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Chromian Andradite and Olivine-Chromite Relations in A Canadian Mineralogist Yol.22, pp. 341-345(1984) CHROMIANANDRADITE AND OLIVINE-CHROMITERELATIONS IN A CHROMITITELAYER FROM THE JIJAL COMPLEX.NORTHWESTERN PAKISTAI\I M. QASIM JAN National Centreof Excellenceand Departmentof Geology, Universityof Peshowar,Peshawar, Pakistan BRIAN F. WINDLEY AND ROBERT N. WILSON Departmentof Geology,The University,Leicester, England LEI 7RH AssrRAcr alpine'ultramafic rocks up to 4 km thick. The Green chromianandradite occurs associatedwith granulites are ultrabasic to intermediate, with garnet chrysotilein a chromititelayer within the duniteof the Ji- + clinopyroxene+ plagioclase* quartz + rutile jal alpine ultramafic complex(northern Pakistan). The .-.r hornblende + epidote as the most common garnetcontains, on average,10.1 wt. tlo Cr2O3(range 9,2 assemblage.Jan & Howie suggestedthat the - tt.6qo) andhas a formulacar.*(cro.urrier+,.r*eb.r) granuliteswere metamorphosedal 690-770"C and Si2.9sOl2.The garnet formed during retrograde 12-14 kbar. Retrograde paragonite (with 100 greenschist-faciesmetamorphism of the Jijal complex. Na/(Na+K) up to 98.7)formed during the uplift of the granulitesat 510-550"C, 8-9.5 kbar (Jan et al. Keywords; chromianandradite, dunite, Jijal complex, 1982). Pakistan,retrograde metamorphism. The alpineultramafic rocks consistof diopsidite, SoMlaarne peridotite, dunite and websterite, all devoid of plagioclaseand garnet. Estimatesof temperature On d6crit une andraditeverte chromiflre associ6eau (800-850"C) and pressure (8-12 kbar) suggsst chrysotiledans un niveaude chromitite situ6 dans la dunite metamorphism under granulite-faciesconditions. du complexeultramafique alpin de Jijal (Nord-Ouestdu Field and geochemical data indicate that the Pakistan). grenat (enpoids) Le contientde 9.2 d ll.69o ultramafic rocks are not comagmaticwith the garnet deCr2O3 (!0.190 en moyenne),.et sa composition globale granulites. Jan & Howie (1981)proposed that the est Ca3.o4(Cro.67Fd+1.2eA1s.02)Si2.e8O12. Le grenat se seraitform6 lors du m6tamorphismer6trograde du com- ultramafic rocks were emplaced as plastic material plexedans le faciesschiste vert. in the granulites before the entire masswas obducted (traduit par la Rddaction) against the Indian plate along the Main Mantle Thrust. Mots-cl*: andradite chromifbre, dunite, complexede The ultramafic rocks contain disseminated Jijal, Pakistan,mdtamorphisme r€trograde. chromite as well as chromitite stringers,layers and lenses. Geologists of the Sarhad Development INrnonucttotrt Authority (pers. comm.) report that pyroxenites, dunites and peridotiteshost the chromitites. Based Chromian andradite occurs in a chromitite layer on a study of over 60 concentrations, they estimate in dunite of the Jijal complex, which constitutesa about 0.37 million metric tonnes of reserves.The part of the approximately 24000 km2 Kohistan tec- chromite bodies are randomly distributed; however, tonic zone of the northwesternHimalaya of northern clusteringof bodies does occur locally, such as to Pakistan; the zone somprisesa variety of plutonic the north of Jijal, near Manidara. Many of the and volcanic rocks (mainly calcalkaline) and minor chromite-richzones show folds, faults, shearzones (meta) sediments.Tahtukheli et al. (1979)proposed and boudinage,caused either by ductile emplacement that the zone representsan island arc turned on end or later deformation. The largest chromite-rich zone during the Himalayan orogeny. Our more recent is over 100 m long and consistsof many lensesof work (Coward et al. 1982) suggeststhat the various sizes.The largestlens is 40 x 5 m. This paper CretaceousKohistan island arc becamean Andean- describesthe unusual associationof chromian an- type margin during the Eocene. The Jijal complex dradite and serpentine,which occursin a l$crn-thick is a fault-bounded wedge about 2(P km2 in area layer of chromitite in dunite to the north of Jijal (35" situated on the Main Mantle Thrust, which is an ex- 05'N, 72o 55'E) and south of the main occurrence tension of the Indus-Zangbo suture that marks the of chromitites. locus of subdustion of the Indian plate under the Tibetan mass. The complex was describedin detail PetnocRAPrtv by Jan (1979)and Jan & Howie (1981).It is com- posed principally of garnet granulite and a slab of The chromitite layer containing the chromian an- 341 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/3446107/341.pdf by guest on 25 September 2021 342 THE CANADIAN MINERALOGIST jq.. r'E 6i 0" \, s .,n,.-.g.