American Mineralogist, Volume 7 4, pages I I3-13 I, 1989

Alkaline igneous rocks of Magnet Cove, Arkansas: Metasomatized ijolitexenoliths from Diamond Jo quarry

M.cnrA, J. K. Fr,onn, Mlr,cor,vr Ross959 National Center, U.S. Geological Survey, Reston, Viryinia 22092,U.5.4.

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Ijolite xenoliths occur in garnet-pseudoleucite syenite that forms part of the outer ringof the Magnet Cove alkaline ring-dike complex. Xenoliths were collected from the Dia-mond Jo quarry located in the southern part of the complex. The xenoliths contain abun-dant diopside and Ti-rich andradite garnets and have been extensively metasomatized.Garnet-pseudoleucite syenite and nepheline syenite are identified as possible sources ofthe volatile-rich fluids responsible for the metasomatism of the ijolite xenoliths. Residual,volatile-rich fluids from the ijolites may have also produced autometasomatic alterationbefore the ijolites were entrained as xenoliths in the garnet-pseudoleucite syenite. Non-isotropic hydrogarnet, formed during metasomatism, contains up to 3.5 wto/o F. Aegirine,biotite, and melanite garnet, also formed during metasomatism, contain relatively highamounts of V andlor Ti, indicating that these elements were transported by the metaso-matic fluids. Mineral compositions and geochemical data indicate that the ijolite xenolithsshare many characteristics with the intrusive ijolites that form the inner rings of theMagnet Cove complex.

INrnolucrroN

The Magnet Cove complex is an alkaline ring-dikecomplex that crops out in an area of approximately 12km2 in Hot Spring County, Arkansas. The igneous rocksof the complex intrude deformed Paleozoic sedimentaryrocks of the Ouachita Mountains. Biotite K-Ar and Rb-Sr ages obtained by Zartman (1977) indicate a Cretaceousage of 95-102 Ma for the complex. Fission-track ages(Eby, 1987) indicate two periods of igneous activity atMagnet Cove. The mean sphene age for the emplacementof the silicate rock units is 101.4 + 1.0 Ma, whereas thecarbonatite, which occurs in the core of the complex, wasemplaced about 5 m.y. later, as indicated by a mean apa-tite age of 95.9 + 0.4 Ma. Magnet Cove is one of severalalkaline igneous intrusions in central Arkansas. Morris(1987) described this diverse suite ofrocks and discussedpossible petrologic relationships among them. One pur-pose of the current study of the igneous rocks from Mag-net Cove is to elucidate the relationships among the var-ious rock types and to compare the petrologic evolutionof Magnet Cove to that of the other intrusions of theArkansas alkaline province.

Metasomatism has played an important role in the pet-rogenetic history of these rocks. All of the samples fromthe Diamond Jo quarry examined thus far show extensivereplacement of primary magmatic minerals by secondaryphases. In this paper, which is the first ofa series on theMagnet Cove complex, we describe both the primary andsecondary mineral assemblages found in ijolite xenolithsfrom the garnet-pseudoleucite syenite that forms part ofthe outer syenite ring of the Magnet Cove complex. We

0003-004x/89 /ol024r I 3$02.00

then consider the petrogenetic relationships ofthe ijolitexenoliths to the garnet ijolite and biotite-garnet ijolitethat form the inner rings of the complex.

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The rocks of Magnet Cove are host to numerous min-eral species, and much of the early literature about thecomplex deals with the minerals. The first detailed petro-graphic study and geologic map of Magnet Cove weremade by Williams (1891). Erickson and Blade (1963) re-mapped the Magnet Cove complex and presented a de-tailed lithologic and geochemical study of the igneousrocks and the surrounding country rocks. A generalizedgeologic map of the complex based on their detailed mapis given in Figure l. Erickson and Blade (1963) concludedthat phonolites and trachytes were intruded first, fol-lowed by jacupirangite and the syenites ofthe outer ring,then the inner-ring ijolites, and finally carbonatite, whichis found in the core of the complex. Erickson and Blade(1963) proposed that the igneous rocks of the MagnetCove complex are related by differentiation and fraction-al crystallization of a residual, volatile-enriched phono-lite magma and that this magma was derived by fraction-al crystallization of an undersaturated olivine basaltmagma.

Exposures of fresh rocks are scarce within the complex.However, the opening of the Diamond Jo quarry in thelatter part ofthe l9th century to supply stone ballast forthe Hot Springs Railroad exposed much fresh unweath-ered rock. This quarry, which is located in the southernpart of the complex and within the outer syenite ring (Fig.

I t 3

r t4 FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS

f f i l a c u p i r a n g i t e

E ' " : ; : ; ; i , ' p h e I i n e

f , l c a r n e t - p a e u d o -l e u c i t e s y e n i i e

f f i P n o n o l i r E , T r a c h y t e

I f l c a r n e t - b i o t i t em e l t e i g i t a

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M e t a g e d i m e n t s

Wrror,B-nocK coMPosrrroNs oFM,LcnBr CovB rror,rrps

Whole-rock major-, minor-, and trace-element com-positions of i jolite xenoliths 5-DJ7, l-166, and l-143(Tables l, 2) reflect the modal diversity of the ijolite xe-nolith suite (Table 3). Included in Tables I and 2 areanalyses ofa garnet ijolite and a biotite-garnet ijolite fromthe inner-ring ijolite for preliminary comparison with theijolite xenoliths. Major- and minor-element composi-tions ofseveral additional ijolites from the inner ring arereported by Erickson and Blade (1963, rheir Table l7).Descriptions by Erickson and Blade indicate that thesesamples are similar to the inner-ring ijolites included inthis study. Overall, the inner-ring ijolites are slightly moremagnesian and contain significantly less F and CO, thanthe ijolite xenoliths.

Of the three analyzed ijolite xenoliths, l-143 containsthe highest abundance of clinopyroxene plus garnet andis enriched in Ti, Fe, and Ca and depleted in Si, Al, andNa compared with samples l-166 and 5-DJ7. Xenolithl-143 is also enriched in V, Y, andZr, compared to sam-ples 5-DJ7 and l-166. These are trace elements com-monly found in notable concentrations in garnets fromalkaline igneous rocks (e.g., Deer et al., 1982 and thisstudy). Schnetzler and Philpotts (1970) also reported thatgarnet has high distribution coefrcients for the heavy rare-earth elements (HREEs) compared to other common rock-forming minerals. In the ijolite xenoliths, as the abun-dance of garnet increases from 100/o in 5-DJ7 to an av-erage of 37o/o in I - I 4 3, the HREE concentration increases(Fig. 3). Garnet ijolite 85-I3B-RSS from the inner-ringijolite has REE concentrations (Table 2) and a REE pat-tern similar to those of the ijolite xenoliths, whereas bio-tite-garnet ijolite 85-16-RSS has a much more fraction-ated REE pattern. Ijolites from the Fen complex, Norway(Mitchell and Brunfelt, 1975), Seabrook Lake, Ontario

In

Fig. 1. Generalized geologic map of the Magnet Cove alkaline igneous complex, Hot Spring County, Arkansas, after Ericksonand Blade (1963). The Diamond Jo quarry (solid triangle) is the sampling site of the ijolite xenoliths described in this paper. Theopen and solid circles mark, respectively, the approximate sampling locations ofgarnet ijolites99423/7 and 85-13B-RSS; tfe openand solid squares mark, respectively, the approximate sampling locations of biotite-garnet ijolites 99423/6 and 85- I 6-RSS.

l), was mapped in detail by Owens and Howard (1989),and a simplified version of part of their map is shown inFigure 2. The two rock types exposed on the walls of thequarry are nepheline syenite (originally mapped by Erick-son and Blade, 1963, as nepheline syenite pegmatite) andgarnet-pseudoleucite syenite. The contact between thesetwo rock types is relatively sharp. The garnet-pseudoleu-cite syenite contains abundant xenoliths ofijolite, rangingfrom a few millimeters to 3 m in diameter. Xenoliths ofStanley shale, now metamorphosed to hornfels, are alsofound within the garnet-pseudoleucite syenite. Prelimi-nary petrographic and electron-microprobe data from theDiamond Jo syenites and ijolite xenoliths were given byRoss (1984) and Flohr and Ross (1985a, 1985b).

Mrrrrors oF ANALYSTs

Mineral compositions were obtained by using an automatedAnL-sruqr electron microprobe operating at 15 kV with a beamcurrent of 0. I pA using either an 8- or 9-spectrometer configu-ration. Correction procedures of Bence and Albee (1968) andalpha-factor modifications of Albee and Ray (1970) were used.During analysis of cancrinite and natrolite, the electron beamwas rastered over an area of 4 pm'? in order to minimize loss ofNa due to mobilization. Data collection procedures were out-lined by McGee (1983, 1985), and backgrounds were calculatedusing an interpolation routine described by M. Mangan and J.J. McGee (written comm., 1985). A variety of natural and syn-thetic materials were used as standards. Traverses of clinopy-roxenes were obtained using an ARL-EMx 3-channel electron mi-croprobe. X-ray single-crystal and powder-diffraction data wereobtained on selected crystals. Whole-rock analyses were ob-tained on three ijolite xenoliths (l-166B, 1-143, and 5-DJ7) andtwo inner-ring ijolites (85-13B-RSS and 85-16-RSS) using ana-lytical methods described by Baedecker (1987).

t Any use of trade names in this report is for descriptive pur-poses only and does not imply endorsement by the U.S. Geo-logical Survey.

FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS l l 5

m s s r ' 2 - D J 1 2

^'{ffi3^}-5t7+s ^-e.99,116 - D J 1 0

:r"-"?if#ii "t. n\

;>9'$'ii;:ii:ili::\-G ,.

4 - D J 1 0

I s - D J l O

J : 1 - 1 1 6

2-Dr7 a? ,_ 01 -DJ7 \ _ r i _ ! -

, l;':".o]-',

T A L U S

D I A i l O N D J O O U A R R Y

O C O R E S A M P L EI B U L K S A M P L E

. / coNTAc r

Fig. 2. Geologic map of the Diamond Jo quarry, MagnetCove complex, after Owens and Howard (1989) indicating lo-cations of all core and bulk samples of ijolite xenoliths and sy-enites collected. Samples 84-1-RSS-A and B are grab samplescollected from the talus slope. Contour interval is 10 ft (3.048m). NS, nepheline syenite; GPS, garnet-pseudoleucite syenite;

Ms, Stanley shale; f, fenite; ns, nepheline syenite xenoliths (?);gi, garnet ijolite xenoliths; ms, metamorphosed Stanley shalexenoliths; MCZ, mafic clot zone; CH and TH, crest and toerespectively, of highwall. The solid triangle marks the locationof survey stat ion DJ8 atlat3426'17"N and long 9251'45'W.

(Cullers and Medaris, 1977) and Oka, Quebec (Eby, 1975),have relatively fractionated REE patterns that are similarto the pattern of the biotite-garnet ijolite from MagnetCove. Mitchell and Brunfelt (1975) noted rhar with in-creasing abundance of garnet, the REE patterns of theijolites from the Fen complex became progressively flat-ter, i.e., the light REEs (LREEs) are less enriched relativeto the HREEs. Abundance of garnet in the ijolites fromMagnet Cove thus exercises significant control over theREE patterns of these rocks.

