Palghat-Cauvery Shear Zone System Research Articles

Vestiges of Precambrian subduction in the south Indian shield? - A seismological perspective

Investigation of large scale suture zones in old continental interiors offers insights into the evolution of continents. The Dharwar Craton (DC) and the Southern Granulite Terrain(SGT) of the Indian shield represent large segments of Precambrian middle to lower crust and preserve a geological record spanning from Mesoarchean to Cambrian. This study illuminates the deep structure of the Palghat-Cauvery Shear Zone System (PCSS) and the Palghat-Cauvery Suture Zone (PCSZ) that comprise crustal-scale structures related to multiple episodes of orogeny, crust formation and reworking. We utilize here 3202 high quality P-receiver functions computed using new data from a 23 station seismic network operated by us. Results show a thick (>38 km) mafic (Poisson's ratio >0.25) crust beneath the SGT. The change in crustal thickness is gradual, with a shallower Moho towards the south of PCSZ. We found little evidence for drastic changes in crustal thickness across prominent shear zones like the PCSZ and Moyar-Bhavani. Few seismic stations located along these boundaries have shown evidence for dipping reflectors around 8–20 km depth, with strikes matching well with the trends of surface geological sutures. We opine that these suture zones do not show indications of a terrane boundary. However, a drastic change in the crustal thickness is observed around the prograde metamorphic transition zone or broadly, the “Fermor line”, which separates rocks of Chanockitic (Orthopyroxene bearing granitoid) and non-Charnockitic (Orthopyroxene-free granitoid) mineral assemblage, further north beneath the DC. We suggest that thicknening of crust north of Moyar-Attur Shear Zone (MASZ) and around Fermor line is related to subduction processes operative during the Precambrian.

Element Mobility and Behaviour of Zircon during HT Metasomatism of Ferroan Basic Granulite at Ayyarmalai, South India: Evidence for Polyphase Neoarchaean Crustal Growth and Multiple Metamorphism in the Northeastern Madurai Province

An exposure of polyphase, Neoarchaean–early Palaeoproterozoic basement rocks has been identified in the northeastern part (10°53·44'N, 78°22·88'E) of the Madurai Province. The dominant migmatitic charnockite contains older enclaves of isoclinally folded syenitic biotite gneiss and boudinaged mafic dykes. A highly evolved (Mg# 10) and high field strength element (HFSE) + rare earth element (REE)-enriched ferroan basic dyke body exhibits a distinct mineralogical zonation produced during intense syn-boudinage infiltration metasomatism, coeval with high-grade metamorphism and anatexis of the host charnockite. Core domains that preserve an anhydrous granulite assemblage (domain A: garnet, clinopyroxene and minor plagioclase) grade through a narrow mineralogical transition zone (domain B) into a broad amphibole-rich rind (domain C: ferropargasite + K-feldspar ± plagioclase, quartz). Pseudosection modelling and thermobarometry yields P–T estimates of ∼800 °C, 8 kbar for the granulite-facies metamorphism (M1) and ∼730 °C, 7 kbar for the high-grade metasomatism (M2). The changes in fabric, mineral assemblage and whole-rock chemistry across domains A to C reveal near-isovolumetric–isochoric conditions of infiltration-driven metasomatism and an unusual enrichment in large ion lithophile elements (LILE), REE, and HFSE, causing prolific neoblastesis of apatite and zircon, and attest attainment of chemical equilibrium. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analyses for major constituents and accessory phases in the metasomatic domains provide a new set of equilibrium distribution coefficients for the REE. LA-ICP-MS spot analyses of zircon in well-documented microstructural settings identified the following events: ∼2·67 Ga: intrusions of foid-bearing syenite and ferroan basic dykes during crustal extension; ∼2·6 Ga: high-grade metamorphism M1; ∼2·6 Ga: emplacement of voluminous arc-type intrusions (the protolith of charnockite); ∼2·48 Ga: high-grade metamorphism and anatexis M2 of the charnockite protolith and metasomatism of the ferroan metabasite. A distinct Pan-African overprint (M3, ∼610 Ma, 520 Ma) of the Palaeoproterozoic high-pressure rocks is manifested by hydration related to the exhumation of the deep-seated granulites to mid-crustal levels. This study confirms the continuation of Neoarchaean crust farther south beyond the perceived Palghat Cauvery shear zone system and contradicts the view that this shear zone system represents the Neoproterozoic–Cambrian suture zone along which the Mozambique Ocean was closed.

