Christoph Dobmeier

Crustal architecture and evolution of the Eastern Ghats Belt and adjacent regions of India

Abstract Extending along the east coast of peninsular India, the Eastern Ghats expose a deep section through a composite orogenic belt that once formed part of the Proterozoic mobile belt system within East Antarctica and East India. The critical evaluation of the