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Southern Granulite Terrain, South of the Palghat-Cauvery Shear Zone: Implications for India-Madagascar Connection
Gvridioann liesenrcii, V. 2,No. 3, pp. 463469 01999 Internntio~ialAssocintion f o r Goridionmi Rmnrclr, Jnpnii
GR
ISSN: 1342-937X
Gondwana Research
Southern Granulite Terrain, South of the PalghatCauvery Shear Zone: Implications for IndiaMadagascar Connection A.S. Janardhan Deparfnzent of Geology, University of Mysore; Manasa Gangotri, Mysore 570 006, India (Manirscuipt ueceizmi October 2, 1998; accepted Marcli 2, 1999)
Abstract The present paper correlates tlie southern Madgascar terrain, south of the Ranotsara shear with the granulite terrain of southern India, occurring south of the Palghat-Cauvery (P-C) shear zone. Both the terrains have witnessed high temperature to ultra liigli temperature granulite metamorphism at 550 Ma and are traversed by shear zones and deep crustal faults. The 550 Ma old granulite terrains of Madagascar and southern India have similar lithologies, in particular, sapphirine bearing pelitic assemblages . Graphite deposits and gem occurrences are common to both these terrains. The 550 Ma old southern granulite terrain of southern India comprises of different blocks , the Madurai and tlie Kerala Khoiidalite belt, but all the blocks have similar litliologies with pelite - calc silicate rocks inter-banded with two pyroxene granulite bodies . These litliologies occur amidst an essentially cliarnockitic terrain. The protolith ages of the southern granulite terrain, south of the P-C shear zone ranges between 2400-2100 Ma. The terrain as a whole has witnessed the 550 Ma old granulite event. Tlie granulite metamorphism took place under temperatures of 800 -1000 ‘I C and at pressures of 9.5 to 5 Kbar. Tlie source of heat for the high temperature granulite event of the southern Madagascar terrain has been linked to advective heat transfer along mantle deep faults. The source for the high temperature granulite metamorphism for tlie southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening. Many workers have linked Madagascar to southern liidia by connecting the Ranotsara shear either to the P- C shear zone or to the Achankovil shear zone, further south. The important factor is the litliologies of the Madagascar terrain, south of Ranotsara shear zone and the 550 Ma. old southern Indian granulite terrain are similar in many aspects. It will be more appropriate to kink the Ranotsara shear to the curvilinear lineament bounding the Anaimalai-Kodaikai~alranges and which merges with the southern margin of tlie P-C shear zone. However, north of tlie Ranotsara shear/fault, the northern Madagascar terrain comprises of a dominant Itremo sequence ( < 1850 Ma) and 780 Ma old calc-alkaline intrusives. The latter have similarities with that of Aravallis and the Sirohi, Malaiii sequences occurring further north east . The Rajasthan terrain has witnessed igneous intrusive activity at 1000-800 Ma. if w e can broaden the area of investigations and include the above areas, the Madagascar- India connection can be better understood.
Key words: Southern India, Madagascar, metamorphism, shear zones, correlation.
Introduction The granulite grade continental blocks and the prominent shear zones associated with them, have in recent years, become one of the important criteria in the reconstruction of the Eastern Gondwanaland . The well-exposed granulite terrain of southern India is critical as it forms the central part of the eastern Gondwanaland (Miller et al., 1996). The granulite terrain of southern India (Fig. 1) is also unique in the fact that, in a compact terrain, we get the 2500
Ma early Proterozoic moderate temperature-high pressure Coorg-Niligiri-Shevaroys-Madras belt ; the 1000 Ma high temperature - moderate pressure Eastern Ghats belt and the 550 Ma Ultra high temperature Southern Granulite Terrain (SGT) ,south of the broad Palghat-Cauvery shear zone . These different granulite blocks have characteristics of their own .The early Proterozoic granulite blocks are associated with coeval granite belts, viz., the Closepet and the Krishnagiri. High temperature intrusions do not precede the late Archaean granulite metamorphism. On the other
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Fig. .I. Map of Southern Indian Peninsular shield showing the granulite terrains of southern India, and the Palghat-Cauvery (PCSZ) and Achankovil (ASZ) shear zones. The 550 Ma PanAfrican southern granulite terrain is shaded a n d occurs south of the PCSZ, after Harris et al. (2994). K.C. Karnataka Craton; cg - Closepet granite; NB Niligiri Block; MsD- Madras Block; MiB- Madurai block ;Am- Achankovil metasediments; KKBKerala Khondalite Belt. Map also includes Madagascar and Eastern Antarctic. MZB, Mozambique; RSZ, Ranotsara shear zone; WC, Wanni Complex; HSWC, Highland Complex; LHC, Lutzow-Holm Complex; SR, Sor Rondande Mountains.
hand, Proterozoic massif type anorthosite- syenites are associated with the Eastern Ghats belt. The 550 Ma old Southern Granulite Terrain, is characterized by anorthosite; syenite-alkali granite-carbonatite intrusives along deep seated and prominent shear zones. The main aim of this paper is to correlate the granulite terrains of southern India with Madagascar, Lutz-Holm bay and Rayner Complex of Antarctic; but the thrust is towards Madagascar and (southern) India connection and for this purpose, the SGT lias been chosen, as it has witnessed high temperature metamorphism at 550 Ma ., as part of the global Pan-African tectono thermal event. Description of the various litliologies of the SGT and their metamorphic history is given in some detail towards arriving at the correlation. The Southern Granulite Terrain (SGT),which had witnessed 550 Ma granulite event, occurs well south of the broad Palghat - Cauvery shear zone. The Palghat-Cauvery tract itself contains oldcr rocks like the 3000 Ma Sittampundi layered anorthosite complex and the younger 660 Ma Tiruchengodu granite (Bhaskar Rao et al., 1996).Theactual northern boundary of the 550 Ma old SGT lies along the shear zone, immcdiatcly north of the Anaimalai- Kodaikanal ranges. Along this shear zone, towards its eastern extremity, the Oddanchatram and the Kadavur anorthosite bodies of Proterozoic ages occur. The 550 Ma old component of the
SGT includes the Madurai, Periyar, Nagercoil blocks and the Kerala Khondalite Belt (Santosh and Yoshida, 1996)and is characterized by prominent shear zones and deep faults which host anorthosite, alkali granites, syenites and carbonatite intrusives. Deep seated faults, associated high temperature intrusions a n d high temperature metamorphism are in many ways similar to the southern Madagascar terrain, south of the Bongolava-Ranotsara fault. In fact, in the southern Madagascar terrain, the N-Strending faults which host the anorthosite bodies are considered to be mantle deep and are believed to have acted as conduits for advective heat transfer and based on gravity studies, Pili et al. (1997) have demonstrated possible mantle upwelling by about 10 Km. The protolith ages of the SGT range between 2400 -2100 Ma., while the peak metamorphic ages range between 560 -530 Ma. (Bartlett et a1.,1998; Jayananda et a1.,1995; and Miller et a1 .,1996 ). The peak metamorphic ages of the SGT are similar to the southern Madagascar terrain (Paquette and Nedelec, 1998).
