Palghat-Cauvery Suture Zone Research Articles

Ultrahigh-density carbonic fluids in ultrahigh-temperature metagabbros from the Palghat-Cauvery Suture Zone, Southern India: Phase equilibria modelling and fluid inclusion study

Fluid inclusions trapped in high-grade metamorphic minerals offer direct evidence for the nature and role of fluids involved in lower crustal processes. Here, we report extremely high-density (~1.155 g/cm 3 ) carbonic fluid inclusions preserved within garnet grains in metagabbros from the central segment of the Palghat-Cauvery Suture Zone (PCSZ) in southern India. The metagabbros are composed of coarse-grained garnet, clinopyroxene, orthopyroxene, plagioclase, quartz, and retrograde calcic amphibole. Garnet occurs either as porphyroblastic grains formed during prograde to peak stage, or as fine-grained aggregates around plagioclase possibly formed by post-peak decompressional (near-isobaric) cooling. The peak and retrograde P–T conditions have been estimated as ~940 °C/~11.5 kbar and ~ 840–850 °C/~9.3–10.2 kbar, respectively, based on phase equilibria modelling using pseudosection computation. Fluid inclusions occur as primary and secondary phases within the porphyroblastic garnet and matrix plagioclase. The melting temperatures of these fluid inclusions are in the range of −57.4 to −56.6 °C, indicating nearly pure CO 2 composition. The lowest homogenization temperature from the primary inclusions in garnet (−50.1 °C) translates to extremely high density of 1.155 g/cm 3 . The calculated isochores for the inclusions (~11.3 kbar at 950 °C) are broadly consistent with the peak P–T conditions, suggesting that the fluids are trapped near the peak metamorphic stage under a dry condition. The preservation of ultrahigh-density carbonic inclusions is probably due to isochore-parallel decompressional cooling after the peak metamorphism. The secondary inclusions show slightly lower densities of 1.014–1.053 g/cm 3 (in garnet) and 0.955–0.976 g/cm 3 (in plagioclase), the isochores of which suggest a lower-pressure range of 4.8–5.8 kbar at 800 °C. The high-density pure CO 2 fluids reported in this study as well as in previous studies from the PCSZ, and the isochoric evolution provide robust evidence for the presence of supercritical carbonic fluids in the lower continental crust. • Extremely high-density (~1.155 g/cm 3 ) carbonic fluid inclusions in metagabbros. • Peak and retrograde conditions of ~940 °C/~11.5 kbar and ~ 840–850 °C/~9.3–10.2 kbar. • Garnet aggregates around plagioclase formed by post-peak near-isobaric cooling. • Isochore-parallel decompressional cooling after the peak metamorphism. • Ultrahigh-density fluids are preserved due to isochoric exhumation path.

Multi‐stage metamorphism of ultrahigh‐temperature Mg‐Al granulites during Gondwana assembly: Evidence from southern India

The Southern Granulite Terrane (SGT) in India hosts granulite facies rocks metamorphosed at ultra‐high temperature (UHT) conditions in the various crustal blocks, as well as within the Palghat‐Cauvery Suture Zone (PCSZ), that is considered as a trace of the Late Neoproterozoic—Cambrian Gondwana suture. Here we investigate UHT granulites from the northern margin of the Madurai Block adjacent to the PCSZ where Mg‐Al‐rich granulites are exposed. We identify sodic gedrite + kyanite in these rocks as the high‐pressure prograde stage assemblage, followed by sillimanite‐garnet‐orthopyroxene that formed during pressure decrease and temperature increase. The rare remnant gedrite is also stable at the near‐peak UHT metamorphism until it was replaced by sapphirine. The rocks subsequently underwent decompression that formed sapphirine + cordierite and sapphirine + plagioclase symplectite around sillimanite. Dehydration during decompression generated orthopyroxene‐sillimanite‐quartz assemblage with the appearance of sapphirine, defining the diagnostic mineral assemblage indicative of peak UHT metamorphism (T > 900°C) at relatively high‐pressure (P > 9 kbar). The UHT peak metamorphism in this region is consistent with the results of P–T calculations using conventional geothermometers and phase equilibrium modelling (T up to 1,050°C, P over12 kbar). Zircon and monazite geochronology on the UHT metapelites indicate distinct stages. Detrital zircon grains in the metasediments indicate protolith from ca. 2.5 Ga igneous source and the metamorphic overgrowths yield 206Pb/238U mean ages concentrated at ca. 550–520 Ma. Monazite ages define another younger group 206Pb/238U mean ages at ca. 450 Ma. The prograde high‐pressure granulite‐facies metamorphism and following UHT event correlate with the subduction‐collision tectonics at 550–500 Ma associated with the final stage of amalgamation of the Gondwana supercontinent, while the 420–460 Ma monazite age records a later hydration at the post‐orogenic stage, possibly associated with deep shearing and fluid influx.

