CRUSTAL GROWTH IN SOUTHERN ARIZONA: U-Pb …

CRUSTAL GROWTH IN SOUTHERN ARIZONA: U-PbGEOCHRONOLOGIC AND Sm-Nd ISOTOPIC EVIDENCE FOR

ADDITION OF THE PALEOPROTEROZOIC COCHISE BLOCK TO THEMAZATZAL PROVINCE

JURGEN EISELE* and CLARK E. ISACHSENDepartment of Geosciences, University of Arizona, Tucson, Arizona, 85721

ABSTRACT. Terranes composed of arc-related igneous and sedimentary rocks wereaccreted onto the continental margin of southwestern North America from 1.9 to 1.6Ga. They formed the Yavapai-Mazatzal province in Arizona, of which the so called‘Pinal Schist’ and associated rocks in southeastern Arizona form an integral part. 148U-Pb analyses on magmatic and detrital zircons were performed on 21 samples toconstrain timing of geologic events in Paleoproterozoic time in the area of southeast-ern Arizona. 25 Sm-Nd whole rock analyses were made to infer sources of sedimentaryand igneous rocks. Our results allow us to delineate a suture zone between the Pinaltectonostratigraphic unit (Pinal block), which consists primarily of basinal metaturbid-itic rocks, and a southeastern allochthonous juvenile volcano-sedimentary domain. Thelocation of the suture is constrained by lithologic contrast across the boundary, anincrease in age of detrital zircons, a marked decrease of �Nd(t), and an interveningsubduction complex (Swift and Force, this issue). We propose the name Cochise blockfor the southeastern part, because it represents a faultbound, allochthonous tectonicunit.

In the Cochise block, we document volcanic activity from about 1630 to 1647 Ma.Quartzitic sedimentary rocks were largely derived from almost contemporaneousvolcanic rocks and show detrital zircon ages from 1630 to 1729 Ma, with the majority(85 percent) between 1630 to 1674 Ma. Quartzites and intermediate to felsic metavol-canic rocks show a tight cluster of �Nd(t)-values (2.9-3.8) close to the depleted mantlemodel and therefore represent juvenile additions to the crust. Post-tectonic metaba-salts with �Nd(1500) � 4.5 are interpreted as being derived from a depleted uppermantle source around 1.5 Ga.

In the Pinal block, detrital zircons yield values from 1678 to 1731 Ma. �Nd(t)-values of metapelites and wackes range from �0.2 to 2.3. Values decrease fromsoutheast to northwest, indicating progressive input of more evolved material towardthe craton. Our data suggest that sedimentation within the Pinal block was proximal tothe continental margin and occurred later than 1.68 Ga and ended before the intrusionof granitoids at 1.65 Ga.

We show that the North American crust grew rapidly and progressively by additionof juvenile but evolved metavolcanic arc rocks and derived quartz-rich metasedimen-tary rocks as documented in the Cochise block. Timespans between depleted mantlemodel age and zircon crystallization on the order of 50 my attest to rapid crustalprocessing during this significant time of crustal growth.

1. introduction

The Paleoproterozoic Yavapai-Mazatzal orogen in the southwestern United Statesis characterized by volcanic arc rocks and associated sediments (Condie, 1982; Bick-ford, 1988; Hoffman, 1989; Bowring and Karlstrom, 1990) of juvenile origin (Patchettand Arndt, 1986). Ages in general become younger from northwest to southeast in theMazatzal-Yavapai province and form the basis of subdivision into tectonostratigraphicunits by different authors (Karlstrom and Bowring, 1988; Anderson, 1989; Condie,1992). Karlstrom and Bowring (1988) defined the Pinal block (Dos Cabezas – Pinal

* Current address: Max-Planck-Institut fur Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz,Germany

[American Journal of Science, Vol. 301, November, 2001, P. 773–797]

773

terrane of Condie, 1992), and we here adopt their terminology and definition oftectonic blocks and terranes. The Pinal block represents the southeasternmost andyoungest tectonostratigraphic unit within the Yavapai-Mazatzal Province comprisingthe basement of southern Arizona (fig. 1). Formerly termed the Pinal Schist by manyworkers, this lithologic unit is the largest (albeit widened by Tertiary taphrogeny) andleast understood area in the 1300 km wide Yavapai-Mazatzal orogenic belt, both interms of age and tectonic setting. Few attempts have been made to provide ageodynamic synthesis of the Pinal block, perhaps because of its scattered outcroppingover a large area and sparse geochronological or isotopic data.

This paper presents results of an integrated study of the Paleoproterozoic rocks ofsoutheastern Arizona that (1) place age constraints on the depositional, magmatic,and tectonic history of the region by single-grain U-Pb analysis of detrital andmagmatic zircons from a diverse suite of samples; (2) constrain sediment and magmasources by a combination of U-Pb zircon data and Sm-Nd analyses of metasedimentaryand igneous rocks. These results allow us to identify two distinct tectonic blocks withinthe former Pinal block separated by an important tectonic boundary, previouslyproposed as a convergent plate margin (Swift, 1987). The earlier recognized lithologiccontrast across this region (Condie and DeMalas, 1985; Copeland and Condie, 1986;

Fig. 1. Map of Paleoproterozoic rocks in southern Arizona. Names of sampled locations are given. Inlayshows crustal provinces of North America and their age of accretion (Williams and others, 1991).

774 J. Eisele & C.E. Isachsen—Crustal growth in southern Arizona: U-Pb geochronologic & Sm-Nd

Anderson, 1989) is shown here to represent two distinctly separate tectonostrati-graphic terranes, defined by marked contrast in lithology, age, and Nd isotopicsignature. In keeping with the terminology of Karlstrom and Bowring (1988), wepropose the name Cochise block for a southeastern terrane composed of intermediateto felsic metavolcanic rocks and associated quartz-rich metasediments as distinguishedfrom the Pinal block, which is composed largely of metaturbiditic rocks. The geochro-nologic work constrains the rapid timing of crust extraction from the mantle andintracrustal differentiation to result in growth of the North American continent.

2. geological settingMajor outcrops of the Yavapai-Mazatzal orogenic belt lie in Arizona’s Transition

Zone southwest of the Colorado Plateau, while the herein studied Pinal block ispreserved as scattered outcrops over a much larger area in the Basin and Rangeprovince of Southern Arizona, extending into Sonora, Mexico (fig. 1). Karlstrom andBowring (1988) have identified eight Proterozoic tectonic blocks in Arizona, separatedby north and northeast trending shear zones with generally steeply plunging linea-tions. They emphasize the separate evolution, at least in part, of each terrane. Earlierauthors (Condie, 1982; Condie 1986; Anderson, 1986) had proposed progressiveaddition of volcanic arcs and associated sediments. The tectonic blocks are interpretedto have been assembled during two main periods of convergent tectonism: 1.74 to 1.70Ga (Yavapai orogeny) and 1.65 to 1.63 Ga (Mazatzal orogeny). All rocks of theYavapai-Mazatzal province are metamorphosed to greenschist or amphibolite facies,but, in general, they are referred to herein by their unmetamorphosed equivalents,where decipherable. The tectonic units of the Yavapai province are made up ofsupracrustal rocks of the 1.79 to 1.70 Ga Yavapai Supergroup containing mafic tointermediate volcanic rocks, volcaniclastic, and sedimentary rocks. These are intrudedby calc-alkaline batholiths of 1.75 to 1.69 Ga age. The Mazatzal province comprisesseveral blocks: the Mazatzal, Sunflower, and Pinal blocks and the Cochise block, whichwe propose here. The Pinal block includes not only the Pinal Schist of Ransome (1903)but also widespread exposures of granitoids, metagraywacke, quartzite, and diversemetavolcanic rocks. Foliated schist and phyllite form a subordinate fraction of theheterogeneous block as a whole. Ages in the Mazatzal block range from 1.74 Ga for agabbro and 1.70 to 1.69 Ga for a suite of volcanic rocks to a 1.65 Ga age for apost-tectonic granite, while the Sunflower block contains a 1.64 to 1.63 Ga graniticintrusive suite (Karlstrom and Bowring, 1988 and references therein).

