1. Introduction
The southern Indian granulite terrane is composed of several granulite-facies blocks exposing mid- and lower levels of the continental crust, and dissected by intracontinental Neoproterozoic shear zones (Drury et al. Reference Drury, Harris, Holt, Reeves-Smith and Wightman1984). Recent geochronological data on high-grade metamorphic rocks in the area have confirmed the widespread effect of the c. 550–530 Ma thermal event (e.g. Braun, Montel & Nicollet, Reference Braun, Montel and Nicollet1998; Santosh et al. Reference Santosh, Yokoyama, Biju-Sekhar and Rogers2003, Reference Santosh, Collins, Tamashiro, Koshimoto, Tsutsumi and Yokoyama2006; Santosh, Morimoto & Tsutsumi, Reference Santosh, Morimoto and Tsutsumi2006; Collins et al. Reference Collins, Clark, Sajeev, Santosh, Kelsey and Hand2007). The Madurai Block, one of the largest crustal blocks in southern India, is composed mainly of biotite–hornblende orthogneiss and massive charnockite with supracrustal rocks such as pelitic and mafic granulites, quartzite and calc-silicate rocks. Recent petrological data from the Madurai Block indicate crustal metamorphism at extreme thermal conditions of T ~ 1000°C and P ~ 10 kbar (e.g. Brown & Raith, Reference Brown and Raith1996; Raith, Karmakar & Brown, Reference Raith, Karmakar and Brown1997; Tateishi et al. Reference Tateishi, Tsunogae, Santosh and Janardhan2004; Sajeev, Osanai & Santosh, Reference Sajeev, Osanai and Santosh2004; Tsunogae & Santosh, Reference Tsunogae and Santosh2007). Rocks formed through such ultrahigh-temperature (UHT) metamorphism (e.g. Harley, Reference Harley, Treloar and O'Brian1998, Reference Harley2004, Reference Harley2008; Kelsey, Reference Kelsey2008) have been reported from several granulite terranes elsewhere (e.g. Motoyoshi & Ishikawa, Reference Motoyoshi, Ishikawa and Ricci1997; Sajeev & Osanai, Reference Sajeev and Osanai2004; Bose & Das, Reference Bose and Das2007; Leite et al. Reference Leite, Barbosa, Goncalves, Nicollet and Sabaté2009). Geochronological studies in the sapphirine-bearing granulites from the Madurai Block and the Palghat–Cauvery Shear Zone system, which marks the northern boundary of this block, indicate that the timing of peak metamorphism coincided with the final amalgamation of the Gondwana supercontinent during Late Neoproterozoic to Cambrian times (Santosh et al. Reference Santosh, Collins, Tamashiro, Koshimoto, Tsutsumi and Yokoyama2006; Collins et al. Reference Collins, Clark, Sajeev, Santosh, Kelsey and Hand2007).
In this study, we report new mineralogical and petrological data on sapphirine-bearing pelitic granulites and related rocks from a classic locality at Rajapalaiyam in the southernmost part of the Madurai Block. Although sapphirine-bearing rocks have been previously reported from several localities in this block (e.g. Tsunogae & Santosh, Reference Tsunogae and Santosh2007 and references therein), the present study area at Rajapalaiyam offers excellent exposures which preserve early mineral assemblages enclosed in coarse-grained poikiloblastic garnet within pelitic granulites. The mineral assemblages and reaction textures reported in this study thus aid in the estimation of peak metamorphic conditions, as well as in the construction of exhumational P–T paths in addressing the metamorphic history of a collisional orogen in an important Gondwana crustal fragment. Previous studies reported the diagnostic UHT assemblage of sapphirine + quartz in this locality for the first time from southern India (Tateishi et al. Reference Tateishi, Tsunogae, Santosh and Janardhan2004; Tsunogae & Santosh, Reference Tsunogae and Santosh2006; Braun et al. Reference Braun, Cenki-Tok, Paquette and Tiepolo2007; Tsunogae, Santosh & Dubessy, Reference Tsunogae, Santosh and Dubessy2008), providing robust evidence for T ~ 1000°C UHT metamorphism. Preliminary P–T paths of the Rajapalaiyam area were proposed by Tateishi et al. (Reference Tateishi, Tsunogae, Santosh and Janardhan2004) and Tsunogae & Santosh (Reference Tsunogae and Santosh2006) on the basis of phase analyses in the FMAS system. However, these works presented only limited data within the broad framework of the FMAS petrogenetic grid, and the precise nature of the P–T history of these rocks remained unresolved. We therefore carried out further detailed studies in this area, which led to the identification of several new and important textures. The results from these studies are presented in this paper to evaluate the P–T and exhumation history based on new FMAS petrogenetic grid considerations. For the precise determination of the P–T trajectory, we further adopted additional information such as mineral composition isopleths (e.g. for Al in orthopyroxene) to evaluate the path constructed by mineral equilibrium observations. Our results provide a typical case of formation of orogens in a continent–continent collision setting associated with the final assembly of the Gondwana supercontinent during Late Neoproterozoic to Cambrian times.
2. General geological setting and field relations
Rajapalaiyam is located about 80 km southwest of Madurai City (Fig. 1) in the Tamil Nadu state of southern India. The lithological units in this region comprise massive charnockite and associated lenses or layers of pelitic and calc-silicate rocks. A pink alkali-feldspar granite intrusive of about 5 × 2 km size also occurs in the area. The outcrop examined in this study reveals the contact between the alkali-feldspar granite and host granulites (dominantly quartzo-feldspathic and pelitic gneisses). Sriramguru et al. (Reference Sriramguru, Janardhan, Basava and Basavalingu2002) first reported sapphirine in this locality from pelitic xenoliths (garnet + orthopyroxene + cordierite + spinel + plagioclase + perthite) in the alkali-feldspar granite, and determined peak temperatures of 860°C from the garnet–orthopyroxene geothermometer and 950°C from the feldspar thermometer on mesoperthite. Our preliminary investigations on the pelitic and quartzo-feldspathic granulites from this area revealed the occurrence of an equilibrium sapphirine + quartz assemblage in some porphyroblastic garnets (Tateishi et al. Reference Tateishi, Tsunogae, Santosh and Janardhan2004).
Figure 1. Geological map of the Madurai Block (based on 1:500 000 map of Tamil Nadu: GSI, 1995) with the sample locality (star) of pelitic granulite from Rajapalaiyam discussed in this paper. DC – Dharwar Craton, MRB – Madurai Block, MSB – Madras Block, NNB – Northern Block, NGB – Nilgiri Block, TRB – Trivandrum Block, ACZ – Achankovil Shear Zone, PCSZ – Palghat–Cauvery Shear Zone, BSZ – Bhavani Shear Zone, MSZ – Moyar Shear Zone.
Figure 2 shows the nature of field occurrence of the granulite and intrusive alkali-feldspar granite in the study area. Remnants of pelitic granulite are occasionally enclosed within the granite (Fig. 2a). The matrix quartzo-feldspathic and pelitic granulites occur as layered gneisses with widths varying from several centimetres to decimetres. Foliation of the rock is clearly defined by compositional layering of various rock types (Fig. 2b–d), but lineation is not obvious (Fig. 2e). The boundary between the granite and granulites is clearly defined (Fig. 2b). Thin intrusive layers of the alkali-feldspar granite are also observed in the pelitic granulite, but the granite does not cross-cut the foliation of the matrix granulite (Fig. 2b). Quartzo-feldspathic granulites occur as lenses or layers in biotite- and orthopyroxene-rich pelitic granulites. Rarely, coarse-grained pelitic granulite occurs as layers parallel to the matrix foliation.