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.

Zircon U-Pb ages and Hf isotopic systematics of charnockite gneisses from the Ediacaran–Cambrian high-grade metamorphic terranes, southern India: Constraints on crust formation, recycling, and Gondwana correlations

The Southern Granulite terrane, southern India, comprises a vast exposure of deep crust and forms a key region in the reconstruction of the Gondwana supercontinent. An E-W−trending crustal-scale shear zone, the Palghat-Cauvery suture zone system, which formed during the late Neoproterozoic−Cambrian (0.75−0.50 Ga), marks a prominent terrane boundary separating terranes with a predominantly late Neoarchean (ca. 2.5 Ga) regional granulite metamorphism to the north from those with an Ediacaran−Cambrian (0.63−0.50 Ga) regional granulite metamorphism to the south. Focusing on the younger granulite metamorphic domains, we present here new zircon U-Pb ages and Hf isotopic compositions for 11 charnockite orthogneisses from the Madurai, Trivandrum, and Nagercoil blocks and contribute to the resolution of the age of their magmatic protoliths. This study shows that the charnockite orthogneisses south of the Palghat-Cauvery suture zone relate to a minimum of four distinct episodes of felsic magmatism centered at: ca. 2.62−2.46 Ga, ca. 2.05−1.84 Ga, ca. 1.0−0.9 Ga, and ca. 0.80−0.76 Ga, pertaining to the Siderian, Orosirian, and Tonian Periods. Hafnium isotope analyses of zircon grains from the charnockite gneisses suggest that the protoliths of the ca. 2.05−1.98 Ga gneisses from the Trivandrum and Nagercoil blocks and the ca. 1.0−0.9 Ga gneisses along the southeastern Madurai block involved a significant juvenile magma component, while the protoliths of charnockite gneisses elsewhere in the Madurai block formed mainly through recycling of older crust up to ca. 3.2 Ga. A regional granulite-facies metamorphic imprint during the Ediacaran−Cambrian marked an advanced stage in the amalgamation of the Madurai, Trivandrum, and Nagercoil blocks into the East African orogen and its collision with the Dharwar craton.

Petrology and zircon U–Pb geochronology of metagabbro from the Highland Complex, Sri Lanka: Implications for the correlation of Gondwana suture zones

The basement in the Sri Lankan fragment of Gondwana is composed of upper amphibolite- to granulite-facies rocks that underwent latest Neoproterozoic – Cambrian high-grade metamorphism. A recent tectonic model suggests that the Highland Complex in the central part of the island corresponds to a suture zone formed by the closure of a Neoproterozoic ocean between the Wanni and Vijayan Complexes. In order to test the model and further unravel the complex tectonic evolution of this region, we present new petrological, pressure–temperature, and zircon U–Pb geochronological data on garnet-bearing metagabbros from the Highland Complex. The metagabbros, which occur as blocks or lenses within metasediments of the Highland Complex, contain granulite-facies mineral assemblages of garnet+clinopyroxene+orthopyroxene+ilmenite+hornblende+plagioclase+quartz (sample HC7) and garnet+plagioclase+clinopyroxene+orthopyroxene+pargasite+quartz+rutile+ilmenite (sample HC8). Application of phase equilibria modeling in the system NCFMASHTO indicates peak P–T conditions of 900–950°C/10.5–11kbar and 800–830°C/8.5–9kbar, respectively, and clockwise P–T paths as inferred from decompression textures and mineral compositional isopleths. These conditions are significantly lower than the high-pressure (>17kbar) and ultrahigh-temperature (>1000°C) conditions obtained previously for similar metagabbros from the Highland Complex, whereas our results are consistent with those of similar rocks from the Palghat-Cauvery Suture Zone in India and the Lützow-Holm Complex in East Antarctica. The latest Neoproterozoic to Cambrian metamorphic ages (574±28Ma and 534±14Ma) as well as the available petrological data suggest that the metagabbros in the Highland Complex could be correlated with those in the Lützow-Holm Complex in East Antarctica. The results of this study confirm that the Highland Complex is an extension of the Lützow-Holm Complex, and that both these terranes involve a package of accreted trench sediments and remnant oceanic lithosphere metamorphosed during the final collision related to the Gondwana assembly.