550 Ma old ( Pan- African) Southern Granulite
Terrain Charnockites of tonalitic to granitic composition are by far the most common rocks of the SGT. Swathes of pelitequartzite- calc-silicate lithologies inter-banded with two pyroxene granulites occurring amidst charnockites are characteristic of the SGT. BIF is rare in the SGT. The metasediments are typical of continental margin basinal affinities. Clz n r n o c ki t es : N on- ga r n e ti f e r o us cha r no c k it e (orthopyroxene-biotite-plagioclase-Kfeldspar-quartz) is the most dominant rock type of the Kodaikana1,the Cardamom and Varushanad hill ranges. Perthitic K-feldspar is the dominant mineral. Plagioclase (An 30 %) and quartz constitute 50 YO of the rock . The mafics are orthopyroxene (X Mg -0.60 with a high alumina content of 6.5 YO)and biotite together form 15% of the rock. Garnet in charnockite, when present, occurs as porphyroblasts with almandine (48Y0)and pyrope contents (36%).First generation biotite grains have 4-6% TiO, while the common retrogressive biotite has a lower Ti02'contentof 2%. Pyroxene granulite bodies: The assemblage orthopyroxeneclinopyroxene-plagioclase is typical. Garnet is present only at the contacts of the pyroxene granulite bodies with calcsilicate bands, and also in smaller bodies occurring as enclaves in charnockite. Garnet is homogeneous with an X Mg of 0.74 -0.77. Orthopyroxene at places grows around clinopyroxene, suggesting a temperature increase. Further, symplectitic orthopyroxene (X Mg - 0.54) + calcic plagioclase assemblage after garnet is quite common to the SGT (Wiebe Gondzuiinn Resenrcli, V. 2,No. 3, 1999
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intermixing between pyroxene granulite and calc-silicate bands do have a control on the mineralogy of calc-silicate rocks. When the calc-silicate layers are a few decimeters thick and the intermixing is limited, calc-silicate rocks Metasediiizentay litholugies contain 60-70% calcite, with scapolite and wollastonite up Of the metascdimentary litholgies which occur as broad to 20%. The grains are polygonal with triple point contacts swathes amidst charnockites, pelites are the most abundant and resemble marble. Hedenbergitic clinopyroxene grows and are interbanded with calc-silicate rocks, pyroxene at the expense of the carbonate phases and garnet is granulite bodies. The pelitic assemblages consist of the conspicuously absent. In contrast, if the individual bands following: a) garnet- cordierite- biotite- spinel- sillimaniteare thin and the intermixing between the two lithologies K.feldspar- quartz; b) garnet- cordierite- orthopyroxeneare intense, the modal abundance of calcite decreases to biotite- plagioclase- K.feldspar- quartz; c) sapphirine25-30"/0, with a concomitant increase of wollastonite and cordierite- orthopyroxene- biotite- plagioclase- spinel and scapolite (rich in Me mol.) up to 70%. Grandite garnet is d) cordierite- orthopyroxene- biotite-plagioclase-K.feldspar. found as coronas around the above minerals. The Assemblages a and b are the most common, while c and d assemblage wollastonite-scapolite-grandite garnet indicates are localized and represent quartz absent assemblages. The high temperatures. This is consistent with the high assemblage garnet-cordierite-biotite-K.feldspar-quartz- temperature sapphirine assemblages present in the area. sillimanite form bands of varying width and can be traced Wollastonite laths often contain inclusions of calcite and at several places for almost continuously for more than 5 quartz attesting to its formation after these two minerals. Km. Garnet is common and often contains inclusions of In the T-X,,, diagram this reaction shows a positive slope biotite, sillimanite and quartz . Grain boundary alteration indicating an increase in temperature (Francis Anto et al., to orthopyroxene is rather frequent suggesting that it 1999).Late garnet coronas in calc-silicate rocks generally formed during high temperature decompression path. indicate cooling as recorded from several terrains viz., Cordierite grains occur as rims around the garnet at first Northern Prince Charles Mountains, Antarctica ( Harley and resorb the garnet almost completely. At places, the et al., 1994 ), Prydz Bay, East Antarctica (Motoyoshi et al., cordierite grains are often cut by fibrolite-biotiteintergrowth 1991). In all these cases, development of coronal garnet along their rims, indicating late retrogression. Plagioclase have been attributed to isobaric cooling. Similar reactions (An RO'Z,) occurs as large well twinned plates. K.feldspar have been described from the Eastern Ghats where cooling is often perthitic. Biotite is a common mafic mineral in and domainal variations in fluid composition are invoked pelites. Biotite included in garnet and cordierite has a high (Bhowmik et al., 1995). TiO, of 5-6%. Biotite coexisting with garnet and cordierite has a lower TiO, of 4%, while late retrogressive ones after Metamorphic Pressure-Temperature Estimations garnet and orthopyroxene have 2 70. Two distinct mineral assemblages, sapphirineP-T estimates for the charnockites using the assemblage cordierite-orthopyroxene-biotite-spinel and cordieritegarnet-orthopyroxene-plagioclase-quartz in charnockites orthopyroxene-biotite-plagioclase (sapphirine absent) range from 750-900°C, at pressures ranging from 9.5-5Kbar. assemblages are noticed. Sapphirine, orthopyroxene and G a r ne t - o r tho p y r o x e n e / c 1in o p y ro x e n e - p 1a g i oc 1a s e cordierite grains (up to 0.8 cm across) can be seen even assemblage in two pyroxene granulite often shows in hand specimens. High alumina orthopyroxene (8-12'3'0) disequilibrium textures suggestive of decompression has been reported in sapphirine assemblages from reaction. The symplectitic orthopyroxene-plagioclase Perumalmalai and Ganguvarpatti (Sivasubramanian et al., association after garnet gives pressures of 7 Kbar. The 1991; Raith et al., 1997).This high alumina orthopyroxene TWEQU data for core and rim clearly indicate a drop in with patches of sapphirine and green spinel is commonly pressure from 9.5 to 7 Kbar . found as a breakdown product of garnet. Garnet formation Similar pressure-temperature estimates were obtained prior to orthopyroxene and sapphirine is important .The for the garnets in the Oddanchatram anorthosite body garnet which has given rise to orthopyroxene gives a (Wiebe and Janardhan, 1988). Here, the garnet that lack metamorphic cooling age of 520 Ma (Sm-Nd garnet-whole orthopyroxene-plagioclase symplectites gave high rock (Janardhan and Peucat., unpublished data). Sapphirine pressures of 10.6 Kbar at 92OoC, while garnet rims in forming de-compressional reactions took place at 850 equilibrium with the symplectitic orthopyroxene + calcic 100OT (Raith et al., 1997 ;Francis Anto et al., 1997). plagioclase gave pressures of around 6.6 Kbar at lower Calc-silicate assemblages: Calc-silicate lithologies contain temperatures. The pressures estimated using compositions of spinelcalci te-scapolite-wollastonite-clinopyroxene-garne tplagioclase-K-feldspar-sphene-quartz. The degree of cordierite-biotite inclusions in sapphirine along with and Janardhan, 1988; Ravindra Kumar and Chacko,1994). Similar symplectites are also found in garnetiferous charnockites.
