNd SmEuGd TbDyHoEr Yb

FIG. 6. Chondrite-normalized REE diagram for metasedimentary rocks from the Uncompahgre Formation. Most samples have REE patterns similar to post-Archean shales, except sample S5, which has a rotated pattern compared to other samples in the formation.

low Th and U abundances (Fig. 9). This is likely a provenance Stable shelf shales of the Hondo Group and Uncompahgre feature and reflects the geochemically depleted nature of the Formation are intimately associated with orthoquartzite units, mantle sources of the volcanic arcs (McL.ennan et al., 1993). suggesting that plutonic rocks were an important component

44

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FIG. 8. Plot of Cr/Th vs. Lau/YhN for metapelitic rocks from the Irving Formation. Note the negative correlation, consistent with mixing of mafic and felsic compositions.

This is also consistent with the low Th/U source suggested for the Southern Colorado/Northern New Mexico lead isotope crustal province (Aleinikoff et al., 1993).

0 Hondo Group ?? Uncompahgre Fm

0 Irving Fm 0 Salida Area

17 18 19 206pt-,y204pt,

21 22 23

FIG. 7. Plots of 2oBw/2MPb and mPt@‘?h vs. WpPh for samples from the Irving Group and Sahda area and for relatively unradiogenic samples from the Hondo Group and Uncompahgre Formation. Shown for reference are Stacey-Kramer model Ph evolution curve (major ticks represent 500 m.y. intervals), a line defining lead isotopic evolution between 1.7 and 0.0 Ga taken from the least radiogenic K-feldspar from plutonic rocks in this region (Aleinikoff et al., 1993) and approximately equivalent to p, = 8.1, and K = 1 and 4 from the least radiogenic K-feldspar. Also shown is the regression for igneous rocks from the Early Proterozoic Mojave Ph Province in California (Wooden and Miller, 1990; Wooden and Dewitt, 1991) and the field for igneous rocks from the Amhean Wyoming Province (Wooden and Meuller, 1988).


7.0

6.0

0.0 1 3 10 30

Th @pm)

FIG. 9. Plot of measured Th/U ratios vs. Th abundances for metasedimentary rocks from southwestern USA. The volcanogenic metasediments plot at low ‘IWU ratio. less than 3.0 and relatively low Th abundances. The low TMJ ratio is consistent with a volcanic provenance that was itself derived from a depleted mantle source with low Th/U. The stable shelf sediments show highly variable Th/U ratios, commonly above 4.0. The overall trend for most samples is consistent with weathering of the source rocks having affected the compositions. Some of the samples on the main trend plot towards low Th/U ratio, similar of the volcanogenic sediments, suggesting that much of the ultimate igneous sources may have had low TMJ. The labeled samples with very low ‘IWIJ also have other unusual geochemical characteristics, described in the text. The low Th/U primarily results from addition of U to these samples, likely during early diagenesis.

to the provenance (Folk, 1972). Apart from the few anomalous samples identified above, the REE patterns from the Hondo Group and Uncompahgre Formation are similar to typical post-Archean shales (Taylor and McLennan, 1985). The negative Eu-anomaly indicates that intracrustal differentiation, involving separation of plagioclase, had thoroughly affected the provenance rocks. The ratio Th/Sc is perhaps more sensitive to the average bulk composition of the provenance than are REE patterns (McLennan and Hemming, 1992). Th/ SC ratios of the Hondo and Uncompahgre are somewhat variable, ranging from 0.45-1.4 and averaging 0.84, which is only slightly lower than the upper crustal and average shale value of 1 .O (Taylor and McLennan, 1985 ) .

Thorium-uranium elemental systematics of these rocks are complex. Several of the Hondo and Uncompahgre samples have exceptionally low Th/U ratio (and high U abundances ) and this is likely related to other sedimentological processes (see below). Otherwise, Th/U ratios are highly variable but mostly above the upper crustal value of 3.8 (Taylor and McLennan, 1985) and accordingly are consistent with a weathering history removing U (see below). An interesting feature is that there is a trend of Th/U vs. Th abundances (Fig. 9) that essentially intersects the cluster of data from the volcanogenic sediments. This is consistent with at least part of the ultimate provenance of the Hondo Group sediments having had low Th/U ratios, more similar to the volcanic arcs than to typical upper continental crust (see below for further discussion).

Insight into the mean age of provenance, provided by neodymium isotopic data, can be instrumental in understanding

the nature of sediment sources. The depleted mantle model age ( TDM) of volcanogenic samples from the Irving Formation and Salida Area are uniform, mostly in the range of 1.83- 1.92 Ga, only slightly older than the age of crust formation

in this region ( 1.7-1.8 Ga). Of great interest is the fact that the stratigraphically younger Hondo Group and Uncompahgre Formation have model ages that completely overlap the volcanogenic sedimentary rocks, in the range of 1.7-2.1 Ga. Thus, even though the chemical composition of the younger shales are significantly different and suggest fundamentally different crustal processes (e.g., intracrustal differentiation to generate Eu-anomalies ) , the mean age of the provenance is either similar or only slightly older.

Figure 10 plots cNd (calculated at the time of sedimentation, 1.7 Ga) against Th/Sc ratio. Shown for reference is the location of typical upper crust, intermediate island arc compositions, and MORB at 1.7 Ga. The significant difference in Th/Sc ratio between volcanogenic and stable shelf shales is clear, as is the similarity in neodymium isotopic composition. tNd for volcanogenic sediments is somewhat lower than average (model) upper mantle at this time, but mostly in the range of similarly aged mantle-derived volcanic rocks in this region (Nelson and DePaolo, 1984). In detail, some of the stable shelf shales have somewhat lower cNd( 1.7 Ga) values than the volcanogenic sediments, as low as -0.2. It is likely that these lower values reflect some component of older crust. This could be either substantial amounts of early Proterozoic crust that had incorporated Archean components, such as that found further to the west and northwest in California, Nevada, and Utah where Nd model ages of Early Proterozoic volcanic

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FIG. 10. Plot of e&1.7 Ga) vs. Th/Sc ratio for Early Proterozoic metasedimentary rocks from southwestern U.S.A. Shown for reference are where typical upper crust, arc andesite and MORB would plot at 1.7 Ga. Volcanogenic sediments from the Irving Formation and Salida area mostly plot at low but somewhat variable Th/Sc ratio, suggesting chemically unfractionated sources, and eNd slightly lower than typical mantle, but within the range of volcanic rocks erupted in this region between about 1.8 and 1.7 Ga (see Fig. 2). The stable shelf sediments of the Hondo Group and Uncompahgre Formation plot at higher ‘HI/SC ratios, suggestive of more differentiated sources. eNd for these samples are mostly similar to the volcanogenic sediments, suggestive of a similar aged provenance, but some samples trend towards lower eNd. probably indicating small (<25%) components of older (Archean) neodymium. This older crust contributing Nd may have been added directly to the sediments or indirectly as assimilated components in Proterozoic igneous rocks. See text for further discussion.


and plutonic rocks reach values in excess of 2.3 Ga (Bennett and DePaolo, 1987; DePaolo et al., 1991), or small amounts of Archean crust (e.g., Wyoming Province) derived directly. Simple mass balance calculations, assuming reasonable Nd contents for arc and upper crustal endmembers (Taylor and McLennan, 1985) permits an average of about lo%, and no more than about 25% in any given sample, of an Archean Nd component.