Figure 2. Field occurrences of sapphirine-bearing pelitic granulites and related rocks discussed in this study. (a) Remnants of pelitic granulite (PG) enclosed in pinkish alkali-feldspathic granite (AG). (b) Sharp intrusive contact between alkali-feldspar granite (AG) and pelitic granulite (PG). Thin layer of alkali-feldspar granite is also intruded concordant with the gneiss foliation. (c) Discordant lens of quartzo-feldspathic gneiss (QG) enclosed in orthopyroxene-rich granulite (sample MD6-2J). (d) Thin layering of pelitic granulite (PG) and quartzo-feldspathic gneiss (QG). (e) Close-up photograph of weakly foliated quartzo-feldspathic garnet granulite (PG) from locality MD6-2. (f) Coarse-grained orthopyroxene-bearing pelitic granulite (CPG) occurring parallel to the foliation defined by pelitic granulite (PG) and quartzo-feldspathic gneiss (QG). Hammer length in (b–d) is 40 cm. Pen length in (e, f) is 14 cm.
3. Petrography
Systematic petrographic studies were carried out in rock samples collected from the Rajapalaiyam area, including identification of mineral assemblages and reaction textures, grain size measurements and abundance. Based on these data, we classify the rocks in this locality into four major categories (Table 1):
(a) Type A: quartzo-feldspathic garnet–sillimanite ± orthopyroxene granulite (e.g. sample MD6-2K);
(b) Type B: garnet–orthopyroxene–cordierite granulite (e.g. sample MD6-2E);
(c) Type C: garnet–cordierite granulite (e.g. sample MD6-1Y);
(d) Type D: coarse-grained garnet–orthopyroxene granulite (e.g. sample MD6-2J).
Table 1. Mineral assemblages of representative granulites with approximate abundance; lithologies A to D are discussed in the text
+++ – abundant, ++ – moderate, + – rare, ′ – inclusion in garnet, * – retrograde mineral
1 – Spr+Qtz, 2 – Spl+Qtz, 3 – Opx+Sil+Qtz+Grt, 4 – Opx+Sil corona around Grt, 5 – Crd-bearing corona around Grt, 6 – Opx+Spr corona around Grt
Lithologies (b) to (d) are pelitic gneiss, while (a) is more SiO2-rich, probably psammitic in origin. A summary of their petrological characteristics is given below.
3.a. Type A: quartzo-feldspathic garnet–sillimanite ± orthopyroxene granulite
This leucocratic rock is composed mainly of quartz (40–60%), mesoperthite (20–40%), garnet (10–20%), biotite (2–10%) and sillimanite (2–10%), with accessory sapphirine, zircon, rutile, monazite and apatite. The rock exhibits a weak foliation defined by elongated sillimanite and quartz. Sample MD6-2L2 contains subidioblastic to xenoblastic and poikiloblastic garnet (0.5–6 mm in diameter) with inclusions of sillimanite, sapphirine, quartz, biotite, apatite, zircon and rutile (Fig. 3a). The sapphirine (0.02–0.3 mm) is pale bluish in colour and shows weak pleochroism. It is poikiloblastic and intergrows with fine-grained (< 0.06 mm) quartz. The sapphirine and quartz inclusions show a sharp grain contact and are in textural equilibrium (Figs 3b, 4a). Sillimanite (0.1–2 mm) occurs either as inclusions in poikiloblastic garnet and matrix quartz, or as prismatic aggregates along grain boundaries of garnet, quartz and mesoperthite. Quartz (up to 6.5 mm) and mesoperthite (0.3–1.8 mm) are dominant minerals and commonly form quartzo-feldspathic layers in the rock. Brownish biotite is medium-grained (0.05–2.1 mm) and surrounds garnet, sillimanite and quartz probably as a late phase. Some fine-grained (< 0.3 mm) biotites are also present as inclusions at the rim of garnet grains.
Figure 3. Photomicrographs of examined quartzo-feldspathic and pelitic gneisses from Rajapalaiyam. (a) Porphyroblastic garnet with inclusions of sapphirine and quartz in sample MD6-2L2. Box in the figure is enlarged in Figure 3b. (b) Sapphirine + quartz assemblage in garnet in quartzo-feldspathic garnet-sillimanite gneiss (sample MD6-2L2). (c) Sapphirine + quartz intergrowth enclosed within garnet in quartzo-feldspathic garnet–sillimanite gneiss (sample MD6-2K). (d) Orthopyroxene + sillimanite corona around garnet in sample MD6-2K. Sapphirine + quartz assemblage is present in the corroded garnet. (e) Xenoblastic garnet mantled by retrograde cordierite in sample MD6-2K. (f) Mesoperthite + quartz assemblage in leucocratic portion of pelitic granulite (sample MD6-2E2) (g) Cordierite-, garnet- and biotite-rich melanocratic portion of sample MD6-2E2. (h) Orthopyroxene + cordierite corona around garnet (sample MD6-2E2). Reactant quartz also occurs adjacent to garnet. (i) Coarse-grained cordierite surrounding garnet in sample MD6-1Y. (j) Subhedral Al-rich orthopyroxene enclosed in core of coarse-grained garnet in sample MD6-2J. (k) Sapphirine + orthopyroxene symplectite around garnet in sample MD6-2J. (l) Orthopyroxene + cordierite + sapphirine symplectite around garnet in sample MD6-2J. (m) Spinel and sapphirine associated with retrograde orthopyroxene–cordierite symplectite around garnet in sample MD6-2J2. (n) Close association of greenish spinel and magnetite in sample MD6-1X. (o) Retrograde biotite surrounding orthopyroxene in sample MD6-2E2. Mineral abbreviations from Kretz (Reference Kretz1983); also Spr – sapphirine: Mpth – mesoperthite. For a colour version of this figure see Appendix 2 at http://journals.cambridge.org/geo.
Figure 4. Back-scattered electron images showing detailed textures of sapphirine-bearing quartzo-feldspathic and pelitic granulites from Rajapalaiyam. (a) Equilibrium sapphirine + quartz assemblage enclosed in garnet in sample MD6-2L2. (b) Sapphirine + quartz assemblage enclosed in garnet in sample MD6-2K. (c) Orthopyroxene + sillimanite corona around garnet. Sapphirine + quartz is included in the corroded garnet (sample MD6-2K). (d) Quartz and garnet inclusions in symplectitic sillimanite, suggesting that orthopyroxene + sillimanite formed after garnet + quartz (+ sapphirine) (sample MD6-2K). (e) Irregular spinel included in idioblastic sapphirine (with quartz and biotite), which is in turn included in garnet (sample MD6-2J). No reaction texture can be seen between sapphirine and quartz. (f) Sapphirine + Al-rich orthopyroxene around garnet in sample MD6-2J. (g) Subhedral quartz inclusion in spinel (sample MD6-2L2). The two minerals are apparently in equilibrium. In contrast, a thin film of sapphirine is present between spinel and surrounding quartz, although the film is not always observed. These assemblages occur as a composite inclusion in poikiloblastic garnet as described by Tsunogae & Santosh (Reference Tsunogae and Santosh2006). (h) Ilmenite–spinel assemblage, probably formed by exsolution from Al-bearing opaque phase, in sample MD6-1X. (i) Mesoperthite (Mpth) in feldspathic portion of sample MD6-2M. It is composed of host plagioclase (dark greyish) and lamellar K-feldspar (whitish). Abbreviations as in Figure 3.
