Petrology, P±T Path and Implications for the Tectonothermal Evolution of the Central Indian Tectonic Zone
JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 PAGES 387±420 2003
Garnetiferous Metabasites from the Sausar Mobile Belt: Petrology, P±T Path and Implications for the Tectonothermal Evolution of the Central Indian Tectonic Zone
SANTANU KUMAR BHOWMIK* AND ABHINABA ROY
REGIONAL PETROLOGY LABORATORY, GEOLOGICAL SURVEY OF INDIA, NAGPUR-440 006, INDIA
RECEIVED JUNE 20, 2001; REVISED TYPESCRIPT ACCEPTED AUGUST 29, 2002
A suite of garnetiferous amphibolites and mafic granulites occur of extension, marked by mafic dyke emplacement. The combined as small boudins within layered felsic migmatite gneiss in the P±T path of evolution has a clockwise sense and provides northern part of the Sausar Mobile Belt (SMB), the latter evidence for a major phase of early continental subduction in constituting the southern component of the Proterozoic Central parts of the CITZ. This was followed by a later continent± Indian Tectonic Zone (CITZ). Although the two types of meta- continent collision event during which granulites of the first basites are in various stages of retrogression, textural, composi- phase became tectonically interleaved with younger lithological tional and phase equilibria studies attest to four distinct units. This tectonothermal event, of possibly Grenvillian age, metamorphic episodes. The early prograde stage (Mo) is repre- marks the final amalgamation of the North and the South Indian sented by an inclusion assemblage of hornblende1 ilmenite1 Blocks along the CITZ to produce the Indian subcontinent. plagioclase1 Æ quartz and growth zoning preserved in garnet. The peak assemblage (M1) consists of porphyroblastic garnet clinopyroxene Æ quartz Æ rutile Æ hornblende in mafic KEY WORDS: Central Indian Tectonic Zone; clockwise P±T path; granulites and garnet quartz hornblende in amphibolites continental collision; metabasite and stabilized at pressure±temperature conditions of 9±10 kbar and 750±800C and 8 kbar and 675 C, respectively. This was followed by near-isothermal decompression (M2), and post- decompression cooling (M3) events. In mafic granulites, the INTRODUCTION former resulted in the development of early clinopyroxene2A± In recent reconstructions of East Gondwanaland, the hornblende2A±plagioclase2A symplectites at 8 kbar and 775 C Central Indian Tectonic Zone (CITZ; Radhakrishna, (M2A stage), synchronous with D2 and later anhydrous clin- 1989) has been recognized as an important Proterozoic opyroxene2B±plagioclase2B±ilmenite2B symplectites and coronal collisional zone (Yedekar et al., 1990; Jain et al., 1991; assemblages at 7 kbar, 750 C(M2B stage) and post-dating D2. Mishra et al., 2000). Existing models predict amal- In amphibolites, ilmenite plagioclase quartz Æ horn- gamation of the South Indian Block [SIB; terminol- blende symplectites appeared during M2 at 6Á4 kbar and 700 C. ogy after Eriksson et al. (1999); comprising the During M3, coronal garnet clinopyroxene quartz Æ Singhbhum, Bastar and Dharwar Provinces] and the hornblende-bearing symplectites in metabasic dykes and North Indian Block (NIB; comprising the Aravalli± hornblende3±plagioclase3 symplectites embaying garnet in Bundelkhand Provinces) during the Palaeoproterozoic mafic granulites were formed. P±T estimates show near-iso- by southerly subduction of the NIB below the SIB baric cooling from 7 kbar and 750C to 6 kbar and 650C during along the Central Indian Suture (CIS) to form the M3. It is argued that the decompression in the mafic granulites Indian subcontinent (Yedekar et al., 1990; Jain et al., is not continuous, being punctuated by a distinct heating 1991; Eriksson et al., 1999; Mishra et al., 2000). The (prograde?) event. The latter is also coincident with a period timing and the location of collision have been modelled
*Corresponding author. Present address: Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 Journal of Petrology 44(3) # Oxford University Press 2003; all rights 302, India. E-mail: [email protected] reserved. JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003
with its transcontinental status (Harris, 1993), has obvious global significance in understanding the geodynamic processes of crustal assembly in major parts of East Gondwanaland. This study focuses on a suite of regionally distributed mafic granulites and associated garnetiferous amphi- bolites from the northern periphery of the Sausar Mobile Belt (SMB), the latter constituting the south- ernmost Meso- to Neoproterozoic component of the CITZ (Acharyya & Roy, 2000). The metabasites occur as small boudins and tectonic lenses within inten- sely tectonized felsic gneisses and bear imprints of polyphase deformation and a strong amphibolite-facies overprint. Although polymetamorphic histories and P±T paths have been extensively documented from rare residual domains of high Mg±Al bulk composition in granulites (Currie & Gittins, 1988; Droop, 1989; Harley et al., 1990; Dasgupta et al., 1995), similar polyphase histories have seldom been retrieved from mafic protoliths (e.g. Harley, 1985). The latter are often restricted to high-P granulite (Mengel & Rivers, 1991; Thost et al., 1991; Zhao et al., 2000, 2001) and eclogite-facies domains (O'Brien, 1997; Dirks & Sithole, 1999). In this study, we present detailed textural, mineral- chemical and P±T data for mafic granulites, garneti- ferous amphibolites and associated metabasic dykes to constrain the metamorphic history of the southern seg- ment of the CITZ. Despite pervasive amphibolite- facies tectonothermal reworking, these data document an early high-grade history and define a complex P±T trajectory. The results not only place important con- Fig. 1. Distribution of different lithotectonic components in the Central Indian Tectonic Zone (CITZ) between the North Indian straints on the models for lower crustal geodynamic Block (NIB) and the South Indian Block (SIB) (see text for termino- processes in the SMB but also provide insights into logy). The CITZ is demarcated by the Son±Narmada North Fault multistage exhumation histories of deep crustal rocks. (SNNF) to the north and the Central Indian Shear (CIS) to the south. The box shows the location of the Sausar Mobile belt (SMB) in the CITZ (detailed map shown in Fig. 2). The inset shows the location of CITZ in India. SNSF, Son±Narmada North Fault; BBG, Bhandara±Balaghat granulite. GEOLOGICAL SETTING The CITZ encompasses the terrain between the Son± Narmada North Fault (SNNF) and the Central Indian only on the basis of limited geological and geophysical Shear Zone (Acharyya & Roy, 2000) (Fig. 1). The data from the mobile belt (Yedekar et al., 1990; Jain SMB is a collage of at least three lithotectonic et al., 1991; Mishra et al., 2000). Detailed metamorphic components (Bhowmik & Pal, 2000): (1) the southern characterization of the rocks of the major lithotectonic Bhandara±Balaghat granulite (BBG) domain; (2) the units of the CITZ, without which any tectonic model central domain of the Sausar Group (SG) of rocks; for the mobile belt remains incomplete, has not been (3) the northern Ramakona±Katangi granulite done. Recent studies have revealed the composite (RKG) domain (Fig. 2). These domains differ in litho- character of the CITZ, comprising three distinct logical assemblage, mineralogy and metamorphic supracrustal belts, Mahakoshal, Betul and Sausar history. (Fig. 1), which have a tectono-magmatic history, The BBG domain is bounded to the north by the low- from the Palaeoproterozoic to the Neoproterozoic grade Sausar Group of rocks and to the south by (Acharyya & Roy, 2000). This implies a more complex the cratonic domain of felsic gneisses of the Amgaon evolutionary history for the CITZ, compared with gneissic complex. The BBG domain is lensoidal in existing monocyclic models. Nevertheless, the CITZ, shape, tapering towards both east and west (Fig. 2).
388 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
Fig. 2. Geological map of the Sausar Mobile Belt in central India, showing the different lithotectonic domains (after Bhowmik & Pal, 2000). The components south of the Central Indian Shear Zone (CIS) constitute the cratonic domain (compare the South Indian Block) with respect to the SMB. Also shown are the locations of the granulite occurrences in the Ramakona±Katangi granulite (RKG) domain (see text). The mafic granulites occur in the Khawasa (location 2), Pindrai (location 4) and Katangi areas (location 5) and the garnetiferous amphibolites are located in the Katangi area.
The granulite suite includes cordierite gneiss (Crd Gn), greenschist±amphibolite facies transition zone has iron formation granulite, quartzite, charnockitic been recognized along the southern part of the Sausar gneiss, two-pyroxene granulite, pyroxenite and ender- supracrustals, in close proximity to the granulites of bitic granulite; they occur as pods and lenses within the BBG domain; an upper amphibolite- to upper intensely tectonized felsic gneiss, which is predomi- amphibolite±granulite-facies transition zone has been nantly mylonitic, with strong development of down- recorded in the northern and northwestern margin of dip stretching lineations. The granulites of the BBG the central domain and also from the RKG domain. domain record an early high-temperature granulite In the RKG domain, along the northern periphery metamorphism (T 900C) and strong retrograde of the SMB, a distinct lithological assemblage, includ- metamorphism dominated by both cooling and decom- ing mafic granulite, cordierite granulite and porphyri- pression textures (Bhowmik & Pal, 2000; Ramchandra tic charnockite, occurs as rafts and lenses within & Roy, 2001). layered tectonites of the composite gneiss and tonalitic The SG occupies the central domain and is an gneiss components of the TBG (Bhowmik et al., 1999; intensely deformed and metamorphosed pelite± Bhowmik & Pal, 2000). The composite character of the arenite±carbonate±Mn-oxide ore assemblage, which layered tectonites is exemplified by thin, extremely represents a stable platformal sequence. This is asso- continuous, subparallel layers, centimetres to metres ciated with the Tirodi biotite gneiss (TBG) and in thickness, of alternating amphibolitic and quartzo- granitoids (Narayanaswami et al., 1963). The TBG feldspathic material. Both the composite and tonalite constitutes the basement to the SG (Narayanaswami gneiss components show stromatic migmatite banding et al., 1963; Bhowmik et al., 1999). Basement±cover and are collectively referred to as the felsic migmatite relations are largely tectonized, except in the eastern gneiss (FMG; Bhowmik et al., 1999; Bhowmik & Pal, part of the belt where a polymictic conglomerate over- 2000). The high-grade rocks can be traced for >240 km lying a porphyritic granitoid is still recognizable (Pal & from Ramakona in the west through Khawasa± Bhowmik, 1998). The SG records broadly a Barrovian Katangi in the centre to the east of Bichhiya type of regional metamorphism with metamorphic (location 6, Fig. 2). In places, the boundary between grade consistently increasing from south to north and the granulites and the gneisses is lined with sheet-like also towards the west (Narayanaswami et al., 1963; bodies of a younger phase of granitoids. The largest Pal & Bhowmik, 1998). A low-grade greenschist- to exposure of these rocks is located to the NE of Katangi
389 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003
(Fig. 2). Locally, the granulites also occur in close granulite±felsic migmatite gneiss components with the spatial association with the Sausar supracrustals or Sausar supracrustals. D2 is also coincident with abun- are directly interlayered with the latter. The contacts dant migmatization in the felsic migmatite gneiss between the lithological associations are often marked (FMG), emplacement of basic dykes and voluminous by prominent ductile shear zones. sheet-like intrusions of a suite of tonalites±porphyritic Geochronological data from the SMB are meagre. granodiorites and potassic granites. Basic dykes Sarkar et al. (1986) reported a Rb/Sr whole-rock emplaced during D2 are termed MD1. Both these isochron age of 1525 Æ 70 Ma and a mineral iso- dykes and the granitoids are deformed and have a chron age of 860 Ma from the Tirodi gneiss. According planar S2 fabric. There is a second generation of to these workers, the 1525 Ma event marks the main basic dykes which are relatively massive, have a well- phase of regional amphibolite-facies metamorphism of preserved intergranular texture and appear to have the Sausar Group, leading to the partial melting of the been emplaced late to post-kinematic with respect basal psammopelitic unit. The younger 860 Ma event to D2. These are termed MD2. Subsequent deforma- is interpreted as a terminal thermal overprint on the tions, D3 and D4 produced macro- as well as meso- Sausar rocks. Recently, Lippolt & Hautmann (1994) scale, shallow-plunging upright folds (F3) and folds of 40 39 reported a Ar/ Ar age of 950 Ma from cryptomelane sinistral asymmetry (F4) with WNW±SSE to NW±SE from the Sitapar mines in the Ramakona area axial traces respectively. (location 1 in Fig. 2) and interpreted this age as the Successive phases of overprinting deformation and time of cooling through the cryptomelane closure attendant metamorphism have ultimately imparted an temperature, immediately following the amphibolite- amphibolite-facies fabric in the host gneisses and facies metamorphism of the Sausar Group of rocks. largely erased the older histories of the terrane. The Tourmaline-bearing granites from the Nainpur± imprints of these polyphase metamorphic events have, Lalbara area in the RKG belt, apparently intrusive however, been recorded in the mafic granulites and into Sausar supracrustals, have yielded a Rb±Sr whole- associated garnetiferous amphibolites in the RKG rock isochron age of 1147 Æ 16 Ma (Pandey et al., belt. These occur as scattered small boudins or as con- 1998). Collating available metamorphic, structural cordant bodies of various sizes within the FMG and and geochronological data, Bhowmik et al. (1999) con- can be traced from Khawasa in the west to Katangi in sidered the main phase of deformation (compare with the east (Fig. 2). In many cases, the contact of the Sausar orogeny) that affected the Sausar Mobile Belt metabasics with the FMG is lined with conformable to be an 1000 Ma event. The 1525 Æ 70 Ma whole- bands of deformed porphyritic granodiorite±potassic rock isochron age from the basement TBG was reinter- granite. The largest exposure of these bodies is located preted as evidence of a Pre-Sausar Mesoproterozoic in the Katangi area. In most cases the mafic granulites tectonothermal event. Recent Sm±Nd and Rb±Sr and garnetiferous amphibolites are spatially separated ages for charnockitic gneisses and two-pyroxene granu- from each other. In the majority of the occurrences, the lites from the BBG domain show three distinct clusters rocks are medium to coarse grained, dark greenish to at 2672 Æ 54, 1416 Æ 59 to 1386 Æ 28 Ma and black in appearance and have an amphibole-rich S2 973 Æ 63 to 800 Æ 16 Ma (Ramchandra & Roy, fabric, which becomes pervasive at the edges of the 2001). These ages are correlated with two phases of body. However, in the central part of these bodies, temporally separate Archaean and Mesoproterozoic large ovoid garnet porphyroblasts (6Á4±12 mm in granulite metamorphism and the final overprint of diameter) are scattered throughout the matrix, giving the Sausar orogeny (Ramchandra & Roy, 2001). a spotty appearance to the rock. A summary of the relative chronological order of the different tectonomagmatic events in the RKG domain is presented in Table 1. The SMB records four phases of deformation (D1±D4), the earliest of which is pre- PETROGRAPHY served as relicts in small boudins and the hinges of Mafic granulites have been sampled from Khawasa isoclinal folds. D2, the strongest deformation in this (location 2, Fig. 2), Pindrai (location 4) and Katangi belt, produced recumbent to gently inclined folds with (location 5) areas; garnetiferous amphibolites and a strong axial planar layered tectonite (LS) fabric (S2). metabasic dykes2 are from the Katangi area only. In S2 is visibly derived by the transposition and attenua- total, 15 samples of mafic granulite, 10 samples of tion of a pre-existing S1 foliation. In this deformational garnetiferous amphibolite and four samples of metabasic event, rocks of the RKG domain appear to have been dykes2 have been studied in detail. Textural and mineral emplaced as large allochthonous nappes thrust over the chemical data are reported for three representative SG in the central domain, leading to tectonic imbri- samples of mafic granulite (sample 77C/SB, Khawasa; cation of the basement mafic granulite±cordierite 72D/PS, Pindrai; 45A/PS, Katangi), two representative
390 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
Table 1: Summary of tectonometamorphic events recognized in the RKG domain
Metamorphism Magmatic, migmatization and veining events Deformatiom Fabric
Isobaric cooling and partial Emplacement of leucogranite Terminal sinistral deformation equilibration (M3) stocks of Alesur±Pusda± (D4), producing folds of sinistral Dahoda pluton asymmetry "
Deformation (D3), producing S3 regional, shallow-plunging, " gentle, upright folds Emplacement of basic " dykes MD2
Decompression (M2), pervasive (1) Folial hbl±crd±bt±bearing Emplacement of sheeted D2 deformation, producing S2 low-pressure metamorphism assemblage in MG±Crd Gn± bodies of tonalite±porphyritic recumbent to gently inclined FMG granodiorite±potassic granite isoclinal folds as part of
(2) Partial melting in FMG basic dykes MD1 regional-scale thrust nappe (3) Emplacement of qtz veins and emplacement. Tectonic " formation and segregation of interleaving (stacking) of qtz±fibrolite tabloids granulites and Sausar supracrustals "
Peak metamorphism (M1) D1 gneissic banding in S1 (In MG±Crd Gn±FMG association) MG±FMG Burial and prograde metamorphism (M ) " 0 Deposition of quartzite±pelite±carbonate±Mn-oxide ores of Sausar Group " Formation of basement of Sausar Group of rocks (SR) " Formation of protolith of felsic migmatite gneiss (FMG), comprising tonalite gneiss±composite gneiss±granite gneiss components, cordierite gneiss (Crd Gn) and mafic granulite (MG)
Hbl, hornblende; Crd, cordierite; Bt, biotite; Qtz, quartz.