Despite the range of trace-element compositions in thethree analyzed ijolite xenoliths, these rocks are generallytypical of mafic alkaline rocks (e.g., Gerasimovsky et al.,1968; Gerasimovsky, 1974; Sage, 1987). Relatively highconcentrations of Pb in xenoliths 5-DJ7 and l-166 andlow concentrations of Th. Nb. and Ta. and low Th/U in

all three xenoliths (Table 2) are notable exceptions. Ijo-lites from the inner ring contain significantly less Pb thanthe ijolite xenoliths. Garnet ijolite 85-I3B-RSS sharessome characteristics with the ijolite xenoliths, but is de-pleted in Ba and Rb relative to them, unlike garnet-bio-tite ijolite 85-16-RSS, which contains significantly moreBa. and also Sr. but has a similar amount of Rb. Addi-tional data are needed to fully compare the geochemistryof the ijolite xenoliths and inner-ring ijolites.

Pprnocn-q.pHy AND MTNERAL coMposrrroNs oFTHE IJOLITE XENOLITHS

Nine samples of ijolite xenoliths were collected fromthe Diamond Jo quarry; six are bulk samples (l-143A,l-143B, l-1668, 5-DJ7, 84-l-RSS-A, and 84-l-RSS-B)and three are drill-core samples having diameters of 2.54

l 1 6 FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS

TABLE 1, Whole-rock maior- and minor-element compositions TABLE 2. Whole-rock minor- and trace-element compositions (in(in wt%) of ijolites from the Magnet Cove igneous ppm) of ijolites from the Magnet Cove igneous com-complex, Arkansas plex, Arkansas

l jolite xenoliths,Diamond Jo quarry Inner-ring ijolites

ljolite xenoliths,Diamond Jo quarry Inner-ring ijolites

Sample: 5-DJ7 1 -166 1 -143 85 -13 -n 5 b

85-1 6-RSS

Sample: 1-166 1-143 85-138- 85-16-H 5 5 H 5 5

sio,Tio,Alro3Fer03FeOMnOMgoCaONaroKrOD A| 2 v 5

HrOCO,clFS

Sum

40.5t . o

1 9 . 1c . J2 2

0.213.0

13.1a o

3.60 6 41.400.740 0550 4 5023

1 0 1 . 1

41.12.40

14.25 56'3.270.405.13

16.26.033.201.080.460.280.020.070.21

99.61

34.23 6 1

l o . I

8.1 3'2850 3 07 . 1 6

14.03.402.341.225.990.24

<0.010.040.42

100.0

21 828.810 .82,740.940.5273.200.478

77817187432

1 2 1439

5 0 5280.971 .090 3 1

9804.44.68

<501 252.825.9

1 5 820.51 1 . 03.611 .340.986.510.91

z c . c

21 610.294 6 9

4 7 76.01

37.5 35.42 3 5 . 9

17 9 12.O5.3 8.93 9 3 . 80.19 0.283.8 3.3

14.6 19 .57.3 4.32 .7 1 .70.80 0.721.78 1 .922.0 0.980.140 0.0090.30 0.240.26 0.39

CeNdSmEuTbYbLu

BaRbSrNbTaZlHfYThUSc

CrCoNiCuZnSb

Pb

440 617117 149695 843

12 540.80 3 05

572 17207 0 e e l o

52 3940 1 9 1 5 7

24.5 92.23 8 6 9 319 7 26.04.6 6.81.59 2.460.94 1.315.59 2.030.80 0.204

25 20405 1 1 1 9

678 112021 2241 73 378

714 4213.3 0.6937 261 77 1 .06

Note; Maior oxides, excluding FeO, determined by tcp-AES (inductivelycoupled plasma-atomic emission spectrometry) by M Kavulak, J. Mari-nenko, and R. Rait in the ijolite xenoliths, and by X-ray fluorescence by J.Taggart, A. J. Bartel, and D. Siems in garnet ijolite 85-138-RSS and biotite-garnet ijolite 85-16-RSS. FeO, H,O, CO,, S, F, and Cl determined by varioustechniquesbyE. Brandt,J. H Christie,J. Sharkey, L.Jackson, M Kavulak,J. lVarinenko, and N. Rait.

. Feros calculated from total Fe obtained as Fe,Oa and reported FeO.

cm and lengths ranging from 7.01 to 9.75 cm (4-DJ7,3-DJ7, and 6-DJ7). Sample locations are shown in Figure2. Samples 84-l-RSS-A and -B are grab samples collectedfrom the talus slope.

According to the modal classification of alkaline rockspresented by Sorensen (1974), 4-DJ7, l-1668, l-143A,and 3-DJ7 are melteigites, and the rest of the samples areijolites (Table 3). By definition, ijolites and melteigitescontain between 100/o and 700/o nepheline (Sorensen, 1974).Two of the ijolite xenoliths do not contain any nepheline(Table 3) owing to replacement by cancrinite and natro-lite. For the sake of simplicity and to be consistent withthe terminology used by Erickson and Blade (1963), wewill collectively refer to the suite of samples as ijolites.Photomicrographs (Figs. 4a4d) illustrate the variedmodes and textures found in the ijolite xenoliths and in-ner-ring ijolites.

Despite the variable modes found in the ijolite xeno-liths, mineral compositions indicate that these rocks forma coherent group. The xenoliths experienced varying de-grees of metasomatism resulting in the textural and, tosome extent, the modal variations seen among samples.Metasomatism is also responsible for some of the com-positional differences found in the minerals, both withinindividual samples and among different samples.

Primary igneous minerals are distinguished from later-formed metasomatic minerals by using textural criteria.(Metasomatic minerals are here defined as those mineralsformed as the result of reaction between earlier-crvstal-

ZrlHtNb/TaTh/URb/Ba

2.781 5

721 50.260.27

3.8894

86zo0.890.22

0.72 2.05 1.33 5.990.74 6.30 4.71 2.31

970 1300 810 7003 .6 34 7 .6 3 .6

13.8 16.0 18.9 36.2<50 <70 <70 <80

11 5 .0 18 2647 153 55 823 .20 10 .2 0 .155 0 .19

3 . 1 8 0 . 3 3 1 3 647 <5 <5

54 54 611 8 1 2 5 90 7 7 1 3 0 . 1 8024 0 .075 0 .11

Note: Ba, Rb, Sr, Nb, Y, and Zt delermined by energy-dispersive X-rayfluorescence spectroscopy by J. Evans. V, Cu, and Pb determined byatomic absorption by M Doughton All other elements determined byinstrumental neutron-activation analysis by J. Mee.

lized primary igneous minerals and volatile-rich fluids.These fluids may be residual fluids from the ijolites them-selves or be derived from external sources.) Diopsidicclinopyroxene, nepheline, and garnets of Groups I andIIA (defined in the section entitled Garnets) are identifiedas the primary rock-forming minerals. Primary accessoryminerals, which occur as inclusions within diopside,nepheline, and/or garnets of Groups I and IIA, are pe-rovskite, equant apatite, and magnetite. Progressive re-placement of nepheline by minerals formed during meta-somatism is observed. Total replacement of nephelineoccurs in some of the ijolite xenoliths (Table 3). Replace-ment of nepheline commences by the formation of can-crinite along fractures within the nepheline. In more ad-vanced stages of replacement, intergrown grains ofcancrinite and, usually, subordinate amounts of natroliteand calcite are found, and only small patches of nephelineremain. Magmatic textures are preserved in ijolite xeno-liths 6-DJ7 and 5-DJ7 that contain only partially replacednepheline. In xenolith 6-DJT,laths of diopside are ophit-ically to subophitically enclosed by nepheline, and 5-DJ7

FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS 1t'7

has an intergranular to subophitic texture. As replacementof nepheline by hydrous silicates and calcite progresses,diopside is replaced by biotite. Biotite is never observedreplacing diopside where the latter is in contact with un-altered nepheline. Similarly, low-Ti garnets (Groups IIIand IV) never occur as overgrowths or rims on the high-Ti schorlomite or melanite garnets (Groups I or IIA) wherethese primary garnets are in contact with diopside ornepheline; rather garnets of Groups III and IV formed byreaction between primary garnet and metasomatic fluid.The melanite garnets of Group II are somewhat problem-atic in their petrogenesis, as they rim primary Group Ischorlomite garnets and also form subhedral grains in-tergrown with metasomatically formed phases. Garnetsof Group II may represent a transitional stage of garnetcrystallization. A second generation of apatite forms acic-ular to tabular grains within masses of cancrinite, natro-Iite, and calcite. Aegirine-augite and a second generationof magnetite are associated with metasomatic silicates andcalcite and appear to be contemporaneous with these min-erals.

Clinopyroxenes

Primary clinopyroxenes occur in all the ijolite xeno-liths. In all the samples except 6-DJ7, these pyroxenesare subhedral to euhedral, sector zoned, pleochroic, andpartially replaced by biotite. Fine, oscillatory zoning issuperimposed on the sector zoning in some grains. Mostofthese clinopyroxenes are 2-5 mm long, although grainsup to about 2 cm long are found in the coarsest sample,4-DJ7 (Fig. 4b). (The nomenclature used in describingpyroxenes follows that recommended by the Internation-al Mineralogic Association Subcommittee on Pyroxenes,Morimoto, 1988.) The sector-zoned clinopyroxenes aresubsilicic aluminian ferrian diopsides (Table 4, nos. l4).The overall compositional ranges are as follows: Mg/(Mg

Fig. 3. Chondrite-normalized, rare-earth-elemert patterns forijolite xenoliths ( I - I 43, I - I 66, and 5-DJ7) and inner-ring ijolites(garnet ijolite 85-l3B-RSS and biotite-garnet ijolite 85-16-RSS).