Although physical mixing of nearby Proterozoic crust and Archean crust, such as the Wyoming Province, can explain the neodymium isotopic characteristics it is important to re- emphasize that such a simple mixing model fails to explain the trace element characteristics. In Fig. 10, no reasonable mixing relationship between the various crustal reservoirs can explain the higher Th/Sc ratios seen in the Hondo Group compared to the Irving Formation. For examining provenance, the REE are likely to be to most reliable indicators. To further illustrate this, in Fig. 11 Eu/Eu* is plotted against LaN/Sm, for samples of the Irving and Rinconada Forma- tions. Mixing relationships between two samples in the Rin- conada Formation with extreme neodymium isotopic compositions are shown. It is clear from such a diagram that variations in REE in the Rinconada Formation cannot be explained by simple physical mixing of a source similar to that for the Irving Formation with a second source (possessing negative Eu-anomalies ) The only possible exception is sample R2-4 which is unique for the Rinconada Formation in not having a negative Eu-anomaly. The sources for the Rinconada Formation (and also samples from the other Hondo Group and Uncompahgre Formation) are required to have undergone additional igneous processing, likely intracrustal differentiation, to fractionate plagioclase (thus providing Eu-anomalies )

The lead isotope data further support the suggestion of in- corporating an Archean component and may also permit iden- tification of terranes in this area that have acted as the provenance. On the basis of lead isotope systematics in basement rocks, several Proterozoic-aged lead isotope provinces have been recognized in this region. In addition, the Archean Wy- oming Province and Proterozoic crust immediately adjacent to it appear to also have distinctive lead isotopic characteristics for this region in having much higher 2a7Pb/2”Pb for a given ‘06Pb/‘04Pb compared to any of the Proterozoic igneous or metasedimentary rocks to the south (Wooden and Meuller, 1988; Wooden and Miller, 1990). On a plot of 207Pb/2MPb vs. 206Pb/204Pb (Fig. 7)) samples the Hondo Group (excluding those with rotated REE patterns) generally fall above a line defining lead growth between 1.7 and 0.0 Ga from an initial composition equal to the least radiogenic Early Proterozoic igneous K-feldspar reported for the region (Aleinikoff, 1993). The higher 207Pb/204Pb characteristics are generally consistent with an Archean component. This is further sub- stantiated in Fig. 12 where the difference in ‘“‘Pb/204Pb between samples from the Rinconada Formation and that pre- dicted by the 1.7 Ga reference line (A 207Pb/‘MPb) is plotted against cNd( 1.7 Ga). Although the data are highly scattered and uncertainties are large, they are consistent with mixing between the Proterozoic Pb provinces in Colorado and New Mexico ( Aleinikoff et al., 1993 ) and a small component of the Archean Wyoming Province to the north. The data are

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FE. 11. Plot of Eu/Eu* vs. La&m, for metasedimentary rocks from southwestern U.S.A. Shown for reference is a mixing line between two of the Hondo Group samples with extreme neodymium isotopic compositions. This diagram illustrates that simple mixing between a source similar to the provenance of the Irving Fm and some second component cannot explain the geochemical relationships and an episode of igneous differentiation in the provenance terrane prior to deposition of the Hondo Group is required.

also consistent with mixing between Pb provinces in Colo- rado/New Mexico and the Mojave Province to the west. Al- though the Mojave Province cannot be excluded as an important component in the Hondo Group, it is not considered to be a dominant endmember for two reasons: ( 1) it is characterized by K between 4 and 15 whereas samples from the Hondo Group have K values mostly below 6.0 (and even these values are likely elevated by weathering processes) and values do not correlate with A207Pb/204Pb or eNd( 1.7 Ga) (see below); and (2) the Pb/Nd isotope data would require abrupt and erratic changes in provenance from dominantly Colorado/ New Mexico Province sources to dominantly Mojave Prov- ince that have no stratigraphic trends; this is considered unlikely.

The Th/U systematics of sedimentary rocks are complex due to the general increase in the ratio during weathering and sediment recycling. However, the relationships between 208Pb/2”Pb and 206Pb/204Pb and the Th/U elemental systematics for the Hondo Group may provide some constraints on the provenance components derived from Proterozoic ter- rains. In Fig. 7 it can be seen that the Hondo Group samples (without rotated REE patterns) scatter about the K = 4 line. However, a number of the samples have lower values with some samples approaching K = 3. Although implied K and measured ThlU ratios are not identical, as is often the case, there is a general correlation. Other samples, not measured for Pb isotopic composition, also have Th/U significantly less than 3. Although the high Th/U ratios are clearly related at least in part to a secondary processes, the low Th/U ratios of some samples and the trend of Th/U versus Th abundances noted above (see Fig. 9) suggests that components of the provenance were characterized by low Th/U ratios (or K).

Both the S. Colorado/N. New Mexico lead isotope province ( Aleinikoff et al., 1993) and the Central Arizona lead isotope province (Wooden and Dewitt, 1991) have low K characteristics and thus at least part of the Proterozoic components for the Hondo Group may have been derived locally. In contrast,


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0 -

CO-NM Prov.

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FIG. 12. Plot of Az”Pb/204Pb vs. eNd( 1.7 Ga) for metasedimentary rocks from the Hondo Group. AZ07Pb/204Pb is the difference between the measured unPb/2”Pb and that estimated from the 1.7-0.0 Ga evolution line in Fig. 7. Although uncertainties are large and the data are highly scattered, there appears to be a relationship for a number of the samples consistent with a component of Archean crust with low eNd( 1.7 Ga) and high *07Pb/2@‘Pb such as the Wyoming Province to the north.

the Mojave Province to the west is characterized by high K, mostly in the range 4- 15, thus limiting the importance of this terrane as a provenance component.

