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Journal of Petrology | Volume 44 | Number 3 | Pages 387-420 | 2003
© Oxford University Press 2003
Garnetiferous Metabasites from the Sausar Mobile Belt: Petrology, P–T Path and Implications for the Tectonothermal Evolution of the Central Indian Tectonic Zone
REGIONAL PETROLOGY LABORATORY, GEOLOGICAL SURVEY OF INDIA, NAGPUR-440 006, INDIA
Present address: Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 302, India. E-mail: Santanu{at}gg.iitkgp.ernet.in
RECEIVED JUNE 20, 2001; ACCEPTED AUGUST 29, 2002
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ABSTRACT |
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A suite of garnetiferous amphibolites and mafic granulites occur as small boudins within layered felsic migmatite gneiss in the northern part of the Sausar Mobile Belt (SMB), the latter constituting the southern component of the Proterozoic Central Indian Tectonic Zone (CITZ). Although the two types of metabasites are in various stages of retrogression, textural, compositional and phase equilibria studies attest to four distinct metamorphic episodes. The early prograde stage (Mo) is represented by an inclusion assemblage of hornblende1 + ilmenite1 + plagioclase1 ± quartz and growth zoning preserved in garnet. The peak assemblage (M1) consists of porphyroblastic garnet + clinopyroxene ± quartz ± rutile ± hornblende in mafic granulites and garnet + quartz + hornblende in amphibolites and stabilized at pressure–temperature conditions of 9–10 kbar and 750–800°C 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 former resulted in the development of early clinopyroxene2A–hornblende2A–plagioclase2A symplectites at 8 kbar and 775°C (M2A stage), synchronous with D2 and later anhydrous clinopyroxene2B–plagioclase2B–ilmenite2B symplectites and coronal assemblages at 7 kbar, 750°C (M2B stage) and post-dating D2. In amphibolites, ilmenite + plagioclase + quartz ± hornblende symplectites appeared during M2 at 6·4 kbar and 700°C. During M3, coronal garnet + clinopyroxene + quartz ± hornblende-bearing symplectites in metabasic dykes and hornblende3–plagioclase3 symplectites embaying garnet in mafic granulites were formed. P–T estimates show near-isobaric cooling from 7 kbar and 750°C to 6 kbar and 650°C during M3. It is argued that the decompression in the mafic granulites is not continuous, being punctuated by a distinct heating (prograde?) event. The latter is also coincident with a period of extension, marked by mafic dyke emplacement. The combined P–T path of evolution has a clockwise sense and provides evidence for a major phase of early continental subduction in parts of the CITZ. This was followed by a later continent–continent collision event during which granulites of the first phase became tectonically interleaved with younger lithological units. This tectonothermal event, of possibly Grenvillian age, marks the final amalgamation of the North and the South Indian Blocks along the CITZ to produce the Indian subcontinent.
KEY WORDS: Central Indian Tectonic Zone; clockwise P–T path; continental collision; metabasite
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INTRODUCTION |
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In recent reconstructions of East Gondwanaland, the Central Indian Tectonic Zone (CITZ; Radhakrishna, 1989
) has been recognized as an important Proterozoic collisional zone (Yedekar et al., 1990; Jain et al., 1991; Mishra et al., 2000). Existing models predict amalgamation of the South Indian Block [SIB; terminology after Eriksson et al. (1999); comprising the Singhbhum, Bastar and Dharwar Provinces] and the North Indian Block (NIB; comprising the Aravalli–Bundelkhand Provinces) during the Palaeoproterozoic by southerly subduction of the NIB below the SIB along the Central Indian Suture (CIS) to form the Indian subcontinent (Yedekar et al., 1990; Jain et al., 1991; Eriksson et al., 1999; Mishra et al., 2000). The timing and the location of collision have been modelled only on the basis of limited geological and geophysical data from the mobile belt (Yedekar et al., 1990; Jain et al., 1991; Mishra et al., 2000). Detailed metamorphic characterization of the rocks of the major lithotectonic units of the CITZ, without which any tectonic model for the mobile belt remains incomplete, has not been done. Recent studies have revealed the composite character of the CITZ, comprising three distinct supracrustal belts, Mahakoshal, Betul and Sausar (Fig. 1), which have a tectono-magmatic history, from the Palaeoproterozoic to the Neoproterozoic (Acharyya & Roy, 2000). This implies a more complex evolutionary history for the CITZ, compared with existing monocyclic models. Nevertheless, the CITZ, with its transcontinental status (Harris, 1993), has obvious global significance in understanding the geodynamic processes of crustal assembly in major parts of East Gondwanaland.
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This study focuses on a suite of regionally distributed mafic granulites and associated garnetiferous amphibolites from the northern periphery of the Sausar Mobile Belt (SMB), the latter constituting the southernmost Meso- to Neoproterozoic component of the CITZ (Acharyya & Roy, 2000
). The metabasites occur as small boudins and tectonic lenses within intensely 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, garnetiferous amphibolites and associated metabasic dykes to constrain the metamorphic history of the southern segment 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 constraints on the models for lower crustal geodynamic processes in the SMB but also provide insights into multistage exhumation histories of deep crustal rocks.
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GEOLOGICAL SETTING |
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The CITZ encompasses the terrain between the Son–Narmada North Fault (SNNF) and the Central Indian Shear Zone (Acharyya & Roy, 2000
) (Fig. 1). The SMB is a collage of at least three lithotectonic components (Bhowmik & Pal, 2000): (1) the southern Bhandara–Balaghat granulite (BBG) domain; (2) the central domain of the Sausar Group (SG) of rocks; (3) the northern Ramakona–Katangi granulite (RKG) domain (Fig. 2). These domains differ in lithological assemblage, mineralogy and metamorphic history.
The BBG domain is bounded to the north by the low-grade Sausar Group of rocks and to the south by the cratonic domain of felsic gneisses of the Amgaon gneissic complex. The BBG domain is lensoidal in shape, tapering towards both east and west (Fig. 2). The granulite suite includes cordierite gneiss (Crd Gn), iron formation granulite, quartzite, charnockitic gneiss, two-pyroxene granulite, pyroxenite and enderbitic granulite; they occur as pods and lenses within intensely tectonized felsic gneiss, which is predominantly mylonitic, with strong development of down-dip stretching lineations. The granulites of the BBG domain record an early high-temperature granulite metamorphism (T
900°C) and strong retrograde metamorphism dominated by both cooling and decompression textures (Bhowmik & Pal, 2000; Ramchandra & Roy, 2001).
The SG occupies the central domain and is an intensely deformed and metamorphosed pelite– arenite–carbonate–Mn-oxide ore assemblage, which represents a stable platformal sequence. This is associated with the Tirodi biotite gneiss (TBG) and granitoids (Narayanaswami et al., 1963). The TBG constitutes the basement to the SG (Narayanaswami et al., 1963; Bhowmik et al., 1999). Basement–cover relations are largely tectonized, except in the eastern part of the belt where a polymictic conglomerate overlying a porphyritic granitoid is still recognizable (Pal & Bhowmik, 1998). The SG records broadly a Barrovian type of regional metamorphism with metamorphic grade consistently increasing from south to north and also towards the west (Narayanaswami et al., 1963; Pal & Bhowmik, 1998). A low-grade greenschist- to greenschist–amphibolite facies transition zone has been recognized along the southern part of the Sausar supracrustals, in close proximity to the granulites of the BBG domain; an upper amphibolite- to upper amphibolite–granulite-facies transition zone has been recorded in the northern and northwestern margin of the central domain and also from the RKG domain.