The northwestern boundary of the Pinal block with the Sunflower block is poorlydefined and obscured by 1.4 Ga granitoids but separates higher metamorphic gradesupracrustal rocks consisting of quartzite and rhyolite to the north (for example,Squaw Peak, Four Peaks) from lower grade metapelitic rocks to the south (forexample, Pinal Mountains, Mineral Mountains, Santan Mountains). Northwesternexposures of the Pinal block contain only minor metavolcanic rocks, and metasedimen-tary rocks range from schists and phyllites to turbiditic graywackes and argillites(Condie and DeMalas, 1985). In the southeastern exposures (for example, DosCabezas Mountains, Pinaleno Mountains), which we call Cochise block, the Paleopro-terozoic rocks consist of intermediate to felsic volcanic rocks, volcaniclastic metasedi-mentary rocks, metaconglomerates, arkosites, quartzites, and subordinate schists alongwith granitoids and metabasalts. The clastic rocks in both regions were derived largelyfrom granites and felsic volcanic rocks (Copeland and Condie, 1986). Three sets ofinternal boundaries to the Pinal block have been proposed on the basis of fieldevidence (fig. 1). Condie and DeMalas (1985) draw a northwest trending boundarybetween a western quartzwacke association and eastern quartzite-arkose-volcanic asso-ciation. This boundary was slightly altered to separate a western and eastern ‘assem-blage’ by Copeland and Condie (1986) and finally named Dos Cabezas and Pinal

775isotopic evidence for addition of the Paleoproterozoic Cochise block to the Mazatzal province

“domains” by Condie (1992). Anderson (1989) identifies a northeast trending bound-ary between the Pinal Schist Terrane and the Dos Cabezas volcanic belt. Swift (1987)and Swift and Force (this issue) propose a suture zone with a possibly ophioliticsequence in the Dragoon and Little Dragoon Mountains. We hope that nomenclaturewe use here extending Karlstrom and Bowring’s (1988) subdivision to the southeastwill supersede the various notions proposed by earlier authors.

Comparatively little work has been done in terms of structural analysis, geochro-nology, and isotope geochemistry in the Paleoproterozoic rocks of southern Arizona,although trace-element geochemistry of igneous rocks and sediments has been welldocumented (Condie, 1992; Condie and DeMalas, 1985; Copeland and Condie, 1986;Condie, Bowling, and Vance, 1985). Few published ages, exist and the geologic historyof the Pinal rocks has been constrained until now by a 1680 to 1700 Ma rhyodaciteintrusive in the Johnny Lyon Hills (Silver, 1963) and the U-Pb zircon age of theapparently post-tectonic Johnny Lyon Granodiorite, reported as 1625 � 10 Ma (Silverand Deutsch, 1963). Both lithologies have been redated, and the ages are presentedbelow.

3. methods

Field relationships were determined by reconnaissance field mapping usingpublished maps as a basis and more detailed mapping in selected areas (Dos CabezasMountains). U-Pb ages of zircons were determined by isotope dilution thermalionization mass spectrometry. U-Pb methods are reported in app. 1.

Sm-Nd isotopic analyses on whole rock samples were performed using establishedlaboratory techniques at the University of Arizona (Patchett and Ruiz, 1987).

4. results

Our results are presented roughly from southeast to northwest. Sample locations,lithology, interpreted U-Pb zircon age, Sm-Nd isotopic data, and map reference aregiven in table 1. Complete U-Pb isotopic data from zircons are listed in app. 2. Figure 2shows a schematic stratigraphy for Paleoproterozoic rocks in the Dos Cabezas Moun-tains. Figures 3 to 5 are concordia diagrams for the U-Pb analyses, �Nd-data of allsamples referred to in the text are plotted in figure 6. All isotopic data are summarizedin figure 7.

4.1 Southeastern volcano-sedimentary domain.—Proterozoic rocks of the Dos Cabezasrange were subdivided by Erickson (1969) into three different sections: the western,eastern, and southern terrain (this notation is unrelated to the “terrane”-concept).The supracrustal rocks of the eastern and western terrains contain significant amountsof volcanic and volcaniclastic rocks in contrast to basinal turbiditic sedimentaryprotoliths in the western part of the Pinal block.

The rocks in the eastern terrain form a �6000 m thick tectonostratigraphicsection of fluvial or shallow marine facies sediments and volcaniclastic rocks, contain-ing cross-bedded and massive quartzites and conglomerates, feldspathic quartzite, andbanded tuffs. Figure 2A is a schematic tectonostratigraphic column of the easternterrain of the Dos Cabezas Mountains showing sample locations. The sediments arederived from felsic volcanic rocks and granites (Condie, Bowling, and Vance, 1985).Subrounded, clear to purple detrital zircons in a quartzite and a conglomerate showedconcordant to nearly concordant ages for the sediment sources ranging from 1630 to1674 Ma (fig. 3A, B). The sedimentary sequence is interbedded with felsic volcanicrocks and intruded by hypabyssal rocks of felsic to intermediate and later maficcomposition. A quartz porphyry containing inclusion free, clear zircons yields anupper intercept age defined by the four most concordant grains of 1634 � 1 Ma(MSWD � 0.33; fig. 3C). Quartzite and quartz porphyry have �Nd(t) � 3.6 and 3.2,


respectively (table 1). These values are close to the depleted mantle model, makingthese rocks juvenile additions to the crust.

The western terrain of the Dos Cabezas Mountains is mainly composed ofvolcaniclastic rocks (Condie, Bowling, and Vance, 1985) and includes a tuffaceous unitthat contains abundant mafic and felsic xenoliths. Stratigraphy (after Erickson andDrewes, 1984) and sample locations within this sequence can be seen in figure 2B.Purple, inclusion-bearing, euhedral zircons were separated from a cobble-sized felsicxenolith. A York fit of the four least discordant analyses gives an age of 1641 � 15 Ma(MSWD � 10.82; fig. 3D). The scatter of the data can be explained by complex Pb-loss,perhaps during entrainment of the xenolith and subsequent metamorphism, inaddition to more recent Pb-loss. The tuffaceous host rock contains clear, euhedral tosubrounded zircons with few inclusions. A York fit of six analyses yields an age of1647 � 2 Ma (fig. 3D) equivalent to that of the xenolith. Nd analyses of the tuff, mafic,and felsic xenoliths show �Nd(t) � 3.6 to 3.8 and a model age of 1.7 Ga. It is thus clearthat the volcanic rocks sample a similar aged, juvenile basement of mafic and felsiccomposition. The Sommer granite gneiss, a late synkinematic rock unit in thewesternmost part of the Dos Cabezas Mountains and two adjacent volcanic units weredated at 1.65 and 1.67 Ga, respectively (Erickson and Bowring, 1990), possiblyindicating a southeastward younging volcanic sequence.

Our ages for Proterozoic rocks in the Dos Cabezas mountains indicatethat formation of protoliths and deposition of sediments derived from them

Fig. 2. Schematic stratigraphy of Paleoproterozoic rocks from the Dos Cabezas Mountains; (A) theeastern terrain (this study) and (B) the western terrain of the Dos Cabezas Mountains (Erickson and Drewes,1984).


took place essentially synchronously over approx 20 my. Taking into considerationthe ages reported by Erickson and Bowring (1990), a timespan of 40 my isindicated.