Cryogenian (∼830 Ma) mafic magmatism and metamorphism in the northern Madurai Block, southern India: A magmatic link between Sri Lanka and Madagascar?

The northern Madurai Block, southern India, lies directly south of, and partly deformed by, the Palghat–Cauvery Shear Zone System (PCSS) – a potential suture of the Neoproterozoic Mozambique Ocean. The Kadavur gabbro-anorthosite complex lies south of the PCSS, in the northern Madurai Block, and crystallized at 829 ± 14 Ma (LA-ICPMS zircon data) in a supra-subduction zone setting. The complex contains zircon ε Hf(t) values of −12.5 to −8.6 that represent Palaeoproterozoic T (DM) model ages (2.3–2.5 Ga). These broadly agree with a whole rock neodymium T (DM) model age of 2287 Ma. Oxygen isotope δ 18O ratios range from 5.82‰ and 6.74‰. The parental magma for the gabbro-anorthosites are interpreted to be derived from a juvenile Neoproterozoic mantle contaminated by Mesoarchaean igneous infra-crustal sources. The gabbro-anorthosites intrude quartzites with dominantly Palaeoproterozoic detrital zircons that contain Neoarchaean and Mesoarchaean hafnium model ages. These quartzite zircons contain metamorphic rims that yield an age of 843 ± 23 Ma demonstrating the autochthonous nature of the gabbro-anorthosite complex. Later felsic magmatism is recorded by the 766 ± 8 Ma crystallisation age of the protolith of a felsic gneiss. Cryogenian magmatism in the Madurai Block is interpreted to form part of an extensive arc magmatic province within the southern East African Orogen that can be traced from central Madagascar, through southern India to the Wanni Complex of Sri Lanka. This province is interpreted to have formed above a south/west dipping subduction system as the Mozambique Ocean was subducted under the Neoproterozoic continent Azania.

Spinel + quartz assemblage in granulites from the Achankovil Shear Zone, southern India: Implications for ultrahigh-temperature metamorphism

We report the finding of equilibrium spinel + quartz assemblage enclosed within garnet in garnet–orthopyroxene–cordierite granulites from Pakkandom within the Achankovil Shear Zone, a region which is considered as the trace of an accretionary suture in recent tectonic models on southern India. The spinel + quartz bearing granulites are composed of poikiloblastic garnet and subidioblastic orthopyroxene in the matrix of quartz, plagioclase, biotite, cordierite, and Fe–Ti oxides. Garnet contains numerous inclusions of sillimanite and biotite as well as spinel and quartz. The spinel in direct contact with quartz has moderate X Mg (= Mg/(Fe 2+ + Mg) = 0.44–0.47), and is Zn and Fe 3+ poor ( X Zn = Zn/(Fe 2+ + Mg + Zn) = 0.027–0.036, Fe 3+/(Fe 2+ + Fe 3+) = 0.12–0.17). Spinel is also present in the matrix surrounded by magnetite, but the matrix spinel contains more Zn( X Zn = 0.067–0.072) and does not show any contact relationship with quartz. Such Zn- and Fe 3+-poor spinel in direct contact with quartz has been regarded as a diagnostic evidence of ultrahigh-temperature (UHT) metamorphism. The high-temperature stability of the spinel + quartz is also supported by the results of geothermobarometric calculation of garnet–orthopyroxene assemblages that provides robust evidence for peak UHT metamorphism at 920–980 °C and 8–10 kbar, which was further confirmed by Al-in-Opx and magnetite–ilmenite geothermometers (900–950 °C and ∼1000 °C, respectively). The peak UHT event was followed by decompression down to 4.0–4.2 kbar and 640–670 °C toward the stability of cordierite along a clockwise P–T path. Similar spinel + quartz assemblage enclosed in poikiloblastic garnet has also been reported from the Palghat–Cauvery Shear Zone system, the trace of a major suture zone within the Gondwana amalgam with evidence for prograde high-pressure ( P up to 20 kbar) metamorphism followed by UHT event. The fine-grained spinel + quartz may thus indicate decompression from higher pressure conditions, confirming the recent attributes of the Achankovil Shear Zone being a collisional suture with evidence for subduction–exhumation processes which accompanied the final collision and amalgamation of the Gondwana supercontinent during the latest Neoproterozoic.

