CRUSTAL EVOLUTION AND GEODYNAMICS

Palaeomagnetism and geochemistry of the mafic dyke swarms of south India

Various geological processes involved in the evolution of the crust and mantle are best understood by a study of mafic magmatic suite of rocks. Indian sub-continent has witnessed episodic mafic magmatism throughout most of Archaean to Tertiary periods. Most studies have been restricted to komatiites and tholeiitic suite of Archaean and Deccan basalts, thus leaving a large gap between Archaean and late Phanerozoic and also on other igneous unites of Phanerozoic age in southern Indian shield. The Indian peninsular shield with many occurrences of dykes and intrusives provide ample opportunity for petrological and geochemical study of these rocks and in placing constraints on mantle-crustal processes and their evolution. These data coupled with paleomagnetic and isotopic data can provide infallible guides in integrating the already available data for other regions of the Indian shield and elsewhere, and in constraining the geodynamic processes with reference to global scenario. CESS is involved in the integrated study of various igneous rocks, essentially the mafic dykes in the south Indian shield. Currently, work is in progress around the Cuddapah and other Proterozoic sedimentary basins. Paleomagnetic data from the Indian shield as well as from other neighbouring continental blocks within the Gondwana framework are being reviewed for critical assessment. The data in combination with petrological and geochronological constraints are being evaluated in terms of Mozambique and Grenville orogenic events.

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CRUSTAL EVOLUTION AND GEODYNAMICS

Petrological study of granulites of southern India

The southern Indian peninsular shield is home to a variety of metamorphic rocks, spanning from >3000 m.y. to the widespread Pan-African event of 550 m.y. The most dominating rock type is the granulites, which are exhumed portions of the lower-middle crust. Their development processes form the key to understanding the formation of the lower crust. The southern Indian granu-lite belt has thus attracted the attention of petrologists and geochemists the world over. Three prominent Proterozoic shears, the Moyar-Bhavani, the Palghat-Cauvery and the Achenkovil shears, traverse these terrains, which are also contact zones of different age provinces, contrasting litho-types and metamorphic facies. Since these shear zones are at the most crucial locations in the south Indian granulite belt, detailed structural and petrological studies, along with chemical and isotopic dating, should unravel the frozen information on the Precambrian crustal evolution and lower crustal processes. Detailed petrological investigations of selected critical lower crustal zones around Palakkad and Kerala Khondalite Belt (KKB) were conducted for comparative evaluation of the geological processes. The following is a general summary of the results. Arrested charnockite in southern India is of distinctly later origin than regional metamorphism. It formed largely as local events and not related to a specific time related mechanism. The formation of arrested charnockite between 2600 m.y. (Kabbaldurga) and 540 m.y. (KKB) was essentially a response to a favourable change in the bulk composition (to granitic) by melting/deformation, PT (decompression) and fluid (CO^/low a(H^O) brines) infiltration. These events may have taken place at any time after regional metamorphism and migmatisation events by a combination or independent events. The general indication is that all the arrested charnockite were formed around pressures of 5-6 kbar, corresponding to the depth of formation of 25 km, even though the peak metamorphism in the terrain could have been much higher (750-900°C and 7-10 kbar). Preexisting structures played a major role as conduits for the migration of dehydrating fluids. CO^ was the dominant fluid species in all the terrains. Mineral phase-controlled metasomatic changes with significant element mobility and comparable patterns between KKB and Palakkad were recorded. PT-t paths are similar between KKB and Palakkad and they (and isotope data) indicate contrasting crustal evolution between regions south of Palakkad and regions to the north in the Kamataka craton. CESS has also initiated a multidisciplinary and multi-institutional research project along a transect from Kuppam to 183111 183112 183U3 183114 183115 183116 183117 Distance Gneiss ==" Chamockite "== Gneiss A sample showing gneiss being converted into charnockite Kanyakumari in the deep continental crust of southern Indie Field studies involve characterisation of the lithologic, struc tural and mineralisation parameters. Laboratory investigation envisaged include mineralogic, textural and petrographic stud ies, mineral chemistry and computation of pressure-tempera ture paths, fluid inclusion studies, geochemical studies, geo chronology and ore genesis. Lithologic and structural studies and representative sample col lection were carried out in the area between Kanyakumari an< Virudunagar. The major rock types identified in the area in elude chamockites, garnet-biotite gneisses, khondalites, pyrox ene granulites and calc-silicates, representing granulite facie assemblages. Patch- and vein-types of incipient charnockite developed within gamet-biotite gneisses representing fluid-con trolled granulite formation were also documented from som< localities. Spinel- and cordierite-bearing aluminous granulite; occur in some places, and provide potential assemblages fo recording mineral reactions and exhumation history. Wollasto nite-bearing calc-silicates were found in two localities - a Arakkakulam and Parappadi. The Arakkakulam, calc-silicat< occurs as blocks within massive charnockite, and show spec tacular wollastonite veining. The study also revealed the occur Total Iron Oxides ™D pe0 CZI FeaOo Ilmenite source: Laterite (A); River (B); Estuary (C); Chavara Placer (D) renceofrare metal and rare earth pegmatites in this area. A new Th-, U- and REE-enriched mineral, chevkinite, was documented. Two more important mineralisations were discovered: (1) disseminated molybdenite associated with Puttetti syenite; and (2) gemstone mineralisation, including chrysoberyl, at Kalakkad. Another major programme CESS is engaged in is the detailed field examination of massive chamockite in Kodaikanal-Car-damom hill chamockite massif (K-CHM) of southern India. The study is to understand the petrological characteristics of chamockite massif, their temporal relation to the arrested chamockite formation seen throughout southern India, whether this ensemble is magmatic (C-type) and has the appropriate age to be a potential heat source to Kerala Khondalite belt rocks. Available geochemical data and field description from previous studies have been compiled and their characteristics are being evaluated to prepare a basis for detailed field examination and traverse sampling across the K-CH chamockite massif.