Fig. 3. Concordia diagrams for samples from the Dos Cabezas Mountains. (A) sandy lens in conglomer-ate, eastern terrain; (B) quartzite, eastern terrain; (C) quartz porphyry, eastern terrain; (D) felsic xenolith(filled ellipses) and metatuff host rock (open ellipses) from the western terrain.


Significant volumes of amphibolite constitute post-tectonic late-stage intrusionsderived from a depleted mantle source with �Nd(1500) of 4.5 and a T(DM) of 1.5 Ga.We consider the model age to be a maximum crystallization age, because the Ndsignature may reflect interaction with 1.63 to 1.67 Ga felsic volcanic and sedimentarywall rocks. The amphibolites have the same metamorphic grade as the felsic metavolca-nic rocks and are absent from younger rock units (for example, 1.4 Ga granitoids).Therefore, the mafic rocks in the Dos Cabezas are at least 100 Ma younger thanassociated felsic rocks and as such do not define a bimodal suite as assumed by Condie,Bowling, and Vance (1985), but seem to be genetically and temporally distinct.

The southern terrain in the Dos Cabezas Mountains consists exclusively ofsedimentary rocks, ranging in grain size from sands to pelites. An �Nd(1650) � 1.8 for apsammitic rock is more consistent with the turbidite facies of the Pinal block, incontrast to the relatively mature but isotopically juvenile volcanic arc facies in theeastern terrain.

In the Pinaleno Mountains, the Proterozoic rock sequence mainly comprises thePinaleno gneiss, the protolith of which cannot be clearly determined due to highmetamorphic grade (Thorman, 1981). The Pinaleno granitic gneiss contains clear,euhedral zircons, and because of small size and relatively low Pb concentrations,multiple grain analyses were necessary in some cases. A triple grain analysis gives aconcordant age of 1615 � 5 Ma, while most of the analyzed zircon fractions in thisgneiss are dominated by an approx 1638 Ma inherited component (fig. 4A), that issimilar in age to the volcanic rocks of the Dos Cabezas Mountains. Granite genesispossibly occurred by anatexis of the 1.64 Ga volcano-sedimentary basement. Thecrystallization age of the granitic gneiss is about 20 Ma younger than volcanism in theDos Cabezas Mountains. �Nd(1615) � 3.2 and 3.3 for an amphibolite and a schist,respectively, and are similar to the quartzite and quartz porphyry in the eastern terrainof the Dos Cabezas Mountains.

The rocks in the Santa Teresa Mountains consist of a sequence of quartzites atleast 2100 m thick, intercalated with schist and cordierite gneiss. This sequence hosts agranitic gneiss (Blacet and Miller, 1978) that may be a northwestern extension of thePinaleno gneiss (Conway and Silver, 1988). Zircons from a mature quartzite from thestructurally highest part of the sedimentary pile are rounded, cloudy, and cracked withfrosted surfaces. 207Pb/206Pb-ages define two distinct age groupings: 1641 to 1652 Maand 1697 to 1729 Ma (fig. 4B). Hence, it appears that in addition to detrital inputderived from the young volcanic activity to the southeast, there may have been inputfrom the continental margin to the northwest. This is in contrast to the Dos CabezasMountains, where older zircons were not observed. A sample of biotite schist from theSanta Teresa Mountains yields an �Nd(1630) � 2.9 with a T(DM) � 1.7 Ga, consistentwith derivation from juvenile sources.

Paleoproterozoic rocks in the Galiuro Mountains consist primarily of fine-grainedintermediate to felsic volcanic rocks. A sample of massive, light pink, fine-graineddacitic volcanic rock from Aravaipa Canyon contained small euhedral zircon crystalsand crystal fragments. Four discordant analyses fail to define a discordia, but Pb-Pbages ranging from 1626 to 1646 Ma are consistent with ages for volcanic rocks in thesoutheastern part of the study area. Sm-Nd analyses yield �Nd(1635) � 3.4 andT(DM) � 1.7 Ga for the same sample.

In the Tortilla Mountains, similar volcanic rocks consist of laminated tuff intrudedby dacite sills and diabase dikes and lenses. Preliminary U-Pb zircon data for a sampleof laminated tuff are highly discordant but yield magmatic zircon ages similar to thosein the Dos Cabezas Mountains. The volcanic series has values of �Nd(1640) � 3.0 andT(DM) � 1.7 Ga for the tuff unit, and �Nd(1500) � 4.5 and T(DM) � 1.5 Ga for the


diabase. The intermediate to felsic and the mafic volcanic rocks seem to be temporallyand genetically unrelated, as was seen in the Dos Cabezas volcanic series.

To summarize, the Paleoproterozoic rocks in the Dos Cabezas, Pinaleno, SantaTeresa, Galiuro, and Tortilla Mountains form a single volcano-sedimentary province

Table 1

List of analyzed samples containing name, location, lithology, UTM coordinates, interpreted

�Sample locations were determined using a Lowrance Global Nav 200 global positioning system (GPS),the precision of which is given with 100 m in x- and y-direction and 150 m in z-direction.

†Age as interpreted by the authors from weighted means or York fits for magmatic zircons or ranges of207Pb/206Pb ages.

•‘Z’ at the end of a sample name denotes a zircon separate while the letters in front denote the locationnames as used in figure 7.


deposited between 1635 and 1670 Ma. The volcanic and sedimentary rocks in theseareas have �Nd(t) � 2.9 to 3.8 and thus represent juvenile crust derived from a depletedmantle source. Post-tectonic mafic rocks are derived from a different magmatic source,characterized by higher �Nd(t) of 4.5 (see fig. 6).

U-Pb zircon age, Nd-values, and a reference to the geologic map of the location


Fig. 4. Concordia diagrams for (A) granite gneiss, Pinaleno Mountains; (B) quartzite, Santa TeresaMountains; (C) metaturbidite (filled ellipses) and metarhyolite (open ellipses), Little Dragoon Mountains;(D) granodiorite, Johnny Lyon Hills.


Fig. 5. Concordia diagrams for (A) metagraywacke from the Santa Catalina Mountains (filled ellipses)and Pinal Mountains (open ellipses); (B) psammitic schist, Mineral Mountains; (C) granite from theMaricopa Mountains and Buckeye Hills; (D) granite from Webb Peak.


4.2 Central mixed graywacke-volcanic domain.—Paleoproterozoic rocks in the LittleDragoon Mountains and the Johnny Lyon Hills consist of sericite schist and meta-graywackes similar to the type area of the Pinal Schist in the Pinal Mountains. Primarybedding forms are suggestive of a turbidite origin (Silver, 1955; Cooper and Silver,1964; Swift, 1987). They are intercalated with basalts, rhyolite lenses, and a latesyn-deformational rhyodacite intrusion. Metamorphic foliation strikes consistentlynortheast (Cooper and Silver, 1964). Swift (1987) interpreted the sequence as part of asubduction melange. He proposed that at least some turbidite deposition and subse-quent deformation occurred within a trench above a northwest-dipping subductionzone, where basaltic blocks of oceanic affinity were tectonically emplaced into thesedimentary sequence. The sedimentary pile was intruded by the apparently post-tectonic Johnny Lyon Granodiorite. Reported ages of 1680 to 1700 Ma and 1625 �10Ma for the rhyodacite and granodiorite, respectively (Silver, 1963; Silver and Deutsch,1963) are the most often referenced constraints on the age of deposition anddeformation of Pinal block rocks.

