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Pyroxene-Ilmenite Intergrowths in Garnet Pyroxenite Xenoliths from A

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Pyroxene-Ilmenite Intergrowths in Garnet Pyroxenite Xenoliths from A American Mineralogist, Volume 63, pages 258-265, 1978 Pyroxene-ilmeniteintergrowths in garnetpyroxenite xenoliths from a New York kimberliteand Arizona latites DaNIEI-J. ScHur-2n,1Hnnwent HBI-usre.gnr Department of Geological Sciences Queen' s Uniu e rs ity, Kings ton, Ont ario lNo Rospnr M. Ce,ssrr Department of the Earth Sciences SUNY, Brockport, New York 14420 Abstract Lamellar pyroxene-ilmeniteintergrowths in garnet pyroxenitexenoliths from a kimberlite in lthaca, New York, and from Tertiary latitesnear Prescott,Arizona, may representa link between exsolution intergrowths in pyroxenes from layered basic intrusions and intergrowths in pyroxene xenocrysts from kimberlites presently interpreted to result from eutectic crystalli- zation. Introduction vored (see referencesabove) for four reasons:(l) textural similaritiesto alloys formed by eutecticcrys- Nodules of pyroxene-ilmenite intergrowths from tallization (Boyd, 197l); (2) restricted compositional kimberlites have received much attention because range of the intergrown phases;(3) an apparent low many investigatorsbelieve them to be unique to these solubility of TiO, in pyroxenes at high pressures rocks. They have also been thought to be possible (Akella and Boyd, 1973);and (4) experimentalpro- indicators of the depth of origin of kimberlites.Such duction of texturally similar intergrowthsby eutectic intergrowthsare best known from African pipes (for crystallization(Wyatt et al., 1975;Wyatt, 1977). reviews, see Gurney et al., 1973; Boyd and Nixon, Natural terrestrialpyroxene-ilmenite intergrowths 1973), but have also been described from the Mir are not restrictedto kimberlites,but are alsocommon pipe in Yakutia (Ilupin et al., 1973),and two North in pyroxenesfrom cumulatesin layered basic intru- American localities: the Stockdale pipe, Kansas sions(Morse, 1969:Deer and Abbott, 1965;Haselton (Brookins, 1971;Gurney et al., 1973;McCallister el and Nash, 1975).As noted by Gurney et al. (1973), al., 1975), and kimberlites in the southern Laramie theseintergrowths have a strong textural resemblance Range, Wyoming (McCallum et al., 1975;Smith el to the kimberlitic intergrowths, yet their origin by al.,1976). subsolidusexsolution is generallyaccepted. The origin of theseintergrowths is still uncertain. In this paper we intend to demonstratethat pyrox- The proposedmechanisms of formation are eutectoid ene-ilmenite intergrowths in kimberlites may also transformation of garnet (Ringwood and Lovering, originate by exsolution.Two new intergrowth occur- 1970),exsolution from an ilmenite-structuredpyrox- rencesare described,one from a kimberlite dike in ene(Dawson and Reid, 1970),eutectic crystallization Ithaca, New York, and the other from Tertiary latites (Williams, 19321'MacGregor and Wittkop, 1970; near Prescott,Arizona. As in other kimberlitic inter- Gurney et al., 1973;Boyd and Nixon, 1973;Smith er growths, textural evidencefor an origin by exsolution al., 1976;Wyatt, 1977'and others),and cotecticcrys- is ambiguous in the Ithaca sample.However, unlike tallization (Frick, 1973). Although the intergrowth most kimberlitic occurrences,the intergrowth-bear- texturesare compatiblewith all of thesemechanisms, ing clinopyroxenefrom Ithaca