*"-y- D Ftc. l. Microphotogaph of the main occurrenceof chromian andradite in chromitite. A andradite, O olivine, C chromite, S serpentine. dradite hassharp contactswith its dunitic host-rock. siderations suggest that all the iron is present in The chromitite consistsof chromite (up to 80-9090 trivalent form. The quantities of Mg, Ti, and Al are modal) and 10-2090 olivine. The denselypacked very low, and Mn is below the detection limit in all grains of chromite have euhedralto subhedralshapes but two cases.A simplified formula for the mineral typical of cumulate; olivine forms postcumulus is Ca3.sa-3.05tCr6.61-s.7F93+1.2r-l36Ah.0r-o.d.S\.yt-t.&o, anhedral grains between them. with a mean for ten point-analyses of In Figure 1, there are six groups of bright green Ca3.ea(Cr6.5rFe3+1.2eAle.e2lSiz.saO,z. Because it ap- andradite grains within an area of 0.5 x 0.5 cm. The pearsthat Ca is slightly overestimatedrelative to the andradite forms subhedral to anhedral grains up to trivalent cations,the analyseswere checked by energy 0.1 - 0.2 mm in diameter betweenthe chromite and dispersion using the microprobe facility in the olivine gains. The largest grains are subhedral and Department of Mineralogy and Petrology at the occurin two veinletsup to 0.2 mm wide and I mm University of Kiel, West Germany; these analyses long, associatedwith erey cross-fibre chrysotile. The give an identical result within experimentalerror. Us- largestgrain of andradite has an inclusion of olivine. Garnet and chrysotile are the only secondary minerals. Rare chrysotile grains occur elsewherein TABLEI. REPRESENTATTVECOMPOSITIONS OF GARNET, JIJAL COMPTEX the chomitite. LLf 3 ltlean 2a 5i02 wt.U 35.60 35.64 35.63 35.70 35.62 0.13 Mn€RAr, CsBursrnv Ti02 0.07 0.09 0.14 0.12 0.12 0.07 A1203 0.29 0.30 0..l0 'll.600.21 0.21 0.17 The performed C1203 9.43 9,23 9.53 10.11 1.47 analyseswere at the University of Fe203 20.91 20.67 21.57 19.24 20.48 1.44 Leicesterwith a CambridgeMicroscan 5 microprobe. lh0 0.03 M9o 0.14 0.15 0.10 0.14 0.13 0.04 Results of wavelenglhdispersion analyses of the Ca0 33.87 33.90 33.78 33.87 33.86 0.09 garnet are presentedin Table l. Results of energy- Total 100.31 99.98 100.85 100.91 100.53 dispersionanalyses of shromite and olivine from the o{ygen 12 12 12 12 12 chromitite and dunite and of serpentineare given in 5i 2.985 2.996 2.977 2.976 2.981 Table 2. In the caseof chromite and olivine, the core Ti 0.004 0.006 0.009 0.008 0.008 Al 0.029 0.030 0.010 0.021 0.0e1 and margin of two grains of each mineral were Cr 0,625 0.613 0.629 0.765 0.669 Fe3+ 1.320 1.307 1.356 1.207 1.290 analyzed to check for compositional variations I'ln 0.002 within and among Fains. Mg 0.017 0.019 0.012 0.017 0.016 Ca 3.043 3.053 3.024 3.025 3.037 Gornet End menbers[rean, recalculated by rethod of Rlckwood(1968)] Andradjte 65.2, uvarovlte 33.8, pyrope 0.5' grossular 0.5. The All meanvalues, standard devlatlon and ranges based on l0 compositionis variable with respestto Cr aad point-analyses, obtalned by electron mlcroprobei R.N. lrilson Fe in the ten point-malyses. Charge-balancecon- analyst. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/3446107/341.pdf by guest on 25 September 2021 CHROMIAN ANDRADITE FROM TIIE JIJAL COMPLEX. PAKISTAN 343 TABLE 2. REPRESEIIIATIVECOMPOSITIONS OF CHROMITE,OTIVINE AND Chromite SERPENTINE..JI,]AL COI'IPLEX chr(r,4) Chr(c,4) 0l (r,4) 0l (c,l ) Srp(2) The compositionsof chromite from the dunite and 't1025i02 wt.9 0.33 0.36 41.46 42.29 41.88 chromitite are similar. Both are very rich in Cr, and 0.12 0.l8 0.05 0.04 0.00 Al203 7.57 7.80 0.08 0.28 0.16 there appearsto be no notable chemicalvariations Cr,0 59.09 60.77 0.11 0.2t 0.35 " from grain to grain. However, the chromite from the Fe2U3 '16.903.86 3.63 Fe0 15.17 4.31 3.78 1-.74 slightly lower AllER3*, and higher Mn0 0.49 0.40 0.07 0.t3 0.00 dunite has a Mso 10.44 12,04 52.09 53,92 F**/Mg in the margin than in the core (180 pm Nt0 0.46 0.49 pattern is seenin the chromite Total 98.80 100.35 98.63 101 .14 ,,. oo distance). A similar 0xygen 44 44 7 from the chromitite, but in one crain Al does not SJ 0.011 0.012 1.005 0.998 2.027 vary from core to margin. The variation, however, TI 0.003 0.004 0.001 0.001 0.000 AI 0.301 0.302 0.002 0.008 0.009 is within the limits of analytical error. Both composi- 1.573 1.576 0.002 0.004 0.013 such Fe3+ 0.098 0.090 tions are typical of alpine chromite, exceptthat Fe2+ 0,476 0.416 0.087 o.ots olozo chromite is somewhatmore Fe-rich (Irvine 1967, Mn 0.014 0.011 0.001 '1.8980.003 0.000 l4g 0,524 0.589 1.883 2.836 Thayer 1970).
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