Sample MD6-2K is an orthopyroxene-bearing variety of this rock type. Garnet in the sample varies from subidioblastic (0.8–1.3 mm in diameter) to xenoblastic (1.5–3 mm in diameter). The subidioblastic garnet is surrounded by quartz grains and is free from inclusions and later reaction textures. In contrast, the relatively coarse-grained garnet is poikiloblastic, containing inclusions of sapphirine, quartz, sillimanite and monazite (Fig. 3c). The sapphirine and quartz inclusions show a direct contact relationship (Fig. 4b) similar to sample MD6-2L2. The sapphirine- and quartz-bearing poikiloblastic garnet is rarely mantled by corona of orthopyroxene + sillimanite (Figs 3d, 4c, d). The corona orthopyroxene and sillimanite contain fine-grained inclusions of sapphirine, quartz and garnet as reactant phases (Fig. 4d). Some garnet grains with no sapphirine inclusion are also corroded, but they are usually mantled by coronae of cordierite ± orthopyroxene (Fig. 3e). Matrix minerals are coarse-grained quartz (up to 7 mm) and mesoperthite (up to 2.7 mm). Foliation of the rock is defined by aligned sillimanite (0.1–1.2 mm) and brownish biotite (0.05–1.2 mm) around garnet.
3.b. Type B: garnet–orthopyroxene–cordierite granulite
This rock type is composed mainly of quartz (30–40%), garnet (30–50%), biotite (5–20%), orthopyroxene (5–10%), cordierite (5–10%) and mesoperthite (2–20%). Sillimanite and sapphirine are accessory minerals. A representative sample MD6-2E2 is composed of a quartz–mesoperthite-rich leucocratic layer and a garnet–cordierite–biotite-rich melanocratic layer (Fig. 3f, g). Foliation of the rock is defined by alternation of the layers as well as tabular aggregates of biotite (Fig. 3g). Garnet is the dominant ferromagnesian mineral in this rock and forms very coarse-grained poikiloblastic aggregates (up to 25 mm) containing inclusions of sapphirine, quartz, rutile, apatite, K-feldspar, plagioclase, zircon and sillimanite. Pale bluish sapphirine (up to 0.8 mm) is pleochroic and occurs mostly as inclusions in the core of garnet grains. Similar to samples MD6-2L2 and MD6-2K, sapphirine shows direct contact with fine-grained quartz (< 0.09 mm). The elongated sapphirine inclusions often occur parallel to the matrix foliation. This further supports an earlier origin of the sapphirine prior to garnet growth. The garnet is in part mantled by corona of cordierite + orthopyroxene (Fig. 3h), a texture similar to that in garnet–cordierite gneiss (sample MD6-1Y). Pale brownish to yellowish pleochroic orthopyroxene occurs as medium- to coarse-grained (up to 1.2 mm) porphyroblasts in the matrix of quartz and cordierite. The mineral is often surrounded by aggregates of secondary biotite (0.1–1.9 mm). Biotite in this association is reddish brown in colour, and occurs together with cordierite and quartz as well as orthopyroxene. Sillimanite occurs as rare tiny inclusions (less than 0.02 mm) within garnet or as medium-grained (up to 0.5 mm) subidioblastic mineral in cordierite around garnet, but is not seen coexisting with orthopyroxene.
3.c. Type C: garnet–cordierite granulite
This rock type is generally composed of garnet (10–20%), cordierite (20–30%), quartz (10–20%), perthite (20–30%), sillimanite (5–10%), orthopyroxene (< 5%) and magnetite/ilmenite (< 5%), with accessory biotite, spinel, sapphirine and zircon. Foliation of the rock is sometimes not obvious, but rarely defined by aligned sillimanite aggregates. The petrological and mineralogical characters of this lithology discussed below are similar to those of the cordierite gneisses in the Achankovil Zone located about 10 km south from this locality (Fig. 1) (Sinha-Roy, Mathai & Narayanaswamy, Reference Sinha-Roy, Mathai and Narayanaswamy1984; Santosh, Reference Santosh1987; Ishii, Tsunogae & Santosh, Reference Ishii, Tsunogae and Santosh2006).
Sample MD6-1Y is a typical garnet-cordierite gneiss from Rajapalaiyam, occurring as a xenolith in alkali-feldspar granite. Garnet (0.7–7 mm) is embayed by coarse-grained (up to 10 mm) pale bluish cordierite (Fig. 3i) with or without vermicular orthopyroxene. Garnet contains rare inclusions of quartz, plagioclase, biotite and sapphirine, although sapphirine and quartz do not coexist. Medium-grained (0.1–1.5 mm) orthopyroxene forms aggregates with opaque minerals (mainly ilmenite with thin magnetite lamella) in the matrix of cordierite. Sillimanite (0.2–4 mm) is prismatic and occurs in the perthite- and quartz-rich portion of the rock. Greenish spinel (0.1–1.0 mm) is irregular in shape and is mostly associated with ilmenite and magnetite.
3.d. Type D: coarse-grained garnet–orthopyroxene granulite
This lithology is characterized by coarse-grained orthopyroxene and garnet in hand specimen (Fig. 2f). The most common assemblage of the rock type is defined by garnet (10–30%), quartz (10–40%), biotite (10–30%), orthopyroxene (10–30%) and cordierite (5–10%). Sapphirine, spinel and sillimanite are minor constituents. A representative sample, MD6-2J, comprises mostly coarse-grained garnet (up to 15 mm), quartz (up to 13 mm), biotite (up to 8 mm) and orthopyroxene (up to 7.5 mm), with minor cordierite, spinel, sapphirine, ilmenite and magnetite. Garnet is subidioblastic and partly corroded, and contains inclusions of fine-grained quartz, sapphirine and rare orthopyroxene as earlier phases (Fig. 3j). Matrix orthopyroxene exhibits brownish to pinkish pleochroism, and occurs mostly as subidioblastic grains associated with corroded garnet. Between the coarse-grained garnet and orthopyroxene, fine- to coarse-grained cordierite (up to 3 mm) is often developed, and rarely orthopyroxene + cordierite symplectite occurs. Brownish biotite is closely associated with orthopyroxene, quartz, K-feldspar and garnet, suggesting their formation during the retrograde stage.
Sapphirine in this sample shows three types of occurrence. The first category is fine-grained (< 0.15 mm) and subidioblastic to idioblastic showing sharp grain contact with quartz, both included in garnet (Fig. 4e). The sapphirine within garnet contains rare inclusions of xenoblastic greenish spinel. Greenish biotite adjacent to the quartz is regarded as a product of a retrograde stage, possibly associated with retrograde fluid activity along cracks of garnet (see Fig. 4e). The second type of sapphirine, medium-grained (0.1–0.9 mm) intergrowth with orthopyroxene (Figs 3k, 4f), occurs as fan-like symplectites around poikiloblastic garnet. The third type of sapphirine is a fine-grained (< 0.2 mm) vermicular mineral within orthopyroxene + cordierite symplectite surrounding garnet (Fig. 3l, m). Fine-grained (< 0.2 mm) dark greenish spinel also occurs in cordierite coronae around garnet (Fig. 3m), together with vermicular sapphirine and orthopyroxene.
4. Mineral chemistry
Chemical analyses of minerals were performed by electron microprobe analyser (JEOL JXA8621) at the Chemical Analysis Division of the Research Facility Centre for Science and Technology, University of Tsukuba, for all minerals. The analyses were performed under conditions of 20 kV accelerating voltage and 10 nA sample current, and the data were regressed using an oxide-ZAF correction program supplied by JEOL. Representative compositions of minerals in the analysed samples are given in online Appendix Tables A1 to A6 at http://journals.cambridge.org/geo. Fe3+ of garnet, orthopyroxene, sapphirine and spinel was calculated based on stoichiometry.
