Yellappa T

Alaskan-type of plutonic complexes represent arc magma chambers and they occur throughout the earth's history from Archean to Phanerozoic periods around the globe. The ultramafic rocks from these complexes commonly contain spinels of varied chemistry, especially rich in Cr, Fe, and Ti coupled with low-Al contents when compared to the spinels from oceanic or continental rift domains. An attempt is made here to study Thenmudiyanur Mafic-Ultramafic Complex (TMUC) from the Belur Shear Zone (BLSZ) in the northeastern margin of Southern Granulite Terrane, located about 20 km SW of Tiruvannamalai Town. The important lithologies of TMUC include dunites, peridotites and pyroxenites surrounded by metagabbros and amphibolites. The dunites and peridotites are highly serpentinized with well-preserved cumulate olivines along with magnesite, chrome spinel and clinopyroxenes. The mineral chemistry of olivines represents higher forsterite (Fo 88-90) and Mg# 87-90 [1009Mg/(Mg+Fe 2+)] contents. The gradational contact within the rock types, compositional variations and cumulate nature of mineralogical characteristics suggest fractional crystallization of magma from a single parental source. The chrome spinels from these lithologies represent higher Cr# [1009Cr/(Cr+Al)] that ranges from 65 to 86 along with lower MgO (1.06-4.57 wt%) and MnO (\1 wt%) contents. The presence of chrome spinels together with olivine in dunites as well as peridotites and clinopyroxene mineral chemistry from pyroxenites reveal arc signatures for the evolution of TMUC, which seems to be similar to that of the Alaskantype. Considering the tectonic setting and the age relationships of adjacent lithologies, it is opined that the TMUC might have evolved during the Neoproterozoic period.

Arc-related origin of pyroxenites in association with Alaskan-type tectonics has been described in many mafic-ultramafic complexes across the globe. The Salem Mafic-Ultramafic Complex (SMUC) is one such Neoproterozoic Alaskan-type complex exposed at the northern margin of the Cauvery Suture Zone (CSZ), Southern Granulite Terrane, south India. The Complex consists of mafic and ultramafic sequences along with several occurrences of pyroxenite intrusions of varied thickness in the form of dykes. Similar pyroxenite dykes were also observed in and around the Complex at several locations within the basement hornblende gneiss, trending in the NE-SW and E-W directions. Petrography of these dykes indicated websterite variety with cumulate textures and reveals the dominance of clinopyroxene along with orthopyroxene, primary amphibole, minor plagioclase and oxide minerals like magnetite, ilmenite and spinels. The whole-rock chemistry of 10 representative samples showed enrichment of LIL elements (Sr, K, Rb, Th) and depletion of HFSE (Hf, Ti, Y, Yb) with normalized primitive mantle and N-MORB. The clinopyroxene mineral chemistry represented tholeiitic signatures with high Mg# values (Mg/(Mg + Fe)) up to 0.91, and the two-pyroxene thermobarometry of these pyroxenites yielded re-equilibrium crystallization temperatures of 820-932C with moderate pressures at 11-12 kbar. Various tectonic discrimination plots of clinopyroxene mineral chemistry together with wholerock chemistry favoured their origin under arc settings with the interactions of fluid-related subduction zone metasomatism relevant to Neoproterozoic Alaskan-type tectonics.

