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METAMORPHIC HISTORY OF STAUROLITE-KYANITE-GARNET-CORDIERITE- BEARING PELITES OF -MERCAKA LINEAMENT, SOUTH

J. DEVARAJUand A. S. JANARDHAN Depai*tmentof Geology, University of Mysore, Mannsagangotri, Mysore - 570 006 Email: asjanrdhan 0 hotmail .corn

Sargur Supracrustalsfrom the Coorg granulite block (1976) considered that this unit formed the lower part of his comprise st*-grt-sil-ky-crd-bio assemblages. StauroIite Dharwar Supergroup. from this association is rich in ZnO content up to 3.7%. The Coorg granulite block is bounded in the north by Late bluish kyanite lnantling garnet exposed around Manga1ol.e-Kasargod-Mercara lineament, whicl~hosts Jalsoor, Sicldapura and Mercara localities situated along younger intrusive bodies like that of Angadilnogar syenite the Mangalore-Mercara lineament suggests that the (638 Ma); Thalu~.granite (710 Ma) quoted in Santosh et al. Coorg granulite block had a long middle to deep crustal (19891, and Sulliya syenite bodies (Ravindra and Janardhan, residence. Developn~entof cordieri te around garnet with 198 1). Towards the east, the Coorg Block is bounded by fibrolite reported from this zone took place at a later the southcrn extension of Balehoni~urshear and towards the stage. Estiniation of P-T vary from 550°C-750°C and south bound by Moyar shear (see inset map in Fig. 1). 4.5-8.6 kbars, Thc age of Coorg charnockite is -2600 Ma (Rb-Sr method; Spoonei- and Fairbairn, 1970), and thus forms part of the 25 10 Ma old B.R.Hills-Nilgiri-Shevaroy-Madras Introduction (Chennai) granulite tract. The Coorg massif is separated from Dharwar craton exposes a prominent suite of infra- and other charnockite lnassifs and supracrustals, by shear zones supracrustal rocks. The infracrustal rocks are principally and in this note we are interested in the Mangalore-Mercara made up of TTG suite of gneisses (Peninsular gneisses) linealnent and concentrate 01.1 the'Jalsoor-Mercara localities. that range in age from 3450-2950 Ma. The supracrustal Pseudotachylites and rnylonites are quite conllnon in these rocks have been divided into two units: older Sargur localities, further corroborating that this zone does form part Supracrustals of 3400-3100 Ma and younger Dharwar of the lineamen tlshear. SupracrustaIs (2900-2600 Ma) as established by Swami Gopalakrishna (1984) had mapped the areas around Nath and Ramakrishi~an(1981). This division is widely Byalkuppe and Kushal Nagar to the east of the main Coorg accepled and is currently in vogue. massif, He utilized the pyroxene granulite bodies, which were Extensive literature on Sargur supracrustals mostly deal previously mapped as long linear bodies running for several with their stratigraphic position vis-a-vis Dharwar km as marke~.horizons and demonstrated that the bodies were supracrustals and Peninsular gneisses. Papers dealing with deformed and the area as a whole had witnessed three metamorphic conditions are scarce and are mostly from the defol-mational events. Cl~ari~ockiteformation (incipient Kabbaldurga to Biligirirangan hills terrain, where Sargur charnockite) at first occurred along shears associated with Supracrustals are extensively seen. The general conclusion F, folds as at Hurlikal area. He had also brought out the is that there is a soutl~wardincrease in pressure from 5.5 increase in pressure from 5.5 kbars in the amphi bolite facies to 8 kbars. In this note, we demoilstrate that the P-T increases terrain east of the Cauvery River to 8 kbars prevalent in the on the E to W traverse from Hunsur-Byalkuppe-Coorg granulite terrain (refer to map and field photo in charnockite massif, in a way follows the arcuate boundary Gopalakrishna et al. 1986). of the Dharwar craton. In thc present note we go much further westwards, along the Mangalore-Kasargod-Mercara lineament, Coorg Granulites concentrating on the pelitic bands associated with pyroxene Mercara group of rocks which form part of the Sargur granulite bodies occurring amidst charnockite. We have Supracrustals was described by Iyengar (1976) who stated concentrated on the pelitic bands exposed in areas around that supracrustals of Coorg consist mostly of st-ky-grt-sil - Jalsoor, on the road cuttings from Sulliya to Mercara and bearing pelites, banded magnetite quartzite and pyroxene localities around Siddapura. The pelitic bands in these granulite bodies amidst charnockites and gneisses. Iyengar regions decrease in size when compared to those in the areas *Refer ~rppe!rt/ixfor a601~l~iutio~z~