A petrologic and laser Raman spectroscopic study of sapphirine–spinel–quartz–Mg-staurolite inclusions in garnet from Kumiloothu, southern India: Implications for extreme metamorphism in a collisional orogen

We report the occurrence of sapphirine, spinel, quartz and Mg-staurolite as inclusions in garnet from quartzo-feldspathic gneiss and garnet–biotite gneisses in the northernmost sector of the Madurai Granulite Block along the Palghat–Cauvery Shear Zone System in southern India, suggesting metamorphism under ultrahigh-temperature conditions. Poikiloblastic garnet grains contain numerous inclusions of quartz, rutile, apatite, biotite and zircon with rare associations of sapphirine + spinel and sapphirine + staurolite. We characterize the inclusion population based on detailed petrologic, mineral chemical and laser Raman spectroscopic studies. A direct contact of sapphirine and/or spinel with the quartz inclusions within garnet is not seen, which we attribute to the low modal abundances of sapphirine and spinel, and/or the progress of reactions during retrograde metamorphism. Staurolite occurs as inclusions in garnet and is partially mantled by sapphirine, suggesting the progress of the reaction: St → Grt + Spr + Sil or Ky + H 2 O. Such relict occurrences of staurolite with Mg/(Fe + Mg) of 0.42–0.44 are regarded as evidence of prograde high-pressure metamorphism in the northern Madurai Block. Lack of sillimanite or kyanite within garnet implies that all aluminosilicates have been completely consumed by reactions: Grt + Sil → Spr + Qtz and/or Grt + Sil → Spl + Qtz to form possible sapphirine + quartz and/or spinel + quartz assemblages. Our results confirm that the northern Madurai Block along the Palghat–Cauvery Shear Zone System underwent high-pressure and ultrahigh-temperature metamorphism along a decompressional clockwise P – T – time path associated with the collisional amalgamation of southern India with the Gondwana supercontinent assembly.

The stability and origin of sodicgedrite in ultrahigh-temperature Mg-Al granulites: a case study from the Gondwana suture in southern India

Mg-Al-rich rocks from the Palghat-Cauvery Shear Zone System (PCSZ) within the Gondwana suture zone in southern India contain sodicgedrite as one of the prograde to peak phases, stable during T = 900–990°C ultrahigh-temperature metamorphism. Gedrite in these samples is Mg-rich (Mg/[Fe + Mg] = X Mg = 0.69–0.80) and shows wide variation in Na2O content (1.4–2.3 wt.%, NaA = 0.33–0.61 pfu). Gedrite adjacent to kyanite pseudomorph is in part mantled by garnet and cordierite. The gedrite proximal to garnet shows an increase in NaA and AlIV from the core (NaA = 0.40–0.51 pfu, AlIV = 1.6–1.9 pfu) to the rim (NaA = 0.49–0.61 pfu, AlIV = 2.0–2.2 pfu), suggesting the progress of the following dehydration reaction: Ged + Ky → Na-Ged + Grt + Crd + H2O. This reaction suggests that, as the reactants broke down during the prograde stage, the remaining gedrite became enriched in Na to form sodicgedrite, which is regarded as a unique feature of high-grade rocks with Mg-Al-rich and K–Si-poor bulk chemistry. We carried out high-P-T experimental studies on natural sodicgedrite and the results indicate that gedrite and melt are stable phases at 12 kbar and 1,000°C. However, the product gedrite is Na-poor with only <0.13 wt.% Na2O (NaA = 0.015–0.034 pfu). In contrast, the matrix glass contains up to 8.5 wt.% Na2O, suggesting that, with the progressive melting of the starting material, Na was partitioned into the melt rather than gedrite. The results therefore imply that the occurrence of sodicgedrite in the UHT rocks of the PCSZ is probably due to the low H2O activity during peak P-T conditions that restricted extensive partial melting in these rocks, leaving Na partitioned into the solid phase (gedrite). The occurrence of abundant primary CO2-rich fluid inclusions in this rock, which possibly infiltrated along the collisional suture during the final amalgamation of the Gondwana supercontinent, strengthens the inference of low water activity.