In summary, the volcanogenic sediments appear to be derived entirely from fairly well mixed contemporaneous aged volcanic rocks, with felsic components dominating. The source of stable shelf shales of the Hondo Group and Uncom- pahgre Formation was dominantly from a provenance that had experienced intracrustal differentiation (e.g., intracrustal partial melting) to generate common negative Eu-anomalies. Neodymium model ages are only slightly in excess of the associated volcanics and lead isotopic characteristics are similar to or slightly enriched in 207Pb/2@‘Pb compared to the immediately associated metavolcanic terranes. Contributions from felsic volcanics cannot be specifically evaluated in shales except to note that if significant amounts were present, they must have had negative Eu-anomalies. The role of older Archean recycled crust (likely the Wyoming Province) is minimal (about 10% on average) and in no sample analysed greater than about 25%. On the basis of Th/U elemental and lead isotopic systematics, a low Th/U (K) provenance component, similar to contemporaneous-aged Proterozoic terranes, was likely.

Weathering History

The weathering history of ancient sedimentary rocks may be evaluated in part by examining relationships among alkali and alkaline earth major elements (Nesbitt and Young, 1982, 1984). Data are plotted on a ternary diagram of molecular proportions A1203 - (CaO * + Na20) - K20, where CaO* is the CaO in the silicate fraction only, in Fig. 13. Plotted for reference are average upper crust and shale (Taylor and McLennan, 1985) and several idealized mineral compositions (Nesbitt and Young, 1984). Using molecular proportions, a

Chemical Index of Alteration can be defined, such that CIA = ( A1203/( A1203 + CaO* + Na,O + K,O))* 100 (Nesbitt and Young, 1982). These values are useful indexes of the degree of alteration, and are given in Tables 1, 2, and 3.

Volcanogenic samples have comparatively low CIA values and plot along a linear array between the feldspar tie line and

A Kaolinite, Gibbsite, Chlorite A

Plagioclase

7 Upper@ Crust

CN FIG. 13. Ternary plot of molecular proportions A1203(A) - CaO*

+ Na,O(CN) - K,O(K) for Early Proterozoic metasedimentary rocks from southwestern U.S.A. The volcanogenic metasediments plot on a trend between the feldspar join and average shale, suggesting a mild weathering history of the soume rocks. In contrast, the stable shelf metasediments plot between average shale and illite, suggesting a relatively intense weathering history for the provenance. Sample 34.4 is anomalous in being relatively K-rich and this is consistent with chemical alteration other than, or in addition to, weathering affecting the composition.


average shale on Fig. 13. This suggests a relatively modest weathering history affecting the source of these samples. Sample 34.4 is an exception and plots along the Al,O,-K,O boundary between idealized muscovite and biotite compositions. Such apparent enrichments in K are consistent with hy- drothermal or other alteration, apart from weathering, having affected the composition of this sample (Nesbitt and Young, 1989 ) Given the tectonic environment (see below ) and rapid sedimentation typical of such turbidites, the lack of weathering history probably does not constrain the climatic conditions. For example, recent erosional products from volcanic islands in equitorial regions of the southwest Pacific Ocean have CIA values that average about 55 (McLennan, 1993). Accordingly, the samples from the Irving Formation could well have been deposited in climatic environments that would lead to severe weathering effects if sedimentation rates had been lower.

In contrast, samples from the Hondo Group and Uncom- pahgre Formation have relatively high CIA values and mostly plot between average shale and illite in Fig. 13, consistent with a rather severe weathering history having affected the sources of these sedimentary rocks. Even higher CIA values were noted by Eriksson and Soegaard ( 1985) and Soegaard and Eriksson ( 1989) for samples from the Hondo Group that were originally selected for metamorphic petrology studies. These workers also noted that CIA values were higher for the Rinconada Formation than for the Pilar and Piedra Lumbre Formations. Such a difference, although less dramatic, is also noted in these data.

Differences in trace element distributions also exist between the volcanogenic and stable shelf sediments that is likely related to the differing weathering histories. Apart from sample 34.4, Rb/Sr ratios in the volcanogenic sediments are below 1.0 (average Rb/Sr = 0.34, s.d. = 0.32). whereas in the stable shelf sediments, Rb/Sr is above 1 .O (average Rb/ Sr = 2.6, s.d. = 1.7) in all but three samples. Th/U ratios generally increase with increasing degrees of weathering due to the oxidation and loss of uranium. In Fig. 9, Th/U is plotted against Th abundances. For the volcanogenic sediments, Th/ U is low, typically below 3.0 which is below the typical upper crustal value of 3.8. The significance of these low values was discussed above. In contrast, the stable shelf sediments have variable but generally higher Th/U ratios and Th abundances. The high Th/U, commonly above 4.0, is thought to be related to the weathering history.

Origin of Rotated REE Patterns

The origin of the rotated REE patterns is of great interest. There is growing evidence that neodymium isotopes and Sm/ Nd ratios in some cases may be partially or completely reset during the sedimentary processes of weathering (e.g., Mc- Daniel et al., 1994) and diagenesis (e.g., Ohr et al., 1991; Bock et al., 1994) where a great deal of mineralogical change takes place. Other studies have shown REE transport of up to at least several meters primarily associated with formation/ dissolution of REE-rich phases such as apatite, monazite, rhabdophane, and so forth (e.g., Banfield and Eggleton, 1989; Milodowski and Zalasiewicz, 1991), or associated with black shale formation ( Leventhal, 1990 ) .

For samples studied here, neodymium isotopes are not particularly helpful in constraining the timing or nature of the process that caused the rotated REE patterns. In most elastic sedimentary rocks, the provenance age (i.e., Nd model age) greatly exceeds the age of sedimentation and resetting of neodymium isotopes near the time of sedimentation results in relatively low and uniform ‘43Nd/‘44Nd at the time of sedimentation. However, for these samples, there is very little difference between these two ages with TDM for unaffected stable shelf samples in the range of 2.1- 1.7 Ga, which compares to a likely sedimentation age for the Hondo Group of about 1.70 Ga. In addition, the provenance of the unaffected stable shelf sediments has small but variable mixtures of recycled Archean crust resulting in variable ENd at the time of sedimentation and thus further obscuring any subtle changes associated with weathering or diagenesis. Consequently, it is not possible to resolve fractionation of REE during extraction from the mantle from any subsequent fractionation during sedimentary processes. It is even difficult to exclude metamorphic effects, on the basis of neodymium isotopes alone, since metamorphism may have occurred between 1.65- 1.25 Ga.