In the RKG domain, along the northern periphery of the SMB, a distinct lithological assemblage, including mafic granulite, cordierite granulite and porphyritic charnockite, occurs as rafts and lenses within layered tectonites of the composite gneiss and tonalitic gneiss components of the TBG (Bhowmik et al., 1999; Bhowmik & Pal, 2000). The composite character of the layered tectonites is exemplified by thin, extremely continuous, subparallel layers, centimetres to metres in thickness, of alternating amphibolitic and quartzo-feldspathic material. Both the composite and tonalite gneiss components show stromatic migmatite banding and are collectively referred to as the felsic migmatite gneiss (FMG; Bhowmik et al., 1999; Bhowmik & Pal, 2000). The high-grade rocks can be traced for >240 km from Ramakona in the west through Khawasa–Katangi in the centre to the east of Bichhiya (location 6, Fig. 2). In places, the boundary between the granulites and the gneisses is lined with sheet-like bodies of a younger phase of granitoids. The largest exposure of these rocks is located to the NE of Katangi (Fig. 2). Locally, the granulites also occur in close spatial association with the Sausar supracrustals or are directly interlayered with the latter. The contacts between the lithological associations are often marked by prominent ductile shear zones.
Geochronological data from the SMB are meagre. Sarkar et al. (1986) reported a Rb/Sr whole-rock isochron age of 1525 ± 70 Ma and a mineral isochron age of 860 Ma from the Tirodi gneiss. According to these workers, the 1525 Ma event marks the main phase of regional amphibolite-facies metamorphism of the Sausar Group, leading to the partial melting of the basal psammopelitic unit. The younger 860 Ma event is interpreted as a terminal thermal overprint on the Sausar rocks. Recently, Lippolt & Hautmann (1994) reported a 40Ar/39Ar age of 950 Ma from cryptomelane from the Sitapar mines in the Ramakona area (location 1 in Fig. 2) and interpreted this age as the time of cooling through the cryptomelane closure temperature, immediately following the amphibolite-facies metamorphism of the Sausar Group of rocks. Tourmaline-bearing granites from the Nainpur–Lalbara area in the RKG belt, apparently intrusive into Sausar supracrustals, have yielded a Rb–Sr whole-rock isochron age of 1147 ± 16 Ma (Pandey et al., 1998). Collating available metamorphic, structural and geochronological data, Bhowmik et al. (1999) considered the main phase of deformation (compare with Sausar orogeny) that affected the Sausar Mobile Belt to be an 1000 Ma event. The 1525 ± 70 Ma whole-rock isochron age from the basement TBG was reinterpreted as evidence of a Pre-Sausar Mesoproterozoic tectonothermal event. Recent Sm–Nd and Rb–Sr ages for charnockitic gneisses and two-pyroxene granulites from the BBG domain show three distinct clusters at 2672 ± 54, 1416 ± 59 to 1386 ± 28 Ma and 973 ± 63 to 800 ± 16 Ma (Ramchandra & Roy, 2001). These ages are correlated with two phases of temporally separate Archaean and Mesoproterozoic granulite metamorphism and the final overprint of the Sausar orogeny (Ramchandra & Roy, 2001).
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 preserved as relicts in small boudins and the hinges of isoclinal folds. D2, the strongest deformation in this belt, produced recumbent to gently inclined folds with a strong axial planar layered tectonite (LS) fabric (S2). S2 is visibly derived by the transposition and attenuation of a pre-existing S1 foliation. In this deformational event, rocks of the RKG domain appear to have been emplaced as large allochthonous nappes thrust over the SG in the central domain, leading to tectonic imbrication of the basement mafic granulite–cordierite granulite–felsic migmatite gneiss components with the Sausar supracrustals. D2 is also coincident with abundant migmatization in the felsic migmatite gneiss (FMG), emplacement of basic dykes and voluminous sheet-like intrusions of a suite of tonalites–porphyritic granodiorites and potassic granites. Basic dykes emplaced during D2 are termed MD1. Both these dykes and the granitoids are deformed and have a planar S2 fabric. There is a second generation of basic dykes which are relatively massive, have a well-preserved intergranular texture and appear to have been emplaced late to post-kinematic with respect to D2. These are termed MD2. Subsequent deformations, D3 and D4 produced macro- as well as meso-scale, shallow-plunging upright folds (F3) and folds of sinistral asymmetry (F4) with WNW–SSE to NW–SE axial traces respectively.
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Successive phases of overprinting deformation and attendant metamorphism have ultimately imparted an amphibolite-facies fabric in the host gneisses and largely erased the older histories of the terrane. The imprints of these polyphase metamorphic events have, however, been recorded in the mafic granulites and associated garnetiferous amphibolites in the RKG belt. These occur as scattered small boudins or as concordant bodies of various sizes within the FMG and can be traced from Khawasa in the west to Katangi in the east (Fig. 2). In many cases, the contact of the metabasics with the FMG is lined with conformable bands of deformed porphyritic granodiorite–potassic granite. The largest exposure of these bodies is located in the Katangi area. In most cases the mafic granulites and garnetiferous amphibolites are spatially separated from each other. In the majority of the occurrences, the rocks are medium to coarse grained, dark greenish to black in appearance and have an amphibole-rich S2 fabric, which becomes pervasive at the edges of the body. However, in the central part of these bodies, large ovoid garnet porphyroblasts (6·4–12 mm in diameter) are scattered throughout the matrix, giving a spotty appearance to the rock.
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PETROGRAPHY |
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Mafic granulites have been sampled from Khawasa (location 2, Fig. 2), Pindrai (location 4) and Katangi (location 5) areas; garnetiferous amphibolites and metabasic dykes2 are from the Katangi area only. In total, 15 samples of mafic granulite, 10 samples of garnetiferous amphibolite and four samples of metabasic dykes2 have been studied in detail. Textural and mineral chemical data are reported for three representative samples of mafic granulite (sample 77C/SB, Khawasa; 72D/PS, Pindrai; 45A/PS, Katangi), two representative samples of garnetiferous amphibolite (samples 19A/PS and 12A/PS) and from one metabasic dyke2 (sample 13/PS).
On the basis of microstructures and reaction relations between mineral phases, four distinct metamorphic episodes, reflecting changing P–T conditions, can be determined from the mafic granulites, garnetiferous amphibolites and metabasic dykes. These are: prograde (M0), peak (M1), post-peak decompression (M2) and post-decompressional cooling (M3).
M0 episode
Mafic granulite and amphibolite
In the mafic granulite and amphibolite, the prograde history is preserved only as well-defined inclusion trails of prograde hornblende (hornblende1), ilmenite (ilmenite1), quartz and plagioclase (plagioclase1) within the porphroblastic garnet [Garnet (P)] and/or porphyroblastic clinopyroxene [Clinopyroxene (P)] (Fig. 3a and b). These textural features suggest that the granulites and garnetiferous amphibolites had a prograde history, prior to the peak metamorphism.
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M1 episode
Mafic granulite and amphibolite
In mafic granulite, this is represented by growth of porphyroblastic phases, namely garnet (P) and clinopyroxene (P), together with quartz. The minerals commonly display granoblastic mosaic textures. The diagnostic mineral assemblage is garnet + clinopyroxene + plagioclase + quartz ± rutile ± hornblende. Rarely quartz may be absent from the rock (e.g. sample 72D/PS). Rutile when present is preferentially enclosed in the periphery of the garnet but absent from the matrix. The peak assemblage is commonly preserved in the core of large mafic granulite boudins where alternate garnet + clinopyroxene- and plagioclase + quartz-bearing layers define an early granulite-facies gneissic foliation, S1. The latter is anastomosed by pervasive matrix hornblende parallel to the S2 plane (henceforth referred to as folial hornblende, hornblendeFOL).
In amphibolite, the peak assemblage is represented by porphyroblastic garnet, plagioclase and coarse hornblendeFOL.