The granulite facies rocks of Southern Granulite Terrain (SGT), India represent lower crust-upper mantle products of different tectonothermal events of different origins. They include charnockites, two pyroxene granulites of mafic-felsic, dismembered ophiolite sequence, mafic-ultramafic intrusions and granitoids. The southern part of the Cauvery Suture Zone (CSZ) around Aniyapuram-Mohanur, near Namakkal is dominated by a wide range of mafic-felsic granulites. These are well exposed in close association with dismembered mafic and ultramafic rocks including peridotite, pyroxenite, amphibolite, gabbrogabbronoritic rocks, metapelite, metachert with younger intrusives of pegmatite and granitoids. The petrology of these granulites is characterized by sub-idioblastic fine to coarse-grained clinopyroxene (40-50%) and orthopyroxene (10-20%), idioblastic garnets (20-30%), subhedral to anhedral amphiboles (10-20%) and plagioclases (5%) with accessory phases of apatite, zircon, biotite and opaque minerals. Whole rock geochemistry of 14 representative samples reveals, that the granulites are mafic to slightly intermediate in composition with higher SiO 2 (47-53 wt%) and A1 2 O 3 (10-16 wt%) with low K 2 O contents (<0.4wt%). Trace element ratios are extremely variable with very high K/Rb ratios (540-13447) reflected in very low Rb contents (0.2-7 ppm) and varied Ba/La (1.8-43.40), Rb/Sr (0.0001-0.13) and Sr/Nd (3.23-34.25) concentrations. The geochemical variation plots (Na 2 O+K 2 O-FeO t-MgO and Zr vs. Y) reveal that these granulites belong to calc-alkaline to tholeiitic signatures. On various tectonic discrimination plots (MnO-TiO 2-P 2 O 5 , Ti vs. V, Ti vs. Zr, and Cr vs. Y) these show Island arc origin. Spider diagrams with normalized MORB show enrichment of LILE (Sr, K, Rb, Ba) and depletion of HFSE (Ti, Nb, Ta, Hf) with-ve Nb anomalies. The above results with available age data suggest that the protoliths of the mafic granulites are tholeiitic basalts developed in an island arc environment related to Neoarchean to Paleoproterozoic suprasubduction zone tectonic setting, consistent with dismembered ophiolite sequence of the region and have been subjected to granulite facies metamorphism during Neoproterozoic arc magmatism in the terrain.

The Aniyapuram Mafic and Ultramafic Complex (AMUC) is a dismembered ophiolite of Neoarchean to Paleoproterozoic age, exposed in south-central part of the Cauvery Suture Zone (CSZ), Southern Granulite Terrane (SGT), India. The complex comprises ultramafic rocks like serpentinized peridotites of harzburgite variety and pyroxenites of websterite variety along with metagabbros, amphibolites, plagiogranites and metacherts. The pyroxenites in the complex occur in the form of dykes, veins and lenses in association with serpentinized peridotites and metagabbros. The chemical compositions of olivine from the peridotites show a moderate forsterite content (Fo 88-87) and spinels are enriched in aluminum (Al 2 O 3 [ 60 wt%) with depletion of Cr contents (Cr# \ 0.5). The pyroxenites are composed of orthopyroxenes with dominance of clinopyroxene and sulphide occurrences (up to 62 wt% S) along the grain boundaries of pyroxenes. The whole rock geochemistry of these pyoxenites shows enrichment of LILE and depletion of HFSE with negative Nb anomalies on N-MORB and primitive normalization. The mineral chemistry of clinopyroxenes from the pyroxenites shows tholeiitic in nature with high Mg# ratios [Mg/(Mg + Fe) = 0.70-0.88]. These mineral chemistry results together with whole rock chemistry reveal their origin in Island arc setting of supra-subduction zone (SSZ) tectonics possibly evolved by the interaction of subduction derived Cuids of host serpentinized peridotites. The estimated two-pyroxene thermobarometry of these pyroxenites represents varied re-equilibrium temperatures of 820-980°C and medium to slightly high pressures of 10-12 kbar. With the available age relationship from the complex, the study supports strong evidence for the occurrence of Neoarchean to Paleoproterozoic suprasubduction tectonics and associated lithologies of formation in the terrane.