High-pressure and ultrahigh-temperature metamorphism at Komateri, northern Madurai Block, southern India

We report for the first time the evidence for prograde high-pressure (HP) metamorphism preceding a peak ultrahigh-temperature (UHT) event in the northernmost part of the Madurai Block in southern India. Mg–Al-rich Grt–Ged rocks from Komateri in Karur district contain poikiloblastic garnet with numerous multi-phase inclusions. Although most of the inclusion assemblages are composed of gedrite, quartz, and secondary biotite, rare staurolite + sapphirine and spinel + quartz are also present. The X Mg (=Mg/[Fe+Mg]) of staurolite (0.45–0.49) is almost consistent with that reported previously from Namakkal district in the Palghat–Cauvery Shear Zone system ( X Mg = 0.51–0.52), north of the Madurai Block. The HP event was followed by peak UHT metamorphism at T = 880–1040 °C and P = 9.8–12.5 kbar as indicated by thermobarometric computations in the Grt–Ged rock and associated mafic granulite. Symplectic intergrowth of spinel ( X Mg = 0.50–0.59, ZnO < 1.7 wt.%) and quartz, a diagnostic indicator of UHT metamorphism, probably formed by decompression at UHT conditions. The rocks subsequently underwent retrograde metamorphism at T = 720–760 °C and P = 4.2–5.1 kbar. The P– T conditions and clockwise exhumation trajectory of the Komateri rocks, comparable to similar features recorded from the Palghat–Cauvery Shear Zone system, suggest that the Madurai Block and the Palghat–Cauvery Shear Zone system underwent similar HP and UHT metamorphic history probably related to the continent–continent collision during the final stage of amalgamation of Gondwana supercontinent.

Petrology and fluid inclusions of garnet-clinopyroxene rocks from Paramati in the Palghat-Cauvery Shear Zone System, southern India

We report petrological and fluid inclusion data on garnet-clinopyroxene rocks from Paramati in the Palghat-Cauvery Shear Zone System, southern India. The rocks are composed of garnet, clinopyroxene, and plagioclase with or without orthopyroxene and retrograde pargasite. The peak P-T conditions of 9-12 kbar and 830-880 °C were obtained from early garnet-clinopyroxene-plagioclase-quartz assemblage. Available petrological data from this region suggest that the high-pressure event was followed by heating to the peak ultrahigh-temperature condition and decompression as supported by the occurrence of orthopyroxene + plagioclase symplectite between garnet + clinopyroxene and plagioclase lamella in clinopyroxene. High Ca-Eskola molecule in the integrated clinopyroxene (∼ 8.6%) is also comparable with its high-pressure origin. Primary and secondary fluid inclusions in garnet are dominantly CO2-rich. They were homogenized into the liquid phase in the temperature range of −11.1 to +14.2 °C, corresponding to low-CO2 densities of 0.82 to 0.99 g/cm3. The fluid densities, when computed into isochores, indicate lower pressures (∼ 6-7 kbar at 900 °C) than the P-T conditions estimated by geothermobarometry. The low-density values and the primary nature of the inclusions trapped in garnet, as well as wide variation in homogenization temperatures, indicate density modification of prograde metamorphic fluid during exhumation. The results of this study support prograde high-pressure metamorphism and subsequent decompression along a clockwise P-T path for the evolution of the rocks in this Gondwana suture zone probably related to subduction of crustal materials to the mantle depth (>40 km) and following continent-continent collision during the amalgamation of the Gondwana Supercontinent.

Electron microprobe dating of monazites from an ultrahigh-temperature granulite in Southern India: Implications for the timing of Gondwana assembly

We report here monazite geochronology on a newly discovered high pressure (HP) and ultrahigh-temperature (UHT) granulite locality at Thoppur within the Palghat-Cauvery Shear Zone system in southern India. In situ dating of monazites associated with the UHT minerals using electron probe technique yielded PbO vs. ThO2* apparent isochron age of 544 ± 5 Ma. This is the first study from southern India where older monazite cores or complex age patterns are virtually absent, and all the monazite population in the studied samples crystallized/recrystallized during a single thermal maximum which we correlate with the timing of extreme crustal metamorphism during the final assembly of the Gondwana supercontinent.