samples of garnetiferous amphibolite (samples 19A/PS trails of prograde hornblende (hornblende1), ilmenite and 12A/PS) and from one metabasic dyke2 (ilmenite1), quartz and plagioclase (plagioclase1) (sample 13/PS). within the porphroblastic garnet [Garnet (P)] and/or On the basis of microstructures and reaction porphyroblastic clinopyroxene [Clinopyroxene (P)] relations between mineral phases, four distinct meta- (Fig. 3a and b). These textural features suggest that morphic episodes, reflecting changing P±T conditions, the granulites and garnetiferous amphibolites had a can be determined from the mafic granulites, garneti- prograde history, prior to the peak metamorphism. ferous amphibolites and metabasic dykes. These are: prograde (M0), peak (M1), post-peak decompression (M2) and post-decompressional cooling (M3).
M1 episode Mafic granulite and amphibolite M0 episode In mafic granulite, this is represented by growth of Mafic granulite and amphibolite porphyroblastic phases, namely garnet (P) and clino- In the mafic granulite and amphibolite, the prograde pyroxene (P), together with quartz. The minerals com- history is preserved only as well-defined inclusion monly display granoblastic mosaic textures. The
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M2 episode Mafic granulite Clinopyroxene plagioclase Æ hornblende Æ ilmenite- bearing coronas and symplectites, fine plagioclase± quartz symplectites, coronal ilmenite around enclosed rutile in garnet and folial hornblende represent M2 reaction textures. Depending on grain size and mineral assemblage, the pyroxene±plagioclase symplectites in quartz-absent assemblages can be divided into two types: (1) the coarser clinopyroxene hornblende plagioclase symplectite; (2) relatively finer, rib-like intergrowth of clinopyroxene and plagioclase with minor ilmenite. Clinopyroxene±hornblende±plagioclase of type 1 and clinopyroxene±plagioclase±ilmenite of type 2 symplectites are referred to as clinopyroxene2A± hornblende2A±plagioclase2A and clinopyroxene2B±pla- gioclase2B±ilmenite2B respectively. Type 1 symplectites are generally aligned parallel to hornblendeFOL and constitute a central region, bounded on both sides by type 2 symplectites (Fig. 4a). Hornblende2A often encloses ovoid relicts of clinopyroxene (P) (Fig. 4b). The type 2 symplectites, in contrast, have no preferen- tial alignment. These may occur either as spectacular radial symplectites around garnet against clinopyr- oxene (P) (Fig. 3a) or as delicate worm-like inter- growths in the matrix (Fig. 4c). In the latter, type 2 symplectites overgrow hornblendeFOL (Fig. 4d). Clinopyroxene2B often occurs as coronas around horn- blende2A against garnet (Fig. 4e). Irrespective of the mode of occurrence, type 2 symplectites generally enclose relict titanite. In quartz-bearing assemblages, type 2 symplectites also occur around embayed garnet Fig. 3. Backscatter electron images showing textures of prograde grains against quartz. Plagioclase±quartz intergrowths metamorphism in the mafic granulites (sample 72D/PS). (a) Part of strictly occur along fine fractures in garnet. The matrix a large ovoid garnet (P), containing inclusion trails of hornblende1. foliation is composed of very coarse aggregates of Garnet is rimmed by a shell of radially oriented clinopyroxene2B± hornblendeFOL. In the majority of the cases, the plagioclase2B symplectites. (b) Part of a very large clinopyroxene (P) enclosing hornblende1, ilmenite1 and plagioclase1. hornblendeFOL, closest to the symplectite domain par- tially pseudomorphs clinopyroxene (P). The relict of diagnostic mineral assemblage is garnet clino- pyroxene can often be traced as discontinuous blebs pyroxene plagioclase quartz Æ rutile Æ horn- within the hornblendeFOL. Further away, within the blende. Rarely quartz may be absent from the rock central folial domain, there is no trace of early clino- (e.g. sample 72D/PS). Rutile when present is preferen- pyroxene, but folial hornblende in places contains tially enclosed in the periphery of the garnet but absent exsolution blebs of plagioclase. In certain outcrops, from the matrix. The peak assemblage is commonly the retrogression is so extensive that clinopyroxene preserved in the core of large mafic granulite boudins (P) is preserved only within garnet with its total exclu- where alternate garnet clinopyroxene- and plagio- sion from the matrix. In others, folial hornblende clase quartz-bearing layers define an early granulite- contains discontinuous layers rich in clinopyroxene (P). facies gneissic foliation, S1. The latter is anastomosed Clinopyroxene in these layers often occurs as porphyro- by pervasive matrix hornblende parallel to the S2 clasts, being armoured by finer recrystallized aggre- plane (henceforth referred to as folial hornblende, gates. Textural features such as these indicate that hornblendeFOL). (1) part of the folial hornblende was produced at the In amphibolite, the peak assemblage is represented expense of M1 clinopyroxene during a post-peak trans- by porphyroblastic garnet, plagioclase and coarse position event (D2), (2) type 1 symplectites pre-dated hornblendeFOL. type 2, being broadly coeval with folial hornblende
392 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
Fig. 4. Backscatter electron images showing M2 reaction textures in mafic granulites (sample 72D/PS). (a) Garnet (P) grains are separated by an outer (centre of field of view) coarser clinopyroxene2A ^plagioclase2A ^(hornblende2A) symplectite and an inner radial, finer clinopyroxene2B ^ plagioclase2B (black)^ilmenite2B (white) symplectite. (b) Earlier hornblende2A symplectitic lobe, containing ovoid inclusion of relic clinopyroxene (P) and rimmed by finer, radial clinopyroxene2B symplectites. (c) Development of delicate clinopyroxene2B ^plagioclase2B intergrowth, overgrow- ing a pervasive matrix hornblendeFOL (foliation direction marked S2 in the image). Inset shows the details of the textural relation illustrated in Fig.4d. (d) The symplectite mass occurs as tongues and lobes within hornblendeFOL. Slightly coarser clinopyroxene away from the symplectite domain, but coeval with clinopyroxene2B, occurs as fine to thick coronae around hornblendeFOL. (e) Coronal clinopyroxene around earlier hornblende2A bleb and parallel to the radial clinopyroxene2B symplectites.
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development, and (3) type 2 symplectites are distinctly post-kinematic with respect to D2. Kelyphitic coronas and symplectites have previously been reported from granulite-facies terranes and are interpreted in terms of decompression (Harley, 1989, 1992; Mengel & Rivers, 1991; Thost et al., 1991; Zhao et al., 2001).
Amphibolite
M2 inamphiboliteisrepresentedbytheformationofspec- tacular ilmenite±plagioclase±quartz Æ hornblende- bearing symplectites. Ilmenite±plagioclase±quartz assemblages generally develop around garnet against hornblendeFOL (Fig. 5a). Ilmenite±quartz±hornblende andplagioclase±quartzsymplectiteslocallyoccurwithin the interior of garnet (P) (Fig. 5a and b). The sym- plectites in general contain relicts of titanite. Although the symplectites generally mantle garnet, their local occurrence within the garnet interior, as in the present study, may not be uncommon. Such occurrences have also been reported from the Sostrene Island, Prydz Bay, Antarctica (Thost et al., 1991; Hensen et al., 1995), where fine orthopyroxene±plagioclase±spinel symplec- tites are preferentially located along fracture cleavages in garnet porphyroblasts. The reaction causing garnet decomposition was presumably kinetically favoured along these weak zones, which also facilitated easy passage of Na-rich solutions necessary for growth of minor albite components in the plagioclase symplec- tite. In the present case, the reactions could have initiated along fractures and progressively propagated inside through catalysing aqueous solutions, as implied Fig. 5. Backscatter electron images showing M2 reaction textures by the stability of minor hornblende in the symplectite. in amphibolites (sample19A/PS). (a) S2 fabric defined by alternate gar- net, hornblendeFOL and quartz-rich bands. Disrupted garnet grains were once part of a single large porphyroblast. Delicate ilmenite^ quartz^plagioclase Æ hornblende symplectites develop as a corona M3 episode around garnet and as a reaction front within the garnet interior. The Mafic granulite spectacular preservation of symplectites in the folial domain and as cor- onae around hornblende implies that the symplectites overprinted Early hornblende±plagioclase symplectites and coronas FOL the S2 foliation. Inset shows details of the textural relations presented in (referred to as hornblende3±plagioclase3) and a later (b). (b) Enlarged view of the inset in (a). The presence of idioblastic hornblende inclusion within garnet should be noted. A fine-grained zoisite±epidote±muscovite assemblage mark the M3 1 episode in the mafic granulites. Hornblende ±plagio- ilmenite±plagioclase intergrowth occurs around garnet, and also 3 within its interior. These are surrounded by coarser-grained horn- clase3 symplectites are restricted to haloes around blende symplectite blebs. garnet (P) (Fig. 6a) and are generally mutually exclu- sive with pyroxene±plagioclase symplectites. However, discussion). The present study resembles that of Zhao in places, the two kinds of coronas may mutually et al. (2001) on the Hengshan Complex, North China coexist, being part of a thick double corona around craton. In that study, the hornblende±plagioclase sym- garnet (P). The relatively finer hornblende3±plagio- plectites were given an independent metamorphic clase3 symplectites are located in the inner shell, being status separate from that of clinopyroxene plagio- armoured by an outer, coarser, type 2 clinopyroxene± clase symplectites. In certain outcrops, the hornble- plagioclase symplectite (Fig. 6b). The inner symplec- nde±plagioclase symplectites display a hexagonal tite often encloses relict clinopyroxene. These textures outline, marking the approximate location of the clearly show that the hornblende3±plagioclase3 sym- original surface of a euhedral garnet crystal (Fig. 6a). plectites post-date the clinopyroxene plagioclase The broadly euhedral outline of the symplectites symplectites; also supported by distinct differences in also implies their formation in a static state and their P±T conditions of formation (see subsequent post-dating D2.
394 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
Fig. 7. Backscatter electron images showing reaction textures in metabasic dyke (sample 13/PS). (a) Garnetiferous metabasic dyke (MD2), showing an intergranular texture. Garnet occurs as coronae at the interface of plagioclase and pyroxene. (b) Exsolution lamellae of orthopyroxene in clinopyroxene in MD2. The lamellae are con- centrated at the centre but depleted at the rim. This depletion appears to result from granular exsolution.
Fig. 6. Backscatter electron images showing M3 reaction textures in mafic granulites. (a) Garnet (P) with an idioblastic shell of hornblende3 ^plagioclase3 symplectites occurs within a coarse-grained mosaic of folial hornblende (S2) (sample 45A/PS). The margins of the symplectite reflect the original outline of the garnet (P). The symplec- tites are generally radially oriented with respect to the garnet rim, except in the edges, which are parallel to the foliation direction. In the latter, the symplectites are aligned parallel to the foliation direction. This implies that the symplectites are broadly post-kinematic with respect to D2 but were slightly reoriented during succeeding deforma- tions. (b) Garnet (P) is surrounded by a double corona of an outer clin- opyroxene2B ^plagioclase2B symplectite and an inner hornblende3 ^ plagioclase3 symplectite (sample 77C/SB). Boththese symplectites con- tain quartz inclusions.