Some trace element relations do support a sedimentary/ diagenetic origin for the disturbance of these REE patterns and lead isotopic data are consistent with such an interpretation. The major differences between the samples with rotated REE patterns and the others are that the anomalous samples: ( 1) are all part of the relatively infrequent high-Si group of samples (Fig. 3), have lower absolute abundances of a number of trace elements and are the most carbonaceous; and (2) have enriched MO, V, and U, leading to low Cr/V and Th/U ratios (and K values) and high U/Pb ratios resulting in elevated 206Pb1204Pb. These latter features can be seen in Figs. 14 and 15. In Fig. 14, Cr/V, MO, and Th/U are plotted against La/Yb ratio, the best (but not perfect) index of REE fractionation. In all cases, the anomalous nature of samples PS- 4, PS-5, PL-4, and S-5 can be seen. Figure 15 plots the lead isotopic compositions for these samples along with other, less radiogenic, samples from the same formations (a number of these data are also plotted in Fig. 7). Samples with rotated REE patterns are notable for their radiogenic character and low to very low K values. Not all samples with low K have obviously rotated REE patterns, however, it seems likely that the process responsible for disturbing Th/U (likely a gain in U) and REE patterns is the same.

An important feature that ties V, MO, and U together is that all are influenced by redox conditions during sedimentary processes with more oxidized species generally being more sol- uble (e.g., Garrels and Christ, 1965; Brookins, 1988). This leads to strong enrichments in highly reduced sediments, such as many black shales (e.g., Wedepohl, 1971; Coveney et al., 1991) . The correlation of these elements with REE patterns suggests a similar control and that sedimentation processes similar to modem black shales were operative during deposition of the Piedra Lumbre, Pilar, and Uncompahgre For- mations.

Lead isotopic data are generally consistent with a major episode of U enrichment near the time of sedimentation although they do not constrain the age in any precise manner. In Fig. 15, it can be seen that the samples with rotated REE


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FIG. 14. Plots of Cr/V, MO and Th/U vs. LaNNbN ratios for Early Proterozoic stable shelf sedimentary rocks from southwestern U.S.A. The labeled samples are those that appear to have rotated REE patterns with LREE depletion and HREE enrichment. Note that these samples also have anomalously low Cr/V and ‘HI/U, thought to be related to elevated V and U abundances, and high MO abundances.

patterns and high ‘06Pb/‘“Pb are essentially collinear with age of 1.43 2 0.25 Ga. The substantial scatter in the data other samples of the same formations, but define an evolution (M.S.W.D. = 59) is not unexpected, given the variable initial line that is more shallow than the 1.7 and 0.0 Ga reference lead isotopic compositions and U/Pb ratios expected for sam- line. A regression of the six samples with distinctly low K ples that contain between 0% and 25% Archean component values (and mostly having rotated REE patterns) leads to an (based on neodymium isotope data) and that may have suf-


46

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40

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17 16 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

FIG. 15. Plots of *08Pb~MPb and 2’r’Pb/ZWPb vs. 206Pb/2MPb for Pilar, Piedra Lumbre and Uncompahgre Formation samples. Shown for reference are Stacey-Kramer model Pb evolution curve (major ticks represent 500 m.y. intervals), a line defining Pb isotopic evolution between 1.7 and 0.0 Ga taken from the least radiogenic K-feldspar from plutonic rocks in this region (Aleinikoff et al., 1993) and approximately equivalent to p, = 8.1, and K = 1 and 4 from the least radiogenic K-feldspar. Samples with highest *06Pbpo4Pb for the most part are those with rotated REE patterns, suggesting that U-enrichment associated with REE changes occurred close to the time of sedimentation. Note from the *08Pb/2WPb vs. ‘“*Pb/L”Pb diagram that these samples also have very low K values suggesting that U addition is an important process.

fered variable degrees of U addition sometime in their history. The calculated Pb-Pb age is nearly within error of the sedimentation age ( 1.70 2 0.01 Ga) and the apparently shallower slope could be the result of variable initial lead isotopic compositions for the different samples. This is considered unlikely because the most radiogenic samples would require initial 207Pb/2WPb ratios that are lower than those of any of the HondoKJncomphgre samples or for that matter the Irving Formation samples. On balance, the Pb isotopic data would appear to suggest that a latter processes has affected these samples.

One possiiblity is that there has been some partial or complete resetting of lead isotopes during later metamorphism. On the other hand, the correlation of U addition (and thus changes in Th/U and U/Pb ratios) with MO and V enrichments is best related to sedimentary/diagenetic processes. Thus, the lead isotope data ate consistent with changes in U/PI, and Th/U ratios near the time of sedimentation, but the lead isotopes may well have been partially reset during later metamorphism between 1.65 and 1.25 Ga. The possibility of resetting of lead isotopes

during later metamorphism does not seriously undermine the interpretations regarding provenance obtained from the lead isotopic composition of other samples, as described above. Note from Fig. 15 that the change in slope from the 1.7-0.0 Ga reference line is quite minor and for less radiogenic samples ( *06Pb/ ?% = 18 to 24) lead isotopic compositions are insensitive to such processes. It is only at extreme values of 206Pb/204Pb that any evidence for resetting is apparent.

A second possibility is that the 1.43 Ga linear array represents a mixing line. In this case, one component could be similar to the unaffected stable shelf sediments (with *07Pb/ ‘04Pb higher than the 1.7 Ga reference line). The second component would have to have high 206Pb/2”Pb but relatively low 207Pb/204Pb compared to Proterozoic rocks in this region. Since these features generally correlate with U, V, and MO abundances it is likely that such a component would represent a marine or diagenetic fluid. A final possibility is that the interpretation of geological relationships is incorrect and the age of sedimentation is significantly younger than 1,700 Ma.


10

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. - s-5

1 Lace Nd SmEuGd Dy Er Yb

FIG. 16. Chondrite-normalized REE diagram comparing three of the samples with rotated REE patterns with the average (HA) and range (field defined by dashed lines) of other samples from the Hondo Group. Samples PS-4 and PL-4 can be explained by increasingly progressive depletion of LREE from a composition equivalent to the Hondo Group average (HA). For this mechanism to operate in sample S5, a parent with about 1.2 times the HREE abundance of HA is required to avoid having the REE patterns cross over at about Er and thus present mass balance problems (see text). Such a composition is within the range of the Hondo Group. Also shown is the REE pattern for removed material if the material constitutes 0.1% (for PS- 4 and S5) or 0.05% (for PL-4) of the original rock. The mineral monazite, a common diagenetic phase in sedimentary rocks, commonly has a pattern similar to these.