Gondivniza Resenrch, V: 2, No. 3,1999
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orthopyroxene cores give a maxiinuin of 7.5 Kbar, while lower pressures of 5 Kbar are given by cordierite- bitotite sapphirine associa tion. Temperatures of sapphirine assemblages vary froin 1000 -800 " C. Scapolite- garnet- plagioclase- wollastonite- quartz assemblage in the calc-silicate rocks is amenable for calculation of P-T-X,,, conditions. The (Me)invariant point gives temperatures 850-925°C and X,,, = 0.65-0.9 in the isobaric (6 Kbar) T-X, o2 diagram. The P-X,,,diagram show pressures 5.5-6.5Kbar with X,,, of 0.8-0.6 (Francis Anto et al., 1999) The P-T path of the SGT thus involves a high pressure medium temperature metamorphism followed by high temperature decoiiipression and rapid cooling. The high pressure metamorphism indicative of burial of lithologies must have taken place after the intrusion of Proterozoic massif anorthosite bodies as at Oddanchatram as suggested by the development of garnet in the anorthosite body at their contacts with the pelitic and two pyroxene granulite xenoliths (Wiebc and Janardhan, 1988).The peak pressures were succeeded later by high temperature decompressional reactions, which took place during or slightly after the 550
'
I
Fig. 2.a
t 5w
-
Ma old granulite metamorphism .Thus the composite P-T path shows a clockwise pattern . (Fig. 2 b)
Major Shear Zones and Faults of the SGT The characteristic feature of the SGT is that the entire terrain is dissected by prominent shear zones and crustal deep faults. The SGT is bounded by the prominent shear zone /fault immediately bordering the Kodaikanal hill ranges. Along the eastern part of this shear, inassif anorthosite bodies of Oddanchatram and Kadavur of Proterozoic age are emplaced into a thick continental marginal basin type sequence of quartzites-pelites and rare BIF. The Kodaikanal or the Madurai block, as it is often termed, forms the northern part of the SGT. Recent works (Janardhan and Wiebe,1985; and Bartlett et al., 1998) have brought out the fact that this strip of the northern SGT hosting the Proterozoic anorthosite bodies may represent a component of the 1000 Ma old Eastern Ghats sequence. The tract hosting the anorthosite bodies may be connected to the Eastern Ghats trend across the Cauvery shear (Janardhan and Wiebe,l985).The same shear turns NW if
1000
'1'"c Fig. 2.b
P-T cslimalcs/paths of the 550 Ma S. Madagascar tcrrain, South of Ranotsara shear zone after Nicollct ( 1 990). a ) Vohibory formation: 1 , Sapp-Co bcaring amphibolites; 2. Scrcndibitc-cli ntonilc-clinopyroxcnitcs b) Ampanihy Ihrination: 3 & 4 Anorthosite complcx; star-graphite bcaring gncisscs c) Androyan formation: 5. Cord-grd-opx-sp-cltz bearing gncisses, 6. Scvcn phasc anatcctic gncisscs from Ihoshy.
1
I
500
600 +
+ Cllarnockitc Gt.[]py.p]ng.~r
I 700
I 800
I 900
Temperature ("C) Cl Snliliiririe hcxrirtg rock, GI-Opx-Flag
+ Anot.thositc G ~ . O ~ . X - P I ~ ~ - Q ~Sa-cotd-Ow
4 Anorthosite C ; I - C l ~ x . p 1 ~ ~ - Q z + Gt-OPx (IlnrlcY) P-1 obtoiricd by A n m d Atohail (1985) Pclilc Gt-Cord P-'I obl:iiiicd by I1:inis ( I 95 I ) f. Pclilc Cortl-OIIX 0 Grew (19SZ) 0 Pelitc (it-Silli-I'lng-Qz X I'eli~cGRAIl,
* *
Calculated pressures and tempcraturcs for various assemblages and P-Tpath for the Palni Range Granulites. AI, S O , stability relations from Holdaway ( I 97 I ).
Figs. 2 a & b. P-T paths of the 550 Ma sciutliern Madgascar terrain (data after Nicollet,lYYO) and the Madurai block of the Southern granulite terrain.
Gondzuniia Resenrch, l? 2,No. 3,1999
INDIA-MADAGASCAR CORRELATION
the Perinthatta anorthosite of northern Kerala are connected to the Oddanchatram and tlie Kadavur bodies. It may be pointed out here that the Peralimala and the Kalpatta granite north of the Palght-Cauvery shear give ages of 550 Ma. (Miller et al., 1996). The point to be made here is that the Palghat- Cauvery shear is curvilinear and represents a zone where transpression tectonicsbrought the Eastern Ghats and the SGT in juxtaposition with the Dharwar craton . This may have involved the collision of the Napier granulite block backed by the Rayner Complex ( Drury,1989).The E-W trend of the Cauvery shear is a later imposed one probably during 550 Ma. Apart from anorthosite, a number of alkali granite, syenite and carbonatite intrusives occur and seem to be restricted to deep faults and shear zones (Santosh et al., 1989).The ages of these alkali intrusions range between 790550 Ma. Among the shear zones of the SGT, the most prominent is the Achankovil shear zone , the Kerala Khondalite Belt ( KKB) lies SW of this shear zone. The KKB has been extensively investigated ( see references in Santosh and Yoshida,1995 and 1996) and the kinematics of the Achankovil shear has also been worked out . It is now generally agreed that the eastern part of the Achankovil shear has witnessed a n earlier dextral movement overprinted by sinistral movements, which becomes prominent towards the west ( Sacks et al., 1997).Bartlett et al. (1998) have stated that the KKB comprises of a slightly younger set of sedimentary sequence of 1800 Ma. They further state that this 1800is not that dominant in the terrain north of the Achankovil shear zone. Their conclusion is that the KKB block may be younger and thus different. However, Bartlett et al., (op.cit.) go on to state that it is quite likely that the 800- 550 Ma old alkaline intrusive activity may have erased the 1800 Ma memory in the Madurai block. The 1800 Ma imprint has been recorded by Jayananda et al. (1995) and Miller et al. (1996) have stated that the protolith age of Varuslianad Hills charnockite, north of the Achankovil is 560 Ma. The important aspect is that there is a definite agreement that all the blocks of the southern granulite terrain, south of the Palghat-Cauvery shear have witnessed a common granulite grade metamorphism at 560 -540 Ma.