395 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003
In the terminal stages of this episode, fine needle- Compositions typically show core to rim decreases in shaped intergrowths of zoisite±epidote and muscovite the spessartine and increases in the grossular compo- replaced plagioclase and clinopyroxene of all textural nent. Pyrope remains either relatively constant (sam- types. ple, 72D/PS, Fig. 8a) or shows slight rimward Mafic dyke, MD enrichment (sample 45A/PS, Fig. 8b and c). Of the 2 two profiles, A±B and C±D in Fig. 8b and c, rimward Mafic dykes MD record imprints of M reaction tex- 2 3 MgO enrichment is more conspicuous in profile C±D. tures. This is represented by the development of The bell-shaped MnO profile has commonly been coronal garnet±clinopyroxene±quartz symplectites described and interpreted as an original feature of and exsolution lamellae of orthopyroxene in clinopyr- crystal growth (see Tracy, 1982; Dempster, 1985; oxene and vice versa and hornblende coronas around Chakraborty & Ganguly, 1990) and is consistent with garnet. The studied sample (13/PS) is relatively fine thermodynamic models for compositional evolution grained, with a spectacular intergranular texture, con- during prograde growth (e.g. Loomis, 1986; Spear, sisting of orthopyroxene and clinopyroxene megacrysts 1988). The spikes and irregularities in MgO, MnO and laths of plagioclase (Fig. 7a) and can be classified and CaO in the profile in Fig. 8a can probably be as a metadolerite. Garnet developed preferentially at attributed to retrograde re-equilibration at the contact the contacts of plagioclase against orthopyro- with prograde assemblages, dominantly hornblende . xene and clinopyroxene. Generally pyroxene exsolu- 1 Compared with the coarser garnet (1Á2 cm diameter) tion lamellae are concentrated mostly within the core in Fig. 8a, the growth zoning of the relatively smaller region of the pyroxene megacrysts, and nearly absent garnet (7Á5 mm diameter) in Fig. 8b and c is relatively at their periphery (Fig. 7b). This may be attributed to suppressed. This may be attributed to partial homo- granular exsolution during cooling. genization during prograde metamorphism in the early dynamothermal event, the effect being more pro- nounced in smaller crystals (e.g. Llano uplift, Carlson MINERAL CHEMISTRY & Schwarze, 1997) and also in the garnet in the The chemical compositions of the coexisting mineral amphibolite. In the latter, there is no variation in phases in samples of mafic granulites, amphibolites and composition from core to rim. Accordingly, the a metadolerite were determined using a CAMECA garnet rim and garnet core compositions were selected SX-51 electron microprobe at the laboratory of the for calculation of peak P±T conditions of meta- Geological Survey of India at Faridabad. The operat- morphism in the granulites and amphibolites, ing conditions were set as 1 mm beam diameter, 15 kV respectively. accelerating voltage and 12 nA specimen current. The The outermost garnet rims (referred to as garnet PAP correction scheme was used. Natural and syn- edges), 5 to 50±150 mm wide, show sharp depletions thetic minerals were used as standards. For amphibole, in grossular and pyrope, but enrichments in spessartine Fe2 and Fe3 contents were estimated according to the and almandine contents (e.g. analysis 3A2-427 in recalculation scheme of Leake et al. (1997). For other Table 2), suggesting the effect of late static resorption. minerals, charge balance criteria were employed. Repre- Marked grossular depletion and enrichment of sentative mineral compositions are given in Tables 2±6. almandine and spessartine components can also be locally observed inside garnet (P) and adjacent to fine plagioclase±quartz intergrowths (analysis 3A1-194, Garnet Fig. 8c). MgO remains unaffected. This indicates Mafic granulite and amphibolite partial internal resorption of garnet that has been Representative garnet analyses are given in Table 2. superimposed on the pattern of growth zoning. Such Garnets in granulites and amphibolites are dominantly resetting of the composition of the garnet is also com- almandine rich (42±65%), with lesser amounts of mon in the amphibolites and can be attributed to the grossular (20±38%), pyrope (7±10%), spessartine operation of net transfer reactions and/or diffusion (2±12%) and andradite (2±7%) components. Three during post-peak decompression and cooling. Garnet types of compositional variations are noted: (1) inter- edges also show pronounced variation in compo- nal growth zoning, with consistent variation in garnet sition in contact with clinopyroxene plagioclase, composition from core to rim; (2) change in the com- hornblende±plagioclase, ilmenite±plagioclase±quartz position of garnet rims against symplectites; (3) varia- symplectites and coronas. Garnet in contact with pyro- tions in the chemistry of garnet core and rim between xene±bearing symplectites is distinctly higher in grossu- granulites and amphibolites. lar and lower in almandine content (analysis 2A1-396, Internal compositional zoning in garnet is preserved Table 2) than that against hornblende±plagioclase in the mafic granulites and is illustrated in Fig. 8. and ilmenite±plagioclase±quartz symplectites (analyses
396 Table 2: Representative chemical analyses of garnet
Mafic granulites Amphibolites MD2
Sample: 72D/PS 77C/SB 45A/PS 12A/PS 19A/PS 13/PS BHOWMIK
Site: 2A1-289 2A1-204 2A1-396 1-29 1-9 3A1-293 3A1-203 3A1-194 3A2-427 11A2-417 11A2-426 11A2-407 6A2-484 6A2-469 6A2-451 10 Textural site: C R Edge C Edge C R R* Edge C Edge Edge C C Edge COR (C)
...... AND SiO2 37 37 38 18 37 94 37 71 37 48 37 45 37 78 37 46 37 31 37 93 37 96 37 47 37 10 37 53 37 60 37 79 ...... TiO2 0 04 0 01 0 09 0 07 0 03 0 02 0 08 0 07 0 08 0 01 Ð Ð 0 07 0 09 0 07 0 03
...... ROY Al2O3 20 27 20 61 20 83 20 80 21 00 20 65 20 67 20 55 20 48 21 36 21 01 20 95 20 07 20 56 20 47 20 67 ...... Cr2O3 0 04 Ð Ð Ð Ð Ð 0 07 0 02 0 01 0 03 Ð 0 05 0 01 0 01 0 13 Ð FeO 20.87 21.40 21.57 26.15 26.60 22.83 24.08 26.09 26.24 25.30 29.72 27.19 27.20 27.87 28.11 29.47
MnO 4.56 1.97 4.86 0.74 1.11 4.93 3.02 4.19 5.50 0.97 1.44 2.37 2.23 1.93 2.88 1.39 METABASITES MgO 1.99 2.43 1.64 1.86 1.65 2.00 2.04 2.24 1.86 3.14 2.76 2.56 2.16 2.51 1.96 3.92 CaO 13.49 14.93 13.18 11.46 11.40 11.31 12.83 9.67 8.44 10.98 7.47 8.59 10.27 9.46 9.39 7.02 ......
397 Total 98 63 99 53 100 11 98 79 99 27 99 19 100 57 100 29 99 92 99 72 100 36 99 18 99 11 99 96 100 61 100 29
Cations 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 Si 2.988 3.004 2.997 3.019 2.995 2.991 2.971 2.972 2.987 2.988 3.009 2.998 2.978 2.983 2.984 2.982 FROM Ti 0.003 0.001 0.005 0.004 0.002 0.001 0.005 0.004 0.005 0.001 Ð Ð 0.004 0.005 0.004 0.002 Al 1.910 1.911 1.939 1.963 1.978 1.943 1.915 1.922 1.932 1.983 1.963 1.976 1.898 1.926 1.915 1.923
Cr3 0.002 Ð Ð Ð Ð Ð 0.004 0.001 0.001 0.002 Ð 0.003 0.001 0.001 0.008 Ð SAUSAR Fe3 0.107 0.079 0.057 Ð 0.025 0.072 0.129 0.124 0.083 0.038 0.018 0.024 0.136 0.097 0.102 0.108 Fe2 1.289 1.330 1.368 1.751 1.752 1.452 1.454 1.607 1.674 1.629 1.952 1.796 1.690 1.755 1.765 1.837 ......
Mn 0 309 0 131 0 325 0 050 0 075 0 334 0 201 0 282 0 373 0 065 0 097 0 161 0 151 0 130 0 193 0 093 MOBILE Mg 0.237 0.285 0.193 0.222 0.197 0.238 0.239 0.265 0.222 0.368 0.326 0.305 0.258 0.298 0.232 0.461 Ca 1.156 1.259 1.116 0.984 0.976 0.968 1.081 0.822 0.723 0.926 0.634 0.736 0.883 0.805 0.798 0.593 Sum 8.001 8 8 7.995 8 7.999 7.999 7.999 8 8 7.999 7.999 7.999 8 8.001 7.999 ...... BELT XAdr 0 055 0 040 0 031 Ð 0 012 0 037 0 067 0 065 0 044 0 019 0 009 0 012 0 071 0 051 0 053 0 055 ...... XPrp 0 079 0 095 0 064 0 074 0 066 0 080 0 080 0 089 0 074 0 123 0 108 0 102 0 087 0 100 0 077 0 155 ...... XAlm 0 431 0 443 0 456 0 582 0 584 0 485 0 488 0 540 0 562 0 545 0 649 0 598 0 566 0 587 0 589 0 616 ...... XSps 0 103 0 044 0 108 0 017 0 025 0 112 0 067 0 095 0 124 0 022 0 032 0 054 0 051 0 043 0 065 0 031 ...... XGrs 0 331 0 379 0 341 0 327 0 313 0 287 0 295 0 212 0 196 0 290 0 202 0 233 0 225 0 218 0 214 0 143
C, core; R, rim; COR, corona; R*, composition at the contact of internal plagioclasequartz symplectite in profile CÐD. Table 3: Representative chemical analyses of hornblende
Mafic granulites
Sample: 72D/PS 45A/PS
Site: 2A2-272 2A2-198 2A2-405 2A2-400 3A1-337 3A1-70 3A2-420 3A2-422 3A2-430 3A2-436 3A2-446 JOURNAL Textural site I I SYM FOL I I SYM SYM SYM FOL (C) FOL (R) (Type 1)
...... OF SiO2 40 40 39 94 39 01 40 55 42 39 42 77 43 64 43 28 43 00 43 08 45 44
...... PETROLOGY TiO2 1 03 1 63 1 31 1 48 0 88 1 14 1 23 1 45 1 28 1 43 1 11 ...... Al2O3 14 91 14 37 13 60 12 91 13 86 11 71 10 39 10 78 10 91 10 68 9 33 . . . . . Cr2O3 0 03 Ð Ð Ð 0 04 0 07 Ð Ð Ð 0 05 0 08 FeO 18.49 19.04 22.09 19.16 17.96 18.83 20.10 19.80 20.56 19.64 17.21 MnO 0.33 0.19 0.40 0.35 0.44 0.35 0.36 0.46 0.47 0.50 0.35 MgO 8.19 8.00 6.25 7.91 8.01 8.61 8.34 7.96 8.25 8.49 10.17 CaO 12.39 12.41 12.17 12.07 11.76 11.76 11.71 11.38 11.65 11.49 12.13 ...... VOLU 398 Na2O 1 36 2 03 1 64 1 83 1 44 1 28 1 29 1 43 1 44 1 43 1 08 ...... K2O 0 40 0 44 0 86 0 55 0 45 0 46 0 56 0 50 0 58 0 53 0 47 ME Total 97.53 98.05 97.33 96.81 97.23 96.98 97.62 97.04 98.14 97.32 97.36 44 Oxygen 23 23 23 23 23 23 23 23 23 23 23 Si 6.058 6.028 6.023 6.207 6.376 6.460 6.594 6.582 6.474 6.522 6.790 NUMBER Ti 0.116 0.185 0.152 0.107 0.100 0.130 0.140 0.166 0.145 0.163 0.125 Al(IV) 1.942 1.972 1.977 1.793 1.624 1.540 1.406 1.418 1.526 1.478 1.210 Al(VI) 0.694 0.585 0.499 0.536 0.834 0.546 0.445 0.515 0.411 0.428 0.433 3 Cr3 0.004 Ð Ð Ð 0.005 0.008 Ð Ð Ð 0.006 0.010 Fe3 0.551 0.331 0.499 0.287 0.192 0.367 0.307 0.219 0.421 0.341 0.178 MARCH Fe2 1.768 2.072 2.353 2.166 2.067 2.012 2.233 2.299 2.168 2.146 1.973 Mn 0.042 0.024 0.052 0.045 0.056 0.045 0.046 0.059 0.060 0.064 0.044
Mg 1.830 1.799 1.438 1.804 1.796 1.938 1.878 1.804 1.851 1.916 2.265 2003 Ca 1.991 2.007 2.013 1.980 1.895 1.903 1.896 1.854 1.880 1.864 1.942 ...... Na (M4) 0 005 Ð Ð 0 011 0 056 0 052 0 056 0 083 0 064 0 073 0 031 Na (A) 0.390 0.594 0.491 0.532 0.364 0.323 0.322 0.339 0.356 0.347 0.282 K (A) 0.077 0.085 0.169 0.107 0.086 0.089 0.108 0.097 0.111 0.102 0.090 Sum 15.468 16.329 15.666 16.214 15.901 15.413 15.431 15.435 15.467 15.45 15.373 Na K (A) 0.467 0.647 0.660 0.639 0.450 0.412 0.430 0.436 0.467 0.449 0.372 ...... XMg 0 510 0 460 0 380 0 450 0 460 0 490 0 460 0 440 0 460 0 470 0 530 Amphibolites MD2
Sample: 12A/PS 19A/PS 13/PS
Site: 11A2-280 11A2-425 11A2-428 6A2-483 6A2-442 6A2-449 6A1-52 6A1-45 7 BHOWMIK Textural site: SYM FOL I SYM SYM FOL (C) FOL (R) COR (FI) (CO)
...... SiO2 43 10 45 93 46 81 40 49 42 61 41 69 41 12 42 60 44 06 ...... AND TiO2 0 96 0 75 0 85 1 34 0 97 1 25 1 56 1 09 1 80 ...... Al2O3 13 27 10 74 9 50 12 45 10 96 11 69 11 50 10 86 10 85 ...... ROY Cr2O3 0 02 0 06 0 08 0 02 0 12 0 07 Ð Ð Ð FeO 16.27 16.07 15.42 21.63 22.32 22.81 21.72 21.03 15.13 MnO 0.20 0.11 0.15 0.31 0.21 0.21 0.38 0.28 0.07
MgO 9.32 10.24 11.53 6.66 6.99 7.00 7.19 7.73 11.29 METABASITES CaO 12.19 11.63 11.81 12.45 11.94 11.50 11.75 11.53 11.58 ...... Na2O 0 96 0 84 0 61 1 37 1 04 1 58 1 50 1 47 1 33
399 ...... K2O 0 79 0 48 0 41 0 56 0 24 0 55 0 48 0 48 0 94 Total 97.08 96.84 97.17 97.28 97.40 98.35 97.20 97.07 97.05 Oxygen 23 23 23 23 23 23 23 23 23 FROM Si 6.440 6.813 6.848 6.221 6.482 6.320 6.301 6.499 6.557 Ti 0.108 0.084 0.094 0.155 0.111 0.143 0.180 0.125 0.201
Al(IV) 1.600 1.187 1.152 1.779 1.518 1.680 1.699 1.501 1.443 SAUSAR Al(VI) 0.778 0.691 0.487 0.475 0.448 0.410 0.378 0.452 0.461 Cr3 0.002 0.007 0.009 0.002 0.014 0.008 Ð Ð Ð 3 ......
