Canadian Mineralogist Yol.22, pp. 341-345(1984)

CHROMIANANDRADITE AND -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 chrysotilein a chromititelayer within the duniteof the Ji- + clinopyroxene+ * 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, , Jijal complex, 1982). Pakistan,retrograde metamorphism. The alpineultramafic rocks consistof diopsidite, SoMlaarne , 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. as well as chromitite stringers,layers and lenses. Geologists of the Sarhad Development INrnonucttotrt Authority (pers. comm.) report that pyroxenites, 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

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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-fibrechrysotile. 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.

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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). NI 0.009 0.009

Totll iron was detemined as Feo. ln chrcnite. the ratio of Fe3+ t! Fe2t is adJusted such that R3+:e2+ = 2:1. In ollvlne and serpentlne, Olivine, and its Mg-Fe distribution witk chromite all lron was taken to be Fe2r. Nunber in parentheses lndlcates nunber of point analysesi l'1. Q. Jan analyst. Serpentine total includes 0.1]U of olivine in dunite is very similar Na20, glving 0.010 Na ln the fomula. D dunite, c chromltlte. Chr The composition chromite, 01 olivJne, Srp serpentine. to that in chromitite. However, in the chromitite, the olivine has higher Al, Cr and Mn. Chromium is con- sideredas divalent, following recentinvestigations (for details,see Brown 1980).The ratio 100Me;/Mg ing the recalculationscheme of Rickwood (1968), + Fd+) of the olivine in dunite is 95.6, only slight- 98.790 of the cations can be assignedto the end- ly lower than that in the chromitite (96.2). Both membersandradite (65.2) and (33.8), with values are markedly higher than those reported a small amount of pyrope (0.5) and grossular(0.5). elsewherein Jijal (Jan & Howie 1981)and in other The composition of the garnet is unusual. Most alpine peridotite complexes(the valuesrange between specimensof chromian garnet are a solid solution 89 and 93). However, similar values have been of uvarovite and grossularend-members; only in very reported from occurrencesof somealpine chromilite, few instanceshas chromian andradite beenreported. for example, the Tari-Misaka complex in western Our survey of Mineralogicol Abstracts for the past Japan (Arai 1979) and the Vulcan Peak peridotite, fifteen years reveals only two occurrencesof chro- Oregon (Himmelberg & Loney 1973). mian andradite: Val Malenco, Italy (Amthauer e/ a/. An increasein the forsterite content of olivine witfi 1974) and Nizhny Tagilsk in the Urals (Rost e/ a/. increasingmodal proportion of chromite has been 1979).The CrrO3content of the Malenco garnet is recordedin somecomplexes, such as Vulcan Peak s 1.390,and the estimatedconditions of formation (Himmelberg & Loney 1973) and the eastern are 370oCand 0.5 to 1.5 kbar. The Urals garnethas BushveldComplex (Cameron 1975).Irvine (1967) comparatively higher CrrO3 (up to 3.9q0). More suggestedthat where olivine and chromite crystallize recently, Duke & Bonardi (1982)described a very from a magma, a small-scaleexchange of Mg similar garnet to that of Jijal, from R6aume and Fd* may take place between them. Where Township, in Ontario; it coexistswith 590 (modal) chromite is a minor constituent, it becomes ap- chromite and formed at greenschist-facies preciablyricher in Fd* as the rock cools, whereas temperatures. The Cr2O3 content of this garnet the remains relatively constant in ranges widely (0.53 - 10.54V0);some values are composition. Where the silicate is subordinate, it slightly higher than in the Jijal garnet' but the becomes more magnesian, whereas the chromite average Cr2O, of 29 determinations is about 3Vo changesonly slightly. The higher Fe content of the lower than in Jijal. A significant difference with the chromite in the Jijal dunite layer and the highsl IvIg Jijal garnet is that the R6aumegarnet is much richer content of the olivine in the chromitite are in agree- in Al (2.05 versus0.18 AzO:). ment with this suggestion.However, the amounts of On thermodynamic reasoning, Ganguly (1970 chromite in the dunite are not sufficient to explain predictedthe presenceof a solvusbetween uvarovite the variation from Foe6 in dunite distant from and andradite end-membersof the ugrandite series, chromitite to Foe6 in dunite associated with and suggestedtlat compositions of naturally occur- chromitite. We suggestthat this variation reflectsloss ring garnet are in agteementwith this. However, the of Fe to chromite as a result of ionic diffusion across more recentchemical data of Duke & Bonardi (1982) the layers. The closenessin tJrecompositions of ac- "suggest rather extensive solid solution at low cessoryand dominant chromite and olivine supports temperature (up to at least 4090 uvarovite)"; our this. four compositions agree with this view. Temperature estimates hased on the Mg-Fd+