+ Fe2* + Fe3*) :0 .447 to 0.619, Al rO3: 7.15 to l2 . lwt0/0, TiO, : 1.45 to 3.92 wto/o, NarO : 0.45 to 0.70 wt0/0,and MnO : 0.17 to 0.45 wt0/0. CaO is uniformly high,ranging from22.7 to 24.4 wt0/0. Pleochroic light- to me-diumlilac or pink sectors are consistently higher in Al,Ti, and total Fe and lower in Mg than pleochroic light-to medium-green sectors. No differences in the concen-trations of Ca, Mn, and Na were detected between sec-tors. A traverse of one sector-zoned diopside, 2.6 mmacross, from sample 84-l-RSS-Al (subhedral grain shownin the lower left quadrant of Fig. 4a) was made with themicroprobe. Count data for Al, Ti, and Si were collectedat l5-pm intervals. The data indicate that the composi-tional boundary between sectors is abrupt, with Ti andAl significantly decreasing in concentration from the out-er to the inner sectors and Si slightly increasing. Within

o=a l o0oso

o

go.E6

@ r o

Tlere 3. Modes (in vol%) of iiolites from the Magnet Cove igneous complex, Arkansas

5

l jolite xenoliths from Diamond Jo quarry Inner-ring iiolites

Sample: 4-DJ7- 1-1668-1 , 2 4

5-DJ7- 6-DJ7- 84-1-1 ,5 1 ,3 RSS-A I

84-1- 1-1434-RSS-B2 1.2

1-1438-3 3-DJ7- 99423171 994231611 , 2

ClinopyroxeneGarnetBiotiteSpheneApatiteMagnetitePerovskiteNephelineCancriniteNatroliteCarbonateSulfideOther

No points

55 1 37.82 .8 17 .4

1 6 4 1 5 72 8 0 63 9 3 40 2 00 6 00 0 .6

13.4 5 .40 .2 15 54 2 2 . 60 . 2 1 . 00 .1 0

99.9 100

985 907

33.3 28 71 0 4 1 1 28 2 8 . 80 .8 5 .01 3 2 . 90.2 0.40 0 8

28.2 21 I1 3 7 1 9 . 02 9 0U . C U 50 3 0 . 40 . 1 0 4

99 I 100

957 979

16 .6 16 .539 5 34.411 .6 12 .61 .6 0 .81 . 2 1 . 1

trace 00 3 00.3 0

11.4 20.81 5 . 0 1 1 . 71 8 1 . 90.4 trace0 2 0 . 2

99.9 100

975 931

25.8 09.0 16.22 . 7 1 1 . 80 0n o n2.3 7.15 .4 1 .9

52.4- 54.4+0 0.40 0o.2 4.2u,o J .co.4 0.4

o a a o o o

1030 944

19.526.9

1 . 83.4

trace00.5

t o . /

16.05.00.80

99.9

968

19 .419 .78.41 . 52 .1

trace0

11 .426.39 .5l a

o.2o.2

99.9r 007

10.743.921.52.82.30.100.1

11.7

0.10.4

100

908

t Thin sections were obtained from the F. C. Calkins collection of 1918, curated by the National Museum, Washington, D.C. Sample 9942317 is agarnet ijolite and sample 99423/6 is a biotite-garnet ijolite

. Sum of altered (3 2%) and unaltered (49.2'/"]'nephelinei Sum of altered (50 1%) and unaltered (4 3%) nepheline

I l 8 FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS

a -9 ! :

{

a

'i ii

Fig 4. Photomicrographs of thin sections of ijolites taken inplane-polarized light to illustrate varied modes and textures. Fieldofview is 2. I cm across in all photomicrographs. Xenoliths fromDiamond Jo quarry are shown in (a) and (b). (a) Sample 84-l-RSS-AI: euhedral to subhedral "atoll"-structured schorlomitegarnets (black) with euhedral, tabular, sector-zoned clinopyrox-enes marginally replaced by biotite in a light-colored ground-mass of cancrinite, natrolite, and calcite, with minor melanitegarnet (dark) and acicular apatite (lower-left quadrant). (b) Sam-ple 4-DJ7 -2: clinopyroxene (abundant tabular, subhedral to eu-hedral gray crystals) in a light-colored groundmass of cancrinite,

calcite, and euhedral apatite (especially abundant in lower right,near edge of section). Inner-ring ijolites are shown in (c) and (d).(c) Garnet ijolite 99423/7: laths ofclinop).roxene are interstitialto and included in equant (white) nepheline. Black areas aremagnetite replacing perovskite. Clinopyroxene, together withminor garnet and biotite, is also intergrown with the oxidesforming clots or aggregates. (d) Garnet ijolite 85-l3B-RSS: lathsof diopside (gray) are interstitial to nepheline (white) in lowerleft and upper right and form at aggregate in central part of thephotograph. Large black grains are garnet; smaller black grainsare gamet and sulfide.

both sectors, Ti and Al show minor fluctuations in con-centration.

In 6-DJ7, the finest-grained ijolite xenolith, primaryclinopyroxenes form laths up to 2 mm long, some of whichare twinned. Clinopyroxenes from 6-DJ7 arc also subsil-icic aluminian ferrian diopsides (Table 4, no. 5) but are,on the average, more magnesian than the sector-zoneddiopsides from the other ijolite xenoliths, with Mg/(Mg* Fe2* + Fe3*) : 0.555 to 0.729. These diopsides arealso slightly less aluminous than the sector-zoned diop-sides and have AlrO, ranging from 5.61 to 9.07 wtVo.

They are continuously zoned. Some laths are rimmed byaegirine-augite and are not partially replaced by biotiteas are the diopsides from the other xenoliths.

Pyroxenes, which are the products of metasomatism,are found in several of the ijolite xenoliths. In 5-DJ7(Table 4, no. 6) and 4-DJ7, pale- to medium-green pleo-chroic aegirine-augite is intergrown with and is appar-ently replacing bioite, which itself has replaced primarydiopside. Aegirine-augite having a similar compositionoccurs as single grains or aggregates in the groundmass of3-DJ7. Ti-rich aegirine (Table 4, no. 7) in 4-DJ7 occurs

FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS

TaaLe 4. Clinopyroxenes from the ijolite xenoliths, Diamond Jo quarry

l 1 9

Qamnro. 84-1-RSS-82 5-DJ7-24 6-DJ7-1 5-DJ7-2A 4-DJ7-2

Weight percent41.5 43 9sio,

Tio,AlrosV.O.Fer03.FeO-MnOMgoCaONaro

Sum

SiAI

AITi

Fe3*Fe,*MnMg

MnM9

Na

No pts- averaged

39.93.42

11 .2nd

8.374.39o228 1 1

2290.52

99 03

1.5430.4572.000

0.0530.09900.2440.14200.4621.000

0.0070.0050 9490.0391.000

4 1 . 82519.78no

8 4 54.200.238.81

2 3 20.51

99 49

1.6040.3962.000

0.046o.0720o.2440.13500 5031 000

0 0070 0010.9540 0381 000

I

1 .857 7 70.208235 2 70 3 89 . 1 1

23.10.58

100.4

1.6730.3272.000

0.0220.0530.0060.2360.16800.5151 000

0 0120 0020.9430 0431.000

7

45.52.386.210.12I -31

5 . 1 10.409.99

22.70.87

100.8

1 .7190.2791.995

00 0680.0040.2140.16100.5531.000

0.0130.0100.9180.0641.005

4

50.90 . 1 80.890.099.328.350.467.32

1 Q R

3.3899.39

1.9610.0392.000

0.0010.0050 003o 2700.2690.015o 4210 984

000.7630.2531 .016

51 I3.221 . 5 2u.5b

26.61 .200.170.600.70

13.299.57

1.9770.0232.000

0.0450.0920.017o 7640.0380.0050.0340.995

000.0290.9761.005

5

2.308.800.19

10.54.060.308.25

23.20.59

99.69

Cations1 6030 3972.000

0.0030.0670 0060 3050.1310.0100.4750.997

00n oEo

0.0441.003

o

Nofe. nd : not detected. Column headings as follows: 1, 2-Sector-zoned grain with no. 1 from sector richer in Al and Ti than sector characterizedin no 2; 3, 4-Sector-zoned grain with no 3 from sector richer in Al and Ti than sector characterized in no. 4; S-Lath enclosed within nepheline; 6-Secondary aegirine-augite intergrown with and rimming biotite; 7-Fibrous aegirine intergrown with magnetite, garnet, and calcite in groundmass.

* Fe3* and Fe,* calculated by assuming ideal stoichiometry of 4 cations per 6 oxygens.

as fibrous masses intergrown with pale-pink garnet, cal-cite, cancrinite, apatite, and titanomagnetite and alsoforms clusters of fibers in the cores of apatite. It is theonly pyroxene analyzed containing significant amounts ofY,0.47 to 1.04 wto/o VrO..

In the primary clinopyroxenes, Al primarily substitutesfor Si in the tetrahedral site, while Ti enters the octahe-dral site, resulting in the positive correlation between Aland Ti seen in Figure 5a. Clinopyroxenes from 6-DJ7contain amounts of Ti comparable with the other xeno-lith pyroxenes, but generally have slightly lower amountsof Al. Both Al (Fig. 5b) and Ti increase with decreasingMg/(Mg + Fe2+ * Fe3*) in the primary clinopyroxenes.Diopsides from 5-DJ7 cluster at the low Al and Ti endof the main group of pyroxenes and also are at the lowend of the range of Mg/(Mg * Fe2* + Fe3*) for the group.

Garnets

Garnets are classified into four groups (I to IV) basedon compositions. These groups indicate the crystalliza-tion sequence for the garnets. Representative microprobeanalyses are presented in Tables 5 and 6, and data areplotted in Figures 6 and 7 . A detailed explanation of themethods used to calculate the formulae of all the garnets

(Tables 5, 6) is given in Appendix l. The data plotted inFigure 6 represent the complete data set (Groups I-IV),whereas the data plotted in Figure 7 are only those of theF-bearing garnets of Group III. Andradite garnets con-taining > l5 wt0/0 TiO, are classified as schorlomites, andthose containing < 15 wt0/0, as melanites, following thecriterion of Deer et al. (1982).

Group I garnets are dark-red to black schorlomitescontaining 15.5-18.2 wto/o TiO, and occur in all samplesexcept 5-DJ7. Group I garnets have morphologies rang-ing from poikilitic with an "atoll"' structure (Fig. 4a) toanhedral to subhedral grains up to 7 mm across. Com-positions are uniform among samples, and zoning withinsingle crystals is slight. In contrast, garnets in 5-DJ7 aremelanites containing 8-10 wto/o TiO,. Compositionallythese garnets belong to Group II (melanites); however,because they are morphologically different from otherGroup II garnets, they are differentiated as Group IIA.Garnets ofGroup IIA form anhedral to subhedral grainsup to 5 mm across and commonly contain abundant apa-tite inclusions.

Melanite garnets (Group II) have a much wider com-positional range than garnets of Group I. They form light-red to brown rims, up to 50 pm wide, on some of the

t20 FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS

,, +f*f*++

rIo t 4 - r - R s a - a r BA 4 - D J z X r - r 4 o

- r r -

l i : - -3 j ; " ' - ', r r1*- l r " ' . " " , ,

o . o 2 0 . o 4 0 . o 6 0 . o 6 0 . t o o " t 2T i

, ,+* -++ +#+

r

b .

+'t4fi+'f

1 - 5

F

1 , O

o . 5

o1 . 5

1 . o

o . 5

o

2 . O

o- o . 4 0 . 5 0 . o o . 7 0 , 4 o . 9

Mg,z(Mg + F#++ Fe3+)

Fig. 5. Variation of total Al with (a) Ti and (b) Mgi(Mg +Fe'?+ * Fe3+) in primary clinopyroxenes (cations per 6 oxygens)from ijolite xenoliths and garnet ijolite99423/7. Symbol key forboth plots is given in (a).

schorlomites and occur as small subhedral grains in thegroundmass. Melanites contain slightly more Mn and Caand less Al, Mg, and Fe2* on the average than do theschorlomites.