Fe 0 186 0 186 0 376 0 421 0 537 0 546 0 497 0 394 0 206 MOBILE Fe2 1.847 1.808 1.509 2.358 2.302 2.346 2.286 2.289 1.677 Mn 0.025 0.014 0.019 0.040 0.027 0.027 0.049 0.036 0.009 Mg 2.076 2.264 2.575 1.525 1.585 1.582 1.642 1.757 2.504 BELT Ca 1.952 1.848 1.851 2.049 1.946 1.868 1.929 1.885 1.847 ...... Na (M4) 0 026 0 099 0 079 Ð 0 028 0 070 0 038 0 062 0 095 Na (A) 0.252 0.143 0.094 0.408 0.278 0.394 0.408 0.373 0.289 K (A) 0.151 0.091 0.077 0.110 0.047 0.106 0.094 0.093 0.178 Sum 15.443 15.235 15.170 15.543 15.323 15.5 15.501 15.466 15.467 Na K (A) 0.403 0.234 0.171 0.518 0.325 0.500 0.502 0.466 0.467 ...... XMg 0 530 0 560 0 630 0 390 0 410 0 400 0 420 0 430 0 600
Analyses 2A2-272 and 2A2-198 are Hornblende1 at garnet core and rim, respectively. I, inclusion; SYM (Type 1), Type 1 symplectite; FOL, folial hbl; F and Cl contents in hornblende are <0Á15 wt %. Fe3 is calculated following Leake et al. (1997). FI, Fine; CO, Coarse. Table 4: Representative chemical analyses of plagioclase, zoisite and epidote
Mafic granulites Amphibolites MD2 Mafic granulites
Sample: 77C/SB 45A/PS 12A/PS 19A/PS 13/PS 72D/PS
Pl Pl Pl Pl Pl Pl Pl Pl Pl Pl Pl Pl Zo 4 Pl Ep 4 Pl
Site: 1Ð6 10Ð28 3A2-445 3A2-418 3A2-423 3A2-428 11A2-409 11A2-427 6A2-448 6A2-450 50 51 1A-76 1A-64 Textural site: M SYM (Type 2) M SYM SYM SYM M SYM SYM SYM C R
...... JOURNAL SiO2 52 21 48 25 54 78 51 86 51 29 52 04 54 25 44 81 44 79 45 71 52 65 56 60 39 74 38 27 . . . . . TiO2 0 03 Ð Ð 0 02 Ð Ð 0 01 Ð 0 17 Ð Ð Ð Ð 0 05 ...... Al2O3 30 09 32 86 28 17 30 52 30 68 30 18 28 95 35 30 34 36 34 55 29 83 27 15 32 32 27 50 . . . Cr2O3 0 05 0 01 Ð Ð Ð Ð 0 01 Ð Ð Ð Ð Ð Ð Ð
FeO Ð 0.12 Ð Ð 0.08 0.13 0.12 0.15 0.52 0.24 0.02 0.43 0.46 6.69 OF . .
MnO Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð 0 01 0 18 PETROLOGY MgO 0.02 0.01 Ð 0.02 Ð Ð Ð Ð 0.02 0.04 Ð Ð Ð Ð CaO 12.78 15.82 10.93 13.42 14.00 13.72 12.02 19.45 18.68 18.52 13.23 9.52 25.54 24.60 ...... Na2O 4 22 2 34 5 19 4 13 3 74 3 87 5 08 0 71 1 24 1 09 4 30 6 14 Ð Ð ...... K2O 0 11 0 03 0 04 0 07 0 03 0 02 0 04 0 01 Ð Ð 0 12 0 20 Ð 0 05 BaO n.a. n.a. Ð 0.01 Ð Ð 0.16 Ð 0.02 0.04 Ð Ð Ð Ð Total 99.51 99.46 99.11 100.05 99.82 99.96 Ð 100.43 99.80 100.19 100.15 100.01 98.07 97.34 VOLU
400 (O) 8 8 8 8 8 8 8 8 8 8 8 8 12 O & 0.5 (OH) 12 O & 0.5 (OH)
Si 2.379 2.218 2.488 2.355 2.337 2.364 2.442 2.063 2.077 2.104 2.386 2.543 3.018 2.987 ME . . . .
Ti 0 001 Ð Ð 0 001 Ð Ð Ð Ð 0 006 Ð Ð Ð Ð 0 003 44 Al (IV) 0.621 0.782 0.512 0.645 0.663 0.636 0.558 0.937 0.923 0.896 0.614 0.457 Ð 0.013 Al (VI) 0.995 0.999 0.990 0.988 0.984 0.980 0.979 0.979 0.955 0.979 0.979 0.981 2.984 2.518 NUMBER Cr3 0.002 Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Fe3 Ð 0.005 Ð Ð 0.003 0.005 0.005 0.006 0.020 0.009 0.001 0.016 0.029 0.436 Mn Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð 0.001 0.012 3 Mg 0.001 0.001 Ð 0.001 Ð Ð Ð Ð 0.001 0.003 Ð Ð Ð Ð ......
Ca 0 624 0 779 0 532 0 653 0 683 0 668 0 583 0 960 0 928 0 914 0 643 0 459 2 078 2 057 MARCH Na 0.373 0.209 0.457 0.364 0.331 0.341 0.443 0.064 0.111 0.097 0.378 0.535 Ð Ð K 0.006 0.002 0.002 0.004 0.002 0.001 0.002 Ð Ð Ð 0.007 0.012 Ð Ð
Ba n.a. n.a. Ð Ð Ð Ð 0.003 Ð Ð 0.001 Ð Ð Ð Ð 2003 Sum 5.002 4.995 4.981 5.011 5.003 4.995 5.012 5.009 5.021 5.002 5.008 5.003 8.11 8.026 ...... XK 0 01 Ð Ð 0 004 0 002 0 001 0 002 Ð Ð Ð 0 01 0 01 Ð Ð ...... XNa 0 37 0 21 0 46 0 356 0 325 0 337 0 430 0 063 0 107 0 096 0 37 0 46 Ð Ð ...... XCa 0 62 0 79 0 54 0 639 0 673 0 661 0 565 0 937 0 893 0 903 0 62 0 53 Ð Ð . . XBa Ð Ð Ð Ð Ð Ð 0 003 Ð Ð 0 001 Ð Ð Ð Ð
Ep, epidote; Analyses 3A2-418, 3A2-423 and 3A2-428 are symplectitic plagioclase3; Analyses 11A2-427, 6A2-448 and 6A2-450 are symplectitic plagioclase intergrown with ilmenite and quartz; Pl, plagioclase; Zo, zoisite; Zo, Ep 4 Pl, Zo, Ep replaced Pl. Table 5: Representative chemical analyses of clinopyroxene and orthopyroxene
Mafic granulites MD2
Sample: 72D/PS 77C/SB 45A/PS 13/PS BHOWMIK
Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx Opx Opx Cpx Cpx Cpx Opx
Site: 1Ð10 1A-79 1A-67 1A-63 2A2-406 1Ð4 1Ð26 3A2-404 3A2-407 17 21 19 27 Textural site: (P) SYM SYM SYM SYM (P) SYM (P)ÙGrt (P)ÙGrt MEG LAM MEG LAM *MEG(R) *MEG(R)
(Type 1) (Type 1) (Type 2) (Type 2) (Type 2) AND
...... ROY SiO2 49 68 47 55 48 38 49 41 50 25 50 14 50 33 51 34 51 16 50 32 50 69 52 31 52 62 51 74 51 02 ...... TiO2 0 23 0 75 0 46 0 32 0 28 0 16 0 12 0 26 0 15 0 11 Ð 0 03 0 10 0 05 0 02 ...... Al2O3 2 43 4 33 3 42 2 47 1 86 1 80 1 15 1 67 1 67 0 44 0 37 0 66 0 73 0 60 0 45 ...... Cr2O3 0 02 0 01 Ð 0 19 Ð 0 02 Ð 0 13 Ð Ð 0 02 Ð 0 01 Ð Ð METABASITES FeO 12.75 13.07 13.16 12.15 13.22 15.97 16.60 13.67 13.56 32.55 33.10 11.38 11.25 17.44 29.58 MnO 0.41 0.40 0.38 0.40 0.40 0.18 0.36 0.47 0.83 0.60 0.57 0.03 0.21 0.19 0.45 MgO 10.41 8.89 8.93 9.61 9.58 8.71 8.11 10.11 10.51 15.05 15.28 12.10 12.42 12.94 15.03 401 CaO 23.43 23.80 23.96 23.90 24.48 21.21 22.24 22.61 22.92 0.67 0.80 23.43 23.61 16.92 3.70 ...... Na2O 0 15 0 28 0 17 0 15 0 20 0 27 0 18 0 28 0 33 Ð Ð 0 17 0 20 0 12 0 03
ZnO 0.17 0.20 0.07 Ð 0.31 Ð Ð 0.09 0.07 Ð Ð Ð Ð Ð Ð FROM Total 99.68 99.28 98.93 98.60 100.50 98.46 99.09 100.63 101.20 99.74 100.83 100.11 101.15 100.00 100.28
Cations 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 SAUSAR Si 1.897 1.830 1.871 1.911 1.910 1.963 1.969 1.949 1.926 1.976 1.970 1.971 1.961 1.972 1.978 Ti 0.007 0.022 0.013 0.009 0.008 0.005 0.004 0.007 0.004 0.003 Ð 0.001 Ð 0.002 Ð Al (IV) 0.103 0.170 0.129 0.089 0.084 0.037 0.031 0.051 0.074 0.020 0.017 0.029 0.032 0.026 0.020 MOBILE Al (VI) 0.006 0.026 0.027 0.023 Ð 0.047 0.022 0.024 0.001 Ð Ð Ð Ð Ð Ð Cr3 0.001 Ð Ð 0.006 Ð 0.001 Ð 0.004 Ð Ð 0.001 Ð Ð Ð Ð Fe3 0.095 0.121 0.089 0.054 0.096 0.001 0.015 0.028 0.09 0.023 0.043 0.040 0.054 0.035 0.024
Fe2 0.312 0.300 0.336 0.339 0.325 0.522 0.528 0.406 0.337 1.046 1.033 0.319 0.297 0.527 0.939 BELT Mn 0.013 0.013 0.013 0.013 0.013 0.006 0.012 0.015 0.026 0.020 0.019 0.001 0.007 0.006 0.015 Mg 0.592 0.510 0.514 0.554 0.544 0.508 0.473 0.572 0.591 0.881 0.885 0.679 0.690 0.737 0.870 Ca 0.958 0.981 0.992 0.990 0.997 0.890 0.932 0.920 0.924 0.028 0.033 0.946 0.943 0.683 0.148 Na 0.011 0.021 0.014 0.012 0.014 0.02 0.014 0.020 0.024 Ð Ð 0.012 0.014 0.009 0.002 Zn 0.005 0.006 0.002 Ð 0.009 Ð Ð 0.003 0.002 Ð Ð Ð Ð Ð Ð Sum 4 4 4 4 4 4 4 3.999 3.999 3.997 4.001 3.998 3.998 3.997 3.996 ...... XMg 0 66 0 63 0 61 0 62 0 63 0 51 0 47 0 59 0 64 0 46 0 46 0 68 0 70 0 58 0 48
Cpx, clinopyroxene; Opx, orthopyroxene; (P), porphyroblast; (P)ÙGrt, relict porphyroblast enclosed within garnet; MEG, megacryst; LAM, lamellae; *MEG(R), reintegrated megacryst composition. JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003
Table 6: Mineral phases and equilibria used in NCMASH petrogenetic grid
Phases: Garnet, clinopyroxene, pargasite, tremolite, albite, anorthite, quartz, vapour
Reactions: (1) 3gr 3py 4tr 2ab 21di 2parg 10an 2H2O (q)
(2) 5gr py 2tr 4q 13di 6an 2H2O (parg, ab)
(3) 9gr 3py 19parg 105q 17tr 31an 19ab 2H2O (di) (4) di parg 5q tr ab an (grt, v)
(5) 9gr 3py 14tr 12ab 31di 12parg 50 2H2O (an)
(6) 13gr 5py 2parg 26q 23di 20an 2ab 2H2O (tr) Invariant point: P 9.4 kbar; T 725C
Mineral abbreviations are after Holland & Powell (1998).