Carbonic fluid inclusions in ultrahigh-temperature granulite from Kumiloothu in the northern Madurai Block, southern India

We report fluid inclusion data on ultrahigh-temperature (UHT) granulites from Kumiloothu in the northern Madurai Block situated along the Palghat-Cauvery Shear Zone system in southern India. Three categories of fluid inclusions have been observed: primary inclusions in garnet comprising the dominant category, secondary inclusions in garnet, and rare primary inclusions in rutile and apatite enclosed within garnet. The melting temperatures of all the categories of inclusions lie in the narrow range of −57.0 to −56.6 °C, close to the triple point of pure CO2. Most of the fluid inclusions homogenize into the liquid phase between a temperature range of +11.3 to +30.3 °C, corresponding to low-CO2 densities of 0.59-0.85 g/cm3. Slightly higher density of 0.95 g/cm3 was obtained from a primary inclusion in rutile that homogenized at −3.6 °C. The fluid densities, when computed into isochores, indicate lower pressures (∼ 6.6 kbar at 1000 °C) than the peak P-T conditions (T >1000 °C at ∼ 8 kbar) estimated for this region. The results of this study, together with the primary nature of the inclusions trapped in high-grade minerals, indicates density reversal of peak metamorphic fluid, a common feature for many UHT granulites along the Gondwana suture zone as well as other UHT terranes showing clockwise P-T history.

Metamorphic P-T path of the eastern Trivandrum Granulite Block, southern India: implications for regional correlation of lower crustal fragments

We report here the metamorphic P-T path for granulite-facies metapelites from the eastern part of the Trivandrum Granulite Block (TGB), southern India, and compare with the paths from the western part of the block as well as from other granulite blocks and shear zones in this region for understanding of exhumation history of the southern granulite terrane. The peak temperature condition of the eastern part (900-1000 °C) is almost consistent with that from the western part, suggesting regional ultrahigh-temperature (UHT) metamorphism throughout the TGB. However, there is a slight difference in peak pressure conditions between the eastern part (12 kbar) and the western part (10 kbar) estimated from different Zn content within spinel in equilibrium with quartz. Such P ∼ 12 kbar high-pressure event is consistent with that obtained from granulite-facies rocks further north in the Palghat-Cauvery Shear Zone system (PCSZ) and Madurai Granulite Block (MGB). The results therefore suggest that the TGB, MGB, and PCSZ underwent similar prograde high-pressure metamorphism followed by peak UHT event along a clockwise P-T trajectory, related to the collisional orogeny during the assembly of the Gondwana Supercontinent.

Fluid characteristics of high- to ultrahigh-temperature metamorphism in southern India: A quantitative Raman spectroscopic study

We report the first quantitative compositional data on fluid inclusions in Late Neoproterozoic to Cambrian ultrahigh-temperature (UHT) granulites from major crustal blocks and shear zones in southern India. Fluid inclusions in various high-grade minerals such as garnet, orthopyroxene, plagioclase, cordierite, and quartz from these rocks were studied in terms of petrography, microthermometry and laser Raman spectroscopy. Measured melting temperatures of the inclusions indicate that the trapped fluid phase is dominantly carbonic. Raman analyses confirmed a CO 2-rich composition with only minor dilutants such as N 2 (≤5.0 mol%), H 2O (≤0.50 mol%), and CH 4 (≤0.47 mol%). The range of most CO 2 isochores computed from density measurements in fluid inclusions from the UHT granulites pass below the peak pressure–temperature conditions of metamorphism indicating considerable resetting of fluid densities during decompression. In contrast, garnetiferous granulites from the Salem area along the Palghat-Cauvery Shear Zone System, which underwent high-pressure prograde metamorphism, contain very high-density syn-metamorphic pure CO 2 inclusions. Such dense carbonic fluid inclusions in garnet and plagioclase coexist with carbonate minerals (calcite or dolomite), indicating probable derivation from deep-seated primary magmatic sources. Although models on the origin of CO 2 are debatable, the ubiquitous association of carbonic fluids in the UHT mineral assemblages is in favour of large-scale infiltration of CO 2-rich fluids during extreme crustal metamorphism associated with collisional orogeny and subsequent extensional collapse at the final stage of amalgamation of the Gondwana supercontinent.

CO 2-rich fluid inclusions in staurolite and associated minerals in a high-pressure ultrahigh-temperature granulite from the Gondwana suture in southern India