Petrology and zircon U–Pb geochronology of metagabbros from a mafic–ultramafic suite at Aniyapuram: Neoarchean to Early Paleoproterozoic convergent margin magmatism and Middle Neoproterozoic high-grade metamorphism in southern India

Several mafic–ultramafic complexes occur within the Palghat-Cauvery Suture Zone (PCSZ) in southern India. The PCSZ is regarded in recent models as the zone along which crustal blocks were amalgamated during the Late Neoproterozoic–Cambrian (550–530Ma) Gondwana assembly. Here we report petrologic and zircon U–Pb geochronologic data from gabbros associated with the Aniyapuram mafic–ultramafic suite in the central domain of the PCSZ. Geothermobarometry and pseudosection approach in the system NCFMASHTO for the metagabbro (Grt+Cpx+Opx+Hbl+Pl+Qtz+Ilm+Rt) yield peak P–T condition of 9.8–10.6kbar and 730–790°C, which was followed by decompression to 6.5–8.0kbar and ca. 750°C as inferred from the formation of Opx+Pl symplectite around garnet, probably along a clockwise P–T path. Zircon U–Pb data analyzed by LA-ICP-MS plot along a well-defined discordia with upper and lower intercepts in the concordia at 2436±22Ma and 731±11Ma respectively, suggesting Neoarchean–Early Paleoproterozoic magmatic emplacement of the protolith and progressive Pb loss related to the Middle Neoproterozoic (Cryogenian) thermal event (or high-grade metamorphism). These results closely compare with the available Neoarchean magmatic ages of mafic–ultramafic complexes (e.g., Sittampundi, Devanur, Agali Hills, and Kanja Malai) and Middle Neoproterozoic magmatic event (e.g., Manamedu and Kadavur) in the PCSZ and adjacent granulite blocks. The 650Ma concordia ages obtained from unzoned zircons might indicate the timing of high-grade metamorphism or post-peak hydration event. The P–T conditions obtained from Aniyapuram are significantly lower than the high-pressure and ultrahigh-temperature conditions of the 550–530Ma final collisional event (P>14 kbar and T>950°C). The Middle Neoproterozoic (ca. 730Ma or 650Ma) high-grade metamorphism in Aniyapuram reported for the first time from the PCSZ is possibly associated with magmatism in arc tectonic setting.

The Neoproterozoic subduction complex in southern India: SIMS zircon U–Pb ages and implications for Gondwana assembly

The Palghat-Cauvery Suture Zone (PCSZ) in southern India defines the trace of the collisional suture developed during the assembly of the Gondwana supercontinent through the closure of the Mozambique Ocean in the Late Neoproterozoic–Cambrian. Here we report Secondary Ion Mass Spectrometry (SIMS) U–Pb ages from zircons in plagiogranites and gabbros from a suprasubduction zone ophiolitic complex at Manamedu, located along the southern periphery of the PCSZ. The morphology and internal structures, together with the high Th/U values of the zircons from the plagiogranite suggest a magmatic crystallization history. The dominant population of zircons in the two plagiogranite samples analyzed in this study yield 206Pb/ 238U ages of 737 ± 23 Ma and 782 ± 24 Ma corresponding to the timing of emplacement of these rocks. The plagiogranite from Manamedu also contains two other zircon populations: the first group shows a discordant population with 207Pb/ 206Pb ages between 2278 and 2527 Ma with an upper intercept age of 2418 ± 65 Ma. Similar Neoarchean–early Paleoproterozoic ages have been widely reported from the surrounding rocks within the PCSZ and also from the Salem Block to the north. We interpret these older zircons as xenocrysts entrained in the plagiogranite during magma ascent and consolidation. The third group of zircons in the plagiogranite yield 206Pb/ 238U age of 513 ± 4.6 Ma, comparing well with the Cambrian ages reported in several recent studies from magmatic and metamorphic rocks in the PCSZ and the crustal blocks to the south, correlating with the tectono-thermal events associated with the collision and post-collisional extension associated with the assembly of the Gondwana supercontinent. Zircons in the two gabbro samples from the Manamedu complex analyzed in this study yield well-defined clusters on the concordia and show weighted mean 206Pb/ 238U ages of 744 ± 11 Ma and 786 ± 7.1 Ma. The internal structure of the zircons from these gabbro samples and their high Th/U values also suggest a magmatic crystallization history. The zircon ages that we obtained from the Manamedu forearc complex also compare with the recently reported zircon age of 825 ± 17 from the gabbro–anorthosite complex and the 766 ± 8 Ma age from oscillatory-zoned euhedral crystals of magmatic zircons in felsic volcanic suite of the Kadavur Dome, to the south of Manamedu. The ages also compare with the 819 ± 26 Ma 206Pb/ 238U age reported from zircons in arc-related rapakivi granite from an adjacent locality within the PCSZ. All these data suggest a prominent mid Cryogenian subduction system along the southern periphery of the PCSZ prior to the destruction of the Mozambique Ocean lithosphere and the final amalgamation of the Gondwana supercontinent.