JOUR.GEOL.SOC.INDIA, VOL.63, MAY 2004 Fig.1. (A) Geological inup of granulite rnassif of Coorg around Jalsoor-Mel-cacaarea. (IIn\ct map 01. \oz~tl~[~~c!i;in ~r~~~ii~li~c l~loch\ with prominent slie;lr zones.

around Byalkuppe, and the pyroxene graniilite bodies also n2icrop1-ohc l'ittecl ~~thSlrN Solar-i\ nrt;~clirnuri~111 rllc no longer occur as long dykes but occur as patches. pods Geology Dcpartmcnt ol' M~\oI.cIjl~i\rcr\itj,. A~il'r;rlyt~c~al C and scl~li~re~lanlidst charnockite. Areas rich in pyroxene co~idition\cmploj ccl \itcr-t I I a1 IS-201111.LI\III~ granulite bodies are shown in the map as linear bodies for ~laturnlrnineral \t:incfard< \~rppE~cdl'1.0111 thc' UIII\CIjrr? 01 the sake of convenience. The thrust of this note is on the Chicago. Keprcsentati\~crnil~cral ;t~i~bl> sc\ 01' (IIC (111 ICI-CII[ pelites and on the paragenetic sequence of tlie pelitic awociation arc given in Table I. assemblages. P-T path and the irnpot-tance of the garnet- kyanite association. (a) Staurolite-Kyanitc-Garncl+SilIin~aniteBsaring Assemblage: Stai11-otitcoccurs as discrcte grains :l\sociatcd Jalsoor-hlercara Area with bluish kyanitc and ~illima~ritc.The photornicrog~.i~ph The area under study occurs mostly along the Mangalore- given in Fig.2. suggests th~~tstiiun)litc i:, socn p;rr;illel Mercara lineament. Charnockites. hbl-bio-gneisses and two- growth associated with kyitnite. Ho\vevct-, rare ~l~clusiolisol' pyroxene granulite bodies are the dominant rock types of staurolite in garnet, suggests its ciu-ly for~nation;~I~LI t11c \t- this region with minor pelitic bands, Banded magnetite ky-grt-bio+sil association could have f'or-nicd hy the quartzite and recrystallized ultramafic bodies. Granite and following possihlc re;lction\: syenite bodies of younger age are also comnlon (Fig. 1). Pelitic assemblages. which form the subject matter of this note. comprise of st-grt-ky-sil-pl-hio with rare antiperthite and ilm: grt-ky-sil-pl-kfs-rt-iIm-qtz-bioand grt-fibroli te-crd- pl-qtz-biokkfs. High ZnO (3.7 wf(jm)-bearing staurcrlitc will1 prljet i111cl Microprobe analyses of the co-existing mineral kyanite has been reported first time in this area. 1 iigh ZIIO associations were carried out through SX50 CAMECA (2.04-4.7wt70) content in staurolitc Pias been repol-~cdby

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Fig.2. Photomicrograph showing (st) staurolite-(ky) kyanite portion in staurolite - kyanite-garnet-sillimanite-bearing assemblages.

Fig.3. Photograpn of specimen showing (ky) kyanite IIUUI~~~II~(grt) garnet. Kyanite is partially altered to paragonite.