3A2-427 and 6A2-451). These compositional variations plagioclase; (5) symplectitic hornblende associated are likely to reflect re-equilibration of garnet under dif- with symplectitic ilmenite±plagioclase±quartz. ferent conditions (Zhao et al., 2001). According to the nomenclature of Leake et al. (1997), Systematic compositional heterogeneity exists in the prograde hornblendes in both granulites and amphi- peak garnet composition between the mafic granulites bolites are ferropargasite to pargasite in composition and the amphibolites. Garnets from the mafic granulites (Fig. 9a). However, there is systematic increase in the have relatively higher grossular and lower almandine ferropargasite component in hornblende1 from amphi- and pyrope contents than those of the amphibolites. In bolite to mafic granulite. Like garnet, hornblende1 in the former, the grossular component varies in the range the mafic granulites also shows systematic variation 30±38 mol %. In contrast, the grossular component in in its composition in relation to its position within the amphibolites is 529%. Such a variation in garnet the host garnet; it shows progressive rimward increase composition is responsible for growth of diverse but in (Na K)A, Ti and Al(IV) (Fig. 9b). This composi- systematic garnet breakdown mineral assemblages tional variation in hbl1 is in consonance with the in the granulites and amphibolites. A highly calcic growth zoning in the enclosing garnet. Progressive clinopyroxene±plagioclase mineral assemblage is con- increase in edenite and Ti-tschermakite substitution spicuous in the mafic granulites and developed from in hornblende is generally attributed to rising tempera- high grossular garnets (grossular 38 mol %). In con- ture of equilibration (Spear, 1981; Ernst & Liu, 1998). trast, ilmenite-bearing mineral assemblages stabilized Compared with hornblende1, folial hornblende is from relatively almandine-rich, grossular-poor garnets distinctly less aluminous, and is more variable in com- in amphibolites. Hornblende±plagioclase symplectites position ranging from ferropargasite through ferroede- formed from intermediate garnet compositions (grossu- nite to edenite in granulites and amphibolites. There is lar 30 mol %) in mafic granulites. consistent core to rim depletion in Ti, Al(IV) and (Na Metadolerite (MD ) K)A contents (Fig. 9c and d). 2 Symplectitic hornblende in type-1 symplectites in the Coronal garnet is enriched in almandine (62 mol %), mafic granulites is compositionally similar to associated moderately enriched in pyrope (15 mol %) and gros- hornblendeFOL. Symplectitic hornblende associated sular (14 mol %) with minor spessartine (3 mol %) and with symplectitic plagioclase is ferroedenite to ferro- andradite (6 mol %) component (Table 2). pargasite. Cores of hornblende3 are enriched in Ti, Al(IV) and (Na K) similar to cores of large horn- Hornblende A blendeFOL (Fig. 9c and d). All these parameters are Mafic granulites and amphibolites depleted in the rim of coarser grains and in smaller Representative chemical analyses of hornblende are symplectite grains. Symplectitic hornblende associated presented in Table 3. Depending on textural type, the with symplectitic ilmenite±plagioclase±quartz in hornblende analyses are subdivided into the following amphibolites varies from edenite to ferropargasite. varieties: (1) prograde hornblende (hornblende1); The general rimward depletion in Ti, Al(IV) and (2) folial hornblende; (3) symplectitic hornblende (Na K)A contents in hornblende of most textural associated with clinopyroxene and plagioclase; (4) types suggests reversal of edenite [(NaA) 1 symplectitic hornblende associated with symplectitic (AlIV) 1 ( ) (SiIV)] and Ti-tschermakite [(TiVI) 1
402 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
Fig. 8. Representative compositional zoning profiles of garnet in mafic granulites. (a) Zoning profile for garnet (P) of 1Á2 cm diameter from sample72D/PS.It showsprogressivecoretorim depletion in MnO,butenrichment in CaO,implyinggrowthzonation.MgOremainsmore orless constant. (b) Profile along A±B (sample 45A/PS, marked in Fig. 6a). The profile of 100 spots is 6Á7 mm long. Distance between the adjacent spots is 33Á4 mm. It shows a progressive depletion in MnO and slight enrichment in CaO from core to rim. (c) Profile along C±D (marked in Fig. 6a). The profile of 100 spots is 7Á4 mm long. Distance between adjacent spots is 37 mm. It shows growth zoning with a systematic increase in MgO and FeO and depletion in MnO from core to rim. The core to rim compositional change in the garnet is marked by a vertical dashed line. Garnet additionallyshows aninternal sharp fall in CaOand a concomitant enrichment in FeO,close toa vein,bearinga plagioclasequartzintergrowth, that implies internal garnet resorption. The garnet rim in contact with the hornblende3±plagioclase3 symplectite shows a sharp reversal in composition with a decrease in CaO and MgO, and enrichment in MnO. This indicates effect of late static metamorphism (M3)(seetext).
(AlIV) 2 (Mg) (SiIV)2] substitution, in response to Matrix plagioclase in the granulites varies in the down-temperature equilibration. range An54±62, whereas in the amphibolites, it is An57. Symplectitic plagioclase is more anorthite rich Metadolerite than matrix-type plagioclase in different types of symplectites. In quartz-bearing domains, plagioclase Coronal hornblende is edenitic (XMg 0Á62) and tita- in type 2 symplectites is bytownite (An ) in composi- niferous (1Á80 wt % TiO2) (Table 3). 79 tion (sample 77C/PS). In quartz-absent domains, plagioclase composition in type 1 and type 2 symplec- Plagioclase tites is not preserved, as it is altered to an assemblage of Mafic granulites and amphibolites zoisite, epidote and locally muscovite (sample 72D/ Representative plagioclase compositions are given in PS). Considering the predominantly calcic alteration Table 4. There are systematic compositional variations assemblages, we assume an anorthitic composition of in plagioclase with textural type, which include the plagioclase symplectites in this domain. This is also following. in accord with a general anorthitic composition of
403 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003
Fig. 9. Compositional variation of hornblende. (a) Si vs XMg plotshowingthecompositionalvariationofhornblende ofdifferenttexturaltypesin mafic granulites, garnetiferous amphibolites and the metabasic dyke. (b) Systematic enrichment of Ti and (NaK)A in hornblende1 from its position in garnet cores to garnet rims, in consonance with growth zoning of the host. (c) Ti vs AlIV plot of hornblende of different textural types. Hornblende2A symplectite and core of folial hornblende are both titaniferous and enriched in AlIV. There is a core to rim depletion inTi and less strongly in AlIV in coarse hornblendeFOL in both mafic granulite and amphibolite. In amphibolite, the hornblende symplectite of finer mode is more depleted inTi and less strongly depleted in AlIV compared with its coarser counterpart. (d) Ti vs (NaK)A plot, showing strong core to rim depletion inTi and minor, but perceptible fall in (NaK)A component in hornblendeFOL.This, combined with depletion in AlIV, implies reverse operation of edenite andTi-Tschermakexchange (see text for details). plagioclase in both granulites and amphibolites (see symplectites in amphibolites, plagioclase is uniformly subsequent discussion). For geothermobarometric anorthitic (An90±94). In contrast, plagioclase3 with calculations (see subsequent discussion), we therefore hornblende3 symplectites is more variable, ranging have taken XAn 0Á90 for symplectitic plagioclase from labradorite (An64±67) (Table 4) to bytownite for this sample. In association with ilmenite±quartz (An83±87) (Bhowmik et al., 1999), reflecting the differential
404 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
contribution of grossular and ferroedenite components Epidote±chlorite from the decomposing garnet and folial hornblende, The pistacite component in epidote, replacing respectively (see discussion on mineral reaction his- pyroxene and plagioclase in the mafic granulites is tory). Exsolution lamellae of plagioclase in folial horn- 15 mol % (Table 4). XMg in chlorite is 0Á45. blende are labradoritic in composition.
Metadolerite MINERAL REACTION HISTORY Plagioclase in the metadolerite is distinctly zoned with Prograde reaction a thicker calcic core (XAn 0Á63) and a thinner, rela- The prograde reaction history in the mafic granulites tively sodic rim (XAn 0Á46) (Table 5). The zoning and amphibolites is best constrained by the inclusion conforms to the original crystal outline. Part of this assemblage of hornblende1 plagioclase1 ilmenite1 Æ normal zoning may be attributed to original igneous quartz within garnet and/or in clinopyroxene (P). zoning. Mafic granulite and amphibolite Pyroxene In the mafic granulites, the presence of included assem- Mafic granulite blages of ferropargasite1, plagioclase1 and ilmenite1 Both porphyroblastic and symplectitic clinopyroxenes within garnet (P) and clinopyroxene (P), the occur- are salite to augite in composition, with low jadeite rence of enclosed rutile within garnet rims and the rare contents, typical of mafic granulites. Clinopyroxene presence of plagioclase1 in the matrix suggest the fol- (P) is low to moderate in alumina (Al2O3 1Á67±2Á54 lowing generalized reaction that produced the M1 wt %) with XMg varying in the range 0Á51± 0Á66 mineral assemblage: (Table 5). Clinopyroxene in type 1 symplectites is hornblende plagioclase ilmenite more aluminous (Al O 3Á42±4Á33 wt %) compared 1 1 1 2 3 garnet P clinopyroxene P with that in type 2 symplectite (Al2O3 1Á86±2Á48 wt %). rutile quartz H2O=melt: 1a Metadolerite This reaction is also supported by chemographic pro- Both orthopyroxene and clinopyroxene megacrysts jections in Al2O3 (CaO Na2O) (FeO MgO) in the metadolerite are low in alumina (Al2O3 SiO2 space. The projections were made using the 0Á40±0Á73 wt %). Clinopyroxene is more magnesian chemical composition of the M1 minerals. (Fig. 10a). (XMg 0Á68) than orthopyroxene (XMg 0Á46) In the amphibolites, porphyroblastic garnet in the (Table 5). There is no perceptible difference in compo- absence of clinopyroxene was produced by reacting sition between pyroxene exsolution lamellae and the hornblende, plagioclase and ilmenite: pyroxene megacrysts, suggesting homogeneity of com- hornblende plagioclase ilmenite garnet P position as a result of diffusion. The core compositions 1 1 1 of the pyroxene megacrysts have been reintegrated titanite quartz H2O: 1b with the assumption that the ratios of lamellae and host Recent experimental studies on mid-oceanic ridge in the core are representative of the entire grain and basalts (MORB) have shown that both reactions (1a) did not change consequent to granular exsolution. The and (1b) are pressure sensitive, with the garnet result shows that magmatic orthopyroxene was essen- clinopyroxene assemblage stabilized at relatively tially of intermediate pigeonite composition (X higher temperature compared with garnet alone (Liu CaCO3 7Á6 mol %, XMg 0Á48). Magmatic clinopyroxene was et al., 1996; Vielzeuf & Schmidt, 2001). These reactions augite (X 35 mol %, X 0Á58) (Table 4). CaCO3 Mg also mark the transition from amphibolite to garneti- ferous amphibolite and mafic granulite with rising pres- Ilmenite sure and temperature. Green & Ringwood (1967) have Coronal ilmenite around rutile in the mafic granulite suggested that the garnet clinopyroxene quartz contains minor pyrophanite component (7 mol %). association in metabasic rocks of intermediate composi- Ilmenite intergrown with plagioclase±quartz symplec- tion is diagnostic of high-pressure (>9 kbar) granulite- facies metamorphism. Rushmer (1993) has also experi- tite in amphibolite is nearly pure FeTiO3. mentally shown that the garnet clinopyroxene plagioclase assemblage, when lacking orthopyroxene, is Titanite indicative of high-pressure metamorphism (12±18 kbar) Titanite in the granulites and amphibolites is slightly of basaltic amphibolite. The exact pressure at which aluminous (Al2O3 1Á31±1Á88 wt %) and ferroan clinopyroxene garnet becomes the dominant residual (FeO 0Á54±1Á55 wt %). assemblage to the exclusion of orthopyroxene is,
405 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003
Fig. 10. Al2O3 SiO2 (FeO MgO) (CaO Na2O) and CaO FeO SiO2 TiO2 diagrams showing possible metamorphic reactions operating in the mafic granulites and the amphibolites of the Sausar Belt. In (a) and (d): A, Al2O3; S, SiO2; F, FeO MgO; C, CaO Na2O. Mineral abbreviations are after Kretz (1983). however, controlled by the protolith composition (Green There is a phase of pervasive syn-D2 growth of & Ringwood, 1967; Patin~o-Douce & Beard, 1995). hornblendeFOL. The relict occurrence of clinopyroxene (P) within hornblende implies that the foliation Retrograde reactions FOL was mainly produced by pseudomorphous replace- Both the mafic granulites and amphibolites were over- ment of clinopyroxene during syntectonic hydration: printed by strong retrograde metamorphism, during which the M garnet and locally clinopyroxene (P) 1 garnet P clinopyroxene P H2O became armoured by coronitic assemblages and by hornblendeFOL: 2b the development of hornblendeFOL. In the mafic granulites, the earliest phase of The compositional similarity of hornblende2A and post-peak retrograde evolution is manifested by the hornblendeFOL and the broad alignment of the sym- development of a coarse-grained clinopyroxene ± 2A plectites along the S2 planes imply synchronous opera- hornblende2A±plagioclase2A symplectite assemblage. tion of reactions (2a) and (2b). Reactions (2a) and Hornblende2A contains relict clinopyroxene (P) and is (2b) are generally considered to have progressed in relatively high in Ti, Al IV, (Na K)A and Al VI. response to decompression (Harley, 1989; Mengel & Clinopyroxene2A is markedly aluminous (Al2O3 Rivers, 1991). 3Á4±4Á3 wt %). This reflects the general reaction The critical petrographic feature of development of type 2 clinopyroxene2B±plagioclase2B±ilmenite2B sym- garnet P clinopyroxene P H2O plectites overgrowing matrix folial hornblende, clinopyroxene2A plagioclase2A and also at the interfaces of garnet (P) and clino- hornblende : 2a 2A pyroxene2A, hornblende2A, the presence of titanite
406 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
inclusions in the symplectites, the relatively low The relatively sodic composition of the plagioclase in alumina (Al2O3 1Á9±2Á5 wt %) content of the symplec- symplectites adjacent to matrix folial hornblende is titic clinopyroxene, the grossular depletion at the considered to reflect the preferential contribution of garnet edges, the anorthitic composition of plagio- the ferroedenite component of the matrix amphibole clase and finally, the crossing tie-lines in the CaO± to form the albite component in plagioclase. In con- FeO±SiO2±TiO2 chemographic projection (Fig. 10b) trast, the bytownitic composition of plagioclase in an imply that these symplectites post-dated D2 and inner halo around garnet is largely controlled by the appeared according to a generalized reaction of decomposition of the grossular component of garnet. the form The chemographic support for reaction (3a) is shown in Fig. 10d. The reaction has also been documented from a number of granulite terranes (Harley, 1989; garnet P clinopyroxene2A hornblende2A= Mengel & Rivers, 1991; Zhao et al., 2001) and is con- hornblendeFOL titanite clinopyroxene2B trolled by fluid composition in addition to P and plagioclase2B ilmenite2B H2O=melt: 2c T. Zhao et al. (2001) recently interpreted the reaction to have progressed in response to cooling, accompany- In silica-rich domains, clinopyroxene±plagioclase sym- ing minor decompression. plectites appeared according to the reaction The post-D2 emplacement of the mafic dykes, MD2, is consistent with its intrusion at the culmination of garnet P quartz clinopyroxene 2B reactions (2c) and (2d). This phase of static recrystal- plagioclase2B: 2d lization, post-dating D2, is also shown by garnetiferous metadolerite, MD , and is manifested by subsolidus Both reactions (2c) and (2d) are strongly pressure 2 re-equilibration involving the development of garnet± dependent and proceed to the right with decompres- quartz±clinopyroxene Æ hornblende-bearing coronas sion (Harley, 1989; Mengel & Rivers, 1991; Zhao et al., and of exsolution lamellae of orthopyroxene in 2001). Reaction (2c), involving the breakdown of a clinopyroxene and vice versa. The generalized hydrous phase, may additionally indicate (1) progres- garnet-forming reaction is sion under fluid-deficient conditions and/or (2) that the decompression is associated with heating (Spear, 1993; Hensen et al., 1995). orthopyroxene clinopyroxene1 plagioclase Æ Ilmenite±plagioclase±quartz Æ hornblende sym- H2O garnet clinopyroxene2 quartz plectites surrounding embayed garnet and in contact with folial hornblende suggest the following reaction in Æ hornblende: 3b the amphibolites: Reaction (3b) proceeds to the right with cooling or garnet P titanite hornblendeFOL ilmenite with increasing pressure (Harley, 1989). plagioclase quartz Æ hornblende: 2e In the last stage of metamorphism, garnet was replaced by chlorite along fractures, and anorthitic The reaction can also be explained in terms of the plagioclase and clinopyroxene by zoisite, epidote and CaO±FeO±SiO2±TiO2 tetrahedron projection system muscovite in mafic granulites. The following general- (Fig. 10c). Ilmenite±plagioclase±quartz symplectites ized reactions mark a late greenschist-facies overprint: developing from garnet and rutile have also been documented from the mafic granulites of the Hengshan Complex and are explained in terms of decompres- plagioclase garnet diopside sion (Zhao et al., 2001). Hornblende±plagioclase symplectites around garnet Ca-Tschermak in clinopyroxene H2O against folial hornblende, which pseudomorph the zoisite=epidote chlorite: 3c idioblastic habit of garnet, and those formed locally at the interface of garnet and type 2 clinopyroxene± plagioclase symplectites imply a relatively static The observed reactions in the mafic granulites and (i.e. post-deformational) hydration reaction in mafic amphibolites suggest that (1) the amphibolites and the granulite: granulites have been subjected to the same meta- morphic event, with both documenting post-peak decompression, and (2) the granulites additionally garnet P clinopyroxene2B hornblendeFol record a post-decompressional cooling history. The quartz H O hornblende plagioclase : 2 3 3 inferred clockwise P±T trajectory will now be further 3a constrained by petrogenetic grid arguments.