U‐Pb zircon geochronology of granites and charnockite from southern India: implications for magmatic pulses associated with plate tectonic cycles within a Precambrian suture zone

AbstractThe Palghat–Cauvery Suture Zone (PCSZ) in southern India marks the trace along which continental blocks were sutured during the Precambrian. We report here U‐Pb geochronological data based on laser ablation ICP mass spectrometry for zircons in two granite plutons and an associated charnockite from within the PCSZ. The weighted mean of 206Pb/238U ages for a rapakivi granite from the southern part of the PCSZ yields 819 ± 26 Ma (error: 2σ; N = 13). On the other hand, a metamorphosed A‐type granite and a charnockite from the central part of the PCSZ yield a markedly older upper intercept age of 2468 ± 36 Ma (error: 2σ) and a lower intercept age of 588 ± 36 Ma (error: 2σ). Our results provide important constraints on the Precambrian crustal evolution in the PCSZ and demonstrate: (1) Neoarchaean to early Palaeoproterozoic magmatism, generating A‐type granites which we correlate with a major global crustal formation event; (2) mid Neoproterozoic magmatism, and generation of the rapakivi granite, probably associated with the subduction of the Mozambique Ocean floor; and (3) latest Neoproterozoic high‐grade metamorphism (generating the charnockite) associated with the collisional amalgamation of the Gondwana supercontinent. Together with the previous geochronological data from the PCSZ and adjacent crustal blocks, our results provide information on magmatic pulses associated with the Archaean cratonization event, as well as the Neoproterozoic Wilson Cycle of the Mozambique Ocean culminating in continent‐continent collision. Copyright © 2011 John Wiley & Sons, Ltd.

Sapphirine + quartz assemblage from the Southern Granulite Terrane, India: diagnostic evidence for ultrahigh‐temperature metamorphism within the Gondwana collisional orogen

AbstractA synthesis of the petrological characters of granulite facies rocks that contain equilibrium sapphirine + quartz assemblage from three localities (Rajapalaiyam, Ganguvarpatti and Panangad) in the Southern Granulite Terrane (SGT), India, provides unequivocal evidence for extreme crustal metamorphism associated with the collisional assembly of the Gondwana supercontinent during the Late Neoproterozoic to Early Cambrian. The reaction microstructures associated with sapphirine + quartz vary among the samples, probably suggesting different tectonic conditions during the metamorphic evolution. Sapphirine and quartz in pelitic granulites from Rajapalaiyam in the Madurai Block occur as an intergrowth in poikiloblastic garnet, whereas the two minerals show host‐inclusion relationship (quartz inclusion in coarse‐grained sapphirine) in a Mg‐Al‐rich rock from Ganguvarpatti in the same block. The sapphirine–quartz association and related microstructures from the two localities are consistent with isobaric cooling from sapphirine + quartz to orthopyroxene + sillimanite + quartz fields, possibly along a counterclockwise P‐T path. In contrast, sapphirine + quartz occurs as coronas around Mg‐rich (XMg ∼ 0.58) staurolite within poikiloblastic garnet in an Mg‐Al‐rich rock from Panangad in the Palghat–Cauvery Suture Zone (PCSZ). The texture suggests decompression from the stability field of Mg‐rich staurolite (P > 15 kbar) at T > 1000 °C possibly along a clockwise P‐T trajectory. Such contrasting P‐T paths are consistent with a recent tectonic model of the SGT, according to which the metamorphism in the Madurai Block is related to heat (and fluid) input from the mantle, whereas that in the PCSZ is related to the subduction‐collision tectonics between the Madurai Block and the Dharwar Craton, where high‐pressure and ultrahigh‐temperature (UHT) metamorphic rocks were extruded along the subduction channel. Copyright © 2010 John Wiley & Sons, Ltd.

Neoarchean high-pressure metamorphism from the northern margin of the Palghat–Cauvery Suture Zone, southern India: Petrology and zircon SHRIMP geochronology