India- Madagascar Connection Three major Precambrian crust forming and re-working events have been identified by Tucker et al. (1997). Their isotopic data of the tonalitic enclaves in the late Neoproterozoic granites suggest that Archaean rocks underlie much, if not all, of Madagascar north of the Bongolava-Ranotsara shear zone. They also state that (< 1850 Ma) Mesoproterozoic (Quartzo-schistose-calcaire) QSC sequence of the Itremo group of rocks were intruded Gotidionnn Xesenuch, V. 2,No. 3, 1999
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by gabbro-diorite plutons of 790-780 Ma. The TE signatures of the uncontaminated gabbros suggest that they were emplaced in an arc like setting. These authors further state that the Mozambique ocean closure ( 900 - 640 Ma ) and connected subduction of the lithosphere can be connected to the gabbro-diorite plutonic intrusions into the Itremo QSC group. At the same time in the continental hinterland of southern India, widespread carbonatite- syenite activity took place during extensional tectonics. The 580-520 Ma., Neoproterozoic events include sheet like emplacement of granites and syenitcs (Tucker et al., 1997; Paquette and Nedelec, 1998) and ductile deformation and metamorphism of the Itremo sequence a n d high temperature granulite metamorphism during 560-540 Ma. and intrusive granites (540-520 Ma), south of the BongolavaRanotsara fault. Recent works including that of Tucker et al. (1997);Powell et al. (1980 and 1997);and Windley et al., (1997) agree that the position of India against Madagascar can be constrained by matching Archaean rocks in northern Madagascar and western India and by tracing Neoproterozoic shear zones from one continental block to the other (Yoshida et al., 1997; Bhaskar Rao et al., 1996).This type of matching brings out the possibility that India may have moved along tlie Madagascar-India margin by 1000 km during the Gondwana breakup (Li and Powell, 1997). Madagascar-India fits are based mostly on the matching of the Palghat-Cauvery or the Achankovil shear zone with the Bongolava-Ranotsara shear (Windley et al., 1994; Yoshida et al., 1997; Harris ,1997 and Janardhan ct a1.,1997). Evidences which strongly support this argument is that the granulite terrain south of the Ranotsara shear is in many ways comparable to thc SCT of southern India, immediately south of the shear bounding the northern slopes of the Kodaikanal hill ranges and which liosts the Proterozoic anorthosite bodies. It is quite likely that the Achankovil shear zone can be extended northwestwards m d may join the western extension of the Palghat - Cauvery shear zone. These two terrains have witnesscd high temperature metamorphism (Nicollet,1990 ; Martelat et al., 1997) with temperatures ranging between 800 - 1000"C and at pressures of 7-9 Kbar. Further, these two terrains are known for their graphite and gem stone occurrences. More importantly, the southern Madagascar terrain is traversed by mantle deep faults, which host high temperature intrusions, and along which advective heat transfer has taken place through fluids from the mantle and that the mantle has been uplifted by 10 km (Pili et al., 1997). These authors appeal to transpression tectonics, buckling of the continental lithosphere for the formation of the mantle deep shear zones. In summary, the SGT and high grade terrain south of the Ranotsara shear are similar to one another in several aspects.
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The heat source for the high grade to ultra high temperature m e t a m o r p h i s m of t h e SGT can be connected to high temperature alkaline intrusive activity which had taken place in an extensional setting after a n initial thickening event. However, if one takes into account the Madagascar terrain north of the Bongolava-Rantosara fault, the similarities seem to be much more in common with that of the Aravallis-Delhi sequence a n d particularly w i t h the acid volcanism a n d plutonism of the Malani province. This aspect can be better e x p l a i n e d by Li a n d Powell (1997, figs.1 d-f), w h e r e Madagascar lies farther north a n d close to the Gujarat coast. Further studies are needed to resolve the Madagascar- India connection by broadening the scope of investigations .
Acknowledgements The author wishes to place on record his grateful thanks to P r o f e s s o r s L e w A s h w a l a n d Yoshida w h o w e r e responsible for the author’s visit to Madagascar and for the very fruitful interaction with other scientists w h o participated in the meeting a n d field trip.
References Bartlett, J.M., Dougherty, J.S, Harris, N.B.W., Hawkesworth, C.J. and Santosh, M. (1998) The application of single zircon evaporation and model Nd ages t o the interpretation of polymetamorphic terrains: an example from the Proterozoic mobile belt of south India . Contrib.Mineral.Petrol., v.131, pp. 181-195. Bhaskar Rao, Y.J., Chetty, T.R.K., Janardhan,A.S. and Gopalan, K. (1996) Sm-Nd and Rb-Sr ages and P-T history of the Archaean S i t t a m p u n d i a n d Bhavani layered metaanorthosite complexes in Cauvery shear zone, south India :evidence for Neoprotrozoic reworking o f Archaean crust. Contrib.Mineral.Petrol., v.125, pp. 237-250. Bhowmik, S.K., Dasgupta, S., Hoernes, S.and Bhattacharya, P.K. (1995) Extremely high temperature calc granulites from Eastern ChatsJndia: Evidence for isobaric cooling, fluid buffering and terminal channelised fluid flow. Eur. J. Mineral., v.7, pp. 689-703. Drury, S.A. (1989)Late Proterozoic Tectonics of Southern lndia a n d adjacent p a r t s of G o n d w a n a land. Abst. IGC Washington. Francis Anto, K., Janardhan, AS. and Basavalingu, B. (1999) Metamorphic history of Calc-silicate lithologies from the Kambam Valley,Tamil N a d u a n d its bearing on the Evolution of the Southern Granulite Terrain. Jour. Geol. SOC.India, v. 53, pp. 27-37. Francis Anto K., Janardhan, A S . and Sivasubramanian, P. (1997) A new sapphirine occurrence from the Kainbam Valley, Tamilnadu and its possible relation to the Pan-African tectonothermal event. Curr. Sci., v.73, pp. 792-796. Harris, N.B.W. (1997) The significance o f the Palghat-Cauvery shear zone in southern India for correlations between southwest India and Eastern Madagascar. Abst. in Proterozoic