We report the first detailed fluid inclusion study on Mg–Al rocks from the Gondwana suture zone along the Palghat–Cauvery Shear Zone System in southern India. These rocks underwent high-pressure ( P > 12 kbar) and ultrahigh-temperature ( T ∼ 990 °C) metamorphism during the late Neoproterozoic associated with the collisional assembly of the Gondwana supercontinent. Fluid inclusions of texturally primary and pseudosecondary categories occur in plagioclase, staurolite, and kyanite in these rocks. Inclusions of secondary nature are found in garnet and apatite. All the trapped fluids show melting temperatures in the range of − 58.6 to − 56.6 °C, suggesting a dominantly pure CO 2 composition. The secondary inclusions show high homogenization temperatures ( T h) of + 12.9 to + 30.0 °C, indicating low density of 0.59–0.83 g/cm 3. Although most of the primary and pseudosecondary inclusions also contain moderate to low-density fluids (0.67–0.95 g/cm 3; T h = − 3.5 to + 27.4 °C), rare high-density inclusions (1.00–1.06 g/cm 3; T h = −26.1 to − 14.1 °C) were discovered in matrix plagioclase and porphyroblastic staurolite. The estimated CO 2 isochores for the high-density primary and pseudosecondary inclusions intersect the clockwise P–T trajectory of the rocks at around 8.3–9.0 kbar at 1000 °C, which broadly coincide with the peak metamorphic conditions of the area. Since these fluid inclusions show textural evidence for entrapment during the growth of the host high-grade minerals, we interpret the high-density CO 2 fluids to be traces of a CO 2-rich synmetamorphic fluid that was present during the peak ultrahigh-temperature metamorphism. Despite the preservation of high-density CO 2 inclusions in staurolite and plagioclase, the primary and pseudosecondary inclusions in most of the other minerals show wide density variation which suggests the fluids in these inclusions underwent significant density reversal (density decrease) during decompression of the rocks from high-pressure through the peak ultrahigh-temperature stage along a clockwise P–T trajectory. The finding of pure CO 2 inclusions in these Mg–Al granulites conforms with similar observations from many other granulite-facies terrains with the carbonic fluids probably derived from mantle reservoirs and transferred through deep-seated shear zones that characterize this Gondwana suture.

Sodicgedrite in ultrahigh-temperature Mg-Al-rich rocks from the Palghat-Cauvery Shear Zone system, southern India

We report high-Na gedrite (sodicgedrite) with up to 3.1 wt% Na2O (∼ 0.81 NaA pfu) in ultrahigh-temperature (UHT) Mg-Al-rich rocks from the northern margin of the Pan-African Madurai Block along the Palghat-Cauvery Shear Zone system, southern India. An amount of NaA of the gedrite increases with increasing AlIV. The sum of NaA and AlIV also increases linearly with decreasing Si suggesting the substitution of NaAAlIV ↔ Si. The negative correlation between Fe + Mg + Si and Al is probably due to the tschermak substitution ((Fe,Mg)VISiIV ↔ AlVIAlIV). The relatively coarse-grained nature and porphyroblastic occurrence of the sodicgedrite suggest that the mineral recrystallized at the peak UHT metamorphism. In contrast, retrograde gedrite with cordierite is less sodic with 1.1-1.6 wt% Na2O (NaA = 0.24-0.35 pfu). Our data suggest that high-Na content in gedrite is a possible indicator of UHT metamorphism.

The Mozambique Ocean Suture in Southern India: Age and Significance of Granulites in the Palghat-Cauvery Shear Zone System

The Neoproterozoic oceans that closed as Gondwana amalgamated left a cryptic, yet recognisable, record in the resulting orogens. These consist of Neoproterozoic oceanic or ocean-margin protoliths, relatively high-pressure metamorphic belts separating distinct crustal domains, and Neoproterozoic provenance fronts. The southern Indian Palghat-Cauvery shear zone system, is interpreted as the remains of the Neoproterozoic Mozambique Ocean, because: a) it separates Archaean and Palaeoproterozoic granulites from the extensive Neoproterozoic-Cambrian Southern Granulite Domain; b) it contains mafic gneisses that may have formed within a Neoproterozoic Ocean; c) it preserves distinctive high pressure, ultra-high temperature granulites that were metamorphosed in the Cambrian Malagasy Orogeny, and; d) it isolates southern Indian Proterozoic metasediments that are unlikely to be sourced from north of the shear system.