We report the metamorphic pressure–temperature ( P– T) history of mafic granulites from two localities in southern India, one from Kanja Malai in the northern margin and the other from Perundurai in the central domain of the Palghat–Cauvery Suture Zone (PCSZ). The PCSZ is described in recent models as the trace of the suture along which crustal blocks were amalgamated within the Gondwana supercontinent during Late Neoproterozoic–Cambrian. The mafic granulite from Kanja Malai yields P– T conditions of 750–800 °C and 8–12 kbar reflecting the partially retrograded conditions following a peak high-pressure (HP) metamorphic event. The common Grt + Cpx + Qtz assemblage in these rocks and lack of decompression texture suggest that peak metamorphism was probably buffered by Grt + Cpx + Opx + Pl + Qtz assemblage, following which the rocks were exhumed through a gradual P– T decrease. The mafic granulite from Perundurai (Grt + Cpx + Pl) contains Opx + Pl symplectite commonly occurring between garnet and clinopyroxene, suggesting the progress of reaction: Grt + Cpx + Qtz → Opx + Pl, with the Grt + Cpx + Qtz representing the peak metamorphic assemblage. The reaction microstructures and calculated P– T conditions suggest that the mafic granulites from Perundurai underwent peak HP metamorphism at P > 12 kbar and T = 800–900 °C and subsequent isothermal decompression along a clockwise P– T path, in contrast to the P– T path inferred for Kanja Malai. The contrasting P– T paths obtained from the two localities suggest that whereas Perundurai is a part of the metamorphic orogen developed within the PCSZ during Gondwana assembly, the high-pressure granulites of Kanja Malai belong to a different orogenic regime. In order to evaluate this aspect further, we analyzed zircons in a charnockite and garnet-bearing quartzo-feldspathic gneiss associated with the HP granulites from Kanja Malai which yielded mean 207Pb/ 206Pb magmatic protolith emplacement ages of 2536.1 ± 1.4 Ma and 2532.4 ± 3.7 Ma, and peak metamorphic ages of 2477.6 ± 1.8 Ma and 2483.9 ± 2.5 Ma, respectively. These results closely compare with the available magmatic (2530–2540 Ma) and metamorphic (2470–2480 Ma) ages reported from charnockites in the Salem Block at the southern fringe of the Archean Dharwar craton, immediately north of the PCSZ. The Neoarchean/Paleoproterozoic ages obtained from Kanja Malai correlate with the tectonic history at the margin of the Archean craton. Although no age data are available for the Perundurai mafic granulite, the close correspondence of their P– T data and exhumation path with those reported for Late Neoproterozoic–Cambrian HP–UHT metamorphism within the PCSZ suggest that these rocks form part of the Gondwana-forming orogen.

Fluids in high- to ultrahigh-temperature metamorphism along collisional sutures: Record from fluid inclusions

Petrographic studies and microthermometric investigations on fluid inclusions associated with high- to ultrahigh-temperature metamorphic rocks in three major Precambrian suture zones on the globe demonstrate the dominant occurrence of CO 2-rich fluids. These rocks form part of hot orogens developed along collisional plate boundaries. The sapphirine-quartz-bearing Mg–Al-rich rock from the Palghat-Cauvery Suture Zone, a trace of the Cambrian Gondwana suture zone in southern India, preserves evidence for a prograde high-pressure event and subsequent peak ultrahigh-temperature metamorphism along a clockwise path, and contains abundant CO 2-rich inclusions in corundum, garnet, and sapphirine. Most of the fluid inclusions are either primary or secondary and preserve low-density CO 2-rich fluids (0.569–0.807 g/cm 3). Similar low-density CO 2-rich fluid inclusions (0.853–0.953 g/cm 3) are also present in pelitic granulites from the Limpopo Complex of southern Africa, a Neoarchean granulite-facies orogen formed by continent–continent collision. In contrast, the garnet–orthopyroxene granulite from Tonagh Island in the Neoarchean Napier Complex in East Antarctica contains very high-density primary (1.095–1.129 g/cm 3) and secondary (0.960–1.179 g/cm 3) carbonic inclusions in garnet and quartz. The calculated isochores for the fluid inclusions from the Palghat-Cauvery Suture Zone and the Limpopo Complex yield significantly lower-pressure estimates than those predicted from peak metamorphic conditions. We interpret this as a result of significant density decrease due to rapid decompression along a clockwise P– T trajectory. In contrast, the estimated isochores for primary inclusions in garnet–orthopyroxene granulites from the Napier Complex are consistent with the peak P– T conditions estimated from mineral phase equilibria for the Tonagh Island rocks, suggesting that most of the fluid inclusions in these rocks did not undergo any marked effect of volume change and density decrease. The contrasting fluid densities among the localities investigated in this study are probably related to the nature of the P– T trajectory; the Tonagh Island rocks had a near-isochoric exhumation history whereas the metamorphic orogens in the other two sutures witnessed rapid decompression. Our results suggest that whereas the composition of the syn-metamorphic fluids are preserved in all cases, density reversal occurs within inclusions as a function of the tectonic history and exhumation style.