The exact nature of the process causing the rotated REE patterns is not clear but some insight is gained by comparing the REE patterns of the unaffected samples to the samples with rotated REE patterns (Fig. 16). One possibility is that the change in patterns is due only to progressively increasing removal of LREEs. Although the rotated REE patterns appear to cross the patterns of other samples within a given formation (see Figs. 5, 6) suggesting addition of HREEs, when the whole range of Hondo Group REE patterns is considered, such a process is permitted. In this case, the removed REEs

would be LREE enriched with abundances determined by the mass of the removed material. Monazite is a common diagenetic phase in fine grained sedimentary rocks and such a mineral has appropriate REE characteristics such that removal of 0.05-O. 1% would readily explain the REE patterns. Leaching of REEs by fluids is an alternative mechanism but cannot be evaluated except to note that the net effect is to preferentially remove LREEs.

A second possibility is that the rotated and normal REE patterns do indeed cross over. This pattern of behavior has been observed in samples of true black shales (Leventhal, 1990) and is much more difficult to explain. Mass balance calculations would suggest that simultaneous depletion of LREEs and enrichment of HREEs requires addition or loss of a significant mass. In order to constrain the nature of such a component, various mixing calculations were performed. The parent composition was assumed to be the average for the unaffected samples of the Hondo Group. A set of calculations was performed for each of the samples with rotated REE patterns with the amount of the second component (termed Com- ponent A) varied in iterative steps of 5%. The final mass of Component A was determined for each sample by minimizing the differences in the calculated REE pattern for each of the samples. Figure 17 displays the apparent REE patterns for Component A calculated for each of the samples and normalized to PAAS (McLennan, 1989). This component would have a LREE-depleted pattern compared to shales with no shale normalized Eu anomaly and possibly a negative Ce anomaly. In general shape it is similar to modem seawater, but with lesser Ce depletion. The amount of Component A required is substantial and variable, between 50 and 85%. In Table 5, the implied chemical composition of Component A is shown for each of the calculations. Although the abundances are highly variable, there are some systematics. Alu- minum varies considerably and sympathetically with K and a number of other trace elements (Cr, SC, Rb, Pb, Th, Sn) The A1203/K20 ratios are comparable to that of the clay mineral illite. Thus, the discrepency in these elements could be due either to variability in the parent compositions or to addition

I I I, I I I I I I1 I, I

Hondo Avge.,

cn 1.00 :

2

E

B

&lo : a : 75% Of PL-4 :

- 5o%Of PL-5 . - 80%Of s-5 1, 11 I I,, 1 I I I I I

La Ce Nd SmEuGd Dy Er Yb

FIG. 17. PAAS-normalized REE diagram showing the implied pattern for Component A assuming variable mixtures between average Hondo Group and Component A to produce various samples with rotated RJ3E patterns such that the difference in REE patterns is min- imized. Note that the patterns are all strongly depleted in LREE compared to average post-Archean shales, have GdN/YbN < 1 .O, and that there is some suggestion of a negative Ce anomaly. The general shape of the patterns is not dissimilar to modem seawater (although modem seawater has a strong negative Ce anomaly) and a number of continental groundwaters.

Geochemistry of Early Proterozoic sedimentary rocks 117.

Table 5. Estimates of composition of Component A for mixtures between Hondo Group average and various samples.

SiO,

Honda Group Average

66.9

_____________________._ Component ‘A’ as_______________________ 50% of 80% of 80% of 75% of 75% of

PL-5 s-5 PS-4 PL-4 PS-5

69.2 68.9 80.8 81.7 89.7 Ti&

Z@

z! Na20 K20 p2os L.O.I. Total KzOlNa20 A12o3/Ne3 -4l2qJK20

La Ce Nd Sm EU Cd

2 yb h.ObN hN/!hN GdN/YbN Eu/Eu*

Y

CK V SC G/v La/SC

Rb Ba Pb

&Rb Rb/Sr Ba/Sr

Th

:

I:, SIl MO

&if TWJ

0.78 17.4

5.65 0.06 1.33 0.25 0.64 3.61 0.09 3.20

99.9 5.64

27.2 4.82

38.8 82.7 39.1

7.49 1.50 6.53 6.40 3.80 3.67 7.14 3.26 1.44 0.66

35

;: 14.0

0.78 2.77

152 561

21 66

197 2.30 8.50

12.0 3.02

195 6.63

19.9 7.5 1 .o 2.5

29.4 3.97 3.64 3.23 3.21 0.86 0.43

0.32 18.4

1.47 0.03 0.69 0.11 1.56 3.79 0.01 3.34

98.9 2.43

11.8 4.85

0.43 8.27 1.73 0.03 3.72 0.16

2.45 0.30

12.8 98.8

0.32 7.70 0.35 0.00 0.41 0.26 0.69 1.94 0.14 5.33

97.9 2.81

11.2 3.97

0.39 5.97

0.15 0.99

j.38

0.17 0.48 1.88 1.96 0.04 7.47

100.1 1.04 3.18 3.05

0.45 0.22

i.13 0.11 5.65

98.4

0.88

15.2 5.50 5.39 14.9 9.78 25.0 7.27 7.12 26.8 14.0

8.35 5.33 4.14 11.1 9.74 1.91 1.61 1.33 2.76 2.27 0.35 0.44 0.33 0.70 0.20 1.83 2.58 2.27 3.65 2.40 2.73 4.84 3.91 4.67 2.58 2.28 3.97 3.40 3.01 1.77 2.69 4.61 3.67 3.39 1.79 3.82 0.81 0.99 2.97 3.69 5.01 2.15 2.55 3.40 2.71 0.55 0.45 0.50 0.87 1.09 0.57 0.66 0.58 0.67 0.26

15.4

45

37.0 28.8 24.5 13.8

2:; 1;: 2:: 9.6 6.6 4.7 0.14 0.09 0.06 0.57 0.82 3.17

18; 1.6

6.11

9.5 480

81 11.0 0.52 1.38

225 909

2: 140

0.78 3.16

4.73 1.30

61 2.2

10.1 12

6.6 4.0

27.7

51 247

4.7 8.6

399 6.38

28.7

5:: 3;: 4.4 5.0

2: 4;; 0.63 0.38 5.23 3.43

6.85 3.81 1.02 10.3 5.19 6.94

174 102 99 6.3 4.2 4.1 7.5 10.0 9.4 3.6 3.9 3.3

46.0 28.5 9.7 1.4 2.0 2.1

27.6 24.3 24.1 0.67 0.73 0.15 0.80 1.41 14.6 0.71 0.58 0.22

9:; 0.21

10.7

9.39

0.19 1.4 0.45

29.0 1.9

NOTES: Elements that are near or below detection limits for a significant number of samples (Ni. Co. Tl. Bi, Cu) or are highly variable in composition (Cu, Zn, Cs) are excluded from the calculations. Negative values, derived from mixing calculations. are not reported

of authigenic illite. If authigenic illite is not part of the added component, Component A would be characterized by a major element composition essentially composed of silica and C with high abundances of the trace elements V, U, MO, and possibly moderate levels of P and Ba.