Anticlockwise evolution of ultrahigh-temperature granulites within continental collision zone in southern India

We report three new localities of corundum and sapphirine-bearing hyper aluminous Mg-rich and silica-poor ultrahigh-temperature granulites formed during Late Neoproterozoic-Cambrian times within the Palghat–Cauvery Shear Zone system in southern India. From petrologic characteristics, mineral chemistry and petrogenetic grid considerations, the peak metamorphic conditions of these rocks are inferred to lie around 950–1000 °C (as suggested by Al in orthopyroxene thermometer) at pressures above 10 kbar (as indicated by the equilibrium orthopyroxene–sillimanite–gedrite ± quartz assemblage). These rocks preserve several remarkable reaction textures, the most prominent among which is the triple corona of spinel–sapphirine–cordierite on corundum, with the whole textural assembly embedded within the matrix of gedrite, suggesting the reaction: Ged + Crn = Spl + Spr + Crd. The formation of sapphirine–sillimanite assemblage/symplectite associated with relict corundum and porphyroblasitc cordierite is explained by the reaction: Crd + Crn = Spr + Sil. The association of sapphirine cordierite symplectite with gedrite–sillimanite assemblage as well as with aluminosilicate boundaries indicates the gedrite consuming reaction: Ged + Sil = Spr + Crd. Extensive growth of sapphirine–cordierite observed on the rim of gedrite porphyroblasts with spinel occurring as relict inclusions within the sapphirine indicates the reaction: Ged + Spl = Spr + Crd. The pressure–temperature ( P– T) path defined from the observed mineral assemblages and reaction texture is characterized by anticlockwise trajectory, with a prograde segment of initial heating and subsequent deep burial, followed by retrograde near-isothermal decompression. Such an anticlockwise trajectory is being reported for the first time from southern India and has important tectonic implications since these rocks were developed at the leading edge of the crustal block that was involved in collisional orogeny and subsequent extension during the final phase of assembly of the Gondwana supercontinent. We propose that the rocks were subjected to deep subduction and rapid exhumation, and the extreme thermal conditions were attained either through input from underplated mantle-derived magmas, or convective thinning or detachment of the lithospheric thermal boundary layer during or after crustal thickening.

The timing of ultrahigh-temperature metamorphism in Southern India: U–Th–Pb electron microprobe ages from zircon and monazite in sapphirine-bearing granulites

We report here U–Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat–Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at ∼ 2420 Ma, while those from Mettanganam show ∼ 2300 Ma peak. Younger zircons with peak ages of ∼ 2100 and ∼ 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with ∼ 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at ∼ 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510–450 Ma. While the zircon populations in these rocks suggest multiple sources of Archaean and Palaeoproterozoic age, the monazite data are interpreted to date the timing of ultrahigh-temperature metamorphism in southern India as latest Neoproterozoic to Cambrian in both the Palghat–Cauvery shear zone system and the northern Madurai Block. The data illustrate the extent of Neoproterozoic/Cambrian metamorphism as India joined the Gondwana amalgam at the dawn of the Cambrian.

First report of garnet–corundum rocks from southern India: Implications for prograde high-pressure (eclogite-facies?) metamorphism

Abstract We report here for the first time the occurrence of garnet and corundum in Mg–Al-rich rocks at Sevitturangampatti (Namakkal district) in the Palghat-Cauvery Shear Zone System (PCSS), southern India. The rocks contain several rare mineral assemblages such as garnet–corundum–sillimanite–cordierite–sapphirine–spinel–Mg-rich staurolite, garnet–corundum–sodic gedrite–cordierite–sillimanite/kyanite, garnet–Mg-rich staurolite–sillimanite/kyanite, sodic gedrite–Mg-rich staurolite–corundum–sapphirine, biotite–corundum–sapphirine and sodic gedrite–sapphirine–spinel–cordierite. Both garnet and corundum in these rocks occur as coarse-grained (1 mm to 10 cm) porphyroblasts in the matrix of sillimanite, cordierite and gedrite. Kyanite is common as inclusions in garnet, but matrix aluminosilicates are mainly sillimanite. The presence of rare garnet + corundum, which has so far been reported from kimberlite xenoliths, aluminous eclogites and ultrahigh-pressure metamorphic rocks as well as in high-pressure experiments, suggests that the assemblage is an indicator of an unusually high-pressure event, which has not been recorded in previous studies from southern India. Phase analysis of quartz-absent MAS system also suggests high-pressure stability of the assemblage. The inference of high pressure metamorphism is also supported by the presence of Mg-rich [Mg/(Fe + Mg) = 0.51] staurolite, which has been reported from high-pressure rocks, included from cores of coarse-grained garnet and gedrite. Porphyroblastic occurrence of garnet + corundum as well as staurolite and kyanite inclusions suggests that the area underwent prograde high-pressure metamorphism, probably in the eclogite field. The rocks subsequently underwent continuous heating at 940 to 990 °C, suggesting ultrahigh-temperature (UHT) metamorphism along a clockwise trajectory. Sapphirine + cordierite and spinel + cordierite symplectites between garnet and sillimanite suggest near isothermal decompression after the peak event. Evidence for such high-pressure metamorphism followed by UHT conditions along a single clockwise exhumation path is the first report from southern India. The Namakkal area occurs within the central domain of the PCSS, which defines the probable collisional suture between the Archean craton in the north and the Proterozoic Madurai Block in the south. We propose that the rocks reported in this study testify to metamorphism at mantle depths (eclogite-facies) and their subsequent heating at extreme thermal conditions during collisional orogenic event and subsequent rapid exhumation associated with the final suturing of the Gondwana assembly in the late Pan-African. The high-pressure metamorphism and subsequent UHT event reported in this study have important bearing on the tectonic evolution of the continental deep crust in southern India.