Crustal architecture beneath Madurai Block, southern India deduced from magnetotelluric studies: Implications for subduction–accretion tectonics associated with Gondwana assembly

The Madurai Block in southern India is considered to represent the eroded roots of an arc-accretionary complex that developed during the subduction–collision tectonics associated with the closure of the Mozambique Ocean and final suturing of the crustal fragments within the Gondwana supercontinent in the Late Neoproterozoic–Cambrian. Here we present a magnetotelluric (MT) model covering the main collisional suture (Palghat–Cauvery Suture Zone) in the north into the central part of the Madurai Block in the south comprising data from 11 stations. Together with a synthesis of the available seismic reflection data along a N–S transect further south within the Madurai Block, we evaluate the crustal architecture and its implications on the tectonic development of this region. According to our model, the predominantly south dipping seismic reflectors beneath the Madurai Block define a prominent south-dipping lithological layering with northward vergence resembling a thrust sequence. We interpret these stacked layers as imbricate structures or mega duplexes developed during subduction–accretion tectonics. The layered nature and stacking of contrasting velocity domains as imaged from the seismic profile, and the presence of thick (>20 km) low resistivity layers ‘floating’ within high resistivity domains as seen from MT model, suggest the subduction of a moderately thick oceanic crust. We identify several low resistivity domains beneath the Madurai Block from the MT model which probably represent eclogitised remnants of oceanic lithosphere. Their metamorphosed and exhumed equivalents in association with ultrahigh-temperature metamorphic orogens have been identified from surface geological studies. Both seismic reflections and MT model confirm a southward subduction polarity with a progressive accretion history during the northward migration of the trench prior to the final collisional assembly of the crustal blocks along the Palghat–Cauvery Suture Zone, the trace of the Gondwana suture in southern India.

Diopsidites from a Neoproterozoic–Cambrian suture in southern India

AbstractWe report the occurrence and characteristics of diopsidite dykes and veins from the Palghat-Cauvery Suture Zone (PCSZ) marking the boundary between the Archaean Dharwar craton to the north and the Proterozoic Madurai Block to the south, which is considered as a trace of the Cambrian Gondwana suture zone in southern India. The diopsidites are composed predominantly of coarse crystals of diopside [Mg no. (100 Mg/(Mg+Fetot)) up to 89] surrounded by retrograde calcic amphibole, plagioclase and phlogopite with accessory titanite and calcite. The major, trace and rare earth element characteristics of the diopside crystals suggest their formation in a subduction zone setting. We correlate the petrogenesis of the diopsidites with the tectonics associated with the subduction and closure of the Neoproterozoic Mozambique Ocean prior to the final collisional assembly of the Gondwana supercontinent in Cambrian.

The Manamedu Complex: Geochemical constraints on Neoproterozoic suprasubduction zone ophiolite formation within the Gondwana suture in southern India

Abstract Ophiolites provide important clues on the role of subduction and have been widely investigated to reconstruct the history of development and closure of ocean basins in the geological past. The Manamedu Complex within the Palghat-Cauvery Suture Zone in southern India comprises metamorphosed equivalents of the following lithological units: (1) an ultramafic group comprising dominantly of pyroxenite and highly altered dunite, locally preserving cumulate textures; (2) a gabbroic suite consisting of gabbro, gabbro norite, and anorthosite; (3) sheeted mafic dykes of amphibolite to meta-andesite categories, (4) plagiogranite veins and pools; and (5) a thin layer of ferruginous cherts. Cr vs. Y, V vs. Cr, Ti vs. Zr, TiO 2 –MnO–P 2 O 5 and Fe 2 O 3 –Na 2 O + K 2 O–MgO plots of the gabbros and mafic dyke assemblages show that these are related to island arc tholeiite (IAT) group with tholeiitic to calcalkaline signatures. Chondrite normalized REE patterns of mafic dykes do not show any pronounced fractionation and display slight positive Eu anomalies. The normalized MORB plots of the mafic dykes show depletion of HFSE (negative Nb, Ti, Ta, Hf anomalies) and enrichment of LFSE (positive K, Ba, Rb, Th). The petrological and geochemical characteristics of the major lithological units in Manamedu Complex suggest that these rocks represent the remnants of an oceanic crust, developed from mantle-derived arc magmas probably within a suprasubduction zone tectonic setting. From the geological set up and our field observations, we infer that these rocks were obducted on to the continental margin with the closure of an ocean basin during the Neoproterozoic. The Manamedu Complex may represent the remnants of the Mozambique Ocean crust developed during Rodinia breakup and which was destroyed during the amalgamation of the Gondwana supercontinent in the Latest Neoproterozoic-Cambrian.