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also for thermobarometry (Dale et al., 2000). The a±X relations proposed by Dale et al. (2000) and incorpo- rated in the latest version (v.3.1) of THERMOCALC provide reasonable approximations for tremolite and pargasite activities. These two amphibole end-mem- bers are chosen because they show the greatest varia- tions in mineral chemistry amongst amphiboles with increasing grade of metamorphism from amphibolite to granulite, and participate in a number of important mineral reactions. Accordingly, a simplified six-com- ponent (NCMASH), eight-phase (garnet, clinopyrox- ene, albite, anorthite, pargasite, tremolite, quartz and vapour) system has been chosen to construct the grid. The non-NCMASH components are adjusted for the activities of the phases involved so that the grid is an `activity corrected' grid in the same sense as those used by Harley & Buick (1992) for calc-silicate rocks, and with the same caveats as explained by Fitzsimons & Harley (1994). The phase compatibilities projected from quartz and water are presented in Fig. 11a. Using Schreinemaker's analysis and THERMOCALC (v.3.1), the positions of all the reaction lines are calcu- lated in P±T space (Fig. 11a). The mineral reactions and the P±T co-ordinates of the invariant point are given in Table 6. It should be recognized that this point is the P±T position of the intersection of lines that are the projections of fields or surfaces in the more complex NCFMASH system, and will move in PTXfluid space as the Fe/Mg ratios of the minerals vary. The geometric relations and the constraints imposed by the chemography in the system NCMASH define a single pseudo-invariant point from which emanate Fig. 11. (a) NCMASH petrogenetic grid showing the stability fields of peak and retrograde metamorphism in amphibolites [M(A)] and eight pseudo-univariant reactions, four of which are mafic granulites [M(G)]. On the basis of sequential appearance of related by degenerate equilibria (Fig. 11a, Table 6). mineral assemblages, the grid predicts a qualitative clockwise P±T Figure 11 also shows that the majority of the reactions path of evolution of the mafic granulite. The inset shows the chemo- are pressure sensitive. There are two pseudo-divariant graphic relations of the mineral phases. The mineral abbreviations are after Holland & Powell (1998). (b) The effect of varying fluid and fields marked on either side of the pseudo-invariant bulk composition (XMg) on the pseudo-invariant, pseudo-univariant point. The plagioclase-absent field, with garnet±clino- and pseudo-divariant mineral assemblages is qualitatively shown. The pyroxene±amphibole (tremolite±pargasite)±quartz± arrows indicate the direction of shifting of univariant reactions and pseudo-invariant point (see text for details). vapour is stabilized on the higher-temperature± higher-pressure side of the pseudo- invariant point. This also marks the stability field of the mafic granu- PETROGENETIC GRID IN lites during peak metamorphism, M1 `marked M1(G) in Fig. 11a.' The (Di, V) pseudo-divariant field stabili- NCMASH SYSTEM zing garnet±plagioclase±amphibole±quartz assemblage Any petrogenetic grid that can explain the observed appears on the relatively lower-temperature±lower- reaction textures must involve a Ca-amphibole phase. pressure side of the pseudo-invariant point, defining Notwithstanding the uncertainties associated with a±X the range of P±T conditions relevant to M1 in garneti- relationships for multi-site solid solution amphiboles, ferous amphibolite [field termed M1(A) in Fig. 11a]. recent progress in deriving non-ideal mixing models The relative positions of these assemblage fields, and for this phase using the internally consistent thermo- the change from mineral assemblages with garnet alone dynamic dataset of Holland & Powell (1998) provides to garnet clinopyroxene assemblages at temperatures a useful basis for calculating phase diagrams in com- of 700±750C and pressures of 9 kbar is consistent plex amphibole-bearing systems (e.g. Na2O±CaO± with recent experimental results on MORB (Liu et al., FeO±MgO±Al2O3±SiO2±H2O, NCFMASH) and 1996; Vielzeuf & Schmidt, 2001).
408 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
Table 7: Different thermobarometric formulations and activity models followed in mafic granulite, amphibolite and metabasic dyke
Barometer Thermometer Mineral activity model Formulation
THERMOCALC Average P±T calculation method Grt Pl AX Program Hbl g THERMOCALC Cpx v.3.1 Opx
Conventional thermobarometers GAHS Grt (Anovitz & Essene, 1987) Log10K isopleth of Essene (1989) GADS Pl (Holland & Powell, 1992) GAFS Px (Holland & Powell, 1990) GAEN Grt±Cpx Krogh (1988) Grt±Opx Harley (1984); Lee & Ganguly (1988) Grt±Hbl Graham & Powell (1984); Ravana (2000) Ti-thermometer Ernst & Liu (1998)
Grt, garnet; Pl, plagioclase; Hbl, hornblende; Cpx, clinopyroxene; Opx, orthopyroxene.
The pseudo-invariant point of Fig. 11a is susceptible granulites. Path 1 shows the prograde path of rising P to variation in fluid and bulk-rock composition (e.g. and T in mafic granulites and marks entry into the Fe/Mg), as is shown qualitatively in Fig. 11b (see also granulite field from prograde hornblende1±plagi- Wells, 1979). Decreased X reduces the stability of oclase assemblage through the field of garnetiferous H2O 1 amphibole±garnet-bearing assemblages, forcing the amphibolite. The rising dP/dT of the prograde path is pseudo-invariant point to slide down-temperature also inferred from prograde compositional zoning of along the (Grt, V) equilibrium. Higher XFe bulk garnet and hornblende1. Figure 11a also predicts that compositions, on the other hand, enhances the stability the transition from garnet clinopyroxene quartz of garnetiferous assemblages and cause the pseudo- [M1(G)] to the hornblende3 plagioclase3 [M3(G)] invariant point to shift to higher temperatures along through clinopyroxene2 plagioclase2 Æ the same equilibrium, if there is no Fe±Mg partitioning hornblende2=FOL assemblages [M2(G)] can take place between diopside, tremolite and pargasite. However, during decompression accompanied by cooling, consis- in the case of non-equipartitioning of Fe±Mg, the tent with textural observations. Taken together, a pseudo-invariant point no longer moves along (Grt) clockwise P±T path has been inferred for the Rama- but along a NCFMASH equilibrium, the locus of kona±Katangi granulite domain of the Sausar Belt. which is controlled by the nature of Fe±Mg partition- ing between the amphibole and the clinopyroxene. The available compositional data indicate that the P^T CONDITIONS OF addition of Fe will enhance the stability of the tremo- METAMORPHISM lite-bearing assemblages compared with that of the Metamorphic conditions have been quantified using diopside-bearing ones. Considering that Fe±Mg parti- conventional geothermobarometry as well as the inte- tioning is more pronounced for tremolite±diopside grated thermodynamic approach using the updated and rather than tremolite-pargasite pairs, the NCFMASH expanded thermodynamic dataset of Holland & Powell line the point tracks out lies in the pseudo-divariant (1998). For the latter, the approach of average pressure± field (Tr, Grt) [line (a) in Fig. 11b]. This shifting of temperature calculation has been used. Calculations garnet-forming reactions to lower pressure in Fe-rich have been performed using THERMOCALC v.3.1. bulk compositions is also consistent with experimental Quantitative estimates for the different episodes of results (Green & Ringwood, 1967; Patin~o-Douce & metamorphism (M ±M ) are based on the following Beard, 1995). Nevertheless, there is no change in the 0 3 assumptions: general topology of the petrogenetic grid. This allows the grid to be used qualitatively to constrain the (1) a frozen-in chemical equilibrium during M0 geometry of the P±T path of evolution in the mafic appears to exist between garnet and coexisting
409 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003
Table 8: Independent equilibria used in THERMOCALC calculations
(a) Independent equilibria used for P±T estimates of the peak assemblage (M1) of mafic granulite (1) q cats an (2) 3an 3di py 2gr 3q (3) 3an 3hed 2gr alm 3q (4) 3fact 5py 10gr 15q 3tr 15hed 15an (5) 3tr 5py 10gr 3ab 3parg 18di 12an
(6) 39fact 105parg 360q 84tr 40gr 65alm 105ab 60H2O
(7) 21an 6tr 10py 11gr 27q 6H2O
(8) 21an 6fact 11gr 10alm 27q 6H2O (9) 18di 12an 3parg 3ab 5py 10gr 3tr
(b) Independent equilibria used for P±T estimates of the peak assemblage (M1) of garnetiferous amphibolite (1) 3tr 5alm 3fact 5py
(2) 6tr 21an 10py 11gr 27q 6H2O
(3) 6fact 21an 11gr 10alm 27q 6H2O (4) 3parg py 2gr 18q 3tr 6an 3ab
(c) Independent equilibria used for P±T estimates of the clinopyroxene2A±plagioclase2A±hornblende2A symplectite (M2A) of mafic granulite (1) py 2gr 3di 3cats (2) 2gr alm 3hed 3cats (3) hed 2cats sph gr ilm an (4) 5py 10gr 3fact 15hed 15cats 3tr
(5) 3hed 5cats 2tr 3sph 10di 3ilm 5an 2H2O
(d) Independent equilibria used for P±T estimates of the clinopyroxene2B ±plagioclase2B ±ilmenite2B symplectite (M2B ) of mafic granulite (1) 12di 3ts 2py 4gr 3tr (2) 3di alm 3hed py (3) 3hed 3ilm 3an gr alm 3sph (4) 60di 6fact 15ts 20gr 10alm 21tr
(5) 8hed 2tr 7ilm 5an 10di 5alm 7sph 2H2O
(e) Independent equilibria used for P±T estimates of ilm±pl±qtz symplectite (M2 ) of garnetiferous amphibolite (1) 3ts 2py 4gr 12q 3tr 12an
(2) 5ts 3gr 11q 3tr 12an 2H2O
(3) 6fact 21an 11gr 10alm 27q 6H2O
(4) 6fact 27ilm 21an 2gr 19alm 24sph 6H2O (5) 9ts 4py 8gr 6ab 3tr 6parg 24an
(f) Independent equilibria used for P±T estimates of Hbl±Pl symplectite (M3 ) in mafic granulite (1) 3ts 2py 4gr 12q 3tr 12an
(2) 6tr 21an 10py 11gr 27q 6H2O
(3) 6fact 21an 11gr 10alm 27q 6H2O (4) 3parg py 2gr 18q 3tr 6an 3ab (6) 9ts 4py 8gr 6ab 3tr 6parg 24an
(g) Independent equilibria used for P±T estimates of coronal garnet in mafic dyke (M3 ) (1) py 2gr 3q 3di 3an (2) 2py gr 3q 3an 3en (3) gr 2alm 3q 3an 3fs
Mineral abbreviations are after Holland & Powell (1998).
prograde hornblende1 in mafic granulite (sample in thermal conditions during prograde 72D/PS) as discussed in the mineral chemistry metamorphism. section. Applying garnet±hornblende ther- (2) For calculation of peak metamorphic conditions mometry, this allows calculation of the variation associated with M1, the combination of garnet
410 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
Table 9: THERMOCALC results for the various metamorphic episodes
Sample T (C) P (kbar) X SD(T) SD(P) Correl. Fit NR M H2O
. . . . . Mafic granulites 77C/SB 750 9 1 0 15 54 1 0 0 429 1 04 6 M1 . . . . . 45A/PS 690 8 1 0 25 47 1 0 0 524 0 16 5 M1 . . . . . 72D/PS 740 8 1 0 5 79 1 6 0 440 1 39 5 M2A . . . . . 72D/PS 730 7 0 0 25 86 1 7 0 574 1 36 5 M2B . . . . 77C/SB 675 6 8 Ð 81 1 2 0 642 0 09 M2B . . . . . 45A/PS 650 6 0 0 75 94 1 5 0 502 1 17 4 M3 . . . . . 45A/PS 680 6 1 0 75 81 1 2 0 569 0 95 4 M3 . . . . . Amphibolites 12A/PS 680 8 4 0 10 71 1 1 0 562 0 25 4 M1 . . . . . 19A/PS 700 6 2 0 25 105 1 1 0 452 1 34 5 M2 . . . . . 19A/PS 690 6 6 0 25 102 1 1 0 488 1 31 5 M2 . . . . MD2 13/PS 650 6 4 Ð 275 3 4 0 911 1 58 3 M3
XH O, fluid composition used; ±, reactions without H2O; SD(T), standard deviations on temperature; SD(P), standard devia- tion2 s on pressure; Correl., correlations between the uncertainties on P and T; Fit, a goodness of fit parameter describing the average of n equilibria (Powell & Holland, 1988); NR, the number of independent equilibria calculated by THERMOCALC; M, metamorphic episodes (see text).