Group III garnets (Table 6) are the most unusual be-cause they contain as much as 3.56 wto/o F. They formcolorless to pale-yellow, slightly birefringent over-growths, which range in thickness from a few microme-ters to about 40 um. on melanites and schorlomites. Theseovergrowths are zoned and compositionally range fromandradites to grossulars (grandite series); we refer to themas fluoro-hydrograndites. Initial microprobe analysesyielded low oxide sums even after analyzing for F, sug-gesting the presence of a hydrogarnet component. Thepresence of OH was verified by Anne Hofmeister (Geo-physical Laboratory), who obtained a mid-IR spectrumofa garnet grain from l-1668. This grain was approxi-mately 30 pm across, was composed of nearly all colorlessgarnet with two small red areas (probably melanite gar-net), and appeared optically to be very similar to the col-orless overgrowth on a melanite garnet from I - 1 668 (Fig.8). The IR spectrum represents an average of the entirecrystal, including the small amounts of red melanite gar-net. The amount of OH (calculated as water) indicated bythe IR spectrum is 0.1-O.5 wtolo (A. Hofmeister, written

o . 52 . O 2 . 7 5 3 . O

Fig. 6. Variation of (a) Ti, (b) Al, and (c) Fe3* with Si ingarnets (cations per 12 oxygens) from ijolite xenoliths. In (a),Group III garnets do not lie along the trend defined by GroupsI, il, IIA, and IV and hence are not plotted so as not to obscurethis trend; only the field where Group III garnets would plot isindicated. Groups are defined in the text. Symbol key for allthree plots is given in (a).

comm., 1986), less than that indicated by the low sumsof the microprobe analyses of other colorless garnets (Ta-ble 6). Microprobe data show a strong negative correla-tion between oxide sums and F contents of the fluoro-hydrograndites, suggesting that the OH content increasesas the F content increases. Given that the IR spectrumrepresents an average ofthe analyzed grain and that mi-croprobe data indicate that the fluoro-hydrograndites aresignificantly zoned with respect to F (Table 6, nos. 4, 5)and probably OH, the low amount of HrO indicated bythe IR data is not totally unreasonable or inconsistentwith the microprobe data. The grain from which the IRspectrum was obtained was unfortunately lost before itcould be afialyzed by microprobe.

The garnets of Group IV are low-Ti melanites and con-

+ 1 . 5a)

oE

1 . O

2 . 2 5 2 . 5

s i

a . + G r o u p II G r o u p t lx G r o u p l l AO G r o u p l l l

+lr[ih.;r++1 r+

A G r o u p l V

I ' l

Hi+*;"1

FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS t2l

tain little or no F. These garnets are the most Fe-rich andAl-poor garnets found in the ijolites and occur both asrims on fluoro-hydrograndite overgrowths and as pale-pink skeletal grains in the groundmass of 4-DJ7 (Table5, no. 7). Enrichment of V occurs in the Group IV gar-nets; over I wto/o V2O3 was found in some overgrowths(Table 5, no. 8).

A strong negative correlation exists between Ti and Siin the Group II, IIA, and IV melanites as well as in theless Ti-rich schorlomites (Fig. 6a). No correlation be-tween Al and Si exists in garnets of Groups I, II, and IV,though one does exist in Group III (Fig. 6b). However,there is a general trend of slightly increasing Al with de-creasing Si from Groups IV to II to I. Group IIA mela-nites have Al contents that are similar to those of GroupI and are generally higher than those of the other mela-nites. Within the more Ti-rich schorlomites, Fer* decreaseswith increasing Si, reflecting Fe3* substitution for Si inthe tetrahedral site. In the melanites, there is a strongpositive correlation between Fe3* and Si, indicating in-creasing Fe3" substitution for Ti in the octahedral site ofthe melanites. Zoning, with F both increasing and de-creasing from outer to inner rim, is found in Group IIIfluoro-hydrograndites. The following trends are found inthese garnets: Si decreases and Al increases as F increases,and Al is inversely correlated with Fe3* (Fig. 7). Thesetrends indicate that as the fluoro-hydrogrossular com-ponent increases, the andradite component decreases. TheGroup III garnets show a small variation in Ti content,and no correlation of Ti4t with F was found. Group IVgarnets lie on the extensions of the trends defined byGroups I and II (Fie. 6).

Single-crystal X-ray examination was made of threegarnet crystals from sample 1-1668. A dark-brown, ho-mogeneous-appearing crystal possesses cubic symmetrywith a : 12.144 A, a value consistent with those of Ti-rich melanites (12.104 to 12.125 A) and schorlomites(12.138 to 12.167 A, Deer et al, 1982, p. 626-627). Alight-brown crystal gave a -- 12.048 A, suggestive of aTi-rich andradite (a : 12.029 A, Deer et al., 1982, p.624).The third garnet crystal examined is nearly colorlessand possesses space group symmetry 1a3 dwth a: 12.067A. ttre very light color and fairty large unit-cell edge sug-gest that the crystal contains a significant amount of Fand OH in the structure. The replacement of SiO" groupswith (OH)4 and Fo groups in the garnet crystal structureis expected to expand the lattice; for example, naturallyoccurring grossulars have a dimensions of I 1.790 to 11.94A lDeer er al., 1982, p. 60a-607), whereas synthetic end-member hydrogrossular, Ca.Alr(OH),r, has an 4 dimen-sion of 12.5n A(Weiss and Grandjean, 1964). The X-raydata are consistent with microprobe analyses that verifiedthat sample l-1668 contains schorlomite, melanite, andfl uoro-hydrograndite garnets.

Phlogopites and biotites

Phlogopite and biotite are most commonly observedreplacing clinopyroxene and are rarely seen as aggregates

"l\..-. .' 'aG3t ' j . .

3 . O

u, 2-7

2 . 5 o . 2 5 o . 5F

o , 7 5

. " l l t .- i -t'-.t!.1--'

i . . ! . t

sf-A r't

o - o . z s o . 5 o . 7 5

FG .

jq?..-

o o f f i "

F e 3 *

Fig. 7. Variation of (a) Si and (b) Al with F (per formulaunit) and (c) variation of A1 with Fe3* for Group III fluoro-hydrograndites (cations per 12 anions; see App. I for the methodof cation calculation) from ijolite xenoliths.

within the groundmass of the ijolite xenoliths. The micasare pleochroic in various shades ofgreen and red or brown.Extinction is often patchy. A large range of mica com-positions is found in the ijolite xenoliths from phlogo-pites with Mg/(Mg * Fe2*) as high as 0.783 to biotiteswith Me/(Mg * Fe2*) as low as 0.095. Representativeanalyses are given in Table 7, and five analyses from 84-l-RSS-B are presented to illustrate the range of compo-sitions in a single sample. Compositions are correlatedwith color: in general, brown or red-brown grains are moreMg-rich than green grains. However, deep-red biotites(Table 7, no. 6) in 6-DJ7 contain very high Fe and aredeficient in (Si + Al) in the tetrahedral site, suggestingthat they may contain small amounts of Fe3* in this site,as identified by Dyar and Burns (1986) in other biotites.Ba was detected only in phlogopites from sample 84-l-RSS-B. Most of the micas have <2.5 wto/o TiOr, but somehave as much as 4.6 wto/o TiOr. There is no clear corre-lation of Ti with Me/(Mg * Fe2*), but very Fe-rich bio-tites generally contain the most Ti. As expected (see Spear,1984, and Munoz, 1984, for discussions of the partition-ing of F in micas), F is positively correlated with Me/(Mg* Fe'?*) (Fig. 9) in micas from the ijolite xenoliths, butphlogopites from the inner-ring ijolites, with a very nar-

122

TABLE 5. Garnets from iiolite xenoliths, Diamond Jo quarry

FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS

Group I Group llA Group ll Group lV

Sample: 3-DJ7-2

I

1-1438-3

2

5-DJ7-2A 3-DJ7-2

4

84-1-RSS.B1

5

1-1438-3

o

4-DJ7-2

7

3-DJ7-2

I

25.417.8

sio,Tio,ZtO2Al,03Vro.FerO".FeO.MnOMgo

NaroNb,osF

Sum

AITiZ l

Fe3*Fe*MgMnCa

Fe,tMnCaNa

S iAIFes*

na5 . Z O

0.5917.32.660.461 2 8

31.40 .16nana

100.3

2.1480.3250.5273.000

01 132

0.0400.5740.0930 1 6 1002.000

0.0950.0332.8450.0262.999

5

24.717.50.803570.52

17.61 .870.331 4 2

31.70 . 1 0nono

100.1

2.0960.3570.5473.000

01 . 1 1 70.0380 0350.5740.0560.1 80002.000

0.0770 0242.8820.0162.999

4

00.6530.0210.0s31.0620.0850.126002.000

0.0970.0292.8650.0103.001

4

2602o.2480.1503.000

00.5740.010

1.2430.0620 . 1 1 1002.000

0 0010.0422.9410.0152.9994

34.72.640.071 . 1 80 3 9

27.10.080.31o.24

33.20 . 1 2ndno

100.0

2.9120.08803.000

0.0290.1 670.0030.0261 7130.0060.0300.0220.0042.000

002.9810.0203.001

3

34.63 .170.180.790.44

27,40 .100.290.24

33.50 .13ndnd

100.8

2.8880.0780.0343.000

00.1990 0080.0291.6890.0070.0300.0210.0172.000

0029790.0213.000

5

34.81 .950 . 1 40.851 . 1 8

z I - h

00 . 1 60.24

33.40.09no0 . 1 8

100.51

2.9140.0840.0023.000

00.1230 0070.0791.73600.0300 0 1 10.0142.000

002.9860.0153.001

4

Weight pelcent302 322 31 010.3 6.51 I 090.44 0.26 0 223.89 2.24 2.510.78 0.54 na

17.8 23.8 22j2 .58 1 .14 0 900.41 0.70 0 591.00 0.64 0.89

31.7 32.1 32.70.06 0.09 0.09nd nd nand nd na

99.16 100.2 100 0

Tetrahedral sites2.5470.3870.0663.000

2.7030.2220.0753.000

Octahedral sites00.4110.0120.0361.4290.0320.080002.000

Dodecahedral sites0.0480.0502.8870.0153.000

4No pts averaged

Notej nd : not detected. na : not analyzed. Column headings as follows: 1, 2-Euhedral to subhedral schorlomites rimmed by melanites and fluoro-hydrograndites; 3-Subhedral melanite; 4-Light-red-brown zone surrounding schorlomite no. 1; 5-Light-red melanite in groundmass; 6-Outer zoneof colorfess overgrowth on schorlomite no.2;7-Pale pink skeletal grain in groundmass intergrown with magnetite, aedirine, and calcite; 8-Outerzone of colorless overgrowth on schorlomite no. 1.

* Fe3t and Fe2* calculated by assuming ideal stoichiometry of 8 cations per 12 oxygens.t Oxide sum adjusted for oxygen equivalent of F.

row range of Mg/(Mg + Fe2*), do not show such a cor-relation. Cl was not detected in any of the micas analyzed.

An unusual intergrowth of biotite, pyrrhotite, ilmenite,minor sphene, and a trace of pyrite occurs in sample 84-l-RSS-A2. The biotite, dark olive green to slightly pleo-chroic red brown, is Fe-rich and contains 2.48-2.98 wto/oV,O, (Table 7, no. 7). Sums for analyses of this biotitewere consistently high, probably indicating analyticalerror, but V-free biotites analyzed during the same anal-ysis session gave good results. Biotites from several oftheother xenoliths were analyzed for V, but it was not de-tected.