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distribution between the olivine and chromite, and facies conditions at temperatrues of less than the Cr content of the chromite in the I site (also 380-525"C (dependingon the pressure).We have no taking into considerationthe Fe3* content in this meansof deducinga pressure.Amthauer et al. (1974) site) are approximately950, 800, 750 and 650"C by suggested370"C and 0.5-1.5 kbar for an Italian the methods of Medaris (1975), Evans & Frost chromian-andradite-bearingserpentinite. Because the (1975),Fabrids (1979) and Fujii (1977),respective- Jijal rock doesnot contain any Ca-bearingphase, ly. The last method gives an erroneous estimate of someCa (and, possibly, Si) must have beenadded temperatureand requiresrevision, as suggestedby from an external source during the formation of Fabrids. From various silicate geothermometers,Jan garnet in the serpentinite. & Howie (1981)calculated that the Jijal ultramafic rocks had undergonegranulite-facies metamorphism between800 and 850oC. ACKNOWLEDGEMENTS

Serpentine We thank the Royal Societyfor a GuestResearch The serpentinecomposition is the averageof two Fellowshipwhich enabledMQJ to make a research very similar setsof analytical data. Considering the visit to Leicester,the Natural Environment Research factthat serpentinecontains about l39o H2O, it ap- Council in London for Karakorum ResearchGrant pearsthat the total is low by 2 to 3Vo.The composi- GR3/4242to BFW, and two refereesfor construc- tion of the Jijal mineral is closer to the average tive criticisms. chrysotile than to lizardite or antigorite (cf. Page 1968,Whittaker & Wicks 1970).Chrysotile typical- REFERENCES ly is fibrous, and the Jijal serpentineconsists of slightly curved fibres. The Jijal grains are optically length-slow; this confirms the determination as AMrHewn, G., Kuntz, W., Rost, F. & Scnr-oelvrcR,H. (1974): Chemismus und Genesedes Andradits aus chrysotile. dem Serpentinit des Val Malenco (Bernina- The stability limits of chrysotile are not clearly Gebiet,/Oberitalien). Schweiz. Mineral. Petrog. Mitt. known. Coleman (1971)and Caruso & Chernosky 54, 691-706. (1979)suggested that it forms at lower temperatures than the other two serpentine minerals. Recent ex- ARAI, S. (1979): Polygenetic in chromitite perimentaldata, summarizedby Moody (1976),in- from the Tari-Misaka ultramafic complex, western dicate that chrysotile crystallizes on the low- Japau Rep. Fac. Sci. Shizuoka Univ. 13,105-l 14. temperature side of reactions in the range from 525"C at 7 kbar P(H2O) to 380'C at 0.2 kbar BnowN, G.E., Jn. (1980): Olivines and silicate spinels. P(H2O)(Chernosky 1973). /n Orthosilicates @.H. Ribbe, ed.). Mineral. Soc. Amer., Rev. Mineral. 5, n5-381.

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Chromian andradite and associatedchrysotile oc- Cenuso, L,J, & CuenNosry, J.V., Jn. (1979): The cur in a chromitite seamin dunite of the Jijal com- stability of lizardite. Can. Mineral. 17,757-769. plex. The compositionsof chromite from both the CurnNosrv, (1973): dunite and chromitite layers resemble those from J.V., Jn. The stability of chrysotile, Mg3Si2O5(OH)a,and the free energy of other alpine complexes, except that they are formation of talc, Mg3SiaOle(OH)r. Geol, Soc. somewhatmore Fe-rich. Moreover, the olivine com- Amer. Abstr. Progroms 5, 575. positions are highly magnesian(Foe6). The distribu- tion of Fe and Mg betweenthe two phasesis consis- Colruar, R.G. (1971): Petrologic and geophysical tent with equilibration temperaturesof about 8moc. nature of serpentinites. Geol. Soc. Amer. Bull. t2, During obduction along the Main Mantle Thrust 897-918. QndusSuture), the complexlocally underwentlower- P-T re-equilibration, giving rise to paragonite- Cowano, M.P., Jel, M.Q., REx, D., TenNrv, J., (1982): bearing assemblagescalculated to have formed at Turuwalr, M. & WrNor,nv, B.F. Geo- framework - garnetgranulites tectonic of tle Himalaya of N Pakistan. 510-550'C and 8 9.5 kbar in the J. Geol. Soc. London 139,299-308, (Jan et al. 1982). Further obduction caused even lower-P-T assemblages,and led to local serpefi:nizl,- Dure, J.M. & BoNar.or, M. (1982): Chromian an- tion of the ultramafic rocls. The chromian andradite dradite from R6aume Township, Ontario. Can. - chrysotile assemblagedeveloped under greenschist- Mineral, 20,49-53.

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