Leventhal ( 1990) suggested that organic material may have the required REE characteristics to explain the anomalous patterns but there are no independent data to support this. It is unlikely that redox conditions alone are responsible for the solubility and mobility of REE (e.g., Sverjensky, 1984). In- stead, other chemical changes associated with changing redox conditions, such as sulphur activity and speciation, carbonate/ bicarbonate speciation, pH and so forth, also control mineral stability, solubility, and complexing behavior.

TECTONIC DEVELOPMENT

Sedimentologists have long endeavoured to distinguish the tectonic conditions prevalent during deposition of sediments using the petrography of framework grains in sandstones and through combined geochemistry and neodymium isotopes (e.g., McLennan et al., 1990, 1993). The latter is appropriate to this study of highly metamorphosed shales. McLennan et al. ( 1993) described five major provenance types on the basis of geochemistry, the characteristics of which are summarized in Table 6.

The Irving Formation is likely dominated by sources rep- resentative of Young Undifferentiated Arc terranes, consistent with previous interpretations of arc-related environments sug-


Table 6. Geochemical and Nd isotopic characteristics of provenance types.

Terrane Type ENd(l.7Ga)# Eu!Eu* Th& Th/U other GeochemicaJ Features Description

Old Upper Continental Crust (OUC)

Recycled Scdimentaxy Rocks (RSR)

Young Undifferentiated Volcanic Arc (YUA)

Young Differentiated Volcanic Arc (YDA)

Exotic Components

5 -5 - 0.60 - 0.70 = 1.0 > 3.8 (shales)

Evolved major element composition (e.g., high Si/Al, CIA); High LILE abundances; Uniform compositions

5 -5 -0.60-0.70 2 1.0 Evidence of heavy mineral concenuation in sands from trace elements (e.g.. Zr, Hf for zircon, REE for monazite)

>+3 = 1.0 < 1.0 < 3.0 Unevolved major element compositions (e.g., low .%/AI. CIA); Low LILE abundances; Variable compositions

2 +3 = 0.50 - 0.90 Variable Variable Evolved major element composition (e.g., high %/AI, CIA); High LILE abundances; Variable compositions

Old igneoushnetamorphic/scdimentaq terranes affected by intracrustal differentiation. Stable cratons, old foundations of active settings.

Recycled sediientaryhnetascdimentary rocks specifically identified. If not separately identified, part of OUC Young mantle-derived volcanic/ plutonic arc rocksDominates forearcs, component in continental arcs, back arcs.

Young mantle-derived volcanic/ plutonic arc rocks affected by intracrustal differentiation. Similar environments as WA but more mature arcs or more dissection.

Chemical and/or isotopic signature depends on the nature of the component. For example, very high Mg, Cr, Ni. v and cr/v would be distinctive of Gphiolite sources.

#- &Nd VahzS are CalCulated for diffewxa expected for sediments deposited at 1.7 Ga. Table adapted from McLennan et al. (1993).

gested for many of the turbidite deposits of southwestern United States (e.g., Condie, 1986). The most important evidence is near mantle-like eNd ( 1.7 Ga) , the lack of Eu-anomalies, low Th/U ratios, and relatively unevolved major and trace element compositions (e.g., low Th/Sc). Neodymium model ages are within 0.1 Ga of the sedimentation age (and eruption age of associated volcanics) and accordingly there is little evidence for significant contribution of old upper crust. The EN,, at the time of sedimentation (+3 to +4) may be slightly lower than the average mantle at that time ( cNd = +6) or may simply reflect a less depleted mantle source. Slight contamination of arc lavas either from sediment subduction or during ascent through continental crust underlying the arc could give rise to such differences. Consequently, the volcanic sources were at most only slightly contaminated by old crust and little or no debris was provided from an old craton, such as might be expected in many back-arc settings (e.g., McLennan et al., 1990). If the tectonic setting was a continental arc, crustal contamination was only slight.

The tectonic association of the stable shelf sediments from the Hondo Group and Uncompahgre Formation does not fit simply into any of the terrane associations described in Table 6. On the basis of major and trace element geochemistry, these sediments are most similar to Old Upper Continental Crust, with negative Eu-anomalies, high Th/Sc, and high Th/U (for most but not all samples), and evolved major element compositions. However, such an association is excluded by the relatively radiogenic neodymium isotopic composition. Neo- dymium and lead isotopic data indicate an older Archean cratonic component, likely the Wyoming craton to the north, but this is restricted to no more than about 10% on average. For some elements, these shales have rather variable compositions, notably for Th/U ratios (Fig. 9), and along with negative Eu-anomalies, suggest an affinity with Young Differ- entiated Arc provenance. The high cNd( 1.7 Ga) is also consistent with such a terrane. An arc provenance would seem unlikely for quartzite-shale associations which are generally thought to be deposited as multicycled sediments or under tropical conditions with low relief and sedimentation rates

(Suttner et al., 1981), a setting more consistent with stable cratonic settings. Soegaard and Eriksson (1986) have suggested rapid transition from volcanically active to stable shelf environments to explain the occurrence of quartzites in the Hondo Group although the tectonic conditions permitting such a rapid transition are problematical (see below).

Tectonic Significance of First Cycle Quart&e-Shale Succession

The conventional view for the origin of orthoquartzites is that they are derived from a multicycle sedimentary history. Recently, it has been recognized that first cycle quartzites can be formed under a unique combination of climatic and tectonic conditions. The requirements include prolonged transport (i.e., slow sedimentation rate or extended alluvial storage) associated with low relief and unusually severe, tropical weathering (Suttner et al., 198 1). Such a combination of conditions has led to modem first cycle quartz sands in the Ori- noco River (Johnsson et al., 1988) and parts of the Amazon River (Johnsson and Meade, 1990).