M2 episode
Mafic granulite
Clinopyroxene + plagioclase ± hornblende ± 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–plagioclase2B–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 preferential alignment. These may occur either as spectacular radial symplectites around garnet against clinopyroxene (P) (Fig. 3a) or as delicate worm-like intergrowths in the matrix (Fig. 4c). In the latter, type 2 symplectites overgrow hornblendeFOL (Fig. 4d). Clinopyroxene2B often occurs as coronas around hornblende2A 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 grains against quartz. Plagioclase–quartz intergrowths strictly occur along fine fractures in garnet. The matrix foliation is composed of very coarse aggregates of hornblendeFOL. In the majority of the cases, the hornblendeFOL, closest to the symplectite domain partially pseudomorphs clinopyroxene (P). The relict of pyroxene can often be traced as discontinuous blebs within the hornblendeFOL. Further away, within the central folial domain, there is no trace of early clinopyroxene, but folial hornblende in places contains exsolution blebs of plagioclase. In certain outcrops, the retrogression is so extensive that clinopyroxene (P) is preserved only within garnet with its total exclusion from the matrix. In others, folial hornblende contains discontinuous layers rich in clinopyroxene (P). Clinopyroxene in these layers often occurs as porphyroclasts, being armoured by finer recrystallized aggregates. Textural features such as these indicate that (1) part of the folial hornblende was produced at the expense of M1 clinopyroxene during a post-peak transposition event (D2), (2) type 1 symplectites pre-dated type 2, being broadly coeval with folial hornblende 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).
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Amphibolite
M2 in amphibolite is represented by the formation of spec-tacular ilmenite–plagioclase–quartz ± hornblende- bearing symplectites. Ilmenite–plagioclase–quartz assemblages generally develop around garnet against hornblendeFOL (Fig. 5a). Ilmenite–quartz–hornblende and plagioclase–quartz symplectites locally occur within the interior of garnet (P) (Fig. 5a and b). The symplectites 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 symplectites 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 symplectite. In the present case, the reactions could have initiated along fractures and progressively propagated inside through catalysing aqueous solutions, as implied by the stability of minor hornblende in the symplectite.
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M3 episode
Mafic granulite
Early hornblende–plagioclase symplectites and coronas (referred to as hornblende3–plagioclase3) and a later zoisite–epidote–muscovite assemblage mark the M3 episode in the mafic granulites. Hornblende3–plagioclase3 symplectites are restricted to haloes around garnet (P) (Fig. 6a) and are generally mutually exclusive with pyroxene–plagioclase symplectites. However, in places, the two kinds of coronas may mutually coexist, being part of a thick double corona around garnet (P). The relatively finer hornblende3–plagioclase3 symplectites are located in the inner shell, being armoured by an outer, coarser, type 2 clinopyroxene–plagioclase symplectite (Fig. 6b). The inner symplectite often encloses relict clinopyroxene. These textures clearly show that the hornblende3–plagioclase3 symplectites post-date the clinopyroxene + plagioclase symplectites; also supported by distinct differences in their P–T conditions of formation (see subsequent discussion). The present study resembles that of Zhao et al. (2001)
on the Hengshan Complex, North China craton. In that study, the hornblende–plagioclase symplectites were given an independent metamorphic status separate from that of clinopyroxene + plagioclase symplectites. In certain outcrops, the hornblende–plagioclase symplectites display a hexagonal outline, marking the approximate location of the original surface of a euhedral garnet crystal (Fig. 6a). The broadly euhedral outline of the symplectites also implies their formation in a static state and post-dating D2.
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In the terminal stages of this episode, fine needle-shaped intergrowths of zoisite–epidote and muscovite replaced plagioclase and clinopyroxene of all textural types.
Mafic dyke, MD2
Mafic dykes MD2 record imprints of M3 reaction textures. This is represented by the development of coronal garnet–clinopyroxene–quartz symplectites and exsolution lamellae of orthopyroxene in clinopyroxene and vice versa and hornblende coronas around garnet. The studied sample (13/PS) is relatively fine grained, with a spectacular intergranular texture, consisting of orthopyroxene and clinopyroxene megacrysts and laths of plagioclase (Fig. 7a) and can be classified as a metadolerite. Garnet developed preferentially at the contacts of plagioclase against orthopyroxene and clinopyroxene. Generally pyroxene exsolution lamellae are concentrated mostly within the core region of the pyroxene megacrysts, and nearly absent at their periphery (Fig. 7b). This may be attributed to granular exsolution during cooling.
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MINERAL CHEMISTRY |
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The chemical compositions of the coexisting mineral phases in samples of mafic granulites, amphibolites and a metadolerite were determined using a CAMECA SX-51 electron microprobe at the laboratory of the Geological Survey of India at Faridabad. The operating conditions were set as 1 µm beam diameter, 15 kV accelerating voltage and 12 nA specimen current. The PAP correction scheme was used. Natural and synthetic minerals were used as standards. For amphibole, Fe2+ and Fe3+ contents were estimated according to the recalculation scheme of Leake et al. (1997)
. For other minerals, charge balance criteria were employed. Repre- sentative mineral compositions are given in Tables 2–6.
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Garnet
Mafic granulite and amphibolite
Representative garnet analyses are given in Table 2. Garnets in granulites and amphibolites are dominantly almandine rich (42–65%), with lesser amounts of the grossular (20–38%), pyrope (7–10%), spessartine (2–12%) and andradite (2–7%) components. Three types of compositional variations are noted: (1) internal growth zoning, with consistent variation in garnet composition from core to rim; (2) change in the composition of garnet rims against symplectites; (3) variations in the chemistry of garnet core and rim between granulites and amphibolites.
Internal compositional zoning in garnet is preserved in the mafic granulites and is illustrated in Fig. 8. Compositions typically show core to rim decreases in the spessartine and increases in the grossular component. Pyrope remains either relatively constant (sample, 72D/PS, Fig. 8a) or shows slight rimward enrichment (sample 45A/PS, Fig. 8b and c). Of the two profiles, A–B and C–D in Fig. 8b and c, rimward MgO enrichment is more conspicuous in profile C–D. The bell-shaped MnO profile has commonly been described and interpreted as an original feature of crystal growth (see Tracy, 1982; Dempster, 1985; Chakraborty & Ganguly, 1990) and is consistent with thermodynamic models for compositional evolution during prograde growth (e.g. Loomis, 1986; Spear, 1988). The spikes and irregularities in MgO, MnO and CaO in the profile in Fig. 8a can probably be attributed to retrograde re-equilibration at the contact with prograde assemblages, dominantly hornblende1. Compared with the coarser garnet (1·2 cm diameter) in Fig. 8a, the growth zoning of the relatively smaller garnet (7·5 mm diameter) in Fig. 8b and c is relatively suppressed. This may be attributed to partial homogenization during prograde metamorphism in the early dynamothermal event, the effect being more pronounced in smaller crystals (e.g. Llano uplift, Carlson & Schwarze, 1997) and also in the garnet in the amphibolite. In the latter, there is no variation in composition from core to rim. Accordingly, the garnet rim and garnet core compositions were selected for calculation of peak P–T conditions of metamorphism in the granulites and amphibolites, respectively.
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The outermost garnet rims (referred to as garnet edges), 5 to 50–150 µm wide, show sharp depletions in grossular and pyrope, but enrichments in spessartine and almandine contents (e.g. analysis 3A2-427 in Table 2), suggesting the effect of late static resorption. Marked grossular depletion and enrichment of almandine and spessartine components can also be locally observed inside garnet (P) and adjacent to fine plagioclase–quartz intergrowths (analysis 3A1-194, Fig. 8c). MgO remains unaffected. This indicates partial internal resorption of garnet that has been superimposed on the pattern of growth zoning. Such resetting of the composition of the garnet is also common in the amphibolites and can be attributed to operation of net transfer reactions and/or diffusion during post-peak decompression and cooling. Garnet edges also show pronounced variation in composition in contact with clinopyroxene + plagioclase, hornblende–plagioclase, ilmenite–plagioclase–quartz symplectites and coronas. Garnet in contact with pyroxene–bearing symplectites is distinctly higher in grossular and lower in almandine content (analysis 2A1-396, Table 2) than that against hornblende–plagioclase and ilmenite–plagioclase–quartz symplectites (analyses 3A2-427 and 6A2-451). These compositional variations are likely to reflect re-equilibration of garnet under different conditions (Zhao et al., 2001
).