rim±core of clinopyroxene (P)±matrix plagio- same sample is constrained at 800C. For other clase Æ ilmenite Æ rutile is chosen. samples of mafic granulite, garnet±clinopyroxene (3) For calculation of the conditions of retrograde thermometry and THERMOCALC give temperature metamorphism, M2 and M3, the combination of estimates that range from 750 to 690 C. The relatively re-equilibrated garnet edge and coexisting newly lower temperature estimate for sample 45A/PS is con- formed pyroxene- and hornblende-bearing corona sidered to reflect greater down-temperature Fe±Mg and symplectitic assemblages is used. exchange (Frost & Chacko, 1989). The phase diagram of Liu et al. (1996) and experimental results of Spear The activity models and different formulations used (1981) imply stability of garnet clinopyroxene for conventional geothermobarometry and THER- assemblages at temperatures in excess of 750C. We MOCALC approaches are presented in Table 7. The therefore infer peak metamorphic temperatures in the independent equilibria used for average pressure± mafic granulites as 750±800C. For garnetiferous temperature estimates of the different episodes of meta- amphibolites, application of the garnet±hornblende morphism and the results of P±T calculations are given thermometer (Graham & Powell, 1984; Ravana, in Tables 8 and 9 respectively. The P±T results of 2000) gives a temperature estimate of 660±680C, conventional geothermobarometry are provided in yielding an average of 675 C for M1 metamorphism Table 10. The P±T estimates for different meta- in amphibolites. morphic episodes are used to quantify the P±T trajec- In quartz-bearing domains, the application of the tory, earlier constrained qualitatively by reaction garnet±anorthite±hedenbergite±quartz (GAHS) and history and petrogenetic grid considerations. garnet±anorthite±diopside±quartz (GADS) baro- For mafic granulite sample, 72D/PS, garnet± meters (Essene, 1989) gives peak pressure estimates in hornblende thermometry using coexisting hornblende1 the range 8Á9±10Á3 kbar. Average pressures estimated and garnet (P) in a traverse from core to rim of the by using THERMOCALC for the same assemblages garnet yields a consistent rise in temperature from provide a relatively lower pressure range of 8Á1± 700C at the core to 800C at the rims. This 9Á1 kbar, with an average of 9 kbar. For quartz- progressive rise in temperature during prograde meta- absent domains, the pressure for peak metamorphism morphism M0 is also consistent with that retrieved cannot be directly calculated. Indirect estimates can, from the Ti in hornblende geothermometer (Ernst & however, be produced using the experimentally cali- Liu, 1998) (Table 10). Garnet±clinopyroxene thermo- brated location of garnet±clinopyroxene equilibria metry yields a temperature of 780C. Because for- (Vielzeuf & Schmidt, 2001) or plagioclase-out equili- mation of garnet (P) is equated with peak bria (Liu et al., 1996) in metabasites of appropriate metamorphism (M1), the peak temperature for the bulk composition. For rutile-bearing domains, this
411 Table 10: Results of conventional geothermobarometry
Sample Site position TGP TRK TK TEL PRef PHD PDI PFS TRef M
Mafic granulites JOURNAL
72D/PS Grt(289)±Hbl1(272) 680 660 Prograde
Grt(204)±Hbl1(198) 805 790 M1
Grt(204)±Cpx(P) (1±10) 750 M1 Grt(204)±Cpx(79)±(An 0Á90)± 780 760 11Á0 750 M 2A OF
Hbl2A(405)±Ilm(369) PETROLOGY Grt(396)±Cpx(63)±(An 0Á90)± 670 8Á6 9Á0 750 M2B
Hbl2A(405)±Ilm(369)
77C/SB Grt(1±29)±Cpx(1±4)±Pl(1±6) 755 10Á3 10Á3 750 M1
Grt(1±9)±Cpx(1±26)±Pl(10±28)±Qtz 720 6Á8 7Á0 M2B
45A/PS Grt(338)±Hbl1(337) 625 640 690 M0
Grt(203)±Cpx(404)±Pl1(445)±Hbl(70) 690 725 9Á4 8Á9 750 M1 VOLU
412 Grt(427)±Hbl3(430)±Pl3(428) 550 530 760 5 M3
Grt(427)±Hbl3(422)±Pl3(423) 565 560 775 5 M3 ME
HblFol(436) 800 M2A 44 Amphibolites 12A/PS Grt(281)±Hbl (280) 670 600 710 M 1 0 NUMBER
Grt(11A2-417)±Pl1(11A2-409)± 680 615 710 M1
Hbl1(280)±Qtz Grt(426)±Hbl(425)±Pl(427)±Qtz 525 430 650 M
2 3
19A/PS Grt(484)±Hbl1(483) 660 M0 MARCH Grt(451)±Hbl(449)±Pl(450) 610 580 750 M2
Grt(451)±Hbl(442)±Pl(450) 605 560 710 M2
HblFol(52) 790 M2 2003 HblFol(45) 710 M2
MD2
13/PS Grt(10)±Opx(17)±Pl(51)±Cpx(19)± 825 5Á7 4Á0 6Á4 750 M3 Hbl(24)
Abbreviations used for T ( C) and P (kbar) estimates: TGP, Graham & Powell (1984); TRK, Ravana (2000); TK, Krogh (1988); TEL, Ernst & Liu (1998); PRef/TRef, reference P and T; PHD, GAHS assemblage; PDI, GADS assemblage; PFS, GAFS assemblage; M, metamorphic episode (see text). BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
gives a range of minimum pressures of 10±12 kbar. are in the range of 670±740C. Experimental evidence Given that the plagioclase-out curve was intersected that the clinopyroxene±plagioclase assemblage is stabi- in quartz-absent domains and that the garnet in lized at a temperature above the wet basalt solidus (Liu these domains has the highest grossular content et al., 1996; Vielzeuf & Schmidt, 2001) requires a mini- (XGrs 0Á38), the peak pressure estimate is likely to mum temperature of 750 C for the formation of this exceed 9 kbar (Vielzeuf & Schmidt, 2001) and we ten- assemblage. THERMOCALC gives an average pres- tatively place it at 10 kbar. For amphibolites, the sure estimate of 6Á8 kbar for the quartz-bearing application of THERMOCALC for the garnet± domain, whereas for a quartz-absent domain, applica- hornblende1±matrix plagioclase±quartz assemblage tion of GAHS barometer gives a maximum pressure gives an average pressure estimate of 8Á4 kbar for M1 range of 8Á6±9Á0 kbar. Application of THERMO- metamorphism. CALC to the quartz-absent situation yields an average Summarizing, P±T conditions of 9±10 kbar and 750± pressure estimate of 7 kbar. We therefore infer 7 kbar 800C, and 8 kbar and 675C are taken as representa- and 750C as the P±T condition for the formation of tive of M1 metamorphism in mafic granulites and these symplectites during M2B stage. amphibolites respectively. The estimated peak pressure For amphibolites, the P±T estimates for ilmenite± of 9±10 kbar also places the mafic granulite in the realm plagioclase±quartz Æ hornblende-bearing symplecti- of high-pressure granulites (Green & Ringwood, 1967). tes show a range from 6Á2 to 6Á6 kbar and from 600C The P±T conditions of the formation of the type 1 to 700C. The lower temperature estimates, obtained symplectites in the mafic granulites are estimated using the garnet±hornblende thermometer, are mini- by using the assemblage clinopyroxene2A horn- mumestimatesandpossiblyrepresentblockingtempera- 2 blende2A plagioclase2A. Application of the garnet± tures of Fe ±Mg exchange for the garnet±hornblende clinopyroxene thermometer using garnet rims and pair. We therefore take 6Á4 kbar and 700C as P±T con- aluminous clinopyroxene2A gives a temperature ditions for M2 metamorphism in amphibolites. 780C. The Ti-thermometer applied to horn- In mafic granulites, the P±T estimates for blende2A grains provides a consistent temperature hornblende3A±plagioclase3A symplectites are deter- estimate of 780C. For the same assemblage mined using the composition of garnet edges and THERMOCALC gives an average temperature esti- coexisting hornblende3 and plagioclase3 symplectites. mate of 740C. On the basis of these results, we place Application of THERMOCALC yields an average the earliest symplectite formation at 775C. The P±T range of 6Á0 Æ 0Á1 kbar and 665 Æ 15C. These application of the GAHS barometer yields a pressure temperature estimates are consistent with that of 11 kbar. However, in the absence of free quartz, determined from the hornblende-in curve of Zhao the estimated values only indicate maximum pressure, et al. (2001), which predicts the change from clino- and the actual pressure of symplectite formation pyroxene plagioclase to hornblende plagioclase may be much lower. The average pressure calculated stability at temperatures of 700C. On the basis of with THERMOCALC, for a garnet±clinopyroxene± these constraints, we infer 6 kbar and 675C as P±T amphibole assemblage and considering absence of conditions for the M3 stage in mafic granulites. free quartz, is 8Á1 kbar at an X of In the garnetiferous metadolerite, the coronal garnet± H2O 0Á50. Therefore, we consider 8 kbar and 775C to orthopyroxene±clinopyroxene±plagioclase assemblage approximate the P±T conditions for the formation of yields a T 790C and 680C using the garnet± the clinopyroxene2A hornblende2A plagioclase2A orthopyroxene calibrations of Lee & Ganguly (1988) symplectites during M2A metamorphic episode. and Harley (1984) respectively (result not shown Using core compositions of folial hornblende in both in Table 10). Average temperature calculations give mafic granulites and amphibolites, application of similar or lower temperatures of 675C. Applying the the Ti-thermometer gives a temperature range of pyroxene thermometer of Lindsley (1983), the rein- 660±800C with a maximum of around 750C for the tegrated pyroxene yields temperatures in excess of pervasive S2 foliation-forming event. The lowest tem- 1000 C, which possibly reflect magmatic crystalliza- perature possibly reflects down-temperature resetting. tion. Pressure estimates for coronal garnet formation, The P±T conditions for the type 2 symplectites in obtained using garnet±anorthite±hedenbergite±quartz, the mafic granulites are constrained using the garnet garnet±anorthite±diopside±quartz and garnet± edge±clinopyroxene2B±hornblende2B±plagioclase2B Æ anorthite±ferrosilite±quartz barometers (Essene, 1989), ilmenite2B combination. For a quartz-bearing domain, are in the range 4Á0±6Á4 kbar, with an average of the application of the garnet±clinopyroxene thermo- 5Á5 kbar. THERMOCALC gives an average pressure meter gives a temperature of 720C, which is 45C of 6Á4 kbar. Considering the geological setting, P±T higher than that estimated by THERMOCALC. For conditions of 6 kbar and 750C are taken by the quartz-absent domains, the estimated temperatures authors for this phase of coronal garnet formation.