A dominantly first cycle origin for both quartzites and pel- ites within the Hondo Group (and likely the Uncompahgre Formation as well) is strongly suggested from the sedimentological arguments of Soegaard and Eriksson (1989) and is consistent with the 2.0- 1.7 Ga Nd-model ages and lead isotopic data reported in this study. Detrital zircons of 1.76- 1.71 Ga age within the Hondo Group (Aleinikoff et al., 1985, 1993), Nd-model ages, and lead isotopic compositions all indicate that the ultimate source of these quartzites and shales was dominated by rocks with ages essentially identical to the nearby mafic metavolcanic sequences and the associated plutonic rocks and that a low Th/U terrane, similar to the S. Colorado/N. New Mexico Pb-isotope Cmstal Province was a major contributor to the provenance. Detailed sedimentological study further suggest that deposition of the Hondo Group and Uncompahgre Formation were in shallow marine environments, essentially at sea level.

Models for the tectonic relationship between the quartzite- shale sequence of the Hondo Group and the mafic/felsic


metavolcanic sequences can be divided into two categories: ( 1) There is an unconformable relationship, suggesting rapid transition from an active arc and/or arc-continent collision to a stable-shelf setting; a rifting episode, marked by the Vadito Group, may be an intervening stage (Soegaard and Eriksson, 1989; Condie, 1992; Bauer, 1993); (2) Assuming that these rocks are equivalent to the Mazatzal Terrane in Arizona, an alternative interpretation is that there is an allochthonous relationship with original development of greenstones and quartzite-shales at some distance from each other and later juxtaposition by tectonic processes (e.g., &ambling et al., 1988; Karlstrom and Bowring, 1988).

Assuming that the Hondo Group is indeed 1.70 -C 0.01 Ga in age, the provenance and tectonic requirements and the geochronological constraints are difficult to reconcile with either model. Mafic metavolcanic sequences most closely associated with the Hondo Group are in the range of 1.76- 1.72 Ga and are generally regarded as being part of a collage of arc terranes accreting from the current northwest to southeast. In Arizona, this would correspond to the Yavapi Orogeny (best estimate at 1.70 Ga) . After a brief period of tectonic quiescence (during which the Hondo Group is thought to have been deposited), erogenic activity resumed between about 1.69- 1.64 Ga, equivalent to the Mazatzal Orogeny in Arizona (best estimate at 1.65 Ga), marked by additional mafic metavolcanic sequences, such as those found in the basement uplifts (Zuni and Manzano Mountains) immediately to the west and southwest of current Hondo Group exposures.

These age constraints permit no more than O-30 m.y. to elapse between the final accretion of arc terranes (end of vol- canism) and deposition of the first cycle quartz-arenites. From our understanding of first cycle quartz-arenite formation, tectonic stability (i.e., subdued topography) would be required. Although colliding arc terranes need not be especially thick, if one assumes that an average of only 15-20 km of erosional unloading is required to produce the appropriate topography, it would require sustained regional denudation rates of between 0.5 and >2.0 mm/yr. Although instantaneous denudation rates can greatly exceed 1.0 mm/yr in regions of extreme topography, average denudation rates over the lifetime of mountain belts are normally much less (e.g., Ahnert, 1984).

On the other hand, any model calling for deposition of the Hondo Group at some distance to the south with later juxtaposition, must explain the following characteristics: ( 1) Evi- dence from both neodymium and lead isotopes for an Early Proterozoic provenance with small but variable amounts of older (Archean) crust. This is exactly the expected provenance for a source from the present-day north and is consistent with present-day facies relationships that also indicate a north- erly source. Although an Archean terrane may well have been situated to the south at the time of Hondo/Uncompahgre deposition, its existence is not established. (2) Evidence from lead isotopes for a major provenance component with a history of low Th/U, equivalent to the S. Colorado/N. New Mexico lead isotope Province and similar to the immediately associated metavolcanic terranes.

There are several possible explanations for these discre- pencies. The first is that the age constraints are not as well understood as presently thought. A second possibility is that

perhaps during the Proterozoic, in the absence of land plants and other complex land dwelling organisms, first cycle quartzite successions could be developed under more tectonically active and/or less severe climatic conditions. In either case, there are numerous quartzites and metaquartzites in central and southern North America, many of which may be of first cycle origin (Soegaard and Eriksson, 1989). A thorough understanding of the age and origin of these successions may provide considerable insight into both the tectonic and climatic conditions of North America at this time.

CRUSTAL EVOLUTION

The Archean-Proterozoic transition is recognized as a fundamental benchmark in the chemical evolution of the continental crust. Archean upper crust is generally thought to be of different composition than post-Archean upper crust and these differences are recorded in the REEs and other geochemistry of sedimentary rocks. This differing composition is thought to be related to two major factors (Taylor and Mc- Lennan, 1985, 1995; McLennan and Taylor, 1991): ( 1) In- tracrustal differentiation processes (partial melting and crystal fractionation) involving plagioclase fractionation are thought to be relatively unimportant during the Archean but to dom- inate the crust during the post-Archean. This leads to the common negative Eu-anomaly in post-Archean sediments from a wide variety of tectonic regimes whereas negative Eu-anomalies are relatively uncommon in Archean sedimentary rocks. (2) The conditions of mantle melting (or the composition of mantle sources) to form Archean crust are thought to differ from the post-Archean. A consequence of this is that Archean igneous rocks commonly have HREE-depletion ( GdN/YbN > 2.0) whereas such REE patterns are relatively rare in post- Archean igneous rocks and this difference is also exhibited in sedimentary rocks.

Regardless of the exact tectonic relationships, the transition from volcanically active tectonic settings with turbidite sediments lacking negative Eu-anomalies (e.g., Irving Formation) to a stable tectonic setting with quartzite-shale associations that have negative Eu-anomalies (Hondo and Uncompahgre Groups), invites comment. At the very least, such a transition is superficially similar to that seen in sedimentary rocks at the Archean/Proterozoic transition (e.g., Gibbs et al., 1986; Con- die and Wronkewicz, 1990).

For the stable shelf sediments of the Hondo Group and Uncompahgre Formation, there is very close similarity with typical post-Archean shales, typified by those from Australia (Nance and Taylor, 1976), in terms of REE characteristics (Fig. 18a). However, it is interesting to note that there is greater variability in these samples than there is in the Aus- tralian shales of a variety of ages and tectonic/sedimentary settings.