The Chalk hills of Salem complex also called Salem mafic and ultramafic complex (SMUC), occurs in northern part of Cauvery suture zone (CSZ), southern granulite terrane (SGT), India with the lithological sequence of ultramafic cumulates, mafic intrusions of gabbros and amphibolites with quartzo-feldspathic veins and ultrapotassic dykes. Recently the complex has been described as Neoproterozoic Alaskan-type. The Ti-rich gabbros are in association with serpentinized dunites and pyroxenites in the form of several intrusions at various locations of the complex. The petrography of these gabbros shows sub-alkaline to alkaline in nature with mineralogy of two pyroxenes and plagioclases of calcicsodic assemblages along with magnetite-titanomagnetite-ilmenite (Ti-spinels). The whole rock geochemistry indicates that they are tholeiitic in nature with enrichment of LILE (Rb, Ba, Th, Sr) and depletion of HFSE (Nb, Ta, Ti) with reference to N-MORB. The mineral chemistry of clinopyroxenes from these gabbros show slightly higher Mg# (0.68-0.80), TiO 2 (0.40-0.73 wt%) with lower Al 2 O 3 (4.4-5.5 wt%), Na 2 O (0.53-0.94 wt%) and Cr 2 O 3 (0.1-0.17 wt%) contents. The orthopyroxenes also show low Al 2 O 3 (1.4-2.3 wt%), Cr 2 O 3 (up to 0.13 wt%), and relatively higher MgO (17.11-21.79 wt%). The Ti-bearing oxides (ilmenites) of these gabbros are characterized by higher contents of TiO 2 (38-51 wt%) and FeO (46-56 wt%), similar to that of Ti-Fe rich Alaskan-type of mafic rocks. On various tectonic discrimination diagrams of whole rock and clinipyroxene mineral chemistry of these gabbros reveal that they are derived from subduction related components of arc magmas of titanium rich, evolved through the process of high degree of crystal fractionation. From the two-pyroxene thermometry, the temperature of formation of these gabbros are determined as 893-1014°C and the clinopyroxene barometry results variable pressures of 8-13 kbars. The tectonic setting with the available age relationships from the complex suggest these intrusions are syntectonic and co-genetic of late emplacements along with the ultramafics during Neoproterozoic subduction events. Such occurrences of high Ti-bearing gabbros are common in many Alaskan-type of complexes like Neoproterozoic Arabian Nubian Shield and other parts of the world.

The Salem Mafic-Ultramafic Complex occurs within the northern part of Cauvery Suture Zone (CSZ), Southern Granulite Terrane (SGT), India. The complex occurs as semicircular to elongated linear body with a thick mantle section and comparatively well-developed crustal section comprising different lithological units, also referred to as the Chalk Hills of Salem. The major rock types in the complex comprise ultramafic cumulates of dunite, peridotite, wherlite, pyroxenite, and hornblendite, mafic and felsic intrusions of gabbros and amphibolites, and quartzo-feldspathic alkaline rocks together with several ultrapotassic dykes. Geochemical studies of amphibolites from this complex indicate tholeiitic parent magma with enrichment of LIL elements (Rb, Ba, Th, and Sr) and are relatively more enriched than N-MORB as compared with arc-related rocks. The enrichment of LIL elements over the HFS elements and with distinct Nb, Ta, and Ti depletion relative to other HFS elements suggest the involvement of subduction-related components in the depleted mantle source and that the magmatism occurred in an oceanic convergent realm. The mineral chemistry of dunite and peridotite shows high content of forsterite (86.9-92.2) in olivine and high Cr# value (80.85-98.73) for spinel. The tectonic discrimination plots of spinel and olivine mineral chemistry together with clinopyroxene chemistry of amphibolites reveal arc signature typical of Alaskan type of complex. The U-Pb zircon analysis of quartz monzonite intrusion within the hornblendite from the complex yielded a weighted mean age of 819 ± 2.4 Ma, suggesting that the complex formed during the Neoproterozoic and can be correlated to the Alaskan-type complexes described from the Arabian-Nubian Shield. KEYWORDS

Granitoid intrusions occur widely in the Southern Granulite Terrain (SGT) of India, particularly within the Cauvery Suture Zone (CSZ), which is considered as the trace of the Neoproterozoic Mozambique ocean closure. Here we present the petrological and geochemical features of 19 granite plutons across the three major tectonic blocks of the terrain. Our data show a wide variation in the compositions of these intrusions from alkali feldspathic syenite to granite. The whole rock geochemistry of these intrusions displays higher concentrations of SiO 2 , FeO*, K 2 O, Ba, Zr, Th, LREE and low MgO, Na 2 O, Ti, P, Nb, Y and HREE's. The granitoids are metaluminous to slightly peraluminous in nature revealing both I-type and A-type origin. In tectonic discrimination plots, the plutons dominantly show volcanic arc and syn-collisional as well as post-collisional affinity. Based on the available age data together with geochemical constrains, we demonstrate that the granitic magmatism in the centre and south of the terrain is mostly associated with the Neoproterozoic subduction–collision–accretion–orogeny, followed by extensional mechanism of Gondwana tectonics events. Similar widespread granitic activity has also been documented in the Arabian Nubian shield, Madagascar, Sri Lanka and Antarctica, providing similarities for the reconstruction of the crustal fragments of Gondwana supercontinent followed by Pan-African orogeny.