Our analyses of detrital zircons from a turbidite sample from the northwesternside of Lime Peak in the Little Dragoon Mountains yield concordant ages ranging from

Fig. 6. �Nd -time plot for samples from the Pinal block and the Cochise block. The �Nd (t)-values for thePinal Basin range from -0.2 to 2.3 (gray box). The Cochise block shows a tight cluster of high �Nd (t) � (2.9 -3.8) close to the depleted mantle line. The two intervening evolution lines are from tectonically mixed rocksin the Little Dragoon Mountains, interpreted as a suture zone between the Cochise block and the PinalBasin. Dashed evolution line is for metabasaltic rocks from the Dragoon Mountains interpreted as possibleophiolite fragments within the suture zone. Post-tectonic mafic rocks (heavy lines) have higher �Nd (t)� 4.5and are probably derived from a different mantle source.


1655 to 1723 Ma (fig. 4C). This range of ages is considerably older than magmaticactivity to the southeast and implies provenance from older sources, presumably to thenorthwest. The youngest 207Pb/206Pb age of 1655 � 4 Ma provides a maximum age ofdeposition for this unit. A regression of four 2 to 15 percent discordant, multiple grainanalyses from a rhyolite lens in the turbidite yields an age of 1654 � 6 Ma (MSWD �1.42; fig. 4C). The grains are 100 to 200 �m long, euhedral, clear, mainly inclusionfree, and contained only 16 to 98 ppm Pb. Swift (1987) interpreted rhyolite lenses inthe metaturbidites as intrusive bodies in contradiction to the interpretation of Cooperand Silver (1964) that they represented surficial flows. While our age data cannotresolve this issue, it is clear that turbidite deposition and rhyolite magmatism occurredwithin a few million years. Whole rock Sm-Nd analyses of the metasediment andrhyodacite samples give �Nd(t) � 2.6 and 3.5 and T(DM) � 1.8 and 1.7 Ga, respectively.A metabasalt lense from this area has higher values of �Nd(1650) � 3.9, implyingderivation from a more depleted source. Trace-element characteristics of the basaltsshow their MORB-character (Condie, 1986; Copeland and Condie, 1986) consistentwith the interpretation of tectonic emplacement of fragments of oceanic crust (Swift,1987; Swift and Force, this issue).

Single-grain analyses of cloudy to clear, inclusion bearing zircons from the JohnnyLyon Granodiorite are 3 to 20 percent discordant and define a simple Pb-loss trajectorywith an upper intercept age of 1643 � 5 Ma (fig. 4D), only slightly younger than therhyolite. This age determination revises an age of 1625 �10 Ma reported in a seminalpaper on U-Pb systematics (Silver and Deutsch, 1963) that predated air-abrasiontechniques and single-grain analysis. It is interesting, that the time interval between ouryoungest detrital grain (1655 Ma), rhyolite lens (1654 Ma), and the age of thereportedly post-tectonic Johnny Lyon intrusion is only 10 Ma.

A metaturbidite and a felsic volcanic rock sampled on the northwestern side of theLittle Dragoons yielded �Nd(1650)-values of 1.6 and 2.2 corresponding to a model ageof 1.8 Ga. These values along with the older detrital zircon population seen inmetasediments from the Little Dragoon Mountains lend support to the interpretationof Swift (1987) that these rocks originated as trench deposits above a subduction zoneand as such represent mixed provenance from the southeast and the northwest.

Lithology in the Dragoon Mountains is somewhat unusual for the Pinal block, as itconstitutes the only area with major basalts occurring as pillowed flows and pillowbreccias west of Cochise Stronghold (Copeland and Condie, 1986; Swift and Force, thisissue). The basalt gives a �Nd(1650) of 4.0 and a depleted mantle model age of 1.68 Ga.Trace-element characteristics were reported as typical for volcanic arcs (Copeland andCondie, 1986), but textural and structural evidence also allows an interpretation asophiolitic volcanic rocks (Swift and Force, this issue).

4.3 Northwestern sedimentary domain.—Exposures of Paleoproterozoic rocks mappedas ‘Pinal Schist’ in the Santa Catalina Mountains are scant and largely isolated byyounger, mostly Tertiary plutonism and faulting. A sample from an isolated exposureof quartz wacke on the northern side of the range contains sub-euhedral to well-rounded zircons with minor inclusions and variably frosted surfaces suggestive ofsedimentary reworking. The U-Pb analyses are 0.7 to 7 percent discordant and have arange in near-concordant Pb-Pb ages of 1683 to 1730 Ma. An analysis of a small grainfragment yielded an anomalously old Pb-Pb age of 2448 Ma. The least discordantanalysis with a Pb-Pb age of 1691 Ma is interpreted as a maximum depositional age withother analyses showing detrital input from even older sources (fig. 5A). The range ofages exhibited by this rock clearly reflects a source region older than that recorded inthe southeastern supracrustal rocks. This is also indicated by an �Nd(1680) � 2.3 forthis sample. A schist from the Santa Catalina Mountains yields an �Nd(1700) � 1, whichlikely reflects input of older detritus, as well.


The Paleoproterozoic metasediments in the Pinal Mountains (the type area of thePinal Schist) consist of a metamorphosed turbidite facies composed of sericite-richwackes to coarse conglomeratic wackes. Detritus of sand size dominates and is mainlycomposed of quartz. Trace-element abundances show derivation from felsic volcanicrocks and granite (Condie and DeMalas, 1985), and trace element analyses of the rocksmainly plot in the active continental margin field of Bhatia (1983). Detrital zircons in awacke from the northern part of the Pinal Peak area range in age from 1678 to 1731 Maand show a tight cluster of near-concordant grains around 1700 Ma (fig. 4B). In thisanalysis as well as in the following, we exclude detrital zircons that are more than 10percent discordant, as they could give misleadingly young ages. Provenance of thesediments probably is from the Mazatzal continental margin, recycling a �1.7 Gaintrusive suite. Time of deposition is later than 1678 Ma and thus the Pinal sedimentsare not syndepositional to the Tonto Basin Supergroup, which was deposited between1.72 and 1.70 Ga (Karlstrom and others, 1990). An �Nd(1675) � �0.1 for this samplesuggests input of some material with crustal residence ages greater than those seen inthe zircon population. The sedimentary sequence in the Pinal Mountains is intrudedby the Madera Diorite with a reported age of 1684 � 25 Ma (Keep, 1996). Our datasuggest that both the sediments and the diorite must be younger than 1678 Ma.

In the Mineral Mountains, Theodore and others (1978) mapped pelitic topsammitic schists, intercalated with amphibolitic lenses. A psammitic rock was col-lected from the northern part of the Mineral Mountains and showed 207Pb/206Pb agesof detrital zircons ranging from 1686 to 1698 Ma, values similar to the Pinal Mountains(fig. 4 B).

Sedimentary rocks of the Pinal block in the Santan Mountains are made up ofphyllites, schists, and argillites, subordinate calc-schists, and quartzites, with associatedamphibolite lenses. The sedimentary sequence was intruded by a Paleoproterozoicgranodiorite (Ferguson and Skotnicki, 1996). Epsilon values of �Nd(1675) � �0.2 andT(DM) � 2.0 Ga are similar to the type locality and suggest correlation with the PinalMountains. Strongly foliated, isoclinally folded schist containing minor diabase intru-sions constitutes the Pinal rocks in the Table Top Mountain Wilderness (Peterson,Tosdal, and Hornberger, 1987). �Nd(1675) � 1.0 and a model age of 2.0 Ga supportcorrelation with adjacent areas. Paleoproterozoic rocks in the Maricopa Mountains arecomposed of metasediments, mainly schists, with �Nd(1675) � 1.3 correlating to amodel age of 1.9 Ga. This signature is again consistent with correlation with othersampled locations in the Pinal block.