Ti-free högbomite in spinel- and sapphirine-bearing Mg-Al rock from the Palghat-Cauvery shear zone system, southern India

AbstractWe report a new type of högbomite in a spinel- and sapphirine-bearing silica-deficient Mg-Al rock from Manavadi at the northern margin of the Madurai Block along the Palghat-Cauvery shear zone system, which defines the Archaean-Proterozoic collisional boundary in southern India. Högbomite occurs here as fine- to medium-grained euhedral to subhedral crystals enclosed within or along grain boundaries of greenish spinel. It is compositionally very unique due to the lack of TiO2 (&lt;0.2 wt.%) and enrichment in Al2O3 (66.4—70.8 wt.%) compared to other occurrences. The variety of högbomite described in this study, confirmed by laser Raman spectroscopy, has not been reported elsewhere previously. Högbomite in this study is subdivided into three groups based on the XMg (=Mg/ (FeTotal+Mg)) ratio; 0.54—0.57, 0.61—0.73 and 0.77—0.79. Compositional data suggest a linear element substitution of 3(Fe+Mg) = 2Al for the Ti-free högbomite, a trend that is distinct from previously reported Ti-bearing högbomites. Textures and mineral chemistry data suggest that the following reaction took place at retrograde stage (T &lt;800°C): spinel + H2O + O2 → högbomite + corundum + magnetite, implying that introduction of hydrous and oxidized fluid as well as low bulk-Ti content played an important role in the formation of Ti-free högbomite.

Late Neoproterozoic-Cambrian Felsic Magmatism Along Transcrustal Shear Zones in Southern India: U-Pb Electron Microprobe Ages and Implications for the Amalgamation of the Gondwana Supercontinent

We report U-Pb electron microprobe ages for zircon and monazite from two granitic plutons from southern India, the Vattamalai granite within the Palghat-Cauvery Shear Zone system and the Pathanapuram granite within the Achankovil Shear Zone. A zircon grain from the Vattamalai granite has a core age of 693±132 Ma and is surrounded by a thick overgrowth with an age of 504±104 Ma. Monazites from the Vattamalai granite show a small range of ages between 500-520 Ma. PbO vs. ThO 2* plots of the monazites define a precise isochron age of 517±6.7 Ma (MSWD = 0.25). The oldest zircons in the Pathanapuram pluton are in the range 961-1149 Ma, with younger overgrowths at ~540-560 Ma. Monazite cores from the granite lie in the range of 526-574 Ma, whereas rims and bright overgrowths range from 506-539 Ma. These monazites define two linear arrays in PbO vs. ThO 2* plots with cores yielding an isochron age of 550±25 Ma (MSWD = 0.58) and the rims defining an age of 515±15 Ma (MSWD = 0.68). The age data from the granite plutons indicate multiple thermal imprints in southern India with the latest orogeny during the Late Neoproterozoic-Cambrian (Pan-African). The older zircon cores up to 1149 Ma from the Pathanapuram pluton suggest inherited components of late Mesoproterozoic age, caught up within the granite magma. However, the dominant 570-520 Ma ages obtained from both zircons and monazites closely compare with similar ages for magmatism and metamorphism from throughout the East African Orogen. Late Neoproterozoic-Cambrian felsic magmatism occurred along both the Palghat-Cauvery Shear System and the Achankovil Shear Zone, indicating that these shears were active at this time and may have served as pathways for the emplacement of magmas generated at depth. The magmatism represents part of the various collisional-extensional episodes that marked the final amalgamation of the Gondwana supercontinent.