Geology of Madagascar. IGCP 348/368 workshop., August 1997, p.30. (1994) Crustal Harris, N.B.W., Santosh, M. and Taylor, P.N. evolution in South India: constraints from Nd isotopes. J. Geol., v. 102, pp. 139-150. Harley, S.L., Fritzsimons, I.C.W. and Buick, IS. (1994) Reaction textures in wollastonite-scapolite granulites and their significance for the pressure-tempera ture-fluid histories of high grade terrains. Precamb.Res., v. 66, pp. 309-323. Janardhan, A S . , Anto, F. and Sivasubramanian, P.(1997) Granulite blocks of southern India: Implications for South India- Madagascar Reconstruction. Abst. in Proterozoic Geology o f Madagascar. IGCP 348/368. August 1997. p. 33. Janardhan, A.S. a n d Wiebe, R.A.(1985) Petrology and geochemistry of the Oddanchatram anorthosite a n d associated basic granulites, Tamilnad u. Jour.Geol. Soc. Ind. V.26, pp. 163-176. Jayananda, M., Janardhan, A.S., Sivasubramanian, P. and Peucat, J.J. (1995) Geochronologic and isotopic constraints on granulite formation in the Kodaikanal area, S.India. ’India and Antarctica during the Precambrian’ (Ed. Santosh, M. and Yoshida, M.) Mem. Geol. Soc. India. 34. pp. 373- 390. Li, Z.X. and Powell, C.McA. (1997) Palaeomagnetic constraints on the India Madagascar continental transform during continental breakup. Abst. i n Proterozoic Geology of Madagascar IGCP. 348/368 workshop, August 1997, pp.47-48. Martelat, J.E., Nicollet, C., Lardeaux, J.M., Vidal, G. and Rakotondrazafy, R. (1 997) Lithosplwric tectonic structures developed under high-grade metamorphism in the southern part o f Madagascar. Geodinamica Acta. v. 10, pp. 94-114. Miller, J.S., Santosh, M., Pressley, R.A., Clements, AS, and Rogers, J.J.W.(1996) A Pan-African thermal event in S.1ndia. JSouth East Asian Earth Sci., v. 14, pp. 127-136. Motoyoshi, Y., Thost, D.E. and Hensen, I3.J. (3 991) Reaction textures in calc-silicate granulites from the Bolingan Islands, Prydz Bay, East Antarctica: Tmplication for the retrograde P-T path. J.Metamorphic Geol., v.9, pp. 293-300. Nicollet, C. (1990) Crustal evolution o f the granulites of Madagascar. in ”Granulites and Crustal Evolution”. (Eds. Vielzeuf D, and Vidal Ph.) Kluwer Pub1 Paquette, J. L. and Nedelec, A. (1998) A new insight into PanAfrican tectonics in the east-west Gondwana collision zone by U-Pb zircon dating of granites from central Madagascar. E.P.S.L., v.155, pp.45-56. Powell, C. McA., Johnson, B.D. and Veevers, J.J. (1980)A revised fit of east and west Gondwana. Tectonophys., v.63, pp. 13-29. Powell, C.McA., Li, Z.X., Mueller, D. and Watkeys, M.K. (1997) The India-Madagascar transform boundary: lmplications for Jurassic and Cretaceous Gondwana breakup. Abst. in Proterozoic Geology of Madagascar, August 1997, p.73. Pili, E., Richard, Y., Lardeaux, J.M. and Sheppard, S.M.F. (1 997) Lithospheric shear zones and mantle crust interactions. Tectonophysics, v. 280, pp.15-29. Raith, M., Karmakar, S.and Brown, M. (1997) Ultra high temperature metamorphism and multistage decompression evolution of sapphrine granulites from the Palani hill ranges southern India. J. Metamorphic Geology, v.15, pp. 379-399. Goizd7onnn 12rscnrcl1, V. 2, No. 3, 1999
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Ravindra Kumar, G.R. and Chacko, T. (1994) Geothermoinetry of mafic granulites and metapelites from the Palghat gap region, South India: Petrological evidence for isothermal uplift and rapid cooling. J.Metamorphic Geol., v.12, pp. 479-492. Sacks, P.E., Nambiar, C.G. and Walters, L.J. (1997) Dextral PanAfrican shear along the SW edge of the Achankovil shear belt,S.India. Jour.Geol., v. 105, pp. 275-284. Santosh, M. and Yoshida, M. (1995) (Eds.) India and Antarctica during the Precambrian. Mem. Geol. Soc. India 34. Santosh, M. and Yoshida, M. (1996) Proceedings of the IGCP368 international field w o r k s h o p a n d Proterozoic continental crust of southern India. Trivandrum. Santhosh. M., Iyer, S.S.,Vasconcellos, M.B.A. and Enzweiler, J. (1989) Late Precambrian Alkaline plutons in SW India: Geochronologic and REE constraints on Pan-African magmatism. Lithos., v.24, pp. 65-79. Sivasubramanian, P., Natrajan, R. and Janardhan, AS. (1991) Sapphrine bearing assemblages from Perumalmalai, Palani hills, Tamil Nadu. Jour. Geol. Soc. India, v.38, pp. 532-537.
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Tucker, R.D., Ashwal, L D., Handke, M.J and Hamilton, M.A.(1997)A geochronologic overview of the precambrian rocks of Madagascar: A record from the middle Archaean to the late Neo-proterozoic: I n Protei ozoic Geology of Madagascar. IGCP.348/368 workshop August 1997. p.99. Wiebe, R.A. and Janardhan, A S . (1988) Metamorphisin of the Oddanchatram anorthosite, Tamil Nadu, south India. LPT Technical Report No.88-06, p. 189. Windley, B.F., Razafiniparany,A., Razakamanana, T. and Ackermand, D. (1994) Tectonic framework of the Precambrian of Madagascar and Its Gondwana connections: A review and reappraisal. Geol. Rundsch., v. 83, pp.642-659. Windley, B.F., Brewer, T.S., Collins, A., Kroner, A , Jaeckel, P. and Razakamanana, T (1 997) The Pan-African Orogeny of Madagascar: In Proterozoic Geology of Madagascar. IGCP.348/368. workshop August 1997, p 104. Yoshida, M., Santosh, M. and Rajesh. H.M. (1997) Recent understanding on the Precambrian tectonic framework of peninsular India and its bearing on the juxtaposition with Madagascar : In Proterozoic Geology of Madagascar IGCP. 348/368. Workshop. August 1997. pp.105-106.
GR
ISSN: 1342-937X
Gondwana Research
Southern Granulite Terrain, South of the PalghatCauvery Shear Zone: Implications for IndiaMadagascar Connection A.S. Janardhan Deparfnzent of Geology, University of Mysore; Manasa Gangotri, Mysore 570 006, India (Manirscuipt ueceizmi October 2, 1998; accepted Marcli 2, 1999)
Abstract The present paper correlates tlie southern Madgascar terrain, south of the Ranotsara shear with the granulite terrain of southern India, occurring south of the Palghat-Cauvery (P-C) shear zone. Both the terrains have witnessed high temperature to ultra liigli temperature granulite metamorphism at 550 Ma and are traversed by shear zones and deep crustal faults. The 550 Ma old granulite terrains of Madagascar and southern India have similar lithologies, in particular, sapphirine bearing pelitic assemblages . Graphite deposits and gem occurrences are common to both these terrains. The 550 Ma old southern granulite terrain of southern India comprises of different blocks , the Madurai and tlie Kerala Khoiidalite belt, but all the blocks have similar litliologies with pelite - calc silicate rocks inter-banded with two pyroxene granulite bodies . These litliologies occur amidst an essentially cliarnockitic terrain. The protolith ages of the southern granulite terrain, south of the P-C shear zone ranges between 2400-2100 Ma. The terrain as a whole has witnessed the 550 Ma old granulite event. Tlie granulite metamorphism took place under temperatures of 800 -1000 ‘I C and at pressures of 9.5 to 5 Kbar. Tlie source of heat for the high temperature granulite event of the southern Madagascar terrain has been linked to advective heat transfer along mantle deep faults. The source for the high temperature granulite metamorphism for tlie southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening. Many workers have linked Madagascar to southern liidia by connecting the Ranotsara shear either to the P- C shear zone or to the Achankovil shear zone, further south. The important factor is the litliologies of the Madagascar terrain, south of Ranotsara shear zone and the 550 Ma. old southern Indian granulite terrain are similar in many aspects. It will be more appropriate to kink the Ranotsara shear to the curvilinear lineament bounding the Anaimalai-Kodaikai~alranges and which merges with the southern margin of tlie P-C shear zone. However, north of tlie Ranotsara shear/fault, the northern Madagascar terrain comprises of a dominant Itremo sequence ( < 1850 Ma) and 780 Ma old calc-alkaline intrusives. The latter have similarities with that of Aravallis and the Sirohi, Malaiii sequences occurring further north east . The Rajasthan terrain has witnessed igneous intrusive activity at 1000-800 Ma. if w e can broaden the area of investigations and include the above areas, the Madagascar- India connection can be better understood.
Key words: Southern India, Madagascar, metamorphism, shear zones, correlation.