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CRUSTAL EVOLUTION AND GEODYNAMICS

Evolution of Kodaikanal – Cardamom Hill (K-CHM) charnockite massif, southern India

Recent recalculation of metamorphic PT conditions using more robust gartnet-Al-opx thermobarnometer on the adjacent Kerala Khondalite Belt rocks has indicated the presence of ultrahigh temperature rocks (900-1050ºC and ~9 Kbar) near its boundaries. One research group hypothesised that the entire K-CHM is magmatic in origin and provided heat for the metamorphism of KKB. One line of evidence supporting this hypothesis is based on the distinctive chemical composition recognised from few available K-CHM charnockites geochemical data (high K, Ba, Zr, P, Ti, LREE). It was important, therefore, to understand the petrological characteristics of the charnockite massif, their temporal relationship to the arrested charnockite formation seen throughout south India, and understand whether this ensemble are magmatic (C-type) and have the appropriate age to be potential heat source to Kerala Khondalite belt rocks. The main objectives therefore are:

v Determine from field studies the interrelationship among major rock types, and classify the charnockite types in the Kodaikanal-Cardamom hill charnockite massif.
v Document mineral paragenesis, metamorphic reactions and PT conditions of metamorphism.
v Examine chemical characteristics of charnockites and deduce, based on chemistry, classification of spatial distributions and tectonic settings, for their formation.
v Synthesis of data and tectonic speculation as to which magmatic and metamorphic processes may have operated in producing this vast massif and thermal imprints seen in adjacent Kerala Khondalite Belt (KKB) and draw implications for the evolution of the South Indian granulite-facies terrain.

Initial geochemical and isotopic studies of ninety samples carried out in collaboration with the University of Alberta (Dr. Thomas Chacko and Dr Larry Heaman, personal communication) supports the division of K-CHM charnockites into two distinct groups. One group is strongly comparable to the “C-type charnockite” with high K, Ba, Zr, P and Ti. Preliminary U-Pb zircon and monazite geochronology on four samples indicates that these two groups of charnockite are very different in age.

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CRUSTAL EVOLUTION AND GEODYNAMICS

Lithology and tectono-metamorphic evolution of the Kerala Khondalite Belt (KKB), Southern India

The project essentially helps promotion of bilateral exchanges between CESS and University of Bonn, encourages collaboration and scientific exchanges with the objective of understanding the petrological, geochemical and geochronological work on the major lithologies of the Kerala Khondalite Belt; generation of granitic magmas along the northern boundary of the KKB in the Achankovil Shear Zone.

As part of the project based exchange visits, the Indian Principle Investigator made the first of the five total visits to the University of Bonn. The visit was essentially to hold discussions and initiate the work and begin analytical work on selected samples, utilising the facilities at the University of Bonn (major and trace element composition of whole-rock samples with X-Ray Fluorescence spectroscopy and mineral analyses using electron microprobe). Further, to plan for further joint fieldwork and other technical matters and decide areas for detailed sampling for laboratory studies. During the visit, selected samples of gneiss, charnockite, mafic granulites from Kerala Khondalite Belt and Cardamom charnockite massif were analysed for all the major and trace elements. Several samples of gneiss, charnockite and mafic granulites were also analysed for all the important mineral phases.

The German team visited India during November-December, 2000. Detailed fieldwork was carried out jointly by the Indian and German teams in parts of Trivandrum and Kollam districts of Kerala and in Kanyakumari and Tirunelveli districts of Tamil Nadu. The major goal of the fieldwork was to establish clear field relationship between different rock types of KKB. Geochemical and geochronological studies of the rocks are on. Further work is in progress.

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