Other minerals

Sphene. Sphene occurs as subhedral to euhedral crys-tals in the groundmass of several ijolite xenoliths, as re-

placement rims on perovskite (4-DJ7), and as small an-hedral grains intergrown with biotite and garnet. Sphenecommonly shows pink to colorless pleochroism. Sphenecompositions are varied among the ijolite xenoliths (Ta-ble 8). The low oxide sums may be attributed to the pres-ence of water and/or unanalyzed elements. Fe is reportedas all Fe3* in accordance with the findings of Higgins andRibbe (1976). We consistently found that Fe3* and Al areinversely correlated with Ti, indicating that these two ele-ments are substituting for Ti in the octahedral site.

Oxides. Primary magnetite occurs with diopside andas inclusions in garnet in 6-DJ7 (Table 9, nos. l-3), andsecondary magnetite is intergrown with aegirine in thegroundmass of 4-DJ7. Rims of primary grains are slightlyenriched in Ti relative to cores. Laths of Ti-free magne-tite (Table 9, no. 4) within clinopyroxene in 4-DJ7 are

FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS 123

TABLE 6. Fluoro-hydrograndites of Group lll from ijolite xeno-liths, Diamond Jo quany

Sample: 1-1668-1 1-1438-3 3-DJ7 -2 84-1-RSS-81

sio,Tio,ZrO,Atro3VrO"FerO.'FeO.MnOlVgOCaONaroNbrosF

Sum

Sum(H,O)t

SioHl4Fl4AI

Weight percent31.7 34.7 33.72.47 235 3.360.09 0.05 nd

10.9 904 7 .720.44 0.27 0.28

13.9 17 .0 17 .70 0 00 0 8 0 . 1 6 0 1 30 1 9 0 . 1 8 0 . 2 6

36.3 35 1 35.30.04 0.03 0.05nd nd nd2 . 9 4 1 0 2 1 . 1 0

99 05 99.90 99 60-1.24 0.43 -0.46

97.81 99.47 99 14219 0.s3 0 86

Tetrahedral sites2.471 2778 2.7070.285 0 070 0.11s0.181 0.065 0.0700.063 0 089 0.1083.000 3.000 3 000

Octahedral sites0 939 0.766 0.6230.145 0,142 0.2030 003 0 0020.028 0.017 0.0180.815 1.024 1 .0700.005 0 .011 0009o 022 0.021 0 0310.038 0.016 0.0461 995 1.999 2 000

31.8 33 .61.76 3.470 j2 0 .41

11.2 4.870.21 017

15.0 21 30 0o 12 0 .230.18 0.29

3s.8 34.40.07 0.12nd nd3.13 1 .09

99 39 99.95-1.32 -0 4698.07 99.491 93 0.51

2.481 2.7490.256 0 0680.193 0 0710.070 0.1123 000 3.000

0.961 0.3570.103 0 .2130.005 0.0190.013 0.01 10 .882 1 .3110 008 0,0160 021 0.0350 007 0.0332.000 1.995

2.990 2.9810.01 1 0 0193.001 3.000

8.000 7.995

0.773 0.2821.024 0.270

10.203 11.448

AIl l

Zr

Fe3*MnMg

CaNa

Sum

FOHoNo. pts.

averaged

Notei See App. 1 for detailed explanation of the method used to calculatecations for fluoro-hydrograndites. nd : not detected. Columns are asfollows: 1-Outer zone of overgrowth; 2-lnner zone of overgrowth onschorlomite no. 2, Table 5; the outer zone of this overgrowth is colorlessmelanite (Table 5, no. 6); 3-lnner zone of overgrowth on melanite (Table5, no 4); the outer zone of this overgrowth is the V-rich garnet given inTable 5, no 8; 4,S-Averages of the outer and inner zones of an over-growth on a groundmass melanite (Table 5, no. 5).

- All Fe converted to Fe3*.t HrO calculated by difference from 100%.

partially replaced by pyrrhotite. I-aths ofpyrrhotite occurwithin garnet and clinopyroxene in other samples, indi-cating that replacement of magnetite by the sulfide wascomplete.

Polysynthetically twinned perovskite is purple-brownand occurs as rounded inclusions in clinopyroxene (4-DJ7), as blebs being replaced by garnet (6-DJ7), and as

Fig. 8. Zoned fluoro-hydrograndite overgrowth on melanitegarnet in ijolite xenolith 1-1668. Field of view is 1.5 mm acrossthe long dimension; plane-polarized light. G : melanite garnet,Fg : fluoro-hydrograndite, B : biotite, A : apatite, C : can-cnnlte.

grains rimmed by sphene within the altered groundmassof 4-DJ7. Perovskites in both samples have essentiallyidentical compositions (Table 9, nos. 5, 6) and Iowamounts of all minor elements. Ilmenite intergrown withpyrrhotite and biotite in 84-1-RSS-A2 is Mn-rich (Table9. no. 7) and contains minor amounts of V.

Apatite. Compositions (Table l0) and textural rela-tionships indicate that two generations of apatites arepresent. Apatites of both groups are inclusion-rich. Rep-resentative gmins from both groups were analyzed in threesamples. The early-formed apatites occur as euhedral,equant inclusions in clinopyroxene and schorlomite gar-net and contain less F than late-formed acicular (up to 7mm long) to tabular grains observed in the groundmass.Zoting was not detected in either group.

Cancrinite, natrolite, and nepheline. Cancrinite andnatrolite, in addition to calcite, replace most of the nephe-line and form most of the groundmass of the ijolite xeno-liths. Analyses are given in Table 1 1. Natrolite and can-crinite were also confirmed optically and by using X-raypowder diffraction. Nepheline that is only partially re-placedby cancrinite, however, occurs in 5-DJ7 and 6-DJ7.Nepheline compositions lie within the Morozewicz-Buer-ger convergence field (Tilley, 1954); the analysis in Table

2.9940.0063.000

/ -YY5

Dodecahedral sites2 995 2.9920.005 0.0083 000 3.000

7.999 8.000

0.258 0.2790 281 0 461

1 1 . 4 6 1 1 1 . 2 6 0

0.7251 139

1 0 1 3 6

6

124 FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS

TleLe 7. Biotites and phlogopites from ijolite xenoliths, Diamond Jo quarry

Sample: 84-1-RSS-B 6-DJ7-1 84-1-RSS-A2

sio,Tio,Alr03V,O.FeOMnOMgo

BaONaroK"oF

Sum-F=O

Sum

D I

AI

AITi

FCNlnMg

UABaKNA

Sumr

N4olarMg/(Mg + Fe,.)

Weight percent

1 .8013.7

24.90.529.720 . 1 1no0.149.72U . 3 J

s6.24-0.22

96.02

Cations2.770't.230

4.000

0.04s0.107

1.6430.0351 .1452.975

0.00900.9800.0201.009

7.9840.132

0.411

3 7 31 9 3

15.4na

1 1 . 6o.23

1 8 . 60.070.540.20

1 0 01.45

97 320.61

96.71

2.7501.2504.000

0.0850.107

0.7170.0142.0462929

0.0060.0160.s420.029n ooa

7 9620.338

0.740

36.60.98

15.3na

17.20.40

14.9ndno0.09

1 0 . 10.82

96.39-0.35

96.04

2.7731 2274.000

0.1 390.056

1.0890 0261.6822.992

000.9790.0130.992

7.9840.196

0.607

33.41 .42

14.7na

32.10.744.99nono0.06o R l

0.1397.05-0.0s

97.00

2.7011.2994 000

0.1 020.086

2.1730.0510.6023.014

000.9810.0090.990

I 0040.033

0.217

3 2 73.74

14.0na

34.1U.OJ

2350.06no

o .129.41no

97 .11

2.675| .5ac

4.000

0.0220.230

2.3340.0440 2862.916

0.00500.9810.0191.005

7.921

0.1 09

33.41 9 8

12.0na

35.31 .552 . 1 8nono

0.099.20no

95.70

2.8021 . 1 9 03.992

00.125

2.4770.1 100.2722 984

000.9840 0150.999I . J I J

0.099

34.41 .52

13.82.70

28.80.666.08ndnd

0 . 1 19.54nd

97.61

2.730't 2704.000

0.0210.0910.1711.91 10.0440.718z.Jco

000.9640.0170.981

7.937

0.273,Vofe; Cations calculated to 11 oxygen equivalents assuming all Fe as FeF*. nd : not detected. na : not analyzed. Columns are as follows: 1 to 5-

Single-point analyses ot phlogopite and biotite replacing pyroxene; 6-Bright-red biotite intergrown with magnetiie replacing pyroxene (3-point average);7-Biotite intergrown with ilmenite and pyrrhotite (6-point average).' F per formula unit calculated by assuming (OH + F) : 2.

ll has a composition of Nez4,Ksrroetzoro, indicatingcrystallization or equilibration at temperatures <500"C (Hamilton, 1961). Such low temperatures suggestpostcrystallization re-equilibration. Calcite was analyzedqualitatively using energy-dispersive analysis on the mi-croprobe, and Sr was the only minor element detected.

Single-crystal X-ray examination of cancrinite fromsample l-1668 gave the following crystallographic data:hexagonal, space group P6r, a: 12.63 A, and c : 5.1l8A, indicating that the crystal is composed for the mostpart of layers in AB AB stacking. However, weak reflec-tions appear at nonintegral values along reciprocallatticerow lines parallel to c*, indicating that the cancrinitestructure has some sort of stacking disorder; for example,see Rinaldi and Wenk (1979).

PnrnocnapHy AND MINERAL coMposrrroNs oFTHE INNER-RING IJOLITES

Preliminary study of the inner-ring ijolites is summa-rized here. Sample 99423/6 is from the east side of the

main body of biotite-garnet ijolite, and sample 99423/7is from the main body of garnet ijolite located approxi-mately l.l km north of Diamond Jo quarry Gig. l). Themodes of these samples, which were collected by F. C.Calkins of the U.S. Geological Survey in 1918, are givenin Table 3. Samples of inner-ring ijolite collected as partof this study include garnet ijolite 8 5- I 3B-RSS and bio-tite-garnet ijolite 85-16-RSS (Fig. l). Neither of the bio-tite-garnet ijolites studied contains any clinopyroxene;however, Erickson and Blade (1963) did report a veryminor amount of diopside in one of their samples. Cli-nopyroxene may have been totally altered to biotite inmost of the rock.