Introduction The granulite grade continental blocks and the prominent shear zones associated with them, have in recent years, become one of the important criteria in the reconstruction of the Eastern Gondwanaland . The well-exposed granulite terrain of southern India is critical as it forms the central part of the eastern Gondwanaland (Miller et al., 1996). The granulite terrain of southern India (Fig. 1) is also unique in the fact that, in a compact terrain, we get the 2500
Ma early Proterozoic moderate temperature-high pressure Coorg-Niligiri-Shevaroys-Madras belt ; the 1000 Ma high temperature - moderate pressure Eastern Ghats belt and the 550 Ma Ultra high temperature Southern Granulite Terrain (SGT) ,south of the broad Palghat-Cauvery shear zone . These different granulite blocks have characteristics of their own .The early Proterozoic granulite blocks are associated with coeval granite belts, viz., the Closepet and the Krishnagiri. High temperature intrusions do not precede the late Archaean granulite metamorphism. On the other
464
A.S. JANARDHAN
Fig. .I. Map of Southern Indian Peninsular shield showing the granulite terrains of southern India, and the Palghat-Cauvery (PCSZ) and Achankovil (ASZ) shear zones. The 550 Ma PanAfrican southern granulite terrain is shaded a n d occurs south of the PCSZ, after Harris et al. (2994). K.C. Karnataka Craton; cg - Closepet granite; NB Niligiri Block; MsD- Madras Block; MiB- Madurai block ;Am- Achankovil metasediments; KKBKerala Khondalite Belt. Map also includes Madagascar and Eastern Antarctic. MZB, Mozambique; RSZ, Ranotsara shear zone; WC, Wanni Complex; HSWC, Highland Complex; LHC, Lutzow-Holm Complex; SR, Sor Rondande Mountains.
hand, Proterozoic massif type anorthosite- syenites are associated with the Eastern Ghats belt. The 550 Ma old Southern Granulite Terrain, is characterized by anorthosite; syenite-alkali granite-carbonatite intrusives along deep seated and prominent shear zones. The main aim of this paper is to correlate the granulite terrains of southern India with Madagascar, Lutz-Holm bay and Rayner Complex of Antarctic; but the thrust is towards Madagascar and (southern) India connection and for this purpose, the SGT lias been chosen, as it has witnessed high temperature metamorphism at 550 Ma ., as part of the global Pan-African tectono thermal event. Description of the various litliologies of the SGT and their metamorphic history is given in some detail towards arriving at the correlation. The Southern Granulite Terrain (SGT),which had witnessed 550 Ma granulite event, occurs well south of the broad Palghat - Cauvery shear zone. The Palghat-Cauvery tract itself contains oldcr rocks like the 3000 Ma Sittampundi layered anorthosite complex and the younger 660 Ma Tiruchengodu granite (Bhaskar Rao et al., 1996).Theactual northern boundary of the 550 Ma old SGT lies along the shear zone, immcdiatcly north of the Anaimalai- Kodaikanal ranges. Along this shear zone, towards its eastern extremity, the Oddanchatram and the Kadavur anorthosite bodies of Proterozoic ages occur. The 550 Ma old component of the
SGT includes the Madurai, Periyar, Nagercoil blocks and the Kerala Khondalite Belt (Santosh and Yoshida, 1996)and is characterized by prominent shear zones and deep faults which host anorthosite, alkali granites, syenites and carbonatite intrusives. Deep seated faults, associated high temperature intrusions a n d high temperature metamorphism are in many ways similar to the southern Madagascar terrain, south of the Bongolava-Ranotsara fault. In fact, in the southern Madagascar terrain, the N-Strending faults which host the anorthosite bodies are considered to be mantle deep and are believed to have acted as conduits for advective heat transfer and based on gravity studies, Pili et al. (1997) have demonstrated possible mantle upwelling by about 10 Km. The protolith ages of the SGT range between 2400 -2100 Ma., while the peak metamorphic ages range between 560 -530 Ma. (Bartlett et a1.,1998; Jayananda et a1.,1995; and Miller et a1 .,1996 ). The peak metamorphic ages of the SGT are similar to the southern Madagascar terrain (Paquette and Nedelec, 1998).
550 Ma old ( Pan- African) Southern Granulite
Terrain Charnockites of tonalitic to granitic composition are by far the most common rocks of the SGT. Swathes of pelitequartzite- calc-silicate lithologies inter-banded with two pyroxene granulites occurring amidst charnockites are characteristic of the SGT. BIF is rare in the SGT. The metasediments are typical of continental margin basinal affinities. Clz n r n o c ki t es : N on- ga r n e ti f e r o us cha r no c k it e (orthopyroxene-biotite-plagioclase-Kfeldspar-quartz) is the most dominant rock type of the Kodaikana1,the Cardamom and Varushanad hill ranges. Perthitic K-feldspar is the dominant mineral. Plagioclase (An 30 %) and quartz constitute 50 YO of the rock . The mafics are orthopyroxene (X Mg -0.60 with a high alumina content of 6.5 YO)and biotite together form 15% of the rock. Garnet in charnockite, when present, occurs as porphyroblasts with almandine (48Y0)and pyrope contents (36%).First generation biotite grains have 4-6% TiO, while the common retrogressive biotite has a lower Ti02'contentof 2%. Pyroxene granulite bodies: The assemblage orthopyroxeneclinopyroxene-plagioclase is typical. Garnet is present only at the contacts of the pyroxene granulite bodies with calcsilicate bands, and also in smaller bodies occurring as enclaves in charnockite. Garnet is homogeneous with an X Mg of 0.74 -0.77. Orthopyroxene at places grows around clinopyroxene, suggesting a temperature increase. Further, symplectitic orthopyroxene (X Mg - 0.54) + calcic plagioclase assemblage after garnet is quite common to the SGT (Wiebe Gondzuiinn Resenrcli, V. 2,No. 3, 1999
INDIA-MADAGASCAR CORRELATION
46 5
intermixing between pyroxene granulite and calc-silicate bands do have a control on the mineralogy of calc-silicate rocks. When the calc-silicate layers are a few decimeters thick and the intermixing is limited, calc-silicate rocks Metasediiizentay litholugies contain 60-70% calcite, with scapolite and wollastonite up Of the metascdimentary litholgies which occur as broad to 20%. The grains are polygonal with triple point contacts swathes amidst charnockites, pelites are the most abundant and resemble marble. Hedenbergitic clinopyroxene grows and are interbanded with calc-silicate rocks, pyroxene at the expense of the carbonate phases and garnet is granulite bodies. The pelitic assemblages consist of the conspicuously absent. In contrast, if the individual bands following: a) garnet- cordierite- biotite- spinel- sillimaniteare thin and the intermixing between the two lithologies K.feldspar- quartz; b) garnet- cordierite- orthopyroxeneare intense, the modal abundance of calcite decreases to biotite- plagioclase- K.feldspar- quartz; c) sapphirine25-30"/0, with a concomitant increase of wollastonite and cordierite- orthopyroxene- biotite- plagioclase- spinel and scapolite (rich in Me mol.) up to 70%. Grandite garnet is d) cordierite- orthopyroxene- biotite-plagioclase-K.feldspar. found as coronas around the above minerals. The Assemblages a and b are the most common, while c and d assemblage wollastonite-scapolite-grandite garnet indicates are localized and represent quartz absent assemblages. The high temperatures. This is consistent with the high assemblage garnet-cordierite-biotite-K.feldspar-quartz- temperature sapphirine assemblages