is not a xenocryst,but an origin by eutectic crystallization is currently fa- occursas grains in a garnetpyroxenite xenolith which ' Present address: Department of Geological Sciences,Uni- compositionally resemblesgarnet pyroxenite xeno- versity of Texas at Dallas, Richardson.Texas 75080. liths from the Arizona latites. Ilmenite lamellae in 0003-0o4x/78/0304_0258$02.00 258 SCHULZE ET AL.,, PYROXENE-ILMENITE INTERGROWTHS clino- and orthopyroxenesof the Arizona xenoliths been describedby Martens (1924), Sheldon (1927), resemblethose found in pyroxenesfrom layered in- and Foster (1970). The host rocks and a variety of trusions and can be shown to have originated by xenocrysts,including garnets,clinopyroxenes, chro- exsolution.From the compositionalsimilarity of the mites, and ilmeniteswere studiedby Foster (1970). Ithaca and Arizona xenoliths we propose that an Foster and Reitan (1972) concluded that the host exsolution origin of the intergrowth from the Ithaca rocks are kimberliteson the basisof textures,miner- kimberliteis a distinctpossibility. alogy, and chemical composition. Ultramafic xeno- liths have not been reported previously, though Garnet clinopyroxenite from lthaca, New York Foster (1970) mentionedthe rare occurrenceof a A small garnet clinopyroxenitexenolith containing dark-greendiopside attached to garnets. clinopyroxene-ilmeniteintergrowths was collectedin The intergrowth-bearing garnet clinopyroxenite October 1976 from a 3-foot wide kimberlite dike occurs in a kimberlite sample containing several outcroppingin the CascadillaGorge on the campus smaller phlogopite-bearingand garnet-bearingxeno- of Cornell University, Ithaca, New York. Eighty-two liths as well as inclusionsof a chrome-richspinel with such dikes and severalsmall diatremesare known diopside. The ovoid xenolith has the approximate from 24 localitiesin the central Finger Lakes region, dimensionsof 1.5 X 0.75 X 1.0cm, and consistsof a New York. They are thought to be of Late Jurassicor relatively large almandine-rich garnet with several Early Cretaceousage, and their geologic setting has equantclinopyroxene grains approximately 3 mm in diameter. Chemical analysesof the mineral phases are presentedin Table l. The clinopyroxenecontains Table I Mineral analysesfrom Ithacagarnet clinopyroxenite fine discontinuousilmenite lamellae (Fig. la) which (62-5\ have partly altered to sphene. Grains of ilmenite which differ in composition AverageL234567 from the lamellae occur interstitially and as in- of n n=I4 n=l n=l n=4 n=5 n=7 n=5 clusionsin the garnet. The garnet also has small inclusions of clinopyroxenecontaining somewhat tta2OX 0.1 0.3 0.4 0.4 1.3 r.b 2.3 more TiO, than the clinopyroxenehost to the ilme- M90 9.2 2.2 L.7 5.6 14.0 12.9 L2.2 nite lamellae(Table l). Minor granulargarnet occurs Al203 zL.a 0.3 0.5 0.8 2.2 4.4 L2.9 in the lamellae-bearingclinopyroxene, and (?) secon- sio2 39.0 0.2 0.5 u.r Jz.) 5L.2 4L.2 dary amphiboleis also presentin the xenolith. K2o 0.0 0,0 0.o 0.o 0.0 o.o 1.8 CaO 5-7 0.0 0.0 0.1 22.r 2L.O II.5 Garnet pyroxenitesfrom Chino Valley, PauldenQuad- r r TiO2 o.I 41.3 37.9 90.1 v-z 0.5 2-6 rangle, Arizona Cr203 0.1 0.3 0.3 0.3 0.1 0.1 0.1 Tertiary latites of the Sullivan Buttes Latite de- ltnO 0.6 0.0 0.o o-7 0.I v.a u.r scribed by Krieger (1965) and Krieger et al. (1971) FeO* 24.2 54.2 57.9 45.5 7.9 8.3 L2.6 contain a rich variety of ultramafic xenoliths includ- ing metamorphic eclogites,garnet clinopyroxenites, flflf'""= roo.e