The Cauvery suture zone (CSZ) in southern India has witnessed multiple deformations associated with multiple subduction-collision history, with incorporation of the related accretionary belts sequentially into the southern continental margin of the Archaean Dharwar craton since Neoarchean to Neoproterozoic. The accreted tectonic elements include suprasubduction complexes of arc magmatic sequences, high-grade supracrustals, thrust duplexes, ophiolites, and younger intrusions that are dispersed along the suture. The intra-oceanic Neoarchean-Neoproterozoic arc assemblages are well exposed in the form of tectonic mélanges dominantly towards the eastern sector of the CSZ and are typically subjected to complex and multiple deformation events. Multi-scale analysis of structural elements with detailed geological mapping of the sub-regions and their structural cross sections, geochemical and geochronological data and integrated geophysical observations suggest that the CSZ is an important zone that preserves the imprints of multiple cycles of Precambrian plate tectonic regimes.

Occurrence of two lamproitic dykes intruding the basement granite near Khadka village at the northern margin of the Indravati Basin, Bastar craton is reported. Combined field, microscopy, XRD, EPMA and whole-rock geochemical investigations reveal that these lamproites were subjected to high degree of hydrothermal alteration as well as possible metamorphism. However, relicts of their original textures are well-preserved thereby providing important clues as to the nature of the protolith. Quartz, carbonate, chlorite and phlogopite constitute the bulk mineralogy whereas spinel, apatite and iron oxides are the accessory phases. Chemical composition of the groundmass spinels are strikingly similar to those from the lamproites. The Khadka lamproite dykes display high abundances of compatible elements such as Ni (238–396 ppm), Cr (484–892 ppm), and V (160–200 ppm) as well as high-field strength elements such as Zr (719–2057 ppm) and Nb (92–126 ppm) that resemble those in lamproites. Khadka lamproites also have high whole-rock REE abundances (ΣREE up to 1260 ppm) and display fractionated chondrite-normalized REE patterns (La/Yb= 113–237) which together with their average compatible and incompatible trace elemental ratios (e.g. Nb/Zr, Nb/La, Ba/Rb) are strikingly similar to those of the Mesoproterozoic Krishna lamproites of the Eastern Dharwar craton. Available field evidences suggest the Khadka lamproites to be of at least Palaeoproterzoic age (1.88 Ga) which makes them some of the oldest such rocks as yet documented from the Indian shield.

Ophiolites, the remnants of ancient oceanic lithosphere, have been described from collisional sutures of various ages with only few examples from Archean terranes. Here we report the discovery of a Neoarchean ophiolite suite from the southern margin of the Dharwar Craton in India, tectonically intercalated within a Neoproterozoic suture zone. The metamorphosed and variably dismembered ophiolite suite, exposed around Devanur, comprises altered ultramafic units, websterite, gabbros, mafic dykes, amphibolites, trondhjemites and pegmatites associated with ferruginous metachert. Structural and petrographic studies indicate that the rocks represent a highly sheared and metamorphosed suite emplaced as a thrust sheet. The major and trace element geochemistry of the mafic dykes indicate derivation from basaltic–andesitic magmas with tholeiitic to calc-alkaline characteristics. The rocks display negative Nb anomalies with enrichment of LILE (K, Rb, Ba, Th) and depletion in HFSE (Ti, Nb, Hf, Tb). The tectonic discrimination of these rocks based on various geochemical plots suggests that they were generated in a suprasubduction zone setting. We present new SHRIMP zircon U–Pb data for two trondhjemite samples from this complex, which yield 238U–206Pb ages of 2528 ± 61 and 2545 ± 56 Ma. The Neoarchean age from the trondhjemites obtained in our study is closely comparable to similar ages obtained in recent studies from magmatic zircons in charnockites and orthogneisses in the area. The suprasubduction zone assemblages and arc magmas suggest a Neoarchean ocean closure along the southern margin of the Dharwar Craton.► Discovery of a Neoarchean suprasubduction ophiolite from southern India. ► U–Pb dating of zircons from trondhjemites yield ~ 2.5 Ga. ► Neoarchean ocean closure along the southern margin of the Dharwar Craton.