Notably absent from the detrital zircon population of sampled rocks from thePinal block are any zircons younger than 1678 Ma (fig. 7). This represents a maximumage for deposition of the sedimentary rocks in the Pinal block. The source of detritalzircons ranging in age from 1678 to 1731 Ma is presumed to be from similar aged rocksin Tonto Basin Supergroup of the Mazatzal block. The similarity in detrital zirconpopulation for the Pinal, Mineral, and Santa Catalina Mountains (fig. 7B) supports theinterpretation that turbiditic rocks were deposited in a basin with little along-axisvariation in age of detrital input.

4.4 Northwestern granitoid belt.—Sedimentary rocks of the Pinal block were depos-ited and deformed prior to being intruded by a suite of granitoids. The metasedimen-tary sequence in the Maricopa Mountains was intruded by the Maricopa granite andsubsequently the leucogranite of Cotton Center (Reynolds and DeWitt, 1991). Large,euhedral, clear zircon grains with minor inclusions were analyzed from a granite in thesouthern Maricopa Mountains. Two analyses are concordant and together with a thirdanalysis record an upper intercept age of 1645 � 1 Ma (fig. 4C). Two other analyses lieslightly to the right of this regression, indicating inheritance of an older component.Four analyses of similar zircons from a granite body from the Northern Maricopa


Mountains display complex Pb systematics. The least discordant point, with a Pb-Pb ageof 1655 Ma, suggests that this granite may be slightly older than its southern counter-part.

In the Buckeye Hills, metasedimentary units are intruded by a large granite. Sixpurple, clouded zircons from the granite were strongly air abraded and contained highPb (up to 473 ppm) and U (up to 1535 ppm). Pb-Pb ages of the six analyses range from1643 to 1650. Three of these are concordant and produce a weighted mean age of1647 � 1 Ma. The age of a monzogranite in the Santan Mountains is about 10 Mayounger with an age of 1638 � 1 Ma (Isachsen and others, 1999).

The three analyzed granites are probably part of the same intrusive suite as seen inthe Sunflower block to the northeast (Karlstrom and Bowring, 1988). The time

Fig. 7(A) Map showing proposed tectonostratigraphic units. A transect running roughly from northwestto southeast shows selected sample locations. Location names are as given in figure 1 and table 1. (B) U-Pbages determined along transect. (C) �Nd (t) values along transect.


difference between the youngest detrital grain in the Pinal Mountains and the oldestgranite in the Maricopa Mountains is about 20 Ma. These ages therefore bracketdeposition in the Pinal block and subsequent deformation between 1678 and about1655 Ma.

4.5 Boundary with the Mazatzal block.—Outcrops of Paleoproterozoic age in theWebb Peak area contain quartz-monzonite and diorite plutons at the western base ofthe section, overlain by a younger volcano-sedimentary sequence. The contact couldbe an unconformity between an older basement and its cover or a structural contact.The plutons preserve an older, northwest-striking foliation, while the volcano-sedimentary sequence preserves a younger event with northeast striking foliation(Northrup and Reynolds, 1991). Zircon separates provided by C. J. Northrup include afraction of up to 400 �m long, euhedral, purple zircons from the western quartzmonzonite pluton. Six analyses yield an upper intercept age of 1736 � 1 Ma (MSWD �0.45; fig. 4D). This is the oldest magmatic age found in our study and probablycorrelates with 1.74 Ga intrusives in the Alder Group (for example, 1738 Ma GibsonCreek batholith) of the Tonto Basin Supergroup in the Mazatzal block or to 1.75 to1.74 granodiorites in the Big Bug block of Central Arizona (Karlstrom and Bowring,1988). The northern boundary of the Pinal block can thus be found either within theWebb Peak area or southeast of it.

5. discussion

Detailed single-grain zircon geochronology coupled with Sm-Nd whole rockanalyses allows us to delineate a suture zone within the Pinal block that separatesmetasediments to the northwest from a volcanic arc to the southeast. The location ofthe suture in the Little Dragoon Mountains was recognized by Swift (1987) as remnantsof a northwest dipping subduction zone, and the Dragoon Mountains are proposed tocomprise a ophiolitic suite (Swift and Force, this issue). We propose the name Cochiseblock for the tectonostratigraphic unit southeast and east of this suture, as it representsa faultbound, independent tectonic unit. The Pinal block, as suggested by our data, isrestricted to the metasedimentary rocks and intrusive suite west and northwest of theproposed suture zone (fig. 7A).

Our evidence for the location of the suture zone is a cratonward increase in age ofdetrital zircons, from ages of 1630 to 1674 Ma (Dos Cabezas Mts.) to ages between 1655and 1723 Ma (Little Dragoon Mts.) and 1678 to 1731 Ma (Pinal Mts.) (fig. 7B). Thistransition in detrital zircon ages is also reflected in a decrease of �Nd(t) (figs. 6, 7C).These observations are in keeping with previously reported lithologic differencesacross the area (Condie and DeMalas, 1985; Copeland and Condie, 1986; Anderson,1989, Condie 1992). Anderson (1989) had made the important observation that thecontrasting lithologies represent different tectonic settings. However, a lack of isotopicand age data precluded any firm tectonic evolutionary model.

Detrital zircons in the Pinal block are �1678 Ma, whereas zircons from volcanicrocks in the Dos Cabezas Mountains are �1.65 Ga old. Detrital zircons derived from a1.7 Ga old basement were found in a turbidite sample in the Little Dragoons, andgrains in excess of 1.7 Ga are present in a quartzite sampled from the Santa TeresaMountains. However, 1.7 Ga detritus is not recognized in sediments from the DosCabezas Mountains. The older zircons from the Santa Teresa Mountains suggest thatpart of the Cochise block may have been more proximal to the continental margin, oralternatively, that the sample dated was deposited as molasse subsequent to accretionof the Cochise arc to the continental margin. �Nd(t)-values for samples from theCochise block generally are higher than 2.5, while sediments from the Pinal block areless than 2.0, indicating input of recycled material from the continental margin (figs. 6,7C).


In the Cochise block, we document volcanic activity from 1647 � 2 to 1631 � 1 Ma(Dos Cabezas Mountains) and magmatism to 1615 � 5 Ma (Pinaleno Mountains). Thedepositional environment of synvolcanic sedimentary rocks in the Dos Cabezas Moun-tains is interpreted to have been a proximal shelf area with mature sediments derivedsyntectonically from the rising arc itself. �Nd(t)-values for volcanic rocks and sedimentsrange from 2.7 to 4.0, values close to the depleted mantle model, and, as such, theserocks represent juvenile additions to the crust. The crustal xenoliths analyzed from theDos Cabezas suggest that the basement of the Cochise block was of mafic and felsiccomposition and was not appreciably older than the outcropping supracrustal rocks.�Nd(t)-values of 4.5 for two metabasalt samples from the Dos Cabezas and GaliuroMountains are distinctly higher than the felsic volcanic rocks (fig. 6). We interpretthem as post-tectonic, derived from a depleted mantle source around 1.5 Ga.