The Cambrian collisional suture of Gondwana in southern India: A geophysical appraisal

The southern Indian crustal fragment occupied a central position within the Late-Neoproterozoic-Cambrian Gondwana supercontinent assembly. Here we synthesize the available geophysical data that includes gravity, seismic tomography, deep seismic sounding (DSS) and magnetotellurics (MT) from the Palghat–Cauvery Suture Zone (PCSZ), which is considered as a trace of the Gondwana-forming suture in southern India, as well as the surrounding regions to delineate the crustal architecture and tectonic history of the region. An increased crustal thickness immediately north of the PCSZ is correlated to crustal thickening associated with the subduction–collision processes during continental amalgamation. A prominent gravity low of about −45 m gal beneath Kodaikanal in the central Madurai Block, south of the PCSZ might suggest the deep root of a thick magmatic arc. Deep seismic studies in and around Chennimalai at the central domain of the PCSZ indicate the presence of ca. 10 km thick low velocity (6.0 km/s) layer at mid-crustal depths. The gravity model indicates a high density (2.80 gm/cm3) layer corresponding to these depths. Two-dimensional MT model shows highly resistive (>20,000 Ω-m) felsic upper crust down to 15–16 km all along the profile. The resistivity of the mid-crust is more than 10,000 Ω-m and the resistivity of the lower crustal domains is in the range of 500–3000 Ω-m. The MT model and revised gravity model, constrained by MT, show a southward dipping low resistive zone (<100 Ω-m) and a high density region at a depth range of 15–45 km beneath the Chennimalai dome within the PCSZ. The interpretation of magnetotelluric and revised gravity model confirm the PCSZ to be the trace of a major suture zone, and correlate with a plate tectonic model of subduction–collision–accretion tectonics along this zone related to the final amalgamation of the Gondwana supercontinent.

Sapphirine+quartz corona around magnesian (XMg ~0.58) staurolite from the Palghat-Cauvery Suture Zone, southern India: Evidence for high-pressure and ultrahigh-temperature metamorphism within the Gondwana suture

Abstract We report the first finding of equilibrium sapphirine + quartz assemblage from the Palghat-Cauvery Suture Zone (PCSZ) in southern India providing unequivocal evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions associated with the collisional assembly of the Gondwana supercontinent in the Late Neoproterozoic–Cambrian. The sapphirine and quartz occur as coronas around Mg-rich (XMg ~ 0.58) staurolite within poikiloblastic garnet in an Mg–Al-rich rock, suggesting the progress of the prograde dehydration reaction: staurolite + garnet → sapphirine + quartz + H2O. Although the occurrence of Mg-rich staurolite was previously noted from some localities in the PCSZ, no sapphirine + quartz direct association has yet been reported. The available experimental studies on Mg-rich staurolite indicate that the mineral is stable at P > 15 kbar, and suggest that the complex texture that we report here might correspond to a peak high-pressure regime prior to the UHT event. The symplectic sapphirine + quartz around staurolite probably implies decompression from P > 15 kbar toward the stability of sapphirine + quartz at temperatures of c. 1000 °C along a clockwise P–T path. The prograde high-pressure metamorphism and following UHT event correlate with the subduction–collision tectonics associated with the final stage of amalgamation of Gondwana supercontinent.

Retrogressed eclogite (20 kbar, 1020 °C) from the Neoproterozoic Palghat–Cauvery suture zone, southern India

The Palghat–Cauvery suture zone in southern India separates Archaean crustal blocks to the north and the Proterozoic Madurai block to the south. Here we present the first detailed study of a partially retrogressed eclogite (from within the Sittampundi anorthositic complex in the suture zone) that occurs as a 20-cm wide layer in a garnet gabbro layer in anorthosite. The eclogite largely consists of an assemblage of coexisting porphyroblasts of almandine–pyrope garnet and augitic clinopyroxene. However, a few garnets contain inclusions of omphacite. Rims and symplectites composed of Na–Ca amphibole and plagioclase form a retrograde assemblage. Petrographic analysis and calculated phase equilibria indicate that garnet–omphacite–rutile–melt was the peak metamorphic assemblage and that it formed at ca. 20kbar and above 1000°C. The eclogite was exhumed on a very tight hairpin-type, anticlockwise P–T path, which we relate to subduction and exhumation in the Palghat–Cauvery suture zone. The REE composition of the minerals suggests a basaltic oceanic crustal protolith metamorphosed in a subduction regime. Geological–structural relations combined with geophysical data from the Palghat–Cauvery suture zone suggest that the eclogite facies metamorphism was related to formation of the suture zone. Closure of the Mozambique Ocean led to development of the suture zone and to its western extension in the Betsimisaraka suture of Madagascar.

Anatomy of a Cambrian suture in Gondwana: Pacific-type orogeny in southern India?