Systematic compositional heterogeneity exists in the peak garnet composition between the mafic granulites and the amphibolites. Garnets from the mafic granulites have relatively higher grossular and lower almandine and pyrope contents than those of the amphibolites. In the former, the grossular component varies in the range 30–38 mol %. In contrast, the grossular component in the amphibolites is <29%. Such a variation in garnet composition is responsible for growth of diverse but systematic garnet breakdown mineral assemblages in the granulites and amphibolites. A highly calcic clinopyroxene–plagioclase mineral assemblage is conspicuous in the mafic granulites and developed from high grossular garnets (grossular 38 mol %). In contrast, ilmenite-bearing mineral assemblages stabilized from relatively almandine-rich, grossular-poor garnets in amphibolites. Hornblende–plagioclase symplectites formed from intermediate garnet compositions (grossular 30 mol %) in mafic granulites.
Metadolerite (MD2)
Coronal garnet is enriched in almandine (62 mol %), moderately enriched in pyrope (15 mol %) and grossular (14 mol %) with minor spessartine (3 mol %) and andradite (6 mol %) component (Table 2).
Hornblende
Mafic granulites and amphibolites
Representative chemical analyses of hornblende are presented in Table 3. Depending on textural type, the hornblende analyses are subdivided into the following varieties: (1) prograde hornblende (hornblende1); (2) folial hornblende; (3) symplectitic hornblende associated with clinopyroxene and plagioclase; (4) symplectitic hornblende associated with symplectitic plagioclase; (5) symplectitic hornblende associated with symplectitic ilmenite–plagioclase–quartz.
According to the nomenclature of Leake et al. (1997), prograde hornblendes in both granulites and amphibolites are ferropargasite to pargasite in composition (Fig. 9a). However, there is systematic increase in the ferropargasite component in hornblende1 from amphibolite to mafic granulite. Like garnet, hornblende1 in the mafic granulites also shows systematic variation in its composition in relation to its position within the host garnet; it shows progressive rimward increase in (Na + K)A, Ti and Al(IV) (Fig. 9b). This compositional variation in hbl1 is in consonance with the growth zoning in the enclosing garnet. Progressive increase in edenite and Ti-tschermakite substitution in hornblende is generally attributed to rising temperature of equilibration (Spear, 1981; Ernst & Liu, 1998).
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Compared with hornblende1, folial hornblende is distinctly less aluminous, and is more variable in composition ranging from ferropargasite through ferroedenite to edenite in granulites and amphibolites. There is consistent core to rim depletion in Ti, Al(IV) and (Na + K)A contents (Fig. 9c and d).
Symplectitic hornblende in type-1 symplectites in the mafic granulites is compositionally similar to associated hornblendeFOL. Symplectitic hornblende associated with symplectitic plagioclase is ferroedenite to ferropargasite. Cores of hornblende3 are enriched in Ti, Al(IV) and (Na + K)A similar to cores of large hornblendeFOL (Fig. 9c and d). All these parameters are depleted in the rim of coarser grains and in smaller symplectite grains. Symplectitic hornblende associated with symplectitic ilmenite–plagioclase–quartz in amphibolites varies from edenite to ferropargasite. The general rimward depletion in Ti, Al(IV) and (Na + K)A contents in hornblende of most textural types suggests reversal of edenite [(NaA)-1 (AlIV)-1 = ( ) (SiIV)] and Ti-tschermakite [(TiVI)-1 (AlIV)-2 = (Mg) (SiIV)2] substitution, in response to down-temperature equilibration.
Metadolerite
Coronal hornblende is edenitic (XMg = 0·62) and titaniferous (1·80 wt % TiO2) (Table 3).
Plagioclase
Mafic granulites and amphibolites
Representative plagioclase compositions are given in Table 4. There are systematic compositional variations in plagioclase with textural type, which include the following.
Matrix plagioclase in the granulites varies in the range An54–62, whereas in the amphibolites, it is An57. Symplectitic plagioclase is more anorthite rich than matrix-type plagioclase in different types of symplectites. In quartz-bearing domains, plagioclase in type 2 symplectites is bytownite (An79) in composition (sample 77C/PS). In quartz-absent domains, plagioclase composition in type 1 and type 2 symplectites is not preserved, as it is altered to an assemblage of zoisite, epidote and locally muscovite (sample 72D/PS). Considering the predominantly calcic alteration assemblages, we assume an anorthitic composition of plagioclase symplectites in this domain. This is also in accord with a general anorthitic composition of plagioclase in both granulites and amphibolites (see subsequent discussion). For geothermobarometric calculations (see subsequent discussion), we therefore have taken XAn = 0·90 for symplectitic plagioclase for this sample. In association with ilmenite–quartz symplectites in amphibolites, plagioclase is uniformly anorthitic (An90–94). In contrast, plagioclase3 with hornblende3 symplectites is more variable, ranging from labradorite (An64–67) (Table 4) to bytownite (An83–87) (Bhowmik et al., 1999), reflecting the differential contribution of grossular and ferroedenite components from the decomposing garnet and folial hornblende, respectively (see discussion on mineral reaction history). Exsolution lamellae of plagioclase in folial hornblende are labradoritic in composition.
Metadolerite
Plagioclase in the metadolerite is distinctly zoned with a thicker calcic core (XAn = 0·63) and a thinner, relatively sodic rim (XAn = 0·46) (Table 5). The zoning conforms to the original crystal outline. Part of this normal zoning may be attributed to original igneous zoning.
Pyroxene
Mafic granulite
Both porphyroblastic and symplectitic clinopyroxenes are salite to augite in composition, with low jadeite contents, typical of mafic granulites. Clinopyroxene (P) is low to moderate in alumina (Al2O3 1·67–2·54 wt %) with XMg varying in the range 0·51– 0·66 (Table 5). Clinopyroxene in type 1 symplectites is more aluminous (Al2O33·42–4·33 wt %) compared with that in type 2 symplectite (Al2O3 1·86–2·48 wt %).
Metadolerite
Both orthopyroxene and clinopyroxene megacrysts in the metadolerite are low in alumina (Al2O3 = 0·40–0·73 wt %). Clinopyroxene is more magnesian (XMg = 0·68) than orthopyroxene (XMg = 0·46) (Table 5). There is no perceptible difference in composition between pyroxene exsolution lamellae and the pyroxene megacrysts, suggesting homogeneity of composition as a result of diffusion. The core compositions of the pyroxene megacrysts have been reintegrated with the assumption that the ratios of lamellae and host in the core are representative of the entire grain and did not change consequent to granular exsolution. The result shows that magmatic orthopyroxene was essentially of intermediate pigeonite composition (XCaCO3 = 7·6 mol %, XMg = 0·48). Magmatic clinopyroxene was augite (XCaCO3 = 35 mol %, XMg = 0·58) (Table 4).
Ilmenite
Coronal ilmenite around rutile in the mafic granulite contains minor pyrophanite component (7 mol %). Ilmenite intergrown with plagioclase–quartz symplectite in amphibolite is nearly pure FeTiO3.
Titanite
Titanite in the granulites and amphibolites is slightly aluminous (Al2O3 1·31–1·88 wt %) and ferroan (FeO 0·54–1·55 wt %).
Epidote–chlorite
The pistacite component in epidote, replacing pyroxene and plagioclase in the mafic granulites is 15 mol % (Table 4). XMg in chlorite is 0·45.
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MINERAL REACTION HISTORY |
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Prograde reaction
The prograde reaction history in the mafic granulites and amphibolites is best constrained by the inclusion assemblage of hornblende1 + plagioclase1 + ilmenite1 ± quartz within garnet and/or in clinopyroxene (P).