The sedimentary rocks associated with mafic metavolcanic sequences both in the Colorado/New Mexico region as well as those in Arizona are plotted in Fig. 18b and are compared to sedimentary rocks from typical Archean greenstone belts. In detail, the REE patterns are clearly different. Although the samples from the Irving Formation do not have Eu-anomalies, on a regional scale, Proterozoic volcanogenic sedimentary rocks commonly do possess substantial negative Eu-anoma-

1174 S. M. McLennan et al

1.40 ““,““,“” . . . . . . . . . . . . . .

: (a) R

Honda Grow /

B ~nCOI?7DahWe Fm. .

1.20 - 7

* 1.00 -

w’ 0

. t I i

.

0.60

0.40

.

0 F'ecos/Go/d Hi//

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Gd,,, I YbN FIG. 18. Plot of Eu/Eu* vs. GdNrYbN for various sedimentary rocks.

(a) Metasedimentary rocks from the Hondo Group and Uncompahgre Formation. Shown for comparison is the field for post-Archean shales from Australia (Nance and Taylor, 1976). Although rather scattered, the Hondo Group and Uncompahgre Formation have REE characteristics similar to typical post-Archean shales. (b) Metasedimentary rocks from the Irving Formation and the Salida area (this study) and for volcanogenic turbid&es from the Dubois greenstone belt, Colo- rado (Condie and Nuter, 1981) and the Adler Group, Arizona (Condie et al., 1992). Note that these samples differ from typical Archean greenstone belt sedimentary rock in far more commonly having negative Eu-anomalies and less commonly possessing HREE depletion. Metavolcanic and associated plutonic rocks from the Pecos and Gold Hill metavolcanic sequences (Condie and McCrink, 1982; Roberston and Condie, 1989) are also plotted. Although samples with HREE depletion (GdN/YbN > 2.0) do exist, they are comparatively scarce and it is unlikely that sediments derived from such terranes would possess HREE depletion.

lies. In contrast, such a feature, while existing in some greenstone belt sedimentary rocks, is fairly rare in the Archean. On the other hand, HREE depletion, marked by GdN/YbN > 2.0 is fairly common in Archean greenstone belts and reflects the common presence of the tonalite-trondhjemite-granodiorite (TTG) suite (and equivalent volcanics) and other HREE-depleted igneous rocks in the Archean. Some Early Proterozoic volcanogenic sedimentary rocks from southwestern U.S.A. display HREE depletion, but less commonly than found in the Archean. Sedimentary rocks have not been analysed for REEs in the underlying mafic metavolcanic sequences in the same

area as the Hondo Group samples (Gold Hill, Moppin, Pecos Complexes ) . However, numerous igneous rocks have been analysed and are also plotted in Fig. 18b. Negative Eu-anomalies are common and HREE-depletion is present but not common; sedimentary rocks derived from such sequences would likely be similar.

From these comparisons, we conclude that REE patterns from the volcanogenic turbidites associated with Early Pro- terozoic metavolcanic sequences in southwestern U.S.A. do not compare favorably with Archean greenstone belt turbidites. In addition, Th/U ratios in the Irving Formation and a number of Early Proterozoic volcanogenic sedimentary rocks from Arizona (Condie et al., 1992) have low Th/U < 3.0, a feature that appears to be rare in Archean greenstone belt turbidites ( McLennan and Taylor, 1991). Accordingly, these data are fully consistent with models suggesting the composition of the Archean upper crust differed from that in the post-Archean (e.g., Taylor and McLennan, 1985; McLennan and Taylor, 199 1) .

Nevertheless, it is likely that Proterozoic crustal development at 2.0-1.7 Ga has a number of similarities with that seen at the Archean-Proterozoic transition. A number of workers have noted a relationship between the accumulation phase of supercontinent cycles and major episodes of cmstal growth (Nance et al., 1986; Hoffman, 1989b; McLennan and Taylor, 1991; Unrug, 1992; Taylor and McLennan, 1995 ) In these models, the major episode of crustal growth during the Late Archean is seen as the first such episode of superconti- nental assembly. Throughout the Archean, new crustal additions were of differing compositions (for example in commonly having HREE depletion) ultimately due to higher heat flow at that time (Taylor and McLennan, 1995). At the time of Late Archean supercontinent assembly, new crustal additions were especially large (such that by about 2.6 Ga there was in excess of 50% of present crustal volume) but there were few preexisting cratonic terranes where intracmstal differentiation had generated the upper crustal Eu anomaly. Ac- cordingly, the intracrustal differentiation of the large amounts of new crust to form stable Late Archean cratons resulted in a net change to upper crustal abundances (notably in gener- ating the ubiquitous negative Eu-anomaly that previously was only a localized feature).

In contrast, during the 2.0- 1.7 Ga Early Proterozoic episode of crustal growth the Earth’s heat flow had diminished and arc lavas were likely more similar to modem arcs with HREE depletion being considerably less common. As shown in this study, it is likely that intracrustal differentiation causing upper crustal negative Eu-anomalies and other geochemical changes to the upper crust must have followed very shortly after the addition of substantial arc terranes during the formation of an Early Proterozoic supercontinent (e.g., Hoff- man, 1989b). However, in this case the amount of new crust that was added was considerably less than during the Late Archean and the amount of already stabilized crust, possessing upper crustal negative Eu-anomalies, was much greater. Thus the net effect was probably to change the upper crustal compositions but to a degree far less than during the Late Archean. Because of the highly cannibalistic nature of the sedimentary record (Veizer and Jansen, 1985; McLennan, 1988; McLennan and Hemming, 1992), such a change is only


noticed on a local scale in the sedimentary record, such as in this study.

Acknowledgments-We thank Shelby Boardman, Dave Gonzalez, Charles Harris, and Kris Soegaard for providing additional samples. We are also grateful to Mike Williams for very helpful discussions, during a visit to Stony Brook, regarding Proterozoic geology of northern New Mexico. Pat Oswald-Scaly assisted with spark source mass spectrometry; Diane McDaniel provided several isotope dilution REE analyses. The manuscript was improved by reviews from Vickie Bennett, Ken Ludwig, Joaquin Ruiz, and Scott Samson. SMM is grateful to Al Hofmann for his hospitality at the Max-Planck-In- stitut fur Chemie in Mainz, during the final stages of preparing this paper. This research was supported by the National Science Foun- dation under grant EAR8816386 and EAR8957784 (to SMM) and EAR8 107523 and EAR85078 11 (to KAE) .

Editorial handling: K. R. Ludwig

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