present in the area. sillimanite form bands of varying width and can be traced Wollastonite laths often contain inclusions of calcite and at several places for almost continuously for more than 5 quartz attesting to its formation after these two minerals. Km. Garnet is common and often contains inclusions of In the T-X,,, diagram this reaction shows a positive slope biotite, sillimanite and quartz . Grain boundary alteration indicating an increase in temperature (Francis Anto et al., to orthopyroxene is rather frequent suggesting that it 1999).Late garnet coronas in calc-silicate rocks generally formed during high temperature decompression path. indicate cooling as recorded from several terrains viz., Cordierite grains occur as rims around the garnet at first Northern Prince Charles Mountains, Antarctica ( Harley and resorb the garnet almost completely. At places, the et al., 1994 ), Prydz Bay, East Antarctica (Motoyoshi et al., cordierite grains are often cut by fibrolite-biotiteintergrowth 1991). In all these cases, development of coronal garnet along their rims, indicating late retrogression. Plagioclase have been attributed to isobaric cooling. Similar reactions (An RO'Z,) occurs as large well twinned plates. K.feldspar have been described from the Eastern Ghats where cooling is often perthitic. Biotite is a common mafic mineral in and domainal variations in fluid composition are invoked pelites. Biotite included in garnet and cordierite has a high (Bhowmik et al., 1995). TiO, of 5-6%. Biotite coexisting with garnet and cordierite has a lower TiO, of 4%, while late retrogressive ones after Metamorphic Pressure-Temperature Estimations garnet and orthopyroxene have 2 70. Two distinct mineral assemblages, sapphirineP-T estimates for the charnockites using the assemblage cordierite-orthopyroxene-biotite-spinel and cordieritegarnet-orthopyroxene-plagioclase-quartz in charnockites orthopyroxene-biotite-plagioclase (sapphirine absent) range from 750-900°C, at pressures ranging from 9.5-5Kbar. assemblages are noticed. Sapphirine, orthopyroxene and G a r ne t - o r tho p y r o x e n e / c 1in o p y ro x e n e - p 1a g i oc 1a s e cordierite grains (up to 0.8 cm across) can be seen even assemblage in two pyroxene granulite often shows in hand specimens. High alumina orthopyroxene (8-12'3'0) disequilibrium textures suggestive of decompression has been reported in sapphirine assemblages from reaction. The symplectitic orthopyroxene-plagioclase Perumalmalai and Ganguvarpatti (Sivasubramanian et al., association after garnet gives pressures of 7 Kbar. The 1991; Raith et al., 1997).This high alumina orthopyroxene TWEQU data for core and rim clearly indicate a drop in with patches of sapphirine and green spinel is commonly pressure from 9.5 to 7 Kbar . found as a breakdown product of garnet. Garnet formation Similar pressure-temperature estimates were obtained prior to orthopyroxene and sapphirine is important .The for the garnets in the Oddanchatram anorthosite body garnet which has given rise to orthopyroxene gives a (Wiebe and Janardhan, 1988). Here, the garnet that lack metamorphic cooling age of 520 Ma (Sm-Nd garnet-whole orthopyroxene-plagioclase symplectites gave high rock (Janardhan and Peucat., unpublished data). Sapphirine pressures of 10.6 Kbar at 92OoC, while garnet rims in forming de-compressional reactions took place at 850 equilibrium with the symplectitic orthopyroxene + calcic 100OT (Raith et al., 1997 ;Francis Anto et al., 1997). plagioclase gave pressures of around 6.6 Kbar at lower Calc-silicate assemblages: Calc-silicate lithologies contain temperatures. The pressures estimated using compositions of spinelcalci te-scapolite-wollastonite-clinopyroxene-garne tplagioclase-K-feldspar-sphene-quartz. The degree of cordierite-biotite inclusions in sapphirine along with and Janardhan, 1988; Ravindra Kumar and Chacko,1994). Similar symplectites are also found in garnetiferous charnockites.
Gondivniza Resenrch, V: 2, No. 3,1999
A S . JANARDHAN
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orthopyroxene cores give a maxiinuin of 7.5 Kbar, while lower pressures of 5 Kbar are given by cordierite- bitotite sapphirine associa tion. Temperatures of sapphirine assemblages vary froin 1000 -800 " C. Scapolite- garnet- plagioclase- wollastonite- quartz assemblage in the calc-silicate rocks is amenable for calculation of P-T-X,,, conditions. The (Me)invariant point gives temperatures 850-925°C and X,,, = 0.65-0.9 in the isobaric (6 Kbar) T-X, o2 diagram. The P-X,,,diagram show pressures 5.5-6.5Kbar with X,,, of 0.8-0.6 (Francis Anto et al., 1999) The P-T path of the SGT thus involves a high pressure medium temperature metamorphism followed by high temperature decoiiipression and rapid cooling. The high pressure metamorphism indicative of burial of lithologies must have taken place after the intrusion of Proterozoic massif anorthosite bodies as at Oddanchatram as suggested by the development of garnet in the anorthosite body at their contacts with the pelitic and two pyroxene granulite xenoliths (Wiebc and Janardhan, 1988).The peak pressures were succeeded later by high temperature decompressional reactions, which took place during or slightly after the 550
'
I
Fig. 2.a
t 5w
-
Ma old granulite metamorphism .Thus the composite P-T path shows a clockwise pattern . (Fig. 2 b)
Major Shear Zones and Faults of the SGT The characteristic feature of the SGT is that the entire terrain is dissected by prominent shear zones and crustal deep faults. The SGT is bounded by the prominent shear zone /fault immediately bordering the Kodaikanal hill ranges. Along the eastern part of this shear, inassif anorthosite bodies of Oddanchatram and Kadavur of Proterozoic age are emplaced into a thick continental marginal basin type sequence of quartzites-pelites and rare BIF. The Kodaikanal or the Madurai block, as it is often termed, forms the northern part of the SGT. Recent works (Janardhan and Wiebe,1985; and Bartlett et al., 1998) have brought out the fact that this strip of the northern SGT hosting the Proterozoic anorthosite bodies may represent a component of the 1000 Ma old Eastern Ghats sequence. The tract hosting the anorthosite bodies may be connected to the Eastern Ghats trend across the Cauvery shear (Janardhan and Wiebe,l985).The same shear turns NW if
1000
'1'"c Fig. 2.b
P-T cslimalcs/paths of the 550 Ma S. Madagascar tcrrain, South of Ranotsara shear zone after Nicollct ( 1 990). a ) Vohibory formation: 1 , Sapp-Co bcaring amphibolites; 2. Scrcndibitc-cli ntonilc-clinopyroxcnitcs b) Ampanihy Ihrination: 3 & 4 Anorthosite complcx; star-graphite bcaring gncisscs c) Androyan formation: 5. Cord-grd-opx-sp-cltz bearing gncisses, 6. Scvcn phasc anatcctic gncisscs from Ihoshy.
1
I
500
600 +
+ Cllarnockitc Gt.[]py.p]ng.~r
I 700
I 800
I 900
Temperature ("C) Cl Snliliiririe hcxrirtg rock, GI-Opx-Flag
+ Anot.thositc G ~ . O ~ . X - P I ~ ~ - Q ~Sa-cotd-Ow
4 Anorthosite C ; I - C l ~ x . p 1 ~ ~ - Q z + Gt-OPx (IlnrlcY) P-1 obtoiricd by A n m d Atohail (1985) Pclilc Gt-Cord P-'I obl:iiiicd by I1:inis ( I 95 I ) f. Pclilc Cortl-OIIX 0 Grew (19SZ) 0 Pelitc (it-Silli-I'lng-Qz X I'eli~cGRAIl,
* *
Calculated pressures and tempcraturcs for various assemblages and P-Tpath for the Palni Range Granulites. AI, S O , stability relations from Holdaway ( I 97 I ).
Figs. 2 a & b. P-T paths of the 550 Ma sciutliern Madgascar terrain (data after Nicollet,lYYO) and the Madurai block of the Southern granulite terrain.