Mafic granulite and amphibolite
In the mafic granulites, the presence of included assemblages of ferropargasite1, plagioclase1 and ilmenite1 within garnet (P) and clinopyroxene (P), the occurrence of enclosed rutile within garnet rims and the rare presence of plagioclase1 in the matrix suggest the following generalized reaction that produced the M1 mineral assemblage:
 | (1a) |
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In the amphibolites, porphyroblastic garnet in the absence of clinopyroxene was produced by reacting hornblende, plagioclase and ilmenite:
 | (1b) |
; Vielzeuf & Schmidt, 2001). These reactions also mark the transition from amphibolite to garnetiferous amphibolite and mafic granulite with rising pressure and temperature. Green & Ringwood (1967) have suggested that the garnet + clinopyroxene + quartz association in metabasic rocks of intermediate composition is diagnostic of high-pressure (>9 kbar) granulite- facies metamorphism. Rushmer (1993) has also experimentally shown that the garnet + clinopyroxene + plagioclase assemblage, when lacking orthopyroxene, is indicative of high-pressure metamorphism (12–18 kbar) of basaltic amphibolite. The exact pressure at which clinopyroxene + garnet becomes the dominant residual assemblage to the exclusion of orthopyroxene is, however, controlled by the protolith composition (Green & Ringwood, 1967; Patiño-Douce & Beard, 1995).
Retrograde reactions
Both the mafic granulites and amphibolites were overprinted by strong retrograde metamorphism, during which the M1 garnet and locally clinopyroxene (P) became armoured by coronitic assemblages and by the development of hornblendeFOL.
In the mafic granulites, the earliest phase of post-peak retrograde evolution is manifested by the development of a coarse-grained clinopyroxene2A–hornblende2A–plagioclase2A symplectite assemblage. Hornblende2A contains relict clinopyroxene (P) and is relatively high in Ti, Al(IV), (Na + K)A and Al(VI). Clinopyroxene2A is markedly aluminous (Al2O3 3·4–4·3 wt %). This reflects the general reaction
 | (2a) |
 | (2b) |
; Mengel & Rivers, 1991).
The critical petrographic feature of development of type 2 clinopyroxene2B–plagioclase2B–ilmenite2B symplectites overgrowing matrix folial hornblende, and also at the interfaces of garnet (P) and clinopyroxene2A, hornblende2A, the presence of titanite inclusions in the symplectites, the relatively low alumina (Al2O3 1·9–2·5 wt %) content of the symplectitic clinopyroxene, the grossular depletion at the garnet edges, the anorthitic composition of plagioclase and finally, the crossing tie-lines in the CaO–FeO–SiO2–TiO2 chemographic projection (Fig. 10b) imply that these symplectites post-dated D2 and appeared according to a generalized reaction of the form
 | (2c) |
 | (2d) |
; Mengel & Rivers, 1991; Zhao et al., 2001). Reaction (2c), involving the breakdown of a hydrous phase, may additionally indicate (1) progression under fluid-deficient conditions and/or (2) that the decompression is associated with heating (Spear, 1993; Hensen et al., 1995).
Ilmenite–plagioclase–quartz ± hornblende symplectites surrounding embayed garnet and in contact with folial hornblende suggest the following reaction in the amphibolites:
 | (2e) |
).
Hornblende–plagioclase symplectites around garnet against folial hornblende, which pseudomorph the idioblastic habit of garnet, and those formed locally at the interface of garnet and type 2 clinopyroxene– plagioclase symplectites imply a relatively static (i.e. post-deformational) hydration reaction in mafic granulite:
 | (3a) |
; Mengel & Rivers, 1991; Zhao et al., 2001) and is controlled by fluid composition in addition to P and T. Zhao et al. (2001) recently interpreted the reaction to have progressed in response to cooling, accompanying minor decompression.
The post-D2 emplacement of the mafic dykes, MD2, is consistent with its intrusion at the culmination of reactions (2c) and (2d). This phase of static recrystallization, post-dating D2, is also shown by garnetiferous metadolerite, MD2, and is manifested by subsolidus re-equilibration involving the development of garnet– quartz–clinopyroxene ± hornblende-bearing coronas and of exsolution lamellae of orthopyroxene in clinopyroxene and vice versa. The generalized garnet-forming reaction is
 | (3b) |
).
In the last stage of metamorphism, garnet was replaced by chlorite along fractures, and anorthitic plagioclase and clinopyroxene by zoisite, epidote and muscovite in mafic granulites. The following generalized reactions mark a late greenschist-facies overprint:
 | (3c) |
The observed reactions in the mafic granulites and amphibolites suggest that (1) the amphibolites and the granulites have been subjected to the same metamorphic event, with both documenting post-peak decompression, and (2) the granulites additionally record a post-decompressional cooling history. The inferred clockwise P–T trajectory will now be further constrained by petrogenetic grid arguments.
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PETROGENETIC GRID IN NCMASH SYSTEM |
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 TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYMINERAL CHEMISTRYMINERAL REACTION HISTORY PETROGENETIC GRID IN NCMASH... P-T CONDITIONS OFMETAMORPHISMTECTONIC EVOLUTION OF THE...REFERENCES |
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Any petrogenetic grid that can explain the observed reaction textures must involve a Ca-amphibole phase. Notwithstanding the uncertainties associated with a–X relationships for multi-site solid solution amphiboles, recent progress in deriving non-ideal mixing models for this phase using the internally consistent thermodynamic dataset of Holland & Powell (1998)
provides a useful basis for calculating phase diagrams in complex amphibole-bearing systems (e.g. Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O, NCFMASH) and also for thermobarometry (Dale et al., 2000). The a–X relations proposed by Dale et al. (2000) and incorporated in the latest version (v.3.1) of THERMOCALC provide reasonable approximations for tremolite and pargasite activities. These two amphibole end-members are chosen because they show the greatest variations 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-component (NCMASH), eight-phase (garnet, clinopyroxene, 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 calculated 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.
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The geometric relations and the constraints imposed by the chemography in the system NCMASH define a single pseudo-invariant point from which emanate eight pseudo-univariant reactions, four of which are related by degenerate equilibria (Fig. 11a, Table 6). Figure 11 also shows that the majority of the reactions are pressure sensitive. There are two pseudo-divariant fields marked on either side of the pseudo-invariant point. The plagioclase-absent field, with garnet–clinopyroxene–amphibole (tremolite–pargasite)–quartz–vapour is stabilized on the higher-temperature–higher-pressure side of the pseudo- invariant point. This also marks the stability field of the mafic granulites during peak metamorphism, M1 ‘marked M1(G) in Fig. 11a.’ The (Di, V) pseudo-divariant field stabilizing garnet–plagioclase–amphibole–quartz assemblage appears on the relatively lower-temperature–lower-pressure side of the pseudo-invariant point, defining the range of P–T conditions relevant to M1 in garnetiferous amphibolite [field termed M1(A) in Fig. 11a]. The relative positions of these assemblage fields, and the change from mineral assemblages with garnet alone to garnet + clinopyroxene assemblages at temperatures of
700–750°C and pressures of 9 kbar is consistent with recent experimental results on MORB (Liu et al., 1996; Vielzeuf & Schmidt, 2001).