Garnet ijolite

Garnet ijolite (sample 99423/7, Fig. 4c) is texturallymore similar to the finest-grained ijolite xenolith fromDiamond Jo (sample 6-DJ7) than to the coarser-grainedxenoliths. The silicates and oxides in 99423/7 form ag-gregates in places, with the silicates normally surrounding

FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS r25

Trele 8. Sphenes from ijolite xenoliths, Diamond Jo quarry

Sample: 4-OJ7-2 3-DJ7-2 r-1668-1

Weight percentsio,Tio,ZrOzAl203V,O"Fer03*MnOMgoCaONa.ONbro5F

2 9 434.20.211.350 7 12.74no0 . 1 6

27.90.040.561.07

98.34-0.45

97.89

30.33 8 4

0 1 00.210.380.94no0 . 1 6

28.40 .15nd0.45

99.49

30.2J C . C

0 8 31 . 1 80.291.90nd0 . 1 5

28.40.07nd0.66

99.1 80.28

98.90

Notej nd : not detected. Each analysis reported is a 3-point average.Columns are as follows: 1-Overgrowth on perovskite; 2-Euhedral grainin the groundmass; 3-Subhedral grain in the groundmass within an"atoll" garnet

- All Fe reoorted as Fe3*.

the oxides. The garnet ijolite contains laths of diopside,some of which are twinned. interstitial to. and includedwithin anhedral to equant grains ofnepheline. The diop-sides (Table 12) contain less Al and Ti than do the diop-sides from the ijolite xenoliths, but the same correlationsbetween Al, Ti, and Mg/(Mg * Fe2* * Fe3*) are observed(Figs. 5a, 5b). Garnet is continuously zoned, from red-brown cores of melanite composition with moderate Ticontents to golden-brown rims with a lower Ti content(Table 12, nos. 3, 4). Phlogopites are pleochroic (light todark brown) and some grains have green rims IhaI are

TABLE 9. Oxides from iiolite xenoliths, Diamond Jo quarry

Fig. 9. Variation of F (per formula unit of I I anions) withMg/(Mg + Fe) in phlogopites and biotites from ijolite xenoliths,garnet ijolite 99423/7 , and biotite-garnet ijolite 99423/6. Circledanalyses from sample 84-l-RSS-A are of V-bearing biotites in-tergrown with ilmenite and pyrrhotite.

depleted in Ti relative to the brown cores (Table 12, nos.12, l3). Cores and rims have essentially the same Mg/(Mg + Fe2*) ratio, with an overall range of 0.798 to0.850, and there is no correlation ofthat ratio with F aswas found in micas from the ijolite xenoliths (Fig. 9).However, F does show a strong positive correlation withTi in the phlogopites from the garnet ijolite; such a cor-relation was not found in micas from the ijolite xenoliths.Nepheline is essentially unaltered, unlike nepheline in theijolite xenoliths. Perovskite is common and is intergrownwith and rimmed by both magnetite and garnet. Mag-netite in the garnet ijolite is homogeneous (Table 13, no.l). The overall Nb content of the perovskites (Table 13,nos. 4, 5) ranges from 0.63 to 1.82 wto/o NbrOr, and Nb

Sum- F = O

Sum

- 0 1 999.30

a 8 4 - I - R S S - A r Bl 1 - r 6 6 B. 4 - D J 7+ 6 - D J ? //x g s 1 2 g / 6 / Lt t s 1 2 3 / 7 / a L .

rl+

M g / ( M g + F e )

Magnetite Perovskite llmenite

6-DJ7-1 4-DJ7-2 4-DJ7-2 84-1-RSS-A2Sample:

Weight percentsio,Tio,Alr03LarO"VrO.FerO"*FeO.MnOMgoCaOSrONaroNbros

Sum

No. pts.averageo

no7.970.82

0.77E l (

36.71 .690.07ndnanand

99.52

no1 1 . 60.32na

0 8 044.239.2

2 . 1 40.03nonanano

98.30

no0.04nona

0.0368.030.7

ndnononanano

98.77

2

no54.80 . 1 6u.5/0.54

1 . 2 10.220.23

40.30.280.350.72

99.38

6

no55.20 .140.460.60

0.87o.170.24

40.5o.21o.17o.78

99.34

o

nd8.800.78nao.77

50.136.82 . 1 90.1 10.0snanano

99.60

no50.60.05na0.592.48

37.47.970.06ndnanand

99.15

o

/Vote.'nd : not detected; na: not analyzed. Cr analyzed for but not detected in magnetite and ilmenite; Zr analyzed for but not detected in perovskite.Columns are as follows: 1, 2-Rim and core, respectively, of groundmass grain; 3-lnclusion in garnet; 4-Magnetite with pyrrhotite lath withinclinopyroxene; s-Perovskite being replaced by garnet; 6-Perovskite being replaced by sphene; 7-llmenite intergrown with V-bearing biotite andpyrrhotite.

'Fe3* and Fe,* calculated by assuming ideal stoichiometry of 3 cations per 4 oxygens for magnetites and 2 cations per 3 oxygens for ilmenite; allFe as Fe'?* for oerovskite.

126

TABLE 10. Apatites from ijolite xenolith, Diamond Jo quarry

Sample: 84-1-RSS-81

FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS

TABLE 11, Nepheline and alteration products from ijolite xeno-liths, Diamond Jo quarry

Cancrinite Natrolite Nepheline

Sample 1 -1 66B-1 1 -1 668-1 5-DJ7-2A

Weight percentsio,LarO"CerO.FeOMnOMgoCaOSrOBaONaroPrO.T

clSum

- F = O

Sum

0.690.080 1 40 . 1 7no0 . 1 3

55.30.56no0.03

39.4z - I and

99.25- 1 1 6

98.09

sio"AtrosFeOMnOMgoCaONaroK.o

Sum

34.428.80.07no0.087.67

1 7 10.04

88 .16

4 6 027 10.08no0.05023

14.9026

88.62

40.835.4

0.70ndno0 . 1 1

15 .97.28

100.20

Weight percent0 1 8nd0 1 10 1 3no0 . 1 3

55.00.65no0.03

41 . 13.71nd

101 .04- 1 5 699.48

Nofe. nd : not detected. Columns are as follows: 1-3-point averageof a grain from the groundmass. 2-3-point average of inclusions withinclinopyroxene

is enriched in cores relative to rims. Minor amounts ofpyrrhotite are found, commonly included within perov-skite.

Garnet ijolite 85-l3B-RSS has a variable texture (Fig.4d) in which laths ofdiopside form aggregates. Other partsof the same sample are texturally more similar to sample99423/7 (Fig. 4c) and have similar proportions of pyrox-ene and nepheline. Preliminary microprobe data fromgarnet ijolite 85-l3B-RSS show that this sample is slight-ly more evolved than ijolite 99423/7. The same zoningtrends are found in diopsides from 85-I3B-RSS, butoverall the diopsides are more Fe-rich with a range ofMg/(Mg * Fe2* + Fe3+) from 0.660 to 0.753. Garnets,which are more abundant than in 99423/7, are zonedfrom cores of schorlomite composition to rims of melan-ite composition. Nepheline is essentially unaltered. Pe-rovskite and magnetite are not found, and pyrrhotite ismore abundant than h 99423/7. Biotite, pyrite, calcite,and apatite are observed as minor phases.

Biotite-garnet ijolite

Both samples of biotite-garnet ijolite (sample 99423/6was originally termed "biotite ijolite" by Calkins but wascollected from an area mapped as biotite-garnet ijolite byErickson and Blade, 1963) are coarse grained and textur-ally distinct from the ijolite xenoliths and the inner-ringgarnet ijolites. Pyroxene was not found in either samplestudied. Garnet from sample 99423/6 is intergrown withand rims magnetite and also replaces perovskite. It has awider range of compositions (Table 12, nos. 5-8) thangarnets from the garnet ijolite but exhibits similar opticaland chemical zoning (dark-red, Ti-rich cores to golden-brown, less Ti-rich rims). Anhedral yellow to nearly col-orless garnet is intergrown with calcite and phlogopite.Phlogopites ftom 99423/6 have pleochroic brown coresand green rims, are optically and chemically similar (Ta-

Nofe. nd : not detected.

ble I 2, nos. 9, I 0) to phlogopites from garnet ij olite 99423/7, and have an overall range of Mg/(Mg + Fe2*) from0.850 to 0.867. Brown phlogopite in the biotite-gametijolite is, on the average, sliehtly less Ba-rich and Fe-richthan that in the garnet ijolite. Phlogopites from both ofthe inner-ring ijolites are distinct from micas from theijolite xenoliths (Fig. 9). Nepheline is partially altered toa material that forms very fine grained, brown, feltedmasses and that contains laths of light-green phlogopite(Table 12, no. ll). This phlogopite is more aluminousand contains more Ba, but less F and Ti, than the greenphlogopite that rims groundmass phlogopite, indicatingthat the two green micas are not contemporaneous. Mag-netite rims both perovskite and phlogopite and has a wid-er range of compositions than magrretite from the garnetijolite. It is depleted in Ti where it is in contact withperovskite (Table 13, nos. 2, 3). Perovskite ftom99423/6has a smaller range of NbrO, contents (0.34-1.13 wto/o;Table 13, no. 6) than was found in perovskite from thegarnet ijolite.

Biotite-garnet ijolite 85-16-RSS is texturally like sam-pIe 99423/6, and preliminary microprobe data indicatethat mineral compositions in these two samples are es-sentially the same. Z,eolite, belonging to the thomsonite-gonnardite series, partially replaces nepheline.

DrscussroN

Relationships between the ijolite xenoliths andinner-ring ijolites

Erickson and Blade (1963), on the basis of field rela-tionships, concluded that at least some of the inner-ringijolites from Magnet Cove were younger than the garnet-pseudoleucite syenite and other alkaline syenites that formthe outer ring of the complex. The ijolite xenoliths-whichoccur within garnet-pseudoleucite syenite at Diamond Joquarry and hence must be older than this syenite-werenot considered by Erickson and Blade (1963) in their dis-cussion of age relationships because these xenoliths ap-peared to be megascopically distinct from the inner-ringijolites. Ijolites from other alkaline igneous complexes arealso mineralogically and chemically heterogeneous. Insome complexes, such as Usaki and Homa Mountain,

FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS l2'7

TABLE 12. Silicates from garnet ijolite 99423/7 and biotite-garnet iiolite 9942316, Magnet Cove igneous complex, Arkansas

PyroxeneGarnet'i l i l

Phlogopite

Sample: 99423/6 9942317

1 2 1 3

IV

1 0 'l 'l

sio,Tio,ZrO"Alr03V"O.Fe2O3*'FeO..MnOMgo

BaONaroKrONbro5F

Sum-F=O

Sum

No ptsaveraged

27.014.8u . / o1 .640.42

19 .51 .760.361 .48

32.Ona

0.07nanono

99 79

52 8 48.50 1 8 1 6 0na na

0 71 3.94nd nd

2.97 5.97267 2900 40 0.36

15.0 12.624.8 23.7

33.1 34.76 24 2.68nd nd226 1 .930 34 0.28

23j 26.71 .13 0.O20.30 0.260.92 0.63

32.8 33.2nanonanono

100.4

3

Weight percent30 3 34.1 35.611 3 3.63 0.94

nd nd nd1 81 2.35 2.80o.22 0 29 0.19

20.0 25.0 26.11.48 0.33 0.010.39 0.34 0.301.45 0.72 0.46

32.6 32.8 33 0na na na0.06 nd ndna na na0.06 nd ndnd nd nd

99.67 99.56 99.40

37.7 38 31.46 0.50

36.12.23na na

15.2 15.40.08 0.06

9.24 7.330.38 0.40

21.0 23.1nd nd1 2 2 0 . 9 80.09 0.079.s3 9.39na na1.02 1 .43

96 09 97 290.43 -0.60

u5.bb vb .bv

37.80.38na

17.9nd

38.30.83

na na1s .4 15 6nd nd

na na na0.36 0.69 0.05nd nd nana na nona na no

99 89 100 3 100 2

6.98 6.91 6.420.39 0.37 0.42

22.9 23.6 23.1nd nd nd0.70 0.62 1.070.26 0 .25 0169.70 9.88 I43na na na0 66 0.63 nd

96.15 96.66 96.68-0.28 -0.26

95.87 96.40

Notei nd : not detected. na : not analyzed Columns are as follows: 1 , 2-Core and rim, respectively, of pyroxene lath; 3, 4-Red core and lighter-colored rim, respectively, of garnet; 5 through 8-Darker-red core, lighter-red zone, yellow zone, and colorless rim, respectively, of garnet; 9, 10-Brown core and green rim, respectively, of phlogopite; 1 1 -Green laths of phlogopite in altered nepheline; 1 2, 13-Brown core and green rim, respectively,of phlogopite

. Roman numerals refer to garnet group designations as defined in the text.- Fe3* and Fe,t calculated by assuming 4 cations per 6 oxygens for pyroxenes and by assuming 8 cations per 12 oxygens for garnets; all Fe as

Fe'?* for phlogopites

Kenya (Le Bas, 1977), more than one intrusion of ijoliteoccurred. Progressive crystallization with late-stage deu-teric or metasomatic alteration also accounts for some ofthe observed heterogeneities in ijolites from these com-plexes. Mitchell and Brunfelt (1975) concluded that rocksof the urtite-ijolite-melteigite series at the Fen complex,Norway, are related by low-pressure differentiation of asingle parent magma.