!: Fig.4. Photomicrograph showing garnet-sillimanite-cordierite association. -- JOUR.GE0L.SOC.INDIA. VOL.63. MAY 2004 I Table 1. Representative analyses of co-existing minerals from the pelitic rocks of Jalsoor-Mer&a area

- pp Mercara Byalkuppe Gn(C) Gn (R) . St(C) St(R) Bio (C) Bio(R) PI (C) PI (R) Grt (R) Crd (C) Crd (R) Bio (C) Bio (R) Gn (c)* Crd(c)* Bio(c)* Si02 xo, A1203 CrzO,. FeO MnO MgO CaO Na,O K,O ZnO Total

0 Basis 24 24 24 24 22 2 2 8 8

* Gopnlakrishna. unpublished thesis (1984): ** Gn-PIag pairco-existing with kyanite SHORT COMMUNICATION 559

several workers (see Table 74, page 828-829 in Deer et al. suggests that this assemblage belongs to granulite grade. 1982).In the present study, we can only state that high ZnO This is corroborated by the fact the pelitic bands exhibiting content had possibly stabilized staurolite to higher grades these assemblages in the field are associated with pyroxene of metamorphism, and staurolite along with ky+grt persists granulite bodies and the latter give T of 750" C at P - 7.5 in pelitic bands inter-banded with pyroxene granulite bodies kbars. P-T estimation after Koziol and Newton (1988) give in ail essentially charnockitic terrain. To further strengthen P of 8.6 to 8.2 kbars for core and - 7.5 kars for rim analyses. this point, staurolite from the st-grt-kya association of Late sillimanite after kyanite gives P of 7.8 to 7.5 kbars at T typical amphibolite facies of the Holenarasipura locality of 750°C (Table 2). has ZnO content of 0.11 -0.77 % (see Bouhallier, 1995). We would like to point out here that this type of kyanite P-T conditions of st-grt association after Richardson growth mantling garnet is of significance as these (1968) give temperature of 485°C-450°C at 4.5-4.8 kbars. associations are to be found in shear zones cutting across Pigage and Greenwood (1982) questioned the OH granulite grade terrains as in Napier Complex, Broken Hill cornpotletit of staurolite as given by Richardson (1 968), and and Scourie areas as quoted in Harley (1989). Further, Harley therefore the estimations after Richardson (1 968) can give (1 989) suggests that the presence of kyanite is indicative of us only a rough idea. Average P-T estimation after Holland long residence in the middle to deep crust at least till the and Powell (1997) ranges from 5.3 to 5.8 kbars at T of 700°C time of shearing This fact is co~oboratedby the fact that the (Table 2). Mangalol-e-Mercara lineament cuts or bounds the Coorg granul j te terrain. (b) Garnet-kyanite Association: The garnet-kyanite association is seen from localities around Jalsoor, one and (c) Garnet-cordierite-sillimanite-biotite Assemblage: half ki lolnetres from the Jalsoor town and from Siddapura At places, particularly near Jalsoor, we have come across situated on the Mangalore-Mercara 1inearnent. Evidences of pelitic assemblages containing cordierite. In thin sections, myloni tes and pseudotach yli tes abound in these locations. large porpl~yroblasticgarnet is surrounded by the fibrolite Grt-ky assemblages are encountered in the present area where variety of sillimanite and aggregates of cordieri te, the latter one can see large blades of bluish kyanite mantling garnet, exhibiting characteristic pleochroic haloes around zircon separated at places by a thin rind of leucosome (Fig.3). Grt- inclusions in biotite and feldspars (Fig.4). Large garnets ky-bearing assemblages also contain plagioclase and rare and cordierite stand out. P-T estilnations using Berman K-feldspar with rutile as the main accessory. Grt-ky-rt (1996, TWEEQ program) for grt-crd-bio assemblage

Table 2. Pressure-temperature estimations of pelitic rocks

Area Assemblages P (Kbar) T ("C) C R C R

Mercara H&P grt-st 5.8 5.3 700 700 Present study K&N grt-ky-pl-qtz 8.2 6.8 750 750 grt-sil-pl-qtz 7.8 6.5 750 750 H grt-bio 575 535

Bynlkuppe B grt-crd-bio-qtz 6.1 630 485 Gopalakrishna (1984) H grt-bio 555

Jalsoor K&N grt-ky-pl-qtz 8.5 7.5 750 750 Krishnaraj et al. (1994)

Jalsoor K&N grt-ky-pl-qtz 8.6 750 Present study grt-sil-pl-q tz 7.5 750 B grt-crd-bio-qtz 5.8 5;3 590 560 H grt-bio . 596 545