The pseudo-invariant point of Fig. 11a is susceptible to variation in fluid and bulk-rock composition (e.g. Fe/Mg), as is shown qualitatively in Fig. 11b (see also Wells, 1979). Decreased XH2O reduces the stability of amphibole–garnet-bearing assemblages, forcing the pseudo-invariant point to slide down-temperature along the (Grt, V) equilibrium. Higher XFe bulk compositions, on the other hand, enhances the stability of garnetiferous assemblages and cause the pseudo-invariant point to shift to higher temperatures along the same equilibrium, if there is no Fe–Mg partitioning between diopside, tremolite and pargasite. However, in the case of non-equipartitioning of Fe–Mg, the pseudo-invariant point no longer moves along (Grt) but along a NCFMASH equilibrium, the locus of which is controlled by the nature of Fe–Mg partitioning between the amphibole and the clinopyroxene. The available compositional data indicate that the addition of Fe will enhance the stability of the tremolite-bearing assemblages compared with that of the diopside-bearing ones. Considering that Fe–Mg partitioning is more pronounced for tremolite–diopside rather than tremolite-pargasite pairs, the NCFMASH line the point tracks out lies in the pseudo-divariant field (Tr, Grt) [line (a) in Fig. 11b]. This shifting of garnet-forming reactions to lower pressure in Fe-rich bulk compositions is also consistent with experimental results (Green & Ringwood, 1967; Patiño-Douce & Beard, 1995). Nevertheless, there is no change in the general topology of the petrogenetic grid. This allows the grid to be used qualitatively to constrain the geometry of the P–T path of evolution in the mafic granulites. Path 1 shows the prograde path of rising P and T in mafic granulites and marks entry into the granulite field from prograde hornblende1–plagioclase1 assemblage through the field of garnetiferous amphibolite. The rising dP/dT of the prograde path is also inferred from prograde compositional zoning of garnet and hornblende1. Figure 11a also predicts that the transition from garnet + clinopyroxene + quartz [M1(G)] to the hornblende3 + plagioclase3 [M3(G)] through clinopyroxene2 + plagioclase2 ± hornblende2/FOL assemblages [M2(G)] can take place during decompression accompanied by cooling, consistent with textural observations. Taken together, a clockwise P–T path has been inferred for the Ramakona–Katangi granulite domain of the Sausar Belt.
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P–T CONDITIONS OFMETAMORPHISM |
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Metamorphic conditions have been quantified using conventional geothermobarometry as well as the integrated thermodynamic approach using the updated and expanded thermodynamic dataset of Holland & Powell (1998)
. For the latter, the approach of average pressure–temperature calculation has been used. Calculations have been performed using THERMOCALC v.3.1. Quantitative estimates for the different episodes of metamorphism (M0–M3) are based on the following assumptions:
- a frozen-in chemical equilibrium during M0 appears to exist between garnet and coexisting prograde hornblende1 in mafic granulite (sample 72D/PS) as discussed in the mineral chemistry section. Applying garnet–hornblende thermometry, this allows calculation of the variation in thermal conditions during prograde metamorphism.
- For calculation of peak metamorphic conditions associated with M1, the combination of garnet rim–core of clinopyroxene (P)–matrix plagioclase ± ilmenite ± rutile is chosen.
- For calculation of the conditions of retrograde metamorphism, M2 and M3, the combination of re-equilibrated garnet edge and coexisting newly formed pyroxene- and hornblende-bearing corona and symplectitic assemblages is used.
The activity models and different formulations used for conventional geothermobarometry and THERMOCALC approaches are presented in Table 7. The independent equilibria used for average pressure– temperature estimates of the different episodes of metamorphism and the results of P–T calculations are given in Tables 8 and 9 respectively. The P–T results of conventional geothermobarometry are provided in Table 10. The P–T estimates for different metamorphic episodes are used to quantify the P–T trajectory, earlier constrained qualitatively by reaction history and petrogenetic grid considerations.
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For mafic granulite sample, 72D/PS, garnet– hornblende thermometry using coexisting hornblende1 and garnet (P) in a traverse from core to rim of the garnet yields a consistent rise in temperature from
700°C at the core to 800°C at the rims. This progressive rise in temperature during prograde metamorphism M0 is also consistent with that retrieved from the Ti in hornblende geothermometer (Ernst & Liu, 1998) (Table 10). Garnet–clinopyroxene thermometry yields a temperature of 780°C. Because formation of garnet (P) is equated with peak metamorphism (M1), the peak temperature for the same sample is constrained at 800°C. For other samples of mafic granulite, garnet–clinopyroxene thermometry and THERMOCALC give temperature estimates that range from 750 to 690°C. The relatively lower temperature estimate for sample 45A/PS is considered to reflect greater down-temperature Fe–Mg exchange (Frost & Chacko, 1989). The phase diagram of Liu et al. (1996) and experimental results of Spear (1981) imply stability of garnet + clinopyroxene assemblages at temperatures in excess of 750°C. We therefore infer peak metamorphic temperatures in the mafic granulites as 750–800°C. For garnetiferous amphibolites, application of the garnet–hornblende thermometer (Graham & Powell, 1984; Ravana, 2000) gives a temperature estimate of 660–680°C, yielding an average of 675°C for M1 metamorphism in amphibolites.
In quartz-bearing domains, the application of the garnet–anorthite–hedenbergite–quartz (GAHS) and garnet–anorthite–diopside–quartz (GADS) barometers (Essene, 1989) gives peak pressure estimates in the range 8·9–10·3 kbar. Average pressures estimated by using THERMOCALC for the same assemblages provide a relatively lower pressure range of 8·1–9·1 kbar, with an average of 9 kbar. For quartz-absent domains, the pressure for peak metamorphism cannot be directly calculated. Indirect estimates can, however, be produced using the experimentally calibrated location of garnet–clinopyroxene equilibria (Vielzeuf & Schmidt, 2001) or plagioclase-out equilibria (Liu et al., 1996) in metabasites of appropriate bulk composition. For rutile-bearing domains, this gives a range of minimum pressures of 10–12 kbar. Given that the plagioclase-out curve was intersected in quartz-absent domains and that the garnet in these domains has the highest grossular content (XGrs = 0·38), the peak pressure estimate is likely to exceed 9 kbar (Vielzeuf & Schmidt, 2001) and we tentatively place it at 10 kbar. For amphibolites, the application of THERMOCALC for the garnet–hornblende1–matrix plagioclase–quartz assemblage gives an average pressure estimate of 8·4 kbar for M1 metamorphism.
Summarizing, P–T conditions of 9–10 kbar and 750–800°C, and 8 kbar and 675°C are taken as representative of M1 metamorphism in mafic granulites and amphibolites respectively. The estimated peak pressure of 9–10 kbar also places the mafic granulite in the realm of high-pressure granulites (Green & Ringwood, 1967).
The P–T conditions of the formation of the type 1 symplectites in the mafic granulites are estimated by using the assemblage clinopyroxene2A + horn-blende2A + plagioclase2A. Application of the garnet– clinopyroxene thermometer using garnet rims and aluminous clinopyroxene2A gives a temperature 780°C. The Ti-thermometer applied to hornblende2A grains provides a consistent temperature estimate of 780°C. For the same assemblage THERMOCALC gives an average temperature estimate of 740°C. On the basis of these results, we place the earliest symplectite formation at 775°C. The application of the GAHS barometer yields a pressure of 11 kbar. However, in the absence of free quartz, the estimated values only indicate maximum pressure, and the actual pressure of symplectite formation may be much lower. The average pressure calculated with THERMOCALC, for a garnet–clinopyroxene–amphibole assemblage and considering absence of free quartz, is 8·1 kbar at an XH2O of 0·50. Therefore, we consider 8 kbar and 775°C to approximate the P–T conditions for the formation of the clinopyroxene2A + hornblende2A + plagioclase2A symplectites during M2A metamorphic episode.
Using core compositions of folial hornblende in both mafic granulites and amphibolites, application of the Ti-thermometer gives a temperature range of 660–800°C with a maximum of around 750°C for the pervasive S2 foliation-forming event. The lowest temperature possibly reflects down-temperature resetting.
The P–T conditions for the type 2 symplectites in the mafic granulites are constrained using the garnet edge–clinopyroxene2B–hornblende2B–plagioclase2B ± ilmenite2B combination. For a quartz-bearing domain, the application of the garnet–clinopyroxene thermometer gives a temperature of 720°C, which is 45°C higher than that estimated by THERMOCALC. For quartz-absent domains, the estimated temperatures are in the range of 670–740°C. Experimental evidence that the clinopyroxene–plagioclase assemblage is stabilized at a temperature above the wet basalt solidus (Liu et al., 1996; Vielzeuf & Schmidt, 2001) requires a minimum temperature of 750°C for the formation of this assemblage. THERMOCALC gives an average pressure estimate of 6·8 kbar for the quartz-bearing domain, whereas for a quartz-absent domain, application of GAHS barometer gives a maximum pressure range of 8·6–9·0 kbar. Application of THERMOCALC to the quartz-absent situation yields an average pressure estimate of 7 kbar. We therefore infer 7 kbar and 750°C as the P–T condition for the formation of these symplectites during M2B stage.