Southern India occupies a central position in the Late Neoproterozoic–Cambrian Gondwana supercontinent assembly. The Proterozoic mosaic of southern India comprises a collage of crustal blocks dissected by Late Neoproterozoic–Cambrian crust-scale shear/suture zones. Among these, the Palghat–Cauvery Suture Zone (PCSZ) has been identified as the trace of the Cambrian suture representing Mozambique Ocean closure during the final phase of amalgamation of the Gondwana supercontinent. Here we propose a model involving Pacific-type orogeny to explain the Neoproterozoic evolution of southern India and its final amalgamation within the Gondwana assembly. Our model envisages an early rifting stage which gave birth to the Mozambique Ocean, followed by the initiation of southward subduction of the oceanic plate beneath a thick tectosphere-bearing Archean Dharwar Craton. Slices of the ocean floor carrying dunite–pyroxenite–gabbro sequence intruded by mafic dykes representing a probable ophiolite suite and invaded by plagiogranite are exposed at Manamedu along the southern part the PCSZ. Evidence for the southward subduction and subsequent northward extrusion are preserved in the PCSZ where the orogenic core carries high-pressure and ultrahigh-temperature metamorphic assemblages with ages corresponding to the Cambrian collisional orogeny. Typical eclogites facies rocks with garnet + omphacite + quartz and diagnostic ultrahigh-temperature assemblages with sapphirine + quartz, spinel + quartz and high alumina orthopyroxene + sillimanite + quartz indicate extreme metamorphism during the subduction–collision process. Eclogites and UHT granulites in the orogenic core define P– T maxima of 1000 °C and up to 20 kbar. The close association of eclogites with ultramafic rocks having abyssal signatures together with linear belts of iron formation and metachert in several localities within the PCSZ probably represents subduction–accretion setting. Fragments of the mantle wedge were brought up through extrusion tectonics within the orogenic core, which now occur as suprasubduction zone/arc assemblages including chromitites, highly depleted dunites, and pyroxene bearing ultramafic assemblages around Salem. Extensive CO 2 metasomatism of the ultramafic units generated magnesite deposits such as those around Salem. High temperature ocean floor hydrothermal alteration is also indicated by the occurrence of diopsidite dykes with calcite veining. Thermal metamorphism from the top resulted in the dehydration of the passive margin sediments trapped beneath the orogenic core, releasing copious hydrous fluids which moved upward and caused widespread hydration, as commonly preserved in the Barrovian amphibolite facies units in the PCSZ. The crustal flower structure mapped from PCSZ supports the extrusion model, and the large scale north verging thrusts towards the north of the orogenic core may represent a fold-thrust belt. Towards the south of the PCSZ is the Madurai Block where evidence for extensive magmatism occurs, represented by a number of granitic plutons and igneous charnockite massifs of possible tonalite–trondhjemite–granodiorite (TTG) setting, with ages ranging from ca. 750–560 Ma suggesting a long-lived Neoproterozoic magmatic arc within a > 200 km wide belt. All these magmatic units were subsequently metamorphosed, when the Pacific-type orogeny switched over to collision-type in the Cambrian during the final phase of assembly of the Gondwana supercontinent. One of the most notable aspects is the occurrence of arc magmatic rocks together with high P/T rocks, representing the deeply eroded zone of subduction. The juxtaposition of these contrasting rock units may suggest the root of an evolved Andean-type margin, as in many arc environments the roots of the arc comprise ultramafic/mafic cumulates and the felsic rocks represent the core of the arc. The final phase of the orogeny witnessed the closure of an extensive ocean — the Mozambique Ocean — and the collisional assembly of continental fragments within the Gondwana supercontinent amalgam. The tectonic history of southern India represents a progressive sequence from Pacific-type to collision-type orogeny which finally gave rise to a Himalayan-type Cambrian orogen with characteristic magmatic, metasomatic and metamorphic factories operating in subduction–collision setting.

Prograde and retrograde högbomites in sapphirine + quartz bearing Mg-Al rock from the Palghat-Cauvery Suture Zone, southern India

We report rare hogbomite in a sapphirine + quartz bearing Mg-Al rock from the Palghat-Cauvery Suture Zone (PCSZ), the trace of the Cambrian Gondwana suture in southern India. Fine-grained (∼ 0.8 mm) hogbomite occurs mantled by sapphirine and spinel with or without rutile, and the entire assemblage is included within poikiloblastic garnet. The microstructure suggests the progress of a possible prograde reaction; Grt + Hog → Spr + Spl + Rt + H2O. As the garnet hosting hogbomite also includes equilibrium sapphirine + quartz, the hogbomite probably underwent T > 1000 °C peak metamorphism, in which case this the first report of this mineral formed under ultrahigh-temperature conditions. On the other hand, coarse-grained (∼ 12 mm) hogbomite, which is slightly richer in Fe + Mg (6.7-6.9 pfu) and poor in Al (14.5-14.7 pfu) than the former type (6.4-6.6 and 14.7-15.0 pfu, respectively), occurs in the matrix around spinel and/or sapphirine. The texture suggests retrograde formation of the latter hogbomite.