413 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003
uniform post-peak near-isothermal decompression history of the two types of rocks (Fig. 12). The granu- lites additionally document a qualitative prograde and post-decompressional isobaric cooling history. The post- decompressional cooling at 6 kbar is also recorded by the post-D2 metabasic dykes, MD2, producing garnet±clinopyroxene±quartz symplectites. Although garnet overprinting original igneous textures and mineral assemblages can also be explained by loading subsequent to emplacement at shallow crustal levels (Cox & Indares, 1999), this has been discounted for MD2 based on combined structural, metamorphic and experimental evidence: (1) established post-D2 meta- morphic reaction textures suggest either decompres- Fig. 12. Schematic P±T path of evolution of the metabasic rocks of the sion or heating [reaction (2c)] or cooling [reaction RKG domain of the Sausar Belt. A partial NCMASH petrogenetic grid (3a)] and therefore do not support burial as a possible combined with key metamorphic equilibria illustrates the different epi- cause of garnet formation in these rocks; (2) the persis- sodes of metamorphism and post-peak P±T paths for the mafic granu- lites and amphibolites.The reactions R1(A)^R3(A) mark formation of tent orthopyroxene±clinopyroxene±plagioclase-bear- ilmenite^anorthite^quartz Æ hornblende symplectites in amphibo- ing liquidus mineralogy in the absence of olivine also lites. Reactions R4(G) and R5(G)^R6(G) represent formation of implies original emplacement depth of these rocks in type 2 clinopyroxene^plagioclase Æ ilmenite symplectites in quartz- excess of 7Á5 kbar, as shown by experimentally derived bearing and quartz-absent rocks, respectively. All these reaction curves are plotted using the dataset of Holland & Powell (1998). Also shown liquidus phase relations (Kuehner, 1992). Based on are the positions of the wet basalt solidus and the garnet-in equilibria these evidences, we infer mid-crustal emplacement below and above the wet basalt solidus (after Vielzeuf & Schmidt, and subsequent cooling history of the metabasic 2001) for comparison. The hornblende-in curve representing the changeover from a clinopyroxene±plagioclase symplectite assem- dykes, MD2. blage to a hornblende±plagioclase symplectite assemblage is after Although the mafic granulites exhibit evidence for Zhao (2001). Both amphibolites and granulites show post-peak post-peak decompression followed by cooling at decompression P±T paths (e.g. Path 1). Anhydrous clinopyroxene2B ^ 6 kbar, it has been argued on the basis of textural plagioclase2B symplectites (M2B) can additionally be generated by heating (Path 2) (see text). and mineralogical assemblages and geothermobaro- metric results that polyphase decompression, as seen particularly in sample 72D/PS, is not continuous, TECTONIC EVOLUTION OF THE being separated by a distinct heating (prograde?) event (Path 2, Fig. 12). This raises a number of possi- SAUSAR MOBILE BELT bilities for the exhumation of these deep crustal rocks. The enclaves of garnetiferous metabasites occurring Three alternative models of crustal evolution are pro- extensively within host gneisses in the northern periph- posed here, each of which remain to be tested on the ery of the Sausar Mobile Belt demonstrate that basis of precise geochronology: (1) polycyclic with at high-pressure metamorphism is regionally widespread. least two unrelated events and intervening residence in Despite pervasive retrograde structural and meta- the middle crust; (2) single-cycle with collision and morphic overprinting, the mafic granulites and asso- extension; (3) modified single-cycle but with punctu- ciated garnetiferous amphibolites record shared ated compression±extension stages. prograde and retrograde metamorphic histories. This In the first model the prograde heating and cooling implies that the garnetiferous amphibolites do not P±T trajectories at mid-crustal levels (M2B±M3) represent retrogressed granulites. Geothermobaro- (Path 2, Fig. 13a) are attributed to a second granulite metric results indicate that the amphibolites and the metamorphic event, unrelated in time to the first granulites were equilibrated at different peak pressure high-P granulite event that was followed by decom- conditions. This implies that despite intense tectonic pression (Path 1, Fig. 13a). Evidence in favour of this interleaving during D2 and subsequent deformational type of polyphase history has been proposed from the events, the amphibolites and the granulites together Prydz Bay area in East Antarctica [Carson et al., 1995; appear to preserve a deep crustal section in the RKG Dirks & Hand, 1995; Dirks et al., 1995; Hensen et al., domain of the Sausar Belt. The combined petro- 1995; see also Fitzsimons (2000) for a review of the graphic, mineral chemistry, reaction history, petroge- present status of metamorphism in East Antarctica]. netic grid and geothermobarometric data reveal an Previously, Bhowmik et al. (1999) also interpreted the almost complete metamorphic history of this exposed mafic granulites of the Khawasa area (sample 77C/SB, reworked deep crustal section. These data indicate a this study) to be polycyclic. On the basis of differences
414 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
Fig. 13. Schematic P±T paths pertinent to the tectonothermal evolution of the RKG domain in the CITZ. (a) Model 1: granulites are poly- metamorphosed, bearing imprints of two unrelated tectonothermal events. This is now manifest by two overprinting P±T paths, where a mid-crustal heating-cooling P±T trajectory (Path 2) is superposed on an earlier granulite event (Path 1). (b) Model 2: Early prograde, peak and post-peak isothermal decompression and post-decompressional isobaric cooling segments can be joined by a continuous line. Such a clockwise P±T loop can be modelled in terms of extensional collapse of thickened orogen, during a single tectonothermal event. (c) Model 3: Schematic P±T path for multistage thrusting evolution. The P±T loop signifies that subsequent to the early continental subduction producing peak granulite metamorphism (M1), the granulites have undergone an initial steep decompression metamorphism (M2A) synchronous with D2 deformation. The M2A stage has produced coarse clinopyroxene2A ^hornblende2A ^plagioclase2A symplectite (SYM) and a pervasive folial hornblende. The granulites are juxtaposed at lower pressure (accompanied by slight burial) with a younger lithological association during a second continent^ continent collision event.The second prograde metamorphism (M2B), producing post-D2 anhydrous clinopyroxene2B ^plagioclase2B symplectite, synchronous with emplacement of basic dykes, and a subsequent IBC stage (M3), during which hornblende3 ^plagioclase3 symplectites are produced, markthe metamorphic response of the secondcollision event. in structure and peak metamorphic temperature, the granulite-facies conditions has been previously granulite-facies metamorphism was considered to be reported from a few select areas [e.g. Dabie Mountains older (Pre-Sausar), and unrelated in time to the meta- (Chen et al., 1998); Grenville Province (Indares, 1995); morphism of the Sausar Group of rocks. The overprint Moldanubian Zone, Austria (Cooke et al., 2000)] and of an amphibolite-facies event was taken by the authors has been attributed to relatively short residence times as the imprint of a younger Sausar event. under the peak granulite conditions coupled with a Discrimination between two unrelated metamorphic lack of intergranular fluid phase and a rapid exhuma- events hinges primarily on precise geochronological tion history. This is required because garnet homoge- evidence, which is not yet available in the Sausar nizes by volume diffusion within a temperature Belt. However, garnet in many of the studied granulites interval of 600±750C, depending on the duration of preserves growth zoning. Prograde garnet zoning in high-temperature conditions and the grain size (Spear,
415 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003
1991). On the basis of these arguments, we consider composite thrust sheet, preserve evidence that they that growth zoning is unlikely to be preserved for were equilibrated at different pressures before their granulites residing at mid-crustal depths for a longer tectonic juxtaposition. The metamorphic implication duration as implied by the polycyclic model. is that instead of a smooth line joining different calcu- The second model assumes that the P±T path is lated pressure±temperature domains, the P±T path in continuous with a clockwise sense, and that the differ- this case will have multiple inflections, incorporating ent metamorphic stages are related to a single tectono- thermal and baric adjustments of multiple thrust stack- metamorphic event. The clockwise P±T loop is usually ing (O'Brien & Carswell, 1993). explained by a geodynamic model involving orogenic The composite P±T path depicted in Fig. 13c crustal thickening and is inferred for several high-grade implies that a similar situation may exist in the RKG amphibolite- to granulite-facies terranes (e.g. Thompson domain of the Sausar Belt. Following the early high- & England, 1984). Such a simple assumption has been pressure metamorphism in response to continental widely adopted in many studies of granulite terranes subduction, the rocks have undergone extensive retro- and forms the basis of the model of extensional collapse gression. The retrogression was associated with a major of thickened orogens (Harley, 1989, 1992; Harley & nappe-forming stage due to northward underthrusting Fitzsimons, 1991). of the continental shelf sediments of the Sausar Group In the present study, there exists evidence in favour of rocks beneath the southward-advancing nappe pile of a prograde P±T segment with rising dP/dT and of the high-pressure rocks. The fluids necessary to cause post-peak steep decompression and cooling segments. the retrogression were possibly derived from the struc- This suggests that M0±M1±M2±M3 stages can directly turally underlying devolatilizing sedimentary pile and/ be joined by a smooth line (Fig. 13b). The anhydrous or by syntectonic granitoid intrusives. As successive clinopyroxene±plagioclase assemblage developing after multiple thrust slices pile up one above the other, dur- the retrograde hornblende may even imply minor ing this renewed continent±continent collision stage, heating during decompression (inflected portion), as the retrogressed granulites will undergo slight burial, has been recently proposed from the modern collisional to be followed by heating and decompression. The orogens such as the Himalayas (Ganguly et al., 2000; post-kinematic nature of second generation anhydrous de Sigoyer et al., 2000). Taken together this may clinopyroxene±plagioclase assemblages, formed after suggest that the extensional collapse model is also folial hornblende and garnet (P), implies a second applicable to the RKG domain in the SMB. However, thermal maximum, post-dating a pervasive deforma- the D2 deformation that produced a major thrust tion event. This can generally be modelled in terms of nappe is indicative of a broad compressional regime. P±T evolution in a collisional setting (England & Moreover, the metamorphic stages M2B and M3 imply Thompson, 1984). The slight thermal perturbation, a major mid-crustal static heating±cooling event, necessary for this renewed prograde event, may punctuated by basic dyke emplacement, which follows be caused by (1) radiogenic heat supplied by the a pervasive decompressional event (M2A). This structurally overlying unit over these metabasites requires that the extensional collapse model be mod- (now eroded out), (2) heat carried by the syntectonic ified. A modified model is presented below. felsic granitoids or (3) mantle-scale perturbation as This model requires that the exhumation of deep evidenced by multiple basic dyking events. Features crustal rocks is a multistage process involving repeated such as the preservation of garnet growth zoning, the thrusting events, and is analogous to that presented for interpreted relatively flat heating±cooling path and the European Variscides (Gillet et al., 1985; Mercier emplacement of syntectonic granitoid intrusives imply et al., 1991a, 1991b). In this model it is assumed that transient thermal pulses. Such short-lived thermal subsequent to the high-pressure event there is a major pulses are fairly common during low-pressure events underthrusting beneath the high-pressure unit. The and have been extensively documented elsewhere [e.g. consequence is that the crustal section closest to the Llano Uplift (Bebout & Carlson, 1986; Carlson & newly underlying unit will undergo rapid cooling. As Johnson, 1991; Letargo et al., 1995); the Variscide underthrusting progresses, it will introduce more and belt, O'Brien & Vrana, 1995]. more lower-grade rocks into the metamorphic pile. One important consequence of this model is the pre- These underthrust units will experience prograde sence of multiple stages of extension, alternating with metamorphism, possibly accompanying dehydration periods of compression. This is supported by repeated reactions. The `refrigerated' pile lying above, on the basic dyke emplacement, some of which are syn-D2 other hand, may suffer a retrograde event, possibly and others are post-D2. The Mesoproterozoic promoted by fluids derived from beneath. The net Grenville Orogen, one of the best-documented Pre- result is the formation of a composite thrust sheet. cambrian collisional belts, bears evidence of three dis- The allochthonous components, which constitute the tinct pulses of crustal shortening, which are separated
416 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT
by periods of extension (Rivers, 1997). Consistent with east±west to ENE±WSW alignment, paralleling the this multistage collision±extension model, the footwall orientation of the CITZ. Consequently, the calc-alka- Sausar Group of rocks to the south of the RKG domain line signatures of these rocks as well as their Palaeopro- exhibit a clockwise P±T history and peak metamorph- terozoic ages cannot be incorporated into tectonic ism at 7 kbar, 675C (Pal & Bhowmik, 1998; Bhowmik models of the amalgamation of the two crustal blocks. et al., 1999). Recent geochronological data show imprints of poly- phase tectono-magmatic histories of the BBG domain spanning from Late Archaean through Mesoprotero- Geodynamic significance of the P^T path zoic to Neoproterozoic time (Ramchandra & Roy, in the CITZ 2001). Roy et al. (2002) have also recently documented The metamorphic event documented herein clearly Palaeoproterozoic tranpressional tectonics from the represents a major compressional event such that the Mahakoshal belt, the latter constituting the northern northern edge of the Sausar Belt underwent deep bur- component of the CITZ. All of these recent results ial of a large portion of the continental margin during a imply that the CITZ is a collage of different litho- major continent±continent collisional orogeny. Pre- tectonic domains, many with polycyclic tectono-mag- vious tectonic models predicted plate convergence matic histories. The available evidence seems to sup- and a major, Palaeoproterozoic suture zone (compare port the notion that the assembly of the South Indian the Central Indian Suture Zone) along the southern and the North Indian blocks and intervening micro- periphery of the Sausar Group of rocks, along which continents(?) was a multistage process spanning from the South Indian and the North Indian Blocks were Palaeoproterozoic through Meso- to Neoproterozoic amalgamated (Yedekar et al., 1990; Jain et al., 1991, time. However, the nature of the geodynamic processes 1995; Mishra et al., 2000). The model was based on the at each stage of amalgamation is yet to be constrained. following arguments: (1) the presence of contrasting Recent seismic profiling and gravity modelling lithological association across the CIS, with high- across the SMB reveal the presence of a northerly grade metasediments and granulites towards the dipping reflector on the DSS profile and also of thicker north and low-grade volcanogenic sequences to the and high-density lower crust under the northern part south; (2) the interpretation that the granulites of the of the SMB (Mishra et al., 2000). These geophysical BBG domain represent exhumed oceanic crust of the signatures coincide with the high-pressure RKG North Indian Block that was subducted below the domain documented here, and in combination with South Indian Block during a Palaeoproterozoic conti- metamorphic imprints suggest a possibly northerly nent±continent collision; (3) the calc-alkaline subduction of a southern block in the Sausar Belt. On geochemical signatures of the Dongargarh grani- the basis of the available geological and geochrono- toids, and Dongargarh and Sakoli volcanics, reflecting logical data, we speculate that the high-pressure subduction-related magmatism as a sequel to this metamorphism in the RKG domain is of Grenvillian suturing event. age and urge that this contention be tested by reliable Recent studies, however, indicate a more complex geochronology. The clockwise P±T trajectory in the evolutionary history of the domains across the CIS and mafic granulites possibly marks the final amalgamation contradict the southerly subduction model. In contrast in the Central Indian Tectonic Zone, producing the to the simple high±low grade contrast across the CIS, Indian subcontinent. the high-grade rocks (compare the granulites of the BBG domain) occur as tectonic slices bounded on both sides by low-grade rocks (Bhowmik & Pal, ACKNOWLEDGEMENTS 2000). In addition, the mafic±ultramafic components We thank S. L. Harley and G. Zhao for their helpful of the BBG domain are predominantly concordant and constructive comments on an earlier version of this gabbroic to pyroxenitic intrusive bodies within the paper. The manuscript has benefited greatly from granulite ensemble (Bhowmik & Pal, 2000), and can- stimulating discussions with S. Dasgupta, P. Sengupta, not be taken as evidence for subducted oceanic crust. P. K. Bhattacharyya, H. C. Dasgupta and M. Raith. Moreover, the Dongargarh granitoids and Dongar- S.K.B. acknowledges the DST and the ISIRD research garh and Sakoli volcanics of Palaeoproterozoic age grant of the Indian Institute of Technology, Kharag- (Sarkar et al., 1981) have a distinct north±south struc- pur, for partial funding of the work. A part of the tural orientation, paralleling the structural grain of the research was carried out with financial support by the SIB, but discordant to the east±west- to ENE±WSW- Geological Survey of India (Research Project RP/CR/ trending CITZ. Had these rocks been produced during MP/MAH/1997/002) for which we thank T. Pal for north±south compressional orogeny, stitching the two help during field work, and N. C. Pant and S. Shome cratonic blocks, the rocks would have a broad for help during microprobe analysis and backscatter
417 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003
electron image photography. The final version of Dasgupta, S., Sengupta, P., Ehl, J., Raith, M. & Bardhan, S. (1995). the manuscript was prepared at the Mineralogisch± Reaction textures in a suite of spinel granulites from the Eastern Petrologisches Institut, University of Bonn, for which Ghats Belt, India: evidence for polymetamorphism, a partial pet- S.K.B. acknowledges a post-doctoral research fellow- rogenetic grid in the system KFMASH and the roles of ZnO and Fe2O3. Journal of Petrology 36, 435±461. ship by DAAD. We also acknowledge the help ren- Dempster, T. J. (1985). Garnet zoning and metamorphism of the dered by N. Pal and A. Basu Sarbadhikari with data Barrovian type area, Scotland. Contributions to Mineralogy and processing. Finally we thank R. Powell and T. Holland Petrology 89, 30±38. for providing access to THERMOCALC v. 3.1. de Sigoyer, J., Chavagnac, V., Blicher-Toft, J., Villa, I., Luais, B., Guillot, S., Cosca, M. & Mascle, G. (2000). Dating the Indian continental subduction and collision thickening in the northwest Himalaya: multichronology of the Tso Morari eclogites. Geology REFERENCES 28, 487±490. Acharyya, S. K. & Roy, A. (2000). Tectonothermal history of the Dirks, P. H. G. M. & Hand, M. (1995). Clarifying temperature± Central Indian Tectonic Zone and reactivation of major faults/ pressure paths via structures in granulite from the Bolingen shear zones. Journal of the Geological Society of India 55, 239±256. Islands. Australian Journal of Earth Sciences 42, 157±172. Anovitz, L. M. & Essene, E. J. (1987). Compatibility of geobarom- Dirks, P. H. G. M. & Sithole, T. A. (1999). Eclogites in the Makuti
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