The ijolite xenoliths and garnet ijolite from the innerring of Magnet Cove have similar mineral and chemicalcharacteristics. The trace-element composition of thebiotite-garnet ijolite, in particular its REE pattern and itshigh concentrations ofNb, Ba, and Sr, set it apart fromall the other Magnet Cove ijolites. Additional data arerequired to fully assess the petrogenetic relationship be-tween the ijolite xenoliths and inner-ring ijolite and todetermine if more than one ijolite magma was present.

Clinopyroxenes from garnet ijolite 99423/7 (and also85-l3B-RSS), although more magnesian than those fromthe ijolite xenoliths, lie along the same trend defined byxenolith pyroxenes on the plot of Al vs. Mg/(Mg * Fe2**Fe3*) (Fig. 5b). Two trends are found on the Al-Ti plotof clinopyroxene compositions (Fig. 5a); one is formedby the ijolite xenoliths excluding 6-DJ7, and the secondis formed by 6-DJ7 and inner-ring ijolite 99423/7. Sorneanalyses, particulady from xenoliths 4-DJl and l-143A,are intermediate between the two trends, suggesting that

TABLE 13. Oxides from garnet ijolite 9942317 and biotite-garnetijolite 99423/6, Magnet Cove igenous complex, Ar-kansas

Magnetite Perovskite

Sample: 99423t7

Weight percent2.76 6.29Tio,

ZrO2Alr03V.O.La2Og

FerO..FeO-MnOMgo

SrONaroNbro5

4.26na0.830.77na

E O A

J U . O

2 . 1 51 .45nonanano

99.66

5 6 40 0 60.070.600.25

0.780 .140.23

4U.O

0.310.230.75

100.4

o

0.89 1.460.68 0.70

55.5 57.1nd na0.04 0.050.55 0.570.49 nana na

624 55 12 7 3 2 8 02.83 4 812.13 2.660.05 ndna na

0.96 0.930.08 nd0.22 0 22

40.1 40.3039 na0.39 na1 57 0.85

100.3 100.0

na nand 0.06

99.04 99.08

No- pts.averaged 8

Note; nd : not detected. na : not analyzed. Cr analyzed for but notdetected in both magnetites and perovskites. Columns are as follows: 1-Magnetite replacing and intergrown with perovskite, range of TiO, is 3.73to 5.28 wt%; 2, 3-Rim and core, respectively, of magnetite ad.iacent toperovskite; 4, s-Rim and core, respectively, of grain enclosed by mag-netite; 6-Perovskite being replaced by magnetite

'Fe3* and Fe,* calculated by assuming ideal stoichiometry of 3 cationsper 4 oxygens for magnetites; all Fe as Fe'?* for perovskites.

t28 FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS

a continuum in compositions exists. The more Mg-richdiopside compositions found in inner-ring ijolite 99423/7indicate that it is a less-evolved rock than are the ijolitexenoliths.

Garnets from the inner-ring ijolites belong to GroupsI, II, and IV and, if plotted with the garnets from theijolite xenoliths, would lie along the same trends as thesegarnets (Fig. 6). Inner-ring ijolile 99423/7 contains gar-nets that are zoned to less Ti-rich compositions than mostof the ijolite xenoliths and also contains significantly moreperovskite than do the xenoliths. Perovskite in xenolith6-DJ7 is almost completely replaced by garnet that iszoned to more Ti-rich compositions than garnet in inner-ring ijolite 99423/7 , indicating that with progressive crys-tallization of the ijolites, early-formed perovskite was re-placed by increasingly more Ti-rich garnet.

Magnetites from 6-DJ7 and99423/7 have similar con-centrations of Al, Mn, and V, bnt 99423/7 magnetitescontain more Mg and less Ti, consistent with their co-existing with more magnesian and less titaniferous py-roxenes. Perovskite from the ijolite xenoliths and inner-ring garnet ijolite have similar minor-element concentra-tions. Perovskites from inner-ring biotite-garnet ijolite99423/6 appear unaltered and have compositions similarto perovskites from both the ijolite xenoliths and garnetijolite 99423/7.

Metasomatic minerals

The occurrences of cancrinite, natrolite, calcite, biotite,sodic pyroxenes, and andradite are not unusual as late-stage metasomatic phases in ijolites and associated al-kaline rocks. Le Bas (l 977) provided detailed petrograph-ic descriptions of a variety of ijolites from the alkalineigneous complexes of the Homa Bay area of Kenya, in-cluding those of secondary minerals formed as the resultof reaction between magmatic phases and metasomaticfluids derived from intrusion of carbonatite or other al-kaline siliceous rocks. In ijolite from the Usaki complex,Le Bas (1977) observed late-stage development of calciteand K-feldspar and replacement of nepheline by cancri-nite and natrolite, melanite by andradite, and aegirine-augite by aegirine and biotite. With the exception of thedevelopment of K-feldspar, the formation of these meta-somatic minerals-which Le Bas attributed to reaction ofresidual fluids derived from the later intrusion of a wol-lastonite urtite-parallels that observed in the ijolite xe-noliths from Diamond Jo quarry. Secondary minerals, in-cluding those just described, are also identified (e.9., byLe Bas, 1977) as forming as the result of deuteric altera-tion.

The replacement of nepheline by cancrinite and na-trolite and of diopside by biotite appear to be contem-poraneous reactions. As nepheline reacts with fluids richin CO, and HrO, K is released that is then free to formbiotite. The replacement of diopside releases Ca and mi-nor Si that are then available for the formation of calcite,cancrinite, natrolite, and fluoro-hydrograndite.

Minor amounts of sphene occur in all of the garnet

ijolites, but it is not found in the inner-ring ijolites. Spheneforms subhedral to euhedral grains in the groundmass of4-DJ7 and 6-DJ7, indicating that it crystallized at thesame time as the other metasomatic phases. ln 4-DJ7, asample poor in garnet, sphene replaces perovskite. Spheneis also intergrown with biotite and fluoro-hydrogranite inother ijolites, indicating that it formed after primary cli-nopyroxene ceased crystallizing.

Garnets of the schorlomite-melanite-andradite seriesare described as being both primary and metasomatic inorigin (e.g., Le Bas, 1977; Sorensen, 1974).In the ijolitesfrom Magnet Cove, garnets, with progressively increasingFe3t and decreasing Ti, span the range from magmatic tometasomatic. In the ijoite xenoliths, garnets of Group Iand IIA began crystallizing after nepheline and diopsidebegan to form, and then cocrystallized with diopside.Melanites of Group II may represent an intermediate stagebetween magmatic and metasomatic crystallization, asthey occur both as rims on schorlomites and as grainsintergrown with the hydrous alteration products ofnepheline. Garnets of Groups III and IY are exclusivelyassociated with other metasomatic minerals and crystal-lized with them. Garnets of Group III are not found inthe ijolites from the inner ring. Low-Ti melanites of GroupIV are associated with calcite and biotite in the garnet-biotite ijolites and thus formed with these metasomaticminerals.

The fluoro-hydrograndite formed by the reaction ofearly-formed garnet with F-rich metasomatic fluids. Oc-currences ofF in garnet have been reported from severaldifferent geologic environments. Nash and Wilkinson(1970) described a melanite garnet containing 0.8 wt0/0 Ffrom a Na-rich syenite in the Shonkin Sag laccolith. Oth-er occurrences of F-bearing garnets are reported fromskarns and various types of mineral deposits (Valley etal., I 983; Van Marcke de Lummen, I 986; Dobson, 1982;Yun and Einaudi, 1982; Madel et al., 1987; Gunow etal., 1980). The highest concentration ofF in garnet wasreported by Madel et al. (1987) who described a spessar-tine garnet containing up to 3.7 wto/o F.

Valley et al. (1983) described grossular garnets with upto 0.76 wto/o F from Adirondack calc-silicates. They pro-posed two substitutions by which F and OH could enterthe garnet structure: (F,OH)414- for (SiOo)o and (Al3*'g:-; for [(Mg,Fe)'z*(F,OH) ]. Our analyses show an in-verse correlation of F and Si (Fig. 7), consistent withFX- for (SiO4)4 substitution mechanism. Ca * Na, andin many instances Ca alone, exceeds 3 cations per for-mula unit, with a concomitant and nearly equal deficien-cy of octahedrally coordinated cations (Table 6). Crystal-structure determinations of a synthetic deuteratedgrossular garnet by Forman (1968) verified that (DO)Xcan completely substitute for (SiO")* and also showedthat all three polyhedra, (DO)., CaOr, and AlOu, are larg-er than the equivalent polyhedra in grossular garnet,Ca.AlrSirO,r. It is possible that the introduction of F andOH expands the structure and allows Ca to enter the oc-tahedral site. Although the substitutional mechanism

(Al3+.O'? ) for [(Mg,Fe)'t.(OH,F) ] proposed by Valley etal. (1983) is crystal-chemically feasible, the low amountof Mg, the lack of correlation of Mg content with F, andthe good assumption that no Fe2* is present in the struc-ture lead us to believe that this type of replacement is notimportant in the Diamond Jo fluoro-hydrograndites.

The unusual intergrowth of pyrrhotite, ilmenite, andV-bearing biotite texturally appears to be a replacementassemblage, not a xenolith. The biotites, except for theirV content, fall within the range of compositions of bio-tites that replace primary clinopyroxene and are Fe-rich(Fig. 9, Table 7), indicating relatively late crystallization.The primary minerals were probably magnetite and cli-nopyroxene, given that magnetite is partly replaced bypyrrhotite and clinopyroxene is partly replaced by biotite.

The enrichment of V in minerals formed during late-stage metasomatism of the ijolite xenoliths (Group IVgarnets, aegirine, biotite) indicates that V either was in-troduced with externally derived metasomatic fluids orwas mobilized during metasomatism. We have found(unpub. data) similar enrichments of V, and also Ti, inlate-stage minerals in syenites from Magnet Cove and arecurrently assessing the significance of such enrichmentsin relation to the economically important V and Ti de-posits of the complex.