For amphibolites, the P–T estimates for ilmenite– plagioclase–quartz ± hornblende-bearing symplectites show a range from 6·2 to 6·6 kbar and from 600°C to 700°C. The lower temperature estimates, obtained using the garnet–hornblende thermometer, are minimum estimates and possibly represent blocking temperatures of Fe2+–Mg exchange for the garnet–hornblende pair. We therefore take 6·4 kbar and 700°C as P–T conditions for M2 metamorphism in amphibolites.
In mafic granulites, the P–T estimates for hornblende3A–plagioclase3A symplectites are determined using the composition of garnet edges and coexisting hornblende3 and plagioclase3 symplectites. Application of THERMOCALC yields an average P–T range of 6·0 ± 0·1 kbar and 665 ± 15°C. These temperature estimates are consistent with that determined from the hornblende-in curve of Zhao et al. (2001), which predicts the change from clinopyroxene + plagioclase to hornblende + plagioclase stability at temperatures of 700°C. On the basis of these constraints, we infer 6 kbar and 675°C as P–T conditions for the M3 stage in mafic granulites.
In the garnetiferous metadolerite, the coronal garnet– orthopyroxene–clinopyroxene–plagioclase assemblage yields a T790°C and 680°C using the garnet–orthopyroxene calibrations of Lee & Ganguly (1988) and Harley (1984) respectively (result not shown in Table 10). Average temperature calculations give similar or lower temperatures of 675°C. Applying the pyroxene thermometer of Lindsley (1983), the reintegrated pyroxene yields temperatures in excess of 1000°C, which possibly reflect magmatic crystallization. Pressure estimates for coronal garnet formation, obtained using garnet–anorthite–hedenbergite–quartz, garnet–anorthite–diopside–quartz and garnet–anorthite–ferrosilite–quartz barometers (Essene, 1989), are in the range 4·0–6·4 kbar, with an average of 5·5 kbar. THERMOCALC gives an average pressure of 6·4 kbar. Considering the geological setting, P–T conditions of 6 kbar and 750°C are taken by the authors for this phase of coronal garnet formation.
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TECTONIC EVOLUTION OF THE SAUSAR MOBILE BELT |
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The enclaves of garnetiferous metabasites occurring extensively within host gneisses in the northern periphery of the Sausar Mobile Belt demonstrate that high-pressure metamorphism is regionally widespread. Despite pervasive retrograde structural and metamorphic overprinting, the mafic granulites and associated garnetiferous amphibolites record shared prograde and retrograde metamorphic histories. This implies that the garnetiferous amphibolites do not represent retrogressed granulites. Geothermobarometric results indicate that the amphibolites and the granulites were equilibrated at different peak pressure conditions. This implies that despite intense tectonic interleaving during D2 and subsequent deformational events, the amphibolites and the granulites together appear to preserve a deep crustal section in the RKG domain of the Sausar Belt. The combined petrographic, mineral chemistry, reaction history, petrogenetic grid and geothermobarometric data reveal an almost complete metamorphic history of this exposed reworked deep crustal section. These data indicate a uniform post-peak near-isothermal decompression history of the two types of rocks (Fig. 12). The granulites 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 metamorphic reaction textures suggest either decompression or heating [reaction (2c)] or cooling [reaction (3a)] and therefore do not support burial as a possible cause of garnet formation in these rocks; (2) the persistent orthopyroxene–clinopyroxene–plagioclase-bearing liquidus mineralogy in the absence of olivine also implies original emplacement depth of these rocks in excess of 7·5 kbar, as shown by experimentally derived liquidus phase relations (Kuehner, 1992). Based on these evidences, we infer mid-crustal emplacement and subsequent cooling history of the metabasic dykes, MD2.
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Although the mafic granulites exhibit evidence for post-peak decompression followed by cooling at
6 kbar, it has been argued on the basis of textural and mineralogical assemblages and geothermobarometric results that polyphase decompression, as seen particularly in sample 72D/PS, is not continuous, being separated by a distinct heating (prograde?) event (Path 2, Fig. 12). This raises a number of possibilities for the exhumation of these deep crustal rocks. Three alternative models of crustal evolution are proposed here, each of which remain to be tested on the basis of precise geochronology: (1) polycyclic with at least two unrelated events and intervening residence in the middle crust; (2) single-cycle with collision and extension; (3) modified single-cycle but with punctuated compression – extension stages.
In the first model the prograde heating and cooling P–T trajectories at mid-crustal levels (M2B–M3) (Path 2, Fig. 13a) are attributed to a second granulite metamorphic event, unrelated in time to the first high-P granulite event that was followed by decompression (Path 1, Fig. 13a). Evidence in favour of this type of polyphase history has been proposed from the Prydz Bay area in East Antarctica [Carson et al., 1995; Dirks & Hand, 1995; Dirks et al., 1995; Hensen et al., 1995; see also Fitzsimons (2000) for a review of the present status of metamorphism in East Antarctica]. Previously, Bhowmik et al. (1999) also interpreted the mafic granulites of the Khawasa area (sample 77C/SB, this study) to be polycyclic. On the basis of differences in structure and peak metamorphic temperature, the granulite-facies metamorphism was considered to be older (Pre-Sausar), and unrelated in time to the metamorphism of the Sausar Group of rocks. The overprint of an amphibolite-facies event was taken by the authors as the imprint of a younger Sausar event.
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Discrimination between two unrelated metamorphic events hinges primarily on precise geochronological evidence, which is not yet available in the Sausar Belt. However, garnet in many of the studied granulites preserves growth zoning. Prograde garnet zoning in granulite-facies conditions has been previously reported from a few select areas [e.g. Dabie Mountains (Chen et al., 1998
); Grenville Province (Indares, 1995); Moldanubian Zone, Austria (Cooke et al., 2000)] and has been attributed to relatively short residence times under the peak granulite conditions coupled with a lack of intergranular fluid phase and a rapid exhumation history. This is required because garnet homogenizes by volume diffusion within a temperature interval of 600–750°C, depending on the duration of high-temperature conditions and the grain size (Spear, 1991). On the basis of these arguments, we consider that growth zoning is unlikely to be preserved for granulites residing at mid-crustal depths for a longer duration as implied by the polycyclic model.
The second model assumes that the P–T path is continuous with a clockwise sense, and that the different metamorphic stages are related to a single tectono-metamorphic event. The clockwise P–T loop is usually explained by a geodynamic model involving orogenic crustal thickening and is inferred for several high-grade amphibolite- to granulite-facies terranes (e.g. Thompson & England, 1984). Such a simple assumption has been widely adopted in many studies of granulite terranes and forms the basis of the model of extensional collapse of thickened orogens (Harley, 1989, 1992; Harley & Fitzsimons, 1991).
In the present study, there exists evidence in favour of a prograde P–T segment with rising dP/dT and post-peak steep decompression and cooling segments. This suggests that M0–M1–M2–M3 stages can directly be joined by a smooth line (Fig. 13b). The anhydrous clinopyroxene–plagioclase assemblage developing after the retrograde hornblende may even imply minor heating during decompression (inflected portion), as has been recently proposed from the modern collisional orogens such as the Himalayas (Ganguly et al., 2000; de Sigoyer et al., 2000). Taken together this may suggest that the extensional collapse model is also applicable to the RKG domain in the SMB. However, the D2 deformation that produced a major thrust nappe is indicative of a broad compressional regime. Moreover, the metamorphic stages M2B and M3 imply a major mid-crustal static heating–cooling event, punctuated by basic dyke emplacement, which follows a pervasive decompressional event (M2A). This requires that the extensional collapse model be modified. A modified model is presented below.