Excluding analyses �10 percent discordant, detrital zircons from the Pinal blockindicate that sedimentation was later than 1678 Ma (Pinal Mountains) and 1686 Ma(Mineral Mountains). Sedimentation probably had ceased by the time of granitoidintrusion at about 1655 Ma in the northern part of the basin (northern MaricopaMountains). Our analyses show ages for the deposition of sediments in the Pinal blockabout 20 Ma younger than previously assumed (Karlstrom and Bowring, 1988; Condie,1992). �Nd(t)-values generally increase from northwest to southeast from –0.2 (SantanMountains) to 1.8 (Dos Cabezas Mountains, southern terrain). Basement to the Pinalblock is not known, and a detrital zircon population ranging in age from 1678 to 1731is most consistent with a northwesterly source. Rocks of similar age constitute much ofthe Mazatzal block (Karlstrom and Bowring, 1988). The apparent lack of exposedbasement in the exhumed Pinal block is most consistent with an origin as a sediment-rich continental margin accretionary prism. Thus, the Pinal block is either autochtho-nous or parautochthonous to the continental margin. In contrast, we interpret thejuvenile Cochise arc to be entirely allochthonous with respect to the Mazatzal marginwith an intervening subduction complex, and, as such, it represents a separatetectonostratigraphic terrane.

Sedimentation in the proposed trench, represented by the turbidites and brokenformation in the Little Dragoon Mountains (Swift and Force, this issue), occurred laterthan 1655 Ma, and incorporated blocks of rhyolite dated at 1654 � 6 Ma. The JohnnyLyon Granodiorite intruded the sequence at 1643 Ma � 5 Ma.

Granitoids intruded rocks of the Pinal block from 1657 � 13 Ma (northernMaricopa Mountains) to 1638 � 1 Ma (Santan Mountains; Isachsen and others, 1999).The granites are thought to represent a late stage of magmatism when the sedimentaryrocks in the Pinal block were already accreted onto the continental margin. Magma-tism was possibly related to northwestward subduction, the remnants of which can beseen in the Johnny Lyons Hills (Swift, 1987). Synchronous and slightly youngervolcanism is documented in the Cochise block.

While the volcanic rocks of the Cochise block are thoroughly deformed andmetamorphosed to upper greenschist facies, similar age granites in the Pinal blockshow little deformation. If the Little Dragoon Mountains were the site of a northwestdipping subduction zone responsible for the granitoids northwest of it, a secondsubduction zone presumably existed beneath the Cochise block. An explanation forthe higher degree of deformation in the Cochise block could be that during accretionit offered less resistance to deformation than the thickened crust of the continentalmargin, including the accreted Pinal block.

The geometry of the basement fragments in southern Arizona is intriguing.Metamorphic foliation strikes consistently northeast, but to separate the metavolcanicrocks from the metasedimentary unit, the boundary has to run northwest for 100 to150 km. The northwest trend of the boundary was recognized by Copeland and Condie


(1986) and interpreted as an assemblage boundary. A northwest-trending crustal breakis not seen anywhere in central Arizona, where Paleoproterozoic rocks are notdisrupted by Basin and Range tectonics. Therefore, the major part of left-lateral offsetis probably not a post-accretion strike-slip movement, although minor left-lateralstrike-slip movement along northwest trending shear zones, such as the Apache PassFault, separating Paleoproterozoic outcrops in the Dos Cabezas Mountains, arereported (Erickson, 1969; Drewes 1985). Several syn-accretion tectonic settings couldaccount for a boundary perpendicular to the main direction of convergence, whichprobably was northwest-southeast.

The northwest trend of the boundary could have originated as an oceanictransform fault along the Proterozoic continental margin, the Cochise block couldhave been an indenter to the Pinal block or was smeared out along an irregularcontinental margin. Because of the fragmentary rock record, it is all the more difficultto pinpoint the location of a suture in a setting that once was a dynamic system with anoceanward stepping system of thrust planes. We nevertheless make this attempt (fig.7A) and propose a northeast trend, parallel to the trend of foliation, south of the LittleDragoon Mountains and north of the Pinal Mountains and a northwest offset inbetween. The interpretation of the metabasalts in the Dragoon Mountains is critical, asthey represent the southernmost outcrop analyzed in this study. They are proposed torepresent an ophiolitic suite (Swift and Force, this issue). Therefore, the answer to thequestion if the exposure of volcanic arc rocks continues southwest of the Dos CabezasMountains has to await further investigation.

Out of 148 single grain analyses, 2 grains yielded anomalously old ages: a 1.88 Gagrain in the Mineral Mountain psammite and a 2.45 Ga grain in the Santa Catalinaquartz wacke. Given their poor representation in the detrital record and isotopicsignatures, it is likely that these grains represent anomalously far-traveled input fromthe Mojave Terrane or Wyoming Craton rather than evidence for a basement of thatage.

The timespan between extraction of material from the mantle and crustalconsolidation, calculated as difference between Nd depleted mantle model age andzircon crystallization age, range from 290 to 340 Ma for the sedimentary rocks in thePinal Mountains to 30 to 70 Ma for rocks from the southeastern Cochise block (fig. 6).The U-Pb zircon analyses and Sm-Nd isotopic data from a felsic and a mafic xenolith inthe Dos Cabezas Mountains suggest that felsic volcanic rocks erupted through anapprox 1.65 Ga basement with wall-rock entrainment and assimilation. The successionof sedimentation, deformation, and granitoid intrusion took place in a brief timeinterval of about 20 Ma in the northwestern Pinal block, while volcanism andsedimentation were almost synchronous in the southeastern Cochise arc. The range ofdetrital zircon ages from the Cochise block suggest most volcanic activity within theCochise block occurred within a time span of about 40 Ma.

The overall composition of supracrustal rocks observed within the Cochise blockis evolved. The percentage of mafic material is low, and the rocks are dominated bymature sediments and intermediate to felsic volcanic rocks. All the rocks studiedwithin the Cochise block, including sediments, seem to have been derived from themantle 50 to 100 Ma prior to crystallization. If primary mantle derived material wasmafic, a process of crustal differentiation must have acted to produce juvenile crust offelsic composition.

The significance of these observations is that large volumes of intermediate tofelsic volcanic rocks and granitoids can be derived from the mantle and differentiatedfrom primary mafic material in a relatively rapid two-step process to be added to thecontinental crust. The process envisioned is one of magma generation from a depletedupper mantle above a subduction zone followed by anatexis of the primary material,


resulting in rapid and efficient intracrustal reprocessing and differentiation. The siteof subduction and arc magmatism seems to have changed after short time intervals andmigrated southeast. All of the Arizona Yavapai-Mazatzal collage of terranes, today a 500km wide belt, was formed and accreted onto the continental margin within about 160Ma (Karlstrom and Bowring, 1988). The igneous cycle seen in the PaleoproterozoicCochise block is very similar to the one seen along active plate margins today. Firstmafic, then intermediate to felsic volcanic rocks erupt, followed by post-tectonic basaltsand diabases as the last stage of igneous activity. Quartzite-rhyolite associationsconstitute the uppermost mature sequence of a growing island arc system. Post-tectonic basalts in such areas may carry signatures of continental rift affinity, such asLREE-enrichment, LILE-enrichment, enrichment of some high field strength ele-ments (Th, Hf, Ti, Zr) (Condie, Bowling, and Vance, 1985) and high ratios of Zr/Y,Ce/Sr, and Ti/Y, by derivation from an only partially depleted mantle wedge andassimilation of material with characteristics of mature crust. Nb-Ta depletions charac-teristic for arc basalts remain (Copeland and Condie, 1986).

The lack of approx 1630 to 1650 Ma zircons in the Pinal block metasedimentssuggests that it was largely exhumed during Cochise volcanism and sedimentation.Conversely, the general lack of older detritus in the Cochise block, with the exceptionof the Santa Teresa sample, suggests that the arc evolved independent from thecontinental margin, either separated from it by an expanse of ocean or a deep trench.These observations preclude interpreting the Pinal block as a back arc basin to theCochise block. Thus, it appears that evolved crust and mature sediments of the Cochiseblock formed by intra-arc fractionation and differentiation independent from anactive continental margin. This is in apparent contradiction to the requirement ofcontinental margin crustal processing proposed by Condie and Chomiak (1996) toproduce observed enrichment of incompatible elements in Paleoproterozoic arcterranes of the southwestern United States.