Gondzuniia Resenrch, l? 2,No. 3,1999
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the Perinthatta anorthosite of northern Kerala are connected to the Oddanchatram and tlie Kadavur bodies. It may be pointed out here that the Peralimala and the Kalpatta granite north of the Palght-Cauvery shear give ages of 550 Ma. (Miller et al., 1996). The point to be made here is that the Palghat- Cauvery shear is curvilinear and represents a zone where transpression tectonicsbrought the Eastern Ghats and the SGT in juxtaposition with the Dharwar craton . This may have involved the collision of the Napier granulite block backed by the Rayner Complex ( Drury,1989).The E-W trend of the Cauvery shear is a later imposed one probably during 550 Ma. Apart from anorthosite, a number of alkali granite, syenite and carbonatite intrusives occur and seem to be restricted to deep faults and shear zones (Santosh et al., 1989).The ages of these alkali intrusions range between 790550 Ma. Among the shear zones of the SGT, the most prominent is the Achankovil shear zone , the Kerala Khondalite Belt ( KKB) lies SW of this shear zone. The KKB has been extensively investigated ( see references in Santosh and Yoshida,1995 and 1996) and the kinematics of the Achankovil shear has also been worked out . It is now generally agreed that the eastern part of the Achankovil shear has witnessed a n earlier dextral movement overprinted by sinistral movements, which becomes prominent towards the west ( Sacks et al., 1997).Bartlett et al. (1998) have stated that the KKB comprises of a slightly younger set of sedimentary sequence of 1800 Ma. They further state that this 1800is not that dominant in the terrain north of the Achankovil shear zone. Their conclusion is that the KKB block may be younger and thus different. However, Bartlett et al., (op.cit.) go on to state that it is quite likely that the 800- 550 Ma old alkaline intrusive activity may have erased the 1800 Ma memory in the Madurai block. The 1800 Ma imprint has been recorded by Jayananda et al. (1995) and Miller et al. (1996) have stated that the protolith age of Varuslianad Hills charnockite, north of the Achankovil is 560 Ma. The important aspect is that there is a definite agreement that all the blocks of the southern granulite terrain, south of the Palghat-Cauvery shear have witnessed a common granulite grade metamorphism at 560 -540 Ma.
India- Madagascar Connection Three major Precambrian crust forming and re-working events have been identified by Tucker et al. (1997). Their isotopic data of the tonalitic enclaves in the late Neoproterozoic granites suggest that Archaean rocks underlie much, if not all, of Madagascar north of the Bongolava-Ranotsara shear zone. They also state that (< 1850 Ma) Mesoproterozoic (Quartzo-schistose-calcaire) QSC sequence of the Itremo group of rocks were intruded Gotidionnn Xesenuch, V. 2,No. 3, 1999
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by gabbro-diorite plutons of 790-780 Ma. The TE signatures of the uncontaminated gabbros suggest that they were emplaced in an arc like setting. These authors further state that the Mozambique ocean closure ( 900 - 640 Ma ) and connected subduction of the lithosphere can be connected to the gabbro-diorite plutonic intrusions into the Itremo QSC group. At the same time in the continental hinterland of southern India, widespread carbonatite- syenite activity took place during extensional tectonics. The 580-520 Ma., Neoproterozoic events include sheet like emplacement of granites and syenitcs (Tucker et al., 1997; Paquette and Nedelec, 1998) and ductile deformation and metamorphism of the Itremo sequence a n d high temperature granulite metamorphism during 560-540 Ma. and intrusive granites (540-520 Ma), south of the BongolavaRanotsara fault. Recent works including that of Tucker et al. (1997);Powell et al. (1980 and 1997);and Windley et al., (1997) agree that the position of India against Madagascar can be constrained by matching Archaean rocks in northern Madagascar and western India and by tracing Neoproterozoic shear zones from one continental block to the other (Yoshida et al., 1997; Bhaskar Rao et al., 1996).This type of matching brings out the possibility that India may have moved along tlie Madagascar-India margin by 1000 km during the Gondwana breakup (Li and Powell, 1997). Madagascar-India fits are based mostly on the matching of the Palghat-Cauvery or the Achankovil shear zone with the Bongolava-Ranotsara shear (Windley et al., 1994; Yoshida et al., 1997; Harris ,1997 and Janardhan ct a1.,1997). Evidences which strongly support this argument is that the granulite terrain south of the Ranotsara shear is in many ways comparable to thc SCT of southern India, immediately south of the shear bounding the northern slopes of the Kodaikanal hill ranges and which liosts the Proterozoic anorthosite bodies. It is quite likely that the Achankovil shear zone can be extended northwestwards m d may join the western extension of the Palghat - Cauvery shear zone. These two terrains have witnesscd high temperature metamorphism (Nicollet,1990 ; Martelat et al., 1997) with temperatures ranging between 800 - 1000"C and at pressures of 7-9 Kbar. Further, these two terrains are known for their graphite and gem stone occurrences. More importantly, the southern Madagascar terrain is traversed by mantle deep faults, which host high temperature intrusions, and along which advective heat transfer has taken place through fluids from the mantle and that the mantle has been uplifted by 10 km (Pili et al., 1997). These authors appeal to transpression tectonics, buckling of the continental lithosphere for the formation of the mantle deep shear zones. In summary, the SGT and high grade terrain south of the Ranotsara shear are similar to one another in several aspects.
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The heat source for the high grade to ultra high temperature m e t a m o r p h i s m of t h e SGT can be connected to high temperature alkaline intrusive activity which had taken place in an extensional setting after a n initial thickening event. However, if one takes into account the Madagascar terrain north of the Bongolava-Rantosara fault, the similarities seem to be much more in common with that of the Aravallis-Delhi sequence a n d particularly w i t h the acid volcanism a n d plutonism of the Malani province. This aspect can be better e x p l a i n e d by Li a n d Powell (1997, figs.1 d-f), w h e r e Madagascar lies farther north a n d close to the Gujarat coast. Further studies are needed to resolve the Madagascar- India connection by broadening the scope of investigations .
Acknowledgements The author wishes to place on record his grateful thanks to P r o f e s s o r s L e w A s h w a l a n d Yoshida w h o w e r e responsible for the author’s visit to Madagascar and for the very fruitful interaction with other scientists w h o participated in the meeting a n d field trip.
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Tucker, R.D., Ashwal, L D., Handke, M.J and Hamilton, M.A.(1997)A geochronologic overview of the precambrian rocks of Madagascar: A record from the middle Archaean to the late Neo-proterozoic: I n Protei ozoic Geology of Madagascar. IGCP.348/368 workshop August 1997. p.99. Wiebe, R.A. and Janardhan, A S . (1988) Metamorphisin of the Oddanchatram anorthosite, Tamil Nadu, south India. LPT Technical Report No.88-06, p. 189. Windley, B.F., Razafiniparany,A., Razakamanana, T. and Ackermand, D. (1994) Tectonic framework of the Precambrian of Madagascar and Its Gondwana connections: A review and reappraisal. Geol. Rundsch., v. 83, pp.642-659. Windley, B.F., Brewer, T.S., Collins, A., Kroner, A , Jaeckel, P. and Razakamanana, T (1 997) The Pan-African Orogeny of Madagascar: In Proterozoic Geology of Madagascar. IGCP.348/368. workshop August 1997, p 104. Yoshida, M., Santosh, M. and Rajesh. H.M. (1997) Recent understanding on the Precambrian tectonic framework of peninsular India and its bearing on the juxtaposition with Madagascar : In Proterozoic Geology of Madagascar IGCP. 348/368. Workshop. August 1997. pp.105-106.