This model requires that the exhumation of deep crustal rocks is a multistage process involving repeated thrusting events, and is analogous to that presented for the European Variscides (Gillet et al., 1985; Mercier et al., 1991a, 1991b). In this model it is assumed that subsequent to the high-pressure event there is a major underthrusting beneath the high-pressure unit. The consequence is that the crustal section closest to the newly underlying unit will undergo rapid cooling. As underthrusting progresses, it will introduce more and more lower-grade rocks into the metamorphic pile. These underthrust units will experience prograde metamorphism, possibly accompanying dehydration reactions. The ‘refrigerated’ pile lying above, on the other hand, may suffer a retrograde event, possibly promoted by fluids derived from beneath. The net result is the formation of a composite thrust sheet. The allochthonous components, which constitute the composite thrust sheet, preserve evidence that they were equilibrated at different pressures before their tectonic juxtaposition. The metamorphic implication is that instead of a smooth line joining different calculated pressure–temperature domains, the P–T path in this case will have multiple inflections, incorporating thermal and baric adjustments of multiple thrust stacking (O'Brien & Carswell, 1993).
The composite P–T path depicted in Fig. 13c implies that a similar situation may exist in the RKG domain of the Sausar Belt. Following the early high-pressure metamorphism in response to continental subduction, the rocks have undergone extensive retrogression. The retrogression was associated with a major nappe-forming stage due to northward underthrusting of the continental shelf sediments of the Sausar Group of rocks beneath the southward-advancing nappe pile of the high-pressure rocks. The fluids necessary to cause the retrogression were possibly derived from the structurally underlying devolatilizing sedimentary pile and/or by syntectonic granitoid intrusives. As successive multiple thrust slices pile up one above the other, during this renewed continent–continent collision stage, the retrogressed granulites will undergo slight burial, to be followed by heating and decompression. The post-kinematic nature of second generation anhydrous clinopyroxene–plagioclase assemblages, formed after folial hornblende and garnet (P), implies a second thermal maximum, post-dating a pervasive deformation event. This can generally be modelled in terms of P–T evolution in a collisional setting (England & Thompson, 1984). The slight thermal perturbation, necessary for this renewed prograde event, may be caused by (1) radiogenic heat supplied by the structurally overlying unit over these metabasites (now eroded out), (2) heat carried by the syntectonic felsic granitoids or (3) mantle-scale perturbation as evidenced by multiple basic dyking events. Features such as the preservation of garnet growth zoning, the interpreted relatively flat heating–cooling path and emplacement of syntectonic granitoid intrusives imply transient thermal pulses. Such short-lived thermal pulses are fairly common during low-pressure events and have been extensively documented elsewhere [e.g. Llano Uplift (Bebout & Carlson, 1986; Carlson & Johnson, 1991; Letargo et al., 1995); the Variscide belt, O'Brien & Vrana, 1995].
One important consequence of this model is the presence of multiple stages of extension, alternating with periods of compression. This is supported by repeated basic dyke emplacement, some of which are syn-D2 and others are post-D2. The Mesoproterozoic Grenville Orogen, one of the best-documented Precambrian collisional belts, bears evidence of three distinct pulses of crustal shortening, which are separated by periods of extension (Rivers, 1997). Consistent with this multistage collision–extension model, the footwall Sausar Group of rocks to the south of the RKG domain exhibit a clockwise P–T history and peak metamorphism at 7 kbar, 675°C (Pal & Bhowmik, 1998; Bhowmik et al., 1999).
Geodynamic significance of the P–T path in the CITZ
The metamorphic event documented herein clearly represents a major compressional event such that the northern edge of the Sausar Belt underwent deep burial of a large portion of the continental margin during a major continent–continent collisional orogeny. Previous tectonic models predicted plate convergence and a major, Palaeoproterozoic suture zone (compare the Central Indian Suture Zone) along the southern periphery of the Sausar Group of rocks, along which the South Indian and the North Indian Blocks were amalgamated (Yedekar et al., 1990; Jain et al., 1991, 1995; Mishra et al., 2000). The model was based on the following arguments: (1) the presence of contrasting lithological association across the CIS, with high-grade metasediments and granulites towards the north and low-grade volcanogenic sequences to the south; (2) the interpretation that the granulites of the BBG domain represent exhumed oceanic crust of the North Indian Block that was subducted below the South Indian Block during a Palaeoproterozoic continent–continent collision; (3) the calc-alkaline geochemical signatures of the Dongargarh granitoids, and Dongargarh and Sakoli volcanics, reflecting subduction-related magmatism as a sequel to this suturing event.
Recent studies, however, indicate a more complex evolutionary history of the domains across the CIS and contradict the southerly subduction model. In contrast to the simple high–low grade contrast across the CIS, 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, 2000). In addition, the mafic–ultramafic components of the BBG domain are predominantly concordant gabbroic to pyroxenitic intrusive bodies within the granulite ensemble (Bhowmik & Pal, 2000), and cannot be taken as evidence for subducted oceanic crust. Moreover, the Dongargarh granitoids and Dongargarh and Sakoli volcanics of Palaeoproterozoic age (Sarkar et al., 1981) have a distinct north–south structural orientation, paralleling the structural grain of the SIB, but discordant to the east–west- to ENE–WSW-trending CITZ. Had these rocks been produced during north–south compressional orogeny, stitching the two cratonic blocks, the rocks would have a broad east–west to ENE–WSW alignment, paralleling the orientation of the CITZ. Consequently, the calc-alkaline signatures of these rocks as well as their Palaeoproterozoic ages cannot be incorporated into tectonic models of the amalgamation of the two crustal blocks.
Recent geochronological data show imprints of polyphase tectono-magmatic histories of the BBG domain spanning from Late Archaean through Mesoproterozoic to Neoproterozoic time (Ramchandra & Roy, 2001). Roy et al. (2002) have also recently documented Palaeoproterozoic tranpressional tectonics from the Mahakoshal belt, the latter constituting the northern component of the CITZ. All of these recent results imply that the CITZ is a collage of different litho-tectonic domains, many with polycyclic tectono-magmatic histories. The available evidence seems to support the notion that the assembly of the South Indian and the North Indian blocks and intervening microcontinents(?) was a multistage process spanning from Palaeoproterozoic through Meso- to Neoproterozoic time. However, the nature of the geodynamic processes at each stage of amalgamation is yet to be constrained.
Recent seismic profiling and gravity modelling across the SMB reveal the presence of a northerly dipping reflector on the DSS profile and also of thicker and high-density lower crust under the northern part of the SMB (Mishra et al., 2000). These geophysical signatures coincide with the high-pressure RKG domain documented here, and in combination with metamorphic imprints suggest a possibly northerly subduction of a southern block in the Sausar Belt. On the basis of the available geological and geochronological data, we speculate that the high-pressure metamorphism in the RKG domain is of Grenvillian age and urge that this contention be tested by reliable geochronology. The clockwise P–T trajectory in the mafic granulites possibly marks the final amalgamation in the Central Indian Tectonic Zone, producing the Indian subcontinent.
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ACKNOWLEDGEMENTS |
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We thank S. L. Harley and G. Zhao for their helpful and constructive comments on an earlier version of this paper. The manuscript has benefited greatly from stimulating discussions with S. Dasgupta, P. Sengupta, P. K. Bhattacharyya, H. C. Dasgupta and M. Raith. S.K.B. acknowledges the DST and the ISIRD research grant of the Indian Institute of Technology, Kharagpur, for partial funding of the work. A part of the research was carried out with financial support by the Geological Survey of India (Research Project RP/CR/MP/MAH/1997/002) for which we thank T. Pal for help during field work, and N. C. Pant and S. Shome for help during microprobe analysis and backscatter electron image photography. The final version of the manuscript was prepared at the Mineralogisch–Petrologisches Institüt, University of Bonn, for which S.K.B. acknowledges a post-doctoral research fellowship by DAAD. We also acknowledge the help rendered by N. Pal and A. Basu Sarbadhikari with data processing. Finally we thank R. Powell and T. Holland for providing access to THERMOCALC v. 3.1.
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