6. conclusions

Field observations integrated with U-Pb zircon geochronology and Sm-Nd analy-ses allow us to define a suture zone separating the Cochise block, a juvenile volcanic arcactive predominantly in the interval from 1630 to 1650 Ma, from the Pinal block, acollapsed basinal assemblage of metaturbiditic rocks with continental affinity. Sedimen-tation in the Cochise block was synchronous with volcanism and rocks ranging from1630 to 1674 Ma in age were recycled. Mafic rocks with �Nd(1500) � 4.5 constitutepost-tectonic igneous activity at about 1.5 Ga. The northeast running suture betweenthe two tectonic units seems to have a 100 to 150 km northwestern offset.

Our data suggest that sedimentation in the sampled areas of the Pinal blockoccurred later than 1678, ended before 1655 Ma, and that continental margin materialwas recycled. The age of sedimentation seems to decrease from northwest to southeast,while the �Nd(t)-values increase. Within the Cochise block, the time span fromseparation of material from a depleted upper mantle source to crystallization ofevolved igneous rocks was 50 to 100 Ma. Felsic igneous rocks apparently intruded onlyslightly older (�40 Ma) basement. Time spans from crystallization to sedimentationwere shorter than 35 Ma for most of the Cochise block.

As with other Paleoproterozoic terranes, the Cochise block and the Pinal blockwere notably short-lived geologic features, where large volumes of evolved crust wereproduced rapidly. The location of subduction activity seems to have migrated south-east while juvenile material was added onto the North American continental margin inthe Yavapai-Mazatzal orogenic belt. Petrogenetic processes in the Cochise block wererelated to subduction activity, and magmas were derived from a depleted upper mantlesource. We show that magma extraction from the mantle followed by intracrustal


differentiation in the Cochise block took place in short geologic time and withoutinfluence of older continental material.

acknowledgmentsWe gratefully acknowledge funding by the National Science Foundation. We are

also indebted to Mike Canova, Eric Force, Norman Meader, Clyde Northrup, MarkMarikos, Stephen Richard, and Thomas McGarvin for their help. We are also gratefulfor the reviews of P. Swift, W. R. Dickinson, P. J. Patchett, Kent Condie, Jon Spencer,and an anonymous reviewer.

References

Anderson, P., 1986, Summary of the Proterozoic plate-tectonic evolution of Arizona from 1900 to 1600 Ma inBeatty, B., and Wilkinson, P. A. K., (editors), Frontiers in geology and ore deposits of Arizona and theSouthwest: Arizona Geological Society Digest 16, p. 5–11.

–––––– 1989, Stratigraphic framework, volcanic-plutonic evolution, and vertical deformation of the Protero-zoic volcanic belts of Central Arizona, in Jenney, J. P., and Reynolds, S. J., Geologic evolution of Arizona:Arizona Geological Society Digest 17, p. 57–147.

Bergquist, J. R., 1979, Reconnaissance geologic map of the Blue Jay Peak Quadrangle, Graham County,Arizona: United States Geological Survey Miscellaneous Field Studies Map MF-1083.

Bhatia, M. R., (1983), Plate-tectonics and geochemical composition of sandstones: Journal of Geology, v. 91,p. 611–627.

Bickford, M. E., 1988, The formation of continental crust: Part 1. A review of some principles; Part 2. Anapplication to the Proterozoic evolution of southern North America: Geological Society of AmericaBulletin 100, p. 1375–1391.

Blacet, P. M., and Miller, S. T., 1978, Reconnaissance geologic map of the Jackson Mountain quadrangle,Graham County, Arizona: United States Geological Survey Miscellaneous Field Studies Map MF-939.

Bowring, S. A., and Karlstrom, K. E., 1990, Growth, stabilization and reactivation of Proterozoic lithospherein the southwestern United States: Geology, v. 18, p.1203–1206.

Condie, K. C., 1982, Plate-tectonics model for Proterozoic continental accretion in the southwestern UnitedStates: Geology, v. 10, p. 37–42.

–––––– 1986, Geochemistry and tectonic setting of Early Proterozoic supracrustal rocks in the southwesternUnited States: Journal of Geology, v. 94, p. 845–864.

–––––– 1992, Proterozoic terranes and continental accretion in southwestern North America, in Condie,K. C., editor, Proterozoic crustal evolution: Amsterdam, Elsevier, 509 p.

(continued on p. 796)

Appendix 1

U-Pb methodsSamples weighing between 5 and 20 kg were reduced to less than 35 mesh using a jaw crusher and

pulverizer. Heavy minerals were then separated by standard Wilfley table and heavy liquid techniques.Analyzed zircons were handpicked and, with a few exceptions, air-abraded for 1 to 6 hrs following themethod of Krogh (1982). Following a warm 4N HNO3 wash, the zircons were dissolved in Teflonmicrobombs using the method of Parrish (1987) and spiked with 205Pb-233U-235U tracer. Subsequent toanion column separation, the Pb was loaded onto rhenium filaments with silica gel and phosphoric acid.During the course of analyses, procedural blanks ranged from 2 to 10 pg total Pb and �1 to 1 pg U.

Pb isotopic measurements were performed in dynamic multicollector mode using a Micromass Sector54 solid-source mass spectrometer. 204Pb was measured using a Daly electron multiplier with internalDaly-Faraday gain correction determined by peak switching of 205Pb between the Daly and an off-axisFaraday collector. U isotopic measurements were determined in static multicollector mode. Mass fraction-ation factors for Pb were determined by replicate analyses of NBS standards SRM 981 and SRM 983 to be 0.06percent per atomic mass unit; U fractionation was directly determined. Radiogenic Pb concentrations andU-Pb values were calculated from raw fractionation-corrected mass spectrometer data following Ludwig(1980). Initial Pb compositions were estimated using the two-stage evolutionary model of Stacey andKramers (1975). Ages were calculated using the decay constants recommended by the International Unionof Geological Sciences subcommision on geochronology (Steiger and Jager, 1977), which are 238U �0.155125 � 10�9 year�1 and 235U � 0.98485 � 10�9 year�1. Linear regression and calculation of interceptsand uncertainties follow Ludwig (1980). All ages are quoted at the 2-sigma confidence level. 4 to 15 zircongrains (or multiple grains in some cases) were analyzed per sample and reported ages are ranges of207Pb/206Pb-ages for detrital samples, weighted means of 207Pb/206Pb-ages or York fit data for igneous rocks.


App

end

ix2

U-P

ban

alyt

ical

data


App

end

ix2

(con

tin

ued)

U-P

ban

alyt

ical

data


Condie, K.C., and Chomiak, B., 1996, Continental accretion: contrasting Mesozoic and Early Proterozoictectonic regimes in North America: Tectonophysics, v. 265, p. 101–126.

Condie, K. C., and DeMalas, J. P., 1985, The Pinal schist: an Early Proterozoic quartz wacke association insoutheastern Arizona: Precambrian Research 27, v. p. 337–356.

Condie, K. C., Bowling, P.G., and Vance, R. K., 1985, Geochemistry and origin of Paleoproterozoicsupracrustal rocks, Dos Cabezas Mountains, southeastern Arizona: Geological Society of AmericaBulletin, v. 96, p. 655–662.

Conway, C. M., and Silver, L. T., 1989, Early Proterozoic Rocks (1710–1615 Ma) in central to southeasternArizona, in Jenney, J. P., and Reynolds, S. J., Geologic evolution of Arizona: Arizona Geological SocietyDigest, v. 17, p. 57–147.