es.s se.2 roo.o roo.4 roo.2 e7.3 garnet websterites, phlogopite-bearing websterite, and garnet-amphibolerocks. Most of the xenoliths X the effective detectability linit of Na2O is belongingto the garnet pyroxenitefamily come from apploxitrBtely 0.5 wt c two of the latite units sampled.The xenoliths in this * electron microprobe analyses, total Fe reported group have angular to rounded shapesand range in as FeO diameter from a few mm to 5 cm. They are very fresh ** TiO2 values are not accurate to more than 5 relative wt t and appear to have reactedslightly or not at all with garnet pyroxenite I. garnet the host magma. Of more than l5 specimensexamined in thin section,4 garnetclinopy- 2. Iarge interstitial granular ilmenite in garnet roxenitesand 2 garnet websteriteswere found to con- 3. tiny ilmenite as rods andl dots in garnet tain pyroxeneswith "ilmenite" intergrowths. 4. il-nenite lamellae in clinopyroxene 5. clinopyroxene host to ilnenite lanellae Ga rneI clino py rox enit es 6. clinopyroxene inclusion in garnet These xenoliths consist of relatively coarse clino- 7. anrphibole (chlorine detected but not analysed) pyroxeneporphyroclasts (up to 2 cm in diameter)set 260 SCHULZE ET AL.: PYROXENE.ILMENITE INTERGROWTHS Fig. I (a)llmenitelamellae(highlyreflective)intergrownwithclinopyroxene,fromlthaca,sample62-5.Darkirregularspotsareholes in the section Reflectedlight. (b) "llmenite" lamellaein clinopyroxene,showing two phasesdiffering in reflectivity Sample l-2, Arizona garnet clinopyroxenite.Reflected light. in a matrix of fine-grainedgranular clinopyroxene analysesshow that they are Fe-Ti oxides with TiO, and garnet. Most samplescontain secondaryamphi- contentsranging from lessthan 5 to slightly above40 boles. Clinopyroxene porphyroclasts with lamellar weight percent. Under reflected light, most of the intergrowths of one or all of the minerals garnet, lamellae are seento consistof two phases(Fig. lb), "ilmenite," and orthopyroxenehave been recognized. presumablyone Fe-rich, the other Ti-rich, that prob- Due to their narrow width, we have not beenable to ably originated from the breakdown of an originally obtain good chemical analysesof the "ilmenite" la- homogeneousFe-Ti oxide. Two-phaselamellae were mellae. but calculations from silicate-contaminated also noted from kimberlite xenocrystsby MacGregor and Wittkop (1970)and Mitchell et al. (1973).Apart from occasional fine orthopyroxene exsolution la- garnet Table 2. Mineral analysesfromArizona clinopyroxenite mellae, the fine-grainedgranular clinopyroxenesare lamellae-free. AverageI234 5 6 Four types of intergrowth relationshipscan be dis- of n n=8 n=10 n=17 n=14 n=16 n=16 tinguished(for chemicalanalyses of different phases, seeTable 2): Na2OX o.2 0.2 1.3 nq ( l ) Clinopyroxenewith planar lamellaeor bead- M90 14.6 15.r 9.3 9.2 L4.8 like stringsof garnetand fine lamellaeof "ilmenite" A12O3 2.7 1.9 2!.A 2L.7 3.0 r.7 parallelto (100)(determined on the universalstage). si02 53.3 53.7 39.2 39.1 52.7 f{.1 Although most clinopyroxenegrains show only one Kzo 0.o 0.0 0.0 0.0 0.0 0.0 set of "ilmenite" lamellae, a second set was recog- CaO 2I.3 22.L 5.6 5 .7 20.3 0.8 nized in rare casesunder reflectedlight. Some grains Ti02 o.2 0.t 0.1 0.1 0.2 0.1 show, in addition, very thin orthopyroxene lamellae which are also parallel to (100). Garnet lamellaeare Cr2O3 o.2 0.2 o.2 0.2 0.2 many timesthicker than the "ilmenite" lamellae(Fig.
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