Possible sources of the metasomatic fluids

Alkaline igneous rocks and carbonatites are, by theirvery nature, volatile-rich (Kogarko, 1974). Metasomaticalteration produced by the volatiles associated with theintrusion of these rocks has been well-documented (e.g.,Sorensen, 1974;LeBas,7977, l98l), and is ascribed tothree processes: alkali metasomatism associated with in-trusion of carbonatite, metasomatism resulting from in-trusion of an alkaline silicate magma, and autometaso-matism, or deuteric alteration, in which early magmaticphases react with their own residual fluids to form a suiteof secondary, Iower-temperature minerals. As diferentrock units within an alkaline igneous complex may havebeen emplaced at different times, these three metaso-matic processes may be superimposed on one another (LeBas. 1977).

Metasomatic alteration of the ijolite xenoliths fromDiamond Jo quarry may be the result of one or moresuch processes. Ijolites from other alkaline complexes arevolatile-rich and produce metasomatism of adjacent rockduring intrusion, as described by Le Bas (1977). At thistime, we cannot identify the specific source(s) of themetasomatic fluid(s) responsible for the alteration of theijolite xenoliths. The metasomatism may, at least in part,be due to reaction with residual ijolitic fluids. The ijolitexenoliths from Diamond Jo quarry occur within garnet-pseudoleucite syenite that is in contact with nephelinesyenite (Fig. 2). We (Flohr and Ross, 1987, and unpub.data) have found that the nepheline syenites and garnet-pseudoleucite syenites from Diamond Jo are volatile-rich,with high amounts of F, Cl, CO,, and HrO. Metasoma-tism of the xenoliths may have occurred when they were

t29

entrained in the volatile-rich garnet-pseudoleucite mag-ma. The lack of a chilled margin at the contact of thenepheline syenite and the garnet-pseudoleucite syenite in-dicates that they were emplaced in relatively rapidsuccession, possibly permitting volatile exchange be-tween the two syenites and causing additional metaso-matism of the entrained ijolite xenoliths by fluids fromthe nepheline syenite.

The alteration observed in the inner-ring biotite-garnetijolite is different in several respects from that found inthe ijolite xenoliths. In the biotite-garnet ijolite, nephe-line is altered to thomsonite-gonnardite and minor can-crinite, rather than cancrinite and natrolite; F-free garnet,rather than fluoro-hydrograndite, replaces melanite;sphene is absent, and fluorapatite is rare. Both the ijolitexenoliths and the inner-ring biotite-garnet ijolite containminor amounts of calcite and abundant biotite. Ericksonand Blade (1963) attributed alteration of biotite-garnetijolite to reaction with carbonatite. Biotite-garnet ijolitesare more closely associated with the carbonatite (Fig. 1)than any of the other ijolites studied, making them themost likely to have been altered by the intrusion of thecarbonatite. Nelson et al. (1988) reported high concen-trations of Ba and Sr in carbonatite from Magnet Cove;infiltration of the biotite-garnet ijolite by carbonatiticfluids may have introduced high concentrations of Ba andSr into the biotite-garnet ijolite. Inner-ring garnet ijoliteis not in contact with any known outcrops of carbonatite,and none of the samples examined in this study are ex-tensively altered. Erickson and Blade (1963) reported thatsome of the samples of garnet ijolite examined by themcontain secondary cancrinite, thomsonite, and calcite. Thevariable degree ofalteration ofgarnet ijolite indicates thatalteration by carbonatit ic f luids was not pervasivethroughout the Magnet Cove complex. It is therefore un-likely that the metasomatism observed in the ijolite xeno-liths and also in the syenites from Diamond Jo quarry(Flohr and Ross, unpub. data) is the result ofcarbonatiticmetasomatrsm.

The inner-ring ijolites lack abundant F-rich minerals.The parent magma(s) of these ijolites may have been poorin F relative to the magma(s) from which the ijolite xeno-liths crystallized. Alternatively, F may have been present,but the volatile-rich portion of the magma(s) separatedfrom the crystallizing ijolites and did not react exten-sively with them. The F found in minerals in the late-stage metasomatic groundmass of the ijolite xenoliths mayhave been introduced by syenitic fluids.

AcxNowr,nncMENTs

Henry delinde (Mabelvale, Arkansas), owner of the Diamond Jo quar-

ry, kindly granted access to it. Mike Howard (Arkansas Geological Com-mission) and Don Owens (University of Arkansas, Little Rock) helped tocollect samples Mike Howard also provided us with invaluable guidancein locating sampling sites within the Magnet Cove complex Ted Arm-brustmacher (USGS, Denver) provided the portable core drill and helpedto collect samples. The National Museum, Washington, D C, provided

FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS

130

polished thin sections of a suite of samples from the F. C. Calkins collec-tion. Daphne R Ross (Smithsonian Institution, Department of MineralSciences) fumished X-ray powder-diffraction data. M. Kavulak, J. Mari-nenko, N Rait, J. Mee, J. Evans, M Doughton, E. Brandt, J H. Christie,J. Sharkey, and L. Jackson (USGS) provided whole-rock analyses. JimMcGee (USGS, Reston) gave advice on selection of microprobe standardsand guidance in the use of the instrument. Jim McGee, Ted Armbrust-macher, and two anonymous reviewers gave very thoughtful and thor-ough reviews ofthis manuscript. This work was funded by the Strategicand Critical Minerals Program of the USGS.

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Mnxuscnryr REcETvED Jenr.rrry 14, 1988Mlwr;scnrpr AccEprED Srgreunen 26. 1988

AppnNorx l. C.lLcurA.TroN oF GARNET FoRMULAE

Calculation of formulae of F-free garnets

Fe3* was calculated from electron-microprobe analyses (Table5) by assuming ideal gamet stoichiometry of 8 cations per 12oxygens. Cation-site occupancies were calculated by using thefollowing procedure. Ti-bearing garnets are silica deficient andhave <3 Si per 12 oxygens. Huggins etal. (1977) concluded thatthe preference for filling the tetrahedral site in such garnets is Al> Fe3* > Ti. Waychunas (1987), in a spectroscopic study of avariety of Ti-rich minerals, found no ewidence for significantamounts of Tia* in tetrahedral coordination in silicates. In viewofthese observations, the tetrahedral site was filled in the orderSi, Al, and Fe3*. The octahedral site was filled with all remainingtrivalent and tetravalent cations (Al, Fe3*, V, Ti, Zr) followed bythe divalent cations, according to ionic radius, in the order Mg,Fe2*, Mn, and Ca. The dodecahedral site was filled primarilywith Ca, any remaining divalent cations, and Na. The need toplace Ca in the octahedral site in three ofthe analyses (Table 5,nos. 6, 7, 8) is puzzling and may reflect analytical error. Ca inexcess of 3 cations per 12 oxygens has been reported by otherinvestigators (Huggins et al., 1977; Waychunas, 1987) in Ti-richgarnets in which there appears to be a deficiency in the octahe-dral site, suggesting that minor amounts of Ca may enter theoctahedral site.

Calculation of formulae of fluoro-hydrograndites

The example in Appendix Table I illustrates the method forcalculating the chemical formula of fluoro-hydrograndite froman electron-microprobe analysis. The analysis used in this ex-ample is from Table 6, no. 1 (sample l-1668).

l. Column A gives the original electron-microprobe analysiswith all Fe converted to Fe3*. We assume that the differencebetween 1000/o and the oxide sum (adjusted for the F equivalentof oxygen) is equal to the water content.

2. Calculate the initial atomic ratios, the total number of cat-ions (cation sum) and anions (anion sum), and the sum of cat-ionic charges p(+)1, as shown in column B.

3. Derive the factor K for converting the initial atomic ratiosinto the final atomic ratios by noting that the sum ofthe cationiccharges excluding hydrogen [>(+)] must equal the sum of theanionic charges ofoxygen, F, and hydroxyl,

> ( + ) : 2 ( O ) + F + O H , ( 1 )

and that the garnet formula requires that the total number ofoxygen, F, and hydroxyl anions be equal to 12,

O' + F' + OH' : 12, (2)

where primes indicate the final anionic ratios. The K factor isequal to the quotient of the final anionic to the initial anionicratios (column B),

AppENDrx TABLE 1. Calculation of the chemical formula of fluoro-hydrograndite

sio,Tio,ZrOzAlr03VrO"FerOoMnOMgo

NaroF

Sum

SumHrOSum

K factor

31.72.470.09

1 0 90.44

13 .90.080 .19

36.30.04294

99 051 .24

97 812 . 1 9

100 00

SiTiZrAI

Fe3*M nMg

N a

Cation sum>(+)FOHoAnion sum>(-)

0.5276 2.4710.0309 0.1450.0007 0.0030.2139 1 .0020.0059 0.0280.1741 0.8150.0011 0.0050.0047 0.0220.6473 3.0320.0013 0.006

1$Or5 ?.52g4.7259 22.13501547 0.72502431 1.1392j641 10.1362.5619 1 2.0004.7260 22.1364.6841

and by Equation 2,

K: 12 /(o + F + oH). (4)

Solving Equation I for oxygen (O) and substituting in Equation4, we have

K : 2 4 / ( > ( + ) + F + O H ) . ( 5 )

4. Calculate the final atomic ratios (column C) by multiplyingcolumn B values by K.

5. In the fluoro-hydrograndite crystal structure, some SiOX-groups are replaced by Ff and (OH)l groups, i.e., ArBr-(SiO.)3 ,G,OH)., where A and B occupy the dodecahedral andoctahedral sites, respectively. Each Si vacancy is thus representedby F/4 or OH/4 as indicated in Table 6.

6. Cations and tetrahedral vacancies can now be allocated tothe tetrahedral, octahedral, and dodecahedral sites as given inTable 6. The tetrahedral site is filled in the order Si, vacancies(expressed as F/4 and OH/4 in Table 6), and, last, Al to give asum of 3.000. The octahedral site is filled with the remainingAl, then Ti,Zr,Y3*, Fe3*, and then the divalent ions Mn andMg. In all our analyses offluoro-hydrogandites, the octahedralsite is slightly deficient in total cations after allocation of Mnand Mg while the dodecahedral site contains some excess Ca.The dodecahedral site is filled with Na and then Ca to give atotal of 3.000; the excess Ca is assigned to the octahedral site.The total number ofcations plus the tetrahedral vacancies (shownas F/4 and OH/4 in Table 6) in our analyses is close to 8.000.

Calculation of hydrogalrlet and fluoro-hydrogarnet chemicalformulas by summing to 8.000 cations (filled tetrahedral, octa-hedral, and dodecahedral sites) is not valid for the tetrahedralsites containing vacancies. Summation to 5.000 cations (filledoctahedral and dodecahedral sites) or to 3.000 cations (filleddodecahedral sites) can also lead to significant error because Almay occupy both the tetrahedral and octahedral sites and a va-riety ofcations may occupy both the octahedral and dodecahe-dral sites., ( : (O '+ F '+ OH' ) / (O + F + OH) (3 )