Cooper, J. R., and Silver, L. T., 1964, Geology and ore deposits of the Dragoon quadrangle, Cochise County,Arizona: United States Geological Survey Professional Paper 416, 196 p.

Copeland, P., and Condie, K. C., 1986, Geochemistry and tectonic setting of the lower Proterozoicsupracrustal rocks of the Pinal schist, southeastern Arizona: Geological Society of America Bulletin,v. 97, p. 1512–1520.

Cunningham, D., DeWitt, E., Haxel, G., Reynolds, S. J., and Spencer, J. E., 1987, Geologic Map of theMaricopa Mountains, Central Arizona: Arizona Geological Survey Open-File Report 87–4, p.

Drewes, H., 1974, Geologic map and sections of the Happy Valley quadrangle, Cochise County, Arizona:United States Geological Survey Miscellaneous Investigations Series, MAP I-832.

–––––– 1984, Geologic map and sections of the Bowie Mountain North quadrangle, Cochise County, Arizona:United States Geological Survey Miscellaneous Investigations Series, MAP I-1492.

–––––– 1985, Geologic map and sections of Dos Cabezas quadrangle, Cochise County, Arizona: United StatesGeological Survey Miscellaneous Investigations Series, Map I-1570.

Erickson, R. C., ms, 1969, Petrology and geochemistry of the Dos Cabezas Mountains, Cochise County,Arizona: Ph.D. thesis, Tucson, University of Arizona, 441 p.

Erickson, R. C., and Bowring, S. A., (1990), Petrology and geochronology of the Sommer Gneiss, DosCabezas Mountains, Arizona; dating the Mazatzal Orogeny: Geological Society of America, Abstractswith Programs, v. 22, p. 21.

Erickson, R. C. and Drewes, H. , 1984, Geologic map of the Railroad Pass Quadrangle, Cochise County,Arizona: United States Geological Survey Miscellaneous Field Studies Map MF-1688.

Ferguson, C. A., and Skotnicki, S. J., 1996, Bedrock geology of the Santan Mountains, Pinal and MaricopaCounties, Arizona: Arizona Geological Survey, Open-file Report 96–9, 22 p.

Hoffman, P. F., 1989, Precambrian geology and tectonic history of North America, in Bally, A. W., andPalmer, A. R., (editors), The geology of North America – An overview: Boulder, Colorado, GeologicalSociety of America, p.

Isachsen, C. E., Gehrels, G., Riggs, N. R., Spencer, J. E., Ferguson, C., Skotnicki, S., and Richard, S. M., 1999,U-Pb isotopic data from zircons from eleven granitic rocks in central and western Arizona: ArizonaGeological Survey, Open-file Report 99–5, 27 p.

Karlstrom, K. E., and Bowring, S. A., 1988, Early Proterozoic assembly of tectonostratigraphic terranes insouthwestern North America: Journal of Geology, v. 96, p. 561–576.

Karlstrom, K. E., Doe, M. F., Wessels, R. L., Bowring, S. A., Dann, J. C., and Williams, M. L., 1990,Juxtaposition of Proterozoic Crustal blocks: 1.65–1.60 Mazatzal Orogeny, in Gehrels, G. E., and Spencer,J. E., editors, Geologic excursion through the Sonoran desert region, Arizona and Sonora: ArizonaGeological Survey Special Paper 7, p. 114–123.

Keep, M., 1996, The Pinal schist, southeast Arizona, USA: contraction of a Palaeoproterozoic rift basin:Journal of the Geological Society of London, v. 153, p. 979–993.

Krogh, T. E., 1982, Improved accuracy of U-Pb zircon ages by creation of more concordant systems using anair abrasion technique: Geochimica et Cosmochimica Acta, v. 45, p. 637–649.

Ludwig, K. R., 1980, Calculation of uncertainties of U-Pb data: Earth and Planetary Science Letters, v. 46,p. 212–220.

Northrup, C. J., and Reynolds, S. J., 1991, Proterozoic geology of the Webb Peak area, northeastern GilaBend Mountains, southwestern Arizona, in Karlstrom, K. E., Proterozoic geology and ore deposits ofArizona: Arizona Geological Society Digest, v. 19, p. 251–260.

Parrish, R. R., 1987, An improved micro-capsule for zircon dissolution in U-Pb geochronology: IsotopeGeoscience, v. 66, p. 99–102.

Patchett, P. J., and Arndt, N. T., 1986, Nd isotopes and tectonics of 1.9–1.7 Ga crustal genesis: Earth andPlanetary Science Letters, v. 78, p. 329–338.

Patchett, P. J., and Ruiz, J., 1987, Nd isotopic ages of crust formation and metamorphism in the Precambrianof eastern and southern Mexico: Contributions to Mineralogy and Petrology, v. 96, p. 523–528.

Peterson, J. A., Tosdal, R. M., and Hornberger, M. I., 1987, Geologic map of the Table Top MountainWilderness Study Area, Pinal and Maricopa Counties, Arizona: United States Geological SurveyMiscellaneous Field Studies Map, MF-1951.

Ransome, F. L., 1903, Geology of the Globe copper district, Arizona: United States Geological SurveyProfessional Paper 12, 168 p.

Reynolds, S. J., and DeWitt, E., 1991, Proterozoic geology of the Phoenix region, central Arizona, inKarlstrom, K. E., Proterozoic geology and ore deposits of Arizona: Arizona Geological Society Digest,v. 19, p. 237–250.

Reynolds, S. J., and Skotnicki, S. J., 1993, Geologic Map of the Phoenix South 30 x 60 quadrangle, centralArizona: Arizona Geological Survey Open-File Report, p. 93–18.

Silver, L. T., ms, 1955, The structure and petrology of the Johnny Lyon Hills area, Cochise County, Arizona:(PhD. thesis): Pasadena, California, California Institute of Technology, 407 p.


–––––– 1963, The use of cogenetic uranium-lead isotope systems in zircons in geochronology: Radioactivedating: Vienna, Austria, International Atomic Energy Agency, p. 279–285.

Silver, L. T., and Deutsch, S., 1963, Uranium-lead isotopic variations in zircons: a case study: Journal ofGeology, v. 71, p. 721–758.

Stacey, J. S., and Kramers, J. D., 1975, Approximation of terrestrial lead isotope evolution by a two-stagemodel: Earth and Planetary Science Letters, v. 26, p. 207–221.

Steiger, R. H., and Jager, H., 1977, Subcomission of geochronology: convention on the use of decayconstants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359–362.

Swift, P. N., ms, 1987, Paleoproterozoic turbidite deposition and melange deformation, southeasternArizona: Ph.D. thesis, Tucson, University of Arizona, 134 p.

Swift, P. N., and Force, E. R., Subduction Margin assemblages in the Proterozoic Pinal and Cochise block,southeastern Arizona: American Journal of Science, v. 301, p. 755–772.

Theodore, T. G., Keith, W. J., Till, A. B., and Peterson, J. A., 1978, Preliminary geologic map of the MineralMountain 7 1⁄2 ‘ quadrangle, Arizona: United States Geological Survey Open-file Report 78–468.

Thorman, C. H., 1981, Geology of the Pinaleno Mountains, Arizona: a preliminary report, in Stone, C., andJenney, J. P., editors, Arizona Geological Society Digest 13, p. 5–12.

Williams, H., Hoffman, P. F., Lewry, J. F., Monger, J. W. H., and Rivers, T., 1991, Anatomy of North America:thematic geologic portrayals of the continent: Tectonophysics 187, p. 117–134.


CRUSTAL GROWTH IN SOUTHERN ARIZONA: U-Pb …

Documents

Transcript of CRUSTAL GROWTH IN SOUTHERN ARIZONA: U-Pb …