For H method temperature calculated at 7 kbars For K & N method pressure calculated at 750°C & 600°C K&N-Koziol & Newton (1988); H-Holdaway et al. (1997) B-Bcrman,TWEEQ, 1997: H & P-Holland and Powetl,THERMOCALC, 1997

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I 5 00 do 7 bo sbo Temperature (OC) Grt-St Mercara .Grt-Crd Jalsoor Cirt-Crd Byalkuppe V Metcara Temperature (OC) Grt-Ky Jalsoor 4 Grt-Ky Jalsoor* Grt-Ky Fig.5. TWEEQ for (grt) garnet-crd (cordieritel-bio (biotite)- 0 Grt-Sil Jalsoor 4 Grt-Sil Jalsoor* Grt-Sil Mercara sillimanite (sil) association. Fig.6. P-T path of pelitic rocks of Jalsoor-Mercara areas.

gives P of 5.8 kbars at T 600 "C and rim P of 5.4 kbars at with kyani te mantling garnet is generally encoun tcred in T 560" C (Table 2 and Fig.5). Cordierite formation after shear zones. One of the be4t examples is that garnet-marltled garnet is clearly indicative of later ITD path.The findings by kyanite in pelites at Kodanadu viewpoint, along the of this association around Jalsoor suggest this growth can Moyar shear (Prakash Narashima. 1992). It is generally be attributed to a later stage. agreed that the northern slopes of the Nilgiri hills exhibit high pressures, where Raith et 31. (1999) have obtaincd P of B-T Path 10.6 kbars at Kodanadu viewpoint. Garnet whole rock ages If all the data from the staurolite-bearing, garnet-kyanite from the Nilgiri are generally around 2450 Ma, while the and garnet-cordieri te- bearing associations from these SmINd ages of garnet from the grt-ky association give different areas are plotted, they show an apparent anomalous ages of 2 175 -2 1 15 Ma and the lower ages have anticlockwise P-T path (Fig.6). Similar paths have been given been attributed by Raith et a!. (1999) to resetting of for the Nilgiri charnockite block (Raith et al. 1990; and Mints Sm-Nd system in garnet during the intense defor~nntion and Konilov ( t 998). In general, the tectonic scenario could associated with the Moyar shear zone reactivated at be interpreted as due to crustal thickening of the deformed 550 Ma. A similar scenario can be invoked for the grt-ky continental margins. with the entrainment of continental association near Jalsoor and Siddapura areas, as these marginal basin sediments, folEowed by extensional thinning shear zones also bound the Coorg granulite block. Furthcr-, of the crust. It must be pointed that the P-T path is made up younger granite and syenite intrusions of -780 Ma arc of two segments - an early prograde staurolite-garnet present in the Mangalore-Mercara lineament. association giving rise to garnet-kyanite association (IBC path) and plausibly later garnet-fibrolite-cordierite-biotite association indicative of an ITD path. Granulite terrains Ac~k~~or~~lecf,~er~~~~~t.v:The authors wish to thank Prof. A.Balasubramanian, Chairman, 2nd illso thank Prof. C.Srikantappa that exhibit such hybrid IBC and ITD paths are related to for providing Departmental frlcilities. Dr.Bas:lvalingu i\ thankud extensional thinning of a previously thickened crust. especially for carrying out the ~nicroprobestudy. The work is carried out iinder the CSIR-ES Programme. We are thiinkful to Discussion the referees cotnments, which hacl helped in improvmng thi\ \hart We would like to emphasize that the grt-ky association cornmunicntion.

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References

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(Received: 20 Mu,vclt 2003; Reviseclfinu uccepred: 8 October 2003)

Appendix

Following abbrevations for minerals (after Kretz, 1983) have been used in the paper: st=staurolite, grt=gamet, ky=kyanite, sil=sillirnanite, crd=cordierite, bio=biotite, pl=plagioclase, kfs=k-feldspar, qtz=quartz, rt=rutile, ilrn=ilmenite.

JOUR.GEOL.SOC.INDIA,VOL.63, MAY 2004