4.a. Garnet
Garnet is a dominant mineral in the pelitic and quartzo-feldspathic granulites of the study area, and is essentially a solid solution of pyrope and almandine (X Mg = Mg/(Fe+Mg) = 0.40–0.61) with low contents of grossular (1.1–4.1 mol.%) and spessartine (0–2.9 mol.%). The core of sapphirine + quartz-bearing poikiloblastic garnet without orthopyroxene + sillimanite corona in sample MD6-2K (Type A) shows the highest pyrope content of Pyr57–61Alm38–40Grs1–2Sps0–2 among the examined samples. In contrast, the core of garnet with orthopyroxene + sillimanite corona in the same sample shows slightly lower pyrope content of Pyr54–55Alm43–44Grs1–2Sps0–1. The pyrope content further decreases toward the rim of the garnet grain near the orthopyroxene + sillimanite corona as Pyr52–53Alm45–46Grs1–2Sps0–1.
Garnet in Type D (e.g. sample MD6-2J) shows marked compositional zoning with Pyr52–53Alm43–45Grs1–3Sps1–2 in the core and Pyr45–51Alm43–49Grs4–5Sps1–2 for the rim adjacent to an orthopyroxene + cordierite corona. The garnet rim adjacent to sapphirine + orthopyroxene symplectite in the same sample is also slightly Mg-poorer than the core as Pyr50–52Alm44–45Grs2–3Sps1–2.
4.b. Orthopyroxene
Orthopyroxene is a common mineral in pelitic granulites. It shows marked compositional variation even within thin-section scale, depending on the mineral association and textural relationship. An orthopyroxene inclusion within the core of garnet in sample MD6-2J shows the highest Al2O3 content of up to 10.3 wt% (Al = 0.44 pfu). The core of the subidioblastic coarse-grained (up to 8 mm in length) orthopyroxene associated with corroded garnet in the same sample is also rich in Al2O3 as ~9.6 wt% (Al = 0.41 pfu), while its rim shows a slightly lower Al2O3 content of 5.9–8.2 wt% (Al = 0.25–0.35 pfu). Such a decrease in Al2O3 content from core to rim of orthopyroxene is a common feature among most of the coarse-grained garnet–orthopyroxene granulite samples examined in this study. Another example is sample MD6-2I2, where the Al2O3 content of the core of the orthopyroxene (9.5–9.9 wt%; Al = 0.41–0.42 pfu) is significantly higher than its rim (7.2–7.9 wt%; Al = 0.31–0.34 pfu). In contrast, some fine- to medium-grained pelitic granulites (e.g. Type B) display no marked variation in orthopyroxene chemistry. For example, the Al2O3 content of the orthopyroxene core (7.6–7.7 wt%) in sample MD6-2E is almost consistent with that of its rim (7.5–8.6 wt%), as well as those associated with the symplectitic phase around garnet (7.8–7.9 wt%).
Orthopyroxene (+ sapphirine) symplectite around garnet in sample MD6-2J (Type D) also contains high Al2O3 (up to 9.7 wt%, Al = 0.41 pfu). In contrast, orthopyroxene symplectite with cordierite around poikiloblastic garnet is less aluminous (5.9–7.1 wt% Al2O3, Al = 0.25–0.31 pfu). X Mg ratios of all the above orthopyroxene varieties are almost consistent, showing a range of 0.69–0.76, despite the marked variations in Al2O3 contents.
4.c. Sapphirine
Sapphirine in the examined samples shows three types of occurrence: (1) small grains (< 0.15 mm) showing grain contact with quartz, with both minerals included in garnet (e.g. sample MD6-2L2; Fig. 3b), (2) medium-grained (0.1–0.8 mm) fan-like sapphirine–orthopyroxene symplectite (sample MD6-2J; Fig. 3k), and (3) fine-grained (< 0.2 mm) vermicular grains as orthopyroxene + cordierite + sapphirine symplectite surrounding garnet (sample MD6-2J; Figs 3l, m). The compositions of sapphirines in the three associations are summarized in Figure 5. Sapphirine included within garnet in sample MD6-2J is slightly magnesian (X Mg = 0.79–0.81) with lower Si (1.54–1.56 pfu) and higher Al (8.6–8.8 pfu) contents as compared with those associated with the fan-like orthopyroxene (X Mg = 0.77–0.80, Si = 1.61–1.63 pfu, Al = 8.4–8.6 pfu) and orthopyroxene + cordierite symplectite (X Mg = 0.77–0.79, Si = 1.58–1.64 pfu, Al = 8.2–8.6 pfu) in the same sample. Sapphirine (+ quartz) occurring within garnet in other samples is also Mg–rich as 0.77–0.78 (sample MD6-2E2), 0.79–0.84 (sample MD6-2L2) and 0.82–0.85 (sample MD6-2K), although the Si content of sapphirine in equilibrium with quartz in the samples has a wide range as 1.52 to 1.63 pfu.
Figure 5. A compositional diagram showing sapphirine chemistry. 7:9:3 and 2:2:1 in the figure imply representative compositions of Mg1.75Al0.45Si0.75O10 and Mg2Al4SiO10, respectively.
4.d. Spinel
Spinel occurs either as a texturally earlier phase in sapphirine coexisting with quartz (e.g. sample MD6-2L2) or as a later phase together with cordierite (sample MD6-2J) around garnet or with ilmenite (sample MD6-2L2). It is principally a solid solution of hercynite and Mg-spinel. The spinel inclusion in sapphirine in sample MD6-2J is Fe-rich as X Mg = 0.44–0.47 with minor Cr2O3 (< 0.03 wt%) and ZnO (0.97–1.00 wt%) contents. Fine-grained dark greenish spinel in cordierite corona around garnet is slightly more Mg-rich as X Mg = 0.55–0.56 than the hercynite included within sapphirine. It also shows negligibly small Cr and Zn contents. Spinel associated with quartz and sapphirine in sample MD6-2L2 (Fig. 4g) also shows low ZnO and Cr2O3 contents (< 0.33 and < 0.11 wt%, respectively). In contrast, an isolated spinel inclusion within garnet in sample MD6-2L2 has the highest X Mg ratio of 0.61–0.63, and high Cr2O3 (3.4–3.6 wt%) and ZnO (2.9–3.2 wt%) contents.
4.e. Other minerals
Cordierite occurs around porphyroblastic garnet as a later product. It shows a uniform magnesian composition in all samples, with X Mg in the range of 0.81–0.87. K-feldspar and plagioclase occur as mesoperthite or perthite in most examined samples. K-feldspar is rich in orthoclase and that in pelitic and quartzo-feldspathic gneisses has a broad compositional range of An0–1Ab5–15Or84–95 depending on samples. Plagioclase in pelitic and quartzo-feldspathic samples is relatively more albite-rich (An8–17Ab79–92Or0–3) than that in the alkali-feldspar granite (An20–27Ab75–78Or0–2).
5. Metamorphic reaction textures
We evaluate the mineral assemblages and textures described in the previous section using the (K2O–)FeO–MgO–Al2O3–SiO2(–H2O) ((K)FMAS(H)) system. The textures in the examined samples are divided into three major stages: prograde, peak and retrograde, based on the host–inclusion relationships and grain shape and size of the minerals in the rocks.
5.a. Prograde reactions
Several prograde reaction textures are indicated by the presence of inclusions in coarse-grained poikiloblastic minerals in the matrix, namely garnet and orthopyroxene. Garnet in samples MD6-2J, E2, K and L2 contain equilibrium sapphirine + quartz assemblages (Figs 3b, c, 4a, b), suggesting that the two minerals formed in the stability field of sapphirine and quartz (T > 1000°C; Hensen & Green, Reference Hensen and Green1973; Bertrand, Ellis & Green, Reference Bertrand, Ellis and Green1991; Kelsey et al. Reference Kelsey, White, Holland and Powell2004). The reaction to form the assemblage could be inferred from xenoblastic spinel inclusions in idioblastic sapphirine in equilibrium with quartz in sample MD6-2J (Fig. 4e). This texture could imply that spinel and quartz were once in equilibrium, and that later sapphirine formed between the two minerals. There are two possible FMAS divariant reactions to explain the formation of sapphirine from spinel and quartz:
As sillimanite is absent in the texture, reaction (2) might not be realistic, although it is possible to interpret that sillimanite has been totally consumed by the progress of the reaction. We therefore infer that reaction (1) took place to form the sapphirine–quartz assemblage in sample MD6-2J within the stability field of sapphirine + quartz.
Greenish spinel with a composition similar to that occurring in sapphirine in sample MD6-2J is seen within poikiloblastic garnet in sample MD6-2L2. Although Tsunogae & Santosh (Reference Tsunogae and Santosh2006) did not identify a spinel + quartz association in this sample, our further detailed examination of the spinel + sapphirine + quartz multiphase inclusion in garnet during this study revealed rare inclusions of quartz in spinel without any reaction boundary between them (Fig. 4g). This texture suggests that spinel + quartz was in equilibrium during high-grade metamorphism. Such an equilibrium spinel + quartz assemblage has been reported from several localities in the Madurai Block and elsewhere in UHT terranes, as evidence of peak UHT metamorphism (Tadokoro et al. Reference Tadokoro, Tsunogae, Santosh and Yoshimura2007; Tsunogae et al. Reference Tsunogae, Santosh, Ohyama and Sato2008b; Santosh, Sajeev & Li, Reference Santosh, Sajeev and Li2006; Santosh et al. Reference Santosh, Tsunogae, Li and Liu2007; Shimizu, Tsunogae & Santosh, Reference Shimizu, Tsunogae and Santosh2009). A similar spinel + quartz association has also been recorded in high-T/low-P metamorphic terranes such as the Namaqualand belt (e.g. Waters, Reference Waters, Vielzeuf and Vidal1990), South Africa, although sapphirine is absent in such occurrences.
5.b. Peak assemblage
Based on the petrographic evidence discussed above, particularly the porphyroblastic nature of garnet, orthopyroxene, sillimanite and mesoperthite, we infer that the peak mineral assemblage of this area is represented by garnet + orthopyroxene ± sillimanite ± mesoperthite. Sapphirine + quartz and spinel + quartz assemblages enclosed in garnet porphyroblast are also regarded as the peak assemblages which were stable at the UHT condition.
5.c. Retrograde reactions
Retrograde reactions are observed as corona and symplectite textures that formed around early coarse-grained minerals. One typical example is a complex intergrowth of orthopyroxene and sillimanite around poikiloblastic garnet in which sapphirine + quartz equilibrium occurs (Figs 3d, 4c, d). There are several unique textural characters that are useful for inferring the reaction that formed the orthopyroxene + sillimanite corona: (1) fine-grained garnet and quartz inclusions in orthopyroxene and sillimanite (Fig. 4d), (2) absence of corona texture around garnet without sapphirine inclusion, (3) increase of almandine content in garnet toward its rim. The above evidence suggests the progress of the following FMAS continuous reaction:
The reaction has been reported in previous papers (e.g. Harley, Hensen & Sheraton, Reference Harley, Hensen and Sheraton1990) as evidence of isobaric cooling from the stability of sapphirine + quartz.
One of the most spectacular corona textures can be found as an intergrowth of brownish orthopyroxene and bluish sapphirine around garnet in sample MD6-2J (Figs 3k, 4f). Cordierite is also present between garnet and the intergrowth (Fig. 4f). The fan-like sapphirine–orthopyroxene symplectite has also been reported from Ganguvarpatti (Mohan, Ackermand & Lal, Reference Mohan, Ackermand and Lal1986; Mohan & Windley, Reference Mohan and Windley1993; Sajeev, Osanai & Santosh, Reference Sajeev, Osanai and Santosh2004; Tamashiro et al. Reference Tamashiro, Santosh, Sajeev, Morimoto and Tsunogae2004) and Palni Hills (Raith, Karmakar & Brown, Reference Raith, Karmakar and Brown1997; Prakash & Arima, Reference Prakash and Arima2003) of the central Madurai Block as a retrograde texture. Harley, Hensen & Sheraton (Reference Harley, Hensen and Sheraton1990) also reported the texture from Forefinger Point in the Rayner Complex of Antarctica as evidence of the progress of the following reaction:
Another common example of retrograde reaction texture is cordierite ± orthopyroxene coronae developed around garnet (e.g. Fig. 3e, h). Such textures suggest the following FMAS continuous reactions:
Progress of reactions (5) and (6) is supported by the decrease of Mg content in garnet from the core to the rim. The above two reactions have been reported from many granulite terranes in the world and are often taken as evidence in favour of near-isothermal decompression after the peak metamorphism (Harley, Reference Harley1989, Reference Harley2004).
Fine-grained (< 0.2 mm) dark greenish spinel occurs in cordierite coronae around garnet (Fig. 3i). The texture may suggest the following FMAS continuous reaction:
The close association of greenish spinel and magnetite is common in the pelitic granulites (Figs 3n, 4h) and rare in the alkali-feldspar granite at Rajapalaiyam. The texture suggests a possible origin of the spinel as external granule oxidation–exsolution from magnetite, that is, exsolution involving oxidation and limited migration of the exsolved material to the edge of the grain as suggested by Grew, Hiroi & Shiraishi, (Reference Grew, Hiroi and Shiraishi1990) to explain the origin of the högbomite–magnetite association, a mechanism that was originally proposed by Buddington & Lindsley (Reference Buddington and Lindsley1964) for the formation of ilmenite granules in magnetite. Although such an exsolved spinel occasionally shows a direct grain contact relationship with quartz (e.g. sample MD6-2L3; Fig. 4h), it is regarded as a metastable assemblage. In contrast, as sapphirine associated with quartz in the examined samples (e.g. Fig. 4a, b, e) is not related to Fe–Ti oxides, sapphirine and quartz discussed in this study are regarded to be an equilibrium assemblage.
Brownish biotite is closely associated with orthopyroxene, quartz, K–feldspar and garnet (Fig. 3o), suggesting the progress of the following retrograde reaction (8), probably at the final stage of high-grade metamorphism:
The H2O-bearing fluid probably infiltrated from an external source during exhumation or was derived from crystallized melt (e.g. Carrington & Harley, Reference Carrington and Harley1995; Harley, Reference Harley2008).
6. Geothermobarometry
Several conventional geothermobarometers are applicable for the examined samples to infer the peak and retrograde metamorphic conditions because of the presence of a number of coexisting ferromagnesian minerals. We briefly summarize below the computed P–T conditions from thermobarometers that are appropriate for the assemblages in the studied samples.
6.a. Garnet–orthopyroxene(–plagioclase–quartz)
This thermometer was applied to porphyroblastic garnet and orthopyroxene in several pelitic samples (e.g. MD6-2E2, MD6-2J and MD6-2K). The estimated temperature ranges are 930–990°C, 800–860°C and 940–1000°C, respectively, at 8 kbar using the method of Lee & Ganguly (Reference Lee and Ganguly1988). Application of other methods (e.g. Aranovich & Berman, Reference Aranovich and Berman1997) also gave temperatures below 1000°C, although slightly lower than the results of Al solubility in orthopyroxene discussed in the next section. It has been emphasized in earlier studies that the Fe–Mg exchange geothermometers rarely preserve the peak temperatures due to the effect of later (retrograde) Fe–Mg exchange between the minerals (Fitzsimons & Harley, Reference Fitzsimons and Harley1994; Bégin & Pattison, Reference Bégin and Pattison1994; Harley, Reference Harley, Treloar and O'Brian1998).
Pressure was estimated by the experimental calibration of the garnet–orthopyroxene–plagioclase–quartz barometer by Perkins & Chipera (Reference Perkins and Chipera1985), yielding values of 8.2–8.4 kbar (sample MD6-2J) and 7.8–8.3 kbar (sample MD6-2K) at 900°C. Moecher, Essene & Anovitz (Reference Moecher, Essene and Anovitz1988) improved this geobarometer using new thermodynamic and experimental data. Calculated results using their method are slightly higher than, but almost consistent with, those obtained from Perkins & Chipera's (Reference Perkins and Chipera1985) geothermometer, yielding pressure ranges of 11.2–11.3 kbar (sample MD6-2J) and 9.6–10.1 kbar (sample MD6-2K) at 900°C. The higher pressure range obtained from the Moecher, Essene & Anovitz (Reference Moecher, Essene and Anovitz1988) method is consistent with the sapphirine + quartz + garnet + orthopyroxene + sillimanite association in sample MD6-2K, which is regarded to be stable at P > 9 kbar (e.g. Hensen & Harley, Reference Hensen, Harley, Ashworth and Brown1990).
6.b. Al solubility in orthopyroxene
The Al content and X Mg of orthopyroxene coexisting with garnet, sillimanite (or cordierite) and quartz are potential indicators of P–T conditions (Hensen & Harley, Reference Hensen, Harley, Ashworth and Brown1990). For the estimates, we adopted core compositions of porphyroblastic coarse-grained orthopyroxene or prograde orthopyroxene inclusions in garnet because these are likely to preserve the near-peak compositions. Application of the revised isopleths of Harley (Reference Harley2004), based on new experimental data (e.g. Hollis & Harley, Reference Hollis and Harley2003) on the core of the orthopyroxene in sample MD6-2J (X Mg = 0.70–0.71, X Al = 0.19–0.21), indicates UHT peak metamorphic conditions of 990–1020°C. In contrast, the isopleth of Baldwin et al. (Reference Baldwin, Powell, Brown, Moraes and Fuck2005) yielded slightly lower P–T conditions of 900–950°C and 7.8–8.2 kbar. Recently, Harley (Reference Harley2008) revised the isopleth considering the effect of H2O activity in cordierite. Application of the new method to our orthopyroxene with 10.5 wt% Al2O3 yielded a P–T condition of about 980–990°C at about 8 kbar, which is nearly comparable to the result from the isopleth of Harley (Reference Harley2004).
The core of the coarse-grained orthopyroxene in sample MD6-2I2 also gave high temperatures of 1010–1020°C at 9–10 kbar when the isopleths of Harley (Reference Harley2004) are applied. The highest temperature conditions are recorded in prograde orthopyroxene inclusion in garnet in sample MD6-2J (up to 10.3 wt% Al2O3) as 1050–1070°C at 8.5–9.5 kbar using the same isopleths. Such T > 1000°C temperature conditions obtained through this method are higher than the results from garnet–orthopyroxene geothermometers.
Compositions of orthopyroxene are plotted on a P–T diagram (Fig. 6) with the X Al isopleths of Harley (Reference Harley2004). The systematic compositional varieties of orthopyroxene (based on Al2O3 content) discussed above are unique for the pelitic granulites of the studied area. A comparable study of compositional variation depending on textural association of orthopyroxene has also been reported from a pelitic granulite at Ganguvarpatti in southern India (Sajeev, Osanai & Santosh, Reference Sajeev, Osanai and Santosh2004; Tamashiro et al. Reference Tamashiro, Santosh, Sajeev, Morimoto and Tsunogae2004).
Figure 6. P–T diagram showing X Al isopleths of orthopyroxene after Harley (Reference Harley2004). Numbers in circles indicate X Al × 100 values as discussed in Harley (Reference Harley2004). Black squares imply ranges of X Al values calculated from three variety of orthopyroxene in sample MD6-2J. A – inclusion orthopyroxene in garnet, B – matrix coarse-grained orthopyroxene, C – retrograde orthopyroxene (with cordierite) around garnet. Arrays indicate approximate P–T path inferred from orthopyroxene compositions.
6.c. Ternary feldspar geothermometer
Recent petrological studies on UHT metamorphic rocks identify feldspars with exsolution texture as an important indicator of the minimum temperatures of UHT metamorphism (e.g. Hokada, Reference Hokada2001; Harley, Reference Harley2004, Reference Harley2008). In this study, we applied ternary feldspar geothermometry using mesoperthite and perthite in pelitic and quartzo-feldspathic gneisses to obtain near-peak metamorphic temperatures. The calculation technique involves the determination of early single-phase feldspar and the application of the thermometer following the method described by Hokada (Reference Hokada2001). Examples of mesoperthite and perthite grains applied for the temperature estimates are shown in Figure 7 as a ternary An–Ab–Or diagram with plots of the compositions of host plagioclase and lamellar K-feldspar, together with early single-phase feldspar inferred from host–lamella volume proportions estimated from back-scattered images (e.g. Fig. 4h). Compositions of host and lamella phases are confirmed to be homogeneous within single mesoperthite and perthite grains.
Figure 7. An–Ab–Or diagram showing compositions of host plagioclase and lamella K-feldspar together with early single-phase feldspar inferred from host–lamella volume proportions (see text for details). Ternary miscibility gaps are estimated for 8 kbar using the method of Fuhrman & Lindsley (Reference Fuhrman and Lindsley1988).
Using the ternary feldspar geothermometer of Fuhrman & Lindsley (Reference Fuhrman and Lindsley1988), we obtained a temperature range of about 950–1050°C for the mesoperthite formation. The results are consistent with those obtained from Al-in-orthopyroxene geothermometers.
6.d. Garnet–cordierite geothermobarometers
As cordierite is obviously a product of retrograde metamorphism in the present rocks, this geothermometer mostly provides the post-peak conditions. Compositional data of rim garnet and corona cordierite were applied for the calculation. Several calibrations are available for this thermometer, although most of them yielded unusually low estimates as compared with the temperature estimates from other assemblages discussed in earlier sections. For example, the experimental geothermometer of Perchuk & Lavrent'eva (Reference Perchuk, Lavrent'eva and Saxena1983) and experimentally calibrated geothermometer of Nichols, Berry & Green (Reference Nichols, Berry and Green1992) gave temperature ranges of 650–660°C and 720–740°C, respectively, at 5 kbar when applied to the assemblage in sample MD6-1G. We also attempted pressure estimations using the garnet–cordierite–sillimanite–quartz geobarometer of Nichols, Berry & Green (Reference Nichols, Berry and Green1992) and obtained a pressure range of 4.6–5.3 kbar at 700°C. Obviously, these estimates correlate to the retrograde evolution of these rocks.
7. Discussion
7.a. UHT metamorphism of the Rajapalaiyam area
Petrological data from the pelitic and quartzo-feldspathic granulites of Rajapalaiyam within the Madurai Block of southern India provide several lines of evidence which indicate that this region underwent extreme crustal metamorphism at UHT conditions. One robust piece of evidence is the sapphirine + quartz grain contact within porphyroblastic garnet in many of the pelitic and quartzo-feldspathic gneiss samples examined from this study area. This assemblage was first reported by Tateishi et al. (Reference Tateishi, Tsunogae, Santosh and Janardhan2004) from this area in a single sample, and is further confirmed in this study from more detailed examination of a large sample suite. The sapphirine is in textural equilibrium with quartz and both the minerals occur in sharp grain contact. Available phase equilibrium data suggest that sapphirine + quartz is stable at T > 1030°C at 9.5 kbar (Hensen & Green, Reference Hensen and Green1973) or T > 1050°C at 11 kbar (Bertrand, Ellis & Green, Reference Bertrand, Ellis and Green1991). Recent thermodynamic calculations in the KFMASH system also support the high-temperature nature of this assemblage (T > 1005°C: Kelsey et al. Reference Kelsey, White, Holland and Powell2004; Kelsey, Reference Kelsey2008). Similar sapphirine + quartz intergrowth enclosed in garnet has been reported from several granulite terranes such as In Ouzzal (Ouzegane & Boumaza, Reference Ouzegane and Boumaza1996), Highland Complex (Sajeev & Osanai, Reference Sajeev and Osanai2004) and Lützow Holm Complex (Yoshimura, Motoyoshi & Miyamoto, Reference Yoshimura, Motoyoshi and Miyamoto2003) and in all cases, the association has been taken as clear evidence for peak UHT metamorphism.
Direct grain contact of spinel and inclusion quartz in sample MD6-2L2 (Fig. 4g) has also been regarded as evidence of UHT metamorphism (e.g. Hensen and Harley, Reference Hensen, Harley, Ashworth and Brown1990) at relatively high-pressure conditions. However, available experimental and thermodynamic data indicate that Zn in spinel extends the stability field of spinel + quartz toward higher pressure and/or lower temperature (e.g. Nichols, Berry & Green, Reference Nichols, Berry and Green1992). Harley (Reference Harley2008) thus argues that the occurrence of spinel + quartz alone may not be diagnostic evidence of UHT metamorphism. As discussed in a previous chapter, the spinel grain associated with quartz has very low ZnO and Cr2O3 contents (< 0.33 and < 0.11 wt%, respectively) and therefore we consider this association as additional evidence of UHT metamorphism.
The high Al2O3 content of orthopyroxene (up to 10.3 wt%, sample MD6-2J) enclosed within garnet in pelitic granulite is an additional indicator of UHT peak metamorphism in this area. As the host garnet also contains the sapphirine + quartz assemblage, we consider that the orthopyroxene was occluded by garnet growth while the rock was in the stability field of sapphirine + quartz. This hypothesis is consistent with the estimated temperature conditions of T > 1000°C for the orthopyroxene. Available experimental data also support the high-T stability of Al-rich orthopyroxene (e.g. Hollis & Harley, Reference Hollis and Harley2003).
Further evidence of UHT metamorphism has been confirmed by ternary feldspar geothermometry of mesoperthite. As mesoperthite is a major constituent of the quartzo-feldspathic domain of the pelitic granulites of the present study, we consider the mineral to be useful for calculating the temperature of garnet-absent rocks. The computed temperatures (T = 900–950°C) are slightly lower than the temperature range from Al solubility in orthopyroxene and the stability field of sapphirine + quartz. Harley (Reference Harley2004) discussed this as a common feature where a ternary feldspar geothermometer generally yields a slightly lower temperature (about 100°C) than those estimated from the stability field of sapphirine and quartz. P–T conditions obtained independently from several conventional geothermometers further support the UHT peak metamorphism in this area. For example, the garnet–orthopyroxene geothermometer as calibrated by Lee & Ganguly (Reference Lee and Ganguly1988) yielded temperatures of up to 1000°C.
Combining the various lines of petrological evidence, we infer that the granulites in the Rajapalaiyam area within the southern part of the Madurai Block experienced T > 1000°C (at 8–10 kbar) UHT peak metamorphism. Our estimates are almost consistent with the available P–T data from the central and northern parts of the Madurai Block (e.g. Brown & Raith, Reference Brown and Raith1996; Raith, Karmakar & Brown, Reference Raith, Karmakar and Brown1997; Satish-Kumar, Reference Satish-Kumar2000; Sajeev, Osanai & Santosh, Reference Sajeev, Osanai and Santosh2004; Tsunogae & Santosh, Reference Tsunogae and Santosh2003; Koshimoto, Santosh & Tsunogae, Reference Koshimoto, Santosh and Tsunogae2004; Tamashiro et al. Reference Tamashiro, Santosh, Sajeev, Morimoto and Tsunogae2004; Santosh & Sajeev, Reference Santosh and Sajeev2006; Tsunogae et al. Reference Tsunogae, Santosh, Ohyama and Sato2008b), suggesting that the entire Madurai Block underwent similar UHT metamorphism. Recent studies of relic minerals in garnet within UHT granulites from the northern Madurai Block along the Palghat-Cauvery Shear Zone system led to the identification of high-Mg staurolite providing evidence for prograde high-pressure (P > 12 kbar) near-eclogite-facies metamorphism (e.g. Shimpo, Tsunogae & Santosh, Reference Shimpo, Tsunogae and Santosh2006; Collins et al. Reference Collins, Clark, Sajeev, Santosh, Kelsey and Hand2007; Tsunogae et al. Reference Tsunogae, Santosh, Ohyama and Sato2008b; Kanazawa et al. Reference Kanazawa, Tsunogae, Sato and Santosh2009). Such an indication of a prograde high-pressure event has not been traced from the present locality.
7.b. P–T trajectory of the Rajapalaiyam area
A combination of detailed petrography on reaction textures and available data on petrogenetic grids is employed here to construct the exhumational P–T trajectory of the granulites from Rajapalaiyam. In this study, we adopted the revised petrogenetic grids of Kelsey (Reference Kelsey2008) in the FMAS system to construct a qualitative P–T path for this crustal segment (Fig. 8).
Figure 8. Petrogenetic grid in the FMAS system (modified after Kelsey, Reference Kelsey2008). Arrows indicate an approximate P–T trajectory of sapphirine-bearing granulites from Rajapalaiyam. Isobaric cooling is tentatively inferred to be around 7.5 kbar followed by decompression toward ~6.5 kbar at ~980°C, but the cooling probably took place at P > 9 kbar (see Fig. 6). See text for further discussion.
Prograde history of the Rajapalaiyam rocks is difficult to trace because most of the early mineral assemblages might have recrystallized at the peak UHT metamorphism at temperatures exceeding 1000°C. We therefore examined the inclusion minerals within garnet porphyroblasts. A spinel inclusion within sapphirine in equilibrium with quartz, which in turn is included in poikiloblastic garnet (sample MD6-2J; Fig. 4e), is an important key assemblage to infer the prograde P–T path. The texture suggests that the stability of spinel + quartz pre-dates that of sapphirine + quartz. Such an early spinel + quartz assemblage is probable because of the occurrence of direct grain contact of spinel and quartz in garnet in sample MD6-2E2 (Fig. 4g). The spinel–sapphirine–quartz association was first reported by Tsunogae & Santosh (Reference Tsunogae and Santosh2006) from the present locality and is further confirmed in this study, although Tsunogae & Santosh (Reference Tsunogae and Santosh2006) did not report direct contact of spinel + quartz (cf. Fig. 4g). Although the stability field of spinel + quartz is not present in the FMAS grid of Kelsey (Reference Kelsey2008) (Fig. 8), the mineral pair could be stable in the field of sapphirine + spinel + quartz, suggesting cooling from T ~ 1050°C or a pressure increase at around 1020–1040°C along an anticlockwise P–T path (Fig. 8). Although the possibility exists that the change of spinel + quartz to sapphirine + quartz may simply be due to a change of the chemical system, recent pseudosection-based analysis of similar textures from the UHT rocks in the Palaeoproterozoic khondalite belt of North China Craton has also yielded a well-defined anticlockwise exhumation path (Santosh et al. Reference Santosh, Sajeev, Li, Liu and Itaya2009b).
After the peak UHT metamorphism T > 1000°C, the rocks underwent near-isobaric cooling from the stability field of sapphirine + quartz + garnet to that of orthopyroxene + sillimanite (Fig. 8). One key mineral texture to infer the post-peak metamorphic history is an orthopyroxene + sillimanite corona around garnet in the matrix of quartz and mesoperthite in sample MD6-2K (Figs 3c, 4c). The texture suggests that, after the T > 1000°C peak metamorphism, the rock isobarically cooled at pressure above the spinel invariant point [Spl] in the FMAS petrogenetic grid (Fig. 8). As shown in Figure 6, the invariant point of [Spl] in the FMAS system corresponds to the P–T condition of 1050°C at 10.5 kbar (Harley, Reference Harley2004). We therefore infer a stage of isobaric cooling at around > 10 kbar. In contrast, Kelsey et al. (Reference Kelsey, White, Holland and Powell2004) have calculated the position of [Spl] at about 6.5 kbar and 987°C using thermodynamic computations, in the case where the cordierite is regarded as anhydrous. However, the results of cordierite analysis indicate the possible presence of CO2/H2O in the mineral, suggesting that 6.5 kbar is a minimum pressure value. Independent pressure estimates for the garnet–orthopyroxene assemblage using the method of Moecher, Essene & Anovitz (Reference Moecher, Essene and Anovitz1988) suggest a high-P condition of 9.6–11.3 kbar, which supports the location of the spinel invariant point at P > 9 kbar (Hensen & Harley, Reference Hensen, Harley, Ashworth and Brown1990; Harley, Reference Harley2004, Reference Harley2008). In Figure 8, isobaric cooling is tentatively inferred to be around 7.5 kbar, followed by decompression toward around 6.5 kbar at ~980°C, but the isobaric cooling could have probably taken place at P > 9 kbar.
Various corona textures surrounding porphyroblastic garnet have been documented in this study; these are mostly regarded as evidence of decompression following the higher-pressure peak conditions. Among them are orthopyroxene–cordierite, orthopyroxene–cordierite–spinel, orthopyroxene–sillimanite–sapphirine and sapphirine–orthopyroxene symplectites. Medium-grained aggregates of cordierite around garnet are also regarded as a typical decompression texture subsequent to the high-grade event (e.g. Harley, Reference Harley, Treloar and O'Brian1998). A garnet–cordierite–sillimanite–quartz assemblage records the lowest P–T condition of T = 650–750°C and P = 4.5–5.5 kbar, probably reflecting a retrograde stage. Our detailed petrological study of the Rajapalaiyam granulites thus suggests UHT metamorphism and a multistage exhumation history, which is consistent with previous reports from the central part of the Madurai Block (e.g. Sajeev, Osanai & Santosh, Reference Sajeev, Osanai and Santosh2004; Tamashiro et al. Reference Tamashiro, Santosh, Sajeev, Morimoto and Tsunogae2004). Such an isobaric cooling path followed by decompression is consistent with the P–T trajectory obtained by X Al isopleths of orthopyroxene (Fig. 7).
7.c. ‘Hot orogens’ during the Gondwana assembly
Petrological and geochronological data on granulite facies terranes from various parts of the world indicate that the formation of UHT rocks is associated with major plate tectonic processes, particularly continent–continent collision and subsequent extensional collapse attending the assembly of major supercontinents (e.g. Harley, Reference Harley1989, Reference Harley, Treloar and O'Brian1998, Reference Harley2004, Reference Harley2008; Santosh et al. Reference Santosh, Tsunogae, Li and Liu2007; Santosh & Omori, Reference Santosh and Omori2008a, Reference Santosh and Omorib; Brown, Reference Brown2007; Kelsey, Reference Kelsey2008). Recent fluid inclusion studies have revealed the occurrence of primary CO2-rich fluids trapped within the UHT minerals in various terranes (e.g. Tsunogae et al. Reference Tsunogae, Santosh, Osanai, Owada, Toyoshima and Hokada2002, Reference Tsunogae, Santosh, Dubessy, Osanai, Owada, Hokada, Toyoshima, Satish-Kumar, Motoyoshi, Osanai, Hiroi and Shiraishi2008a; Tsunogae, Santosh & Dubessy, Reference Tsunogae, Santosh and Dubessy2008; Santosh & Omori, Reference Santosh and Omori2008a; Santosh et al. Reference Santosh, Tsunogae, Ohyama, Sato, Li and Liu2008; Ohyama, Tsunogae & Santosh, Reference Ohyama, Tsunogae and Santosh2008). Such CO2-rich fluids, probably derived from sub-lithospheric sources, have been considered to be instrumental in buffering the water activity and generating granulite facies assemblages at high- and ultrahigh-temperature conditions (e.g. Santosh & Omori, Reference Santosh and Omori2008a, Reference Santosh and Omorib and references therein). An evaluation of the tectonic settings under which UHT rocks are generated using modern analogues shows that divergent tectonics, both post-collisional extension and rifting, play a crucial role, particularly in addressing the extreme thermal anomalies and the involvement of CO2 possibly liberated from a carbonated tectosphere (Santosh & Omori, Reference Santosh and Omori2008b). The collisional amalgamation of continents within a supercontinental assembly and the post-collisional extension account well for the generation of metamorphic rocks under extreme thermal conditions and their dry mineral assemblage buffered by CO2 flushed out from a decarbonating tectosphere. The subsequent extensional collapse is also reflected in a series of decompression textures as recorded in the rocks of the present study.
In a recent study, Santosh et al. (Reference Santosh, Maruyama, Komiya, Yamamoto, Kusky, Zhai and Xiao2009a) proposed a general two-fold classification of orogens based on thermal history and temporal constraints. Based on the thermal character, they grouped orogens over the globe into three major categories: cold, hot and ultra-hot orogens. Santosh et al. (Reference Santosh, Maruyama, Komiya, Yamamoto, Kusky, Zhai and Xiao2009a) also proposed two extreme cases of super-cold and super-hot orogens which are unrealistic in this planet. The mineral assemblages and P–T conditions recorded from the present study area in southern India provide a typical example for an ultra-hot orogen according to the above classification. Similar UHT metamorphic assemblages have been widely reported from Late Neoproterozoic–Cambrian orogenic belts in different parts of the world (cf. Kelsey, Reference Kelsey2008; Santosh & Omori, Reference Santosh and Omori2008a, Reference Santosh and Omorib; Harley, Reference Harley2008). This clearly suggests that the Pan-African orogeny witnessed the generation of several hot and ultra-hot orogens in a broad time span during Late Neoproterozoic–Cambrian times. A recent plate tectonic perspective of the architecture of the Neoproterozoic–Cambrian collisional orogens suggests a Pacific-type orogeny involving subduction and closure of the intervening ocean prior to exhumation and final Himalayan-type collisional suturing (Santosh, Maruyama & Sato, Reference Santosh, Maruyama and Sato2009), which probably account for the anticlockwise P–T paths recorded from metamorphic mineral assemblages including the present study. The detailed characterization of hot orogens in Gondwana fragments constitutes an important theme for future research in understanding the crustal evolution history during this important time span in Earth history.
Acknowledgements
We express our sincere thanks to the late Prof. A. S. Janardhan and Ms Preetha Warrier for valuable field support. We also thank the staff at Gondwana Research Office in Trivandrum for their helpful support. Ms Keiko Tateishi is acknowledged for her effort on thin-section preparation, and Dr N. Nishida for his assistance on microprobe analyses. T. Tsunogae thanks the Geology Department of the University of Johannesburg, and M. Santosh thanks Kochi University (Japan) for facilities and support. This is a contribution to the Grant-in-Aid from the Japanese Ministry of Education, Sports, Culture, Science and Technology to Tsunogae (Nos 17340158, 20340148) and Santosh (No. 17403013), and JSPS-INSA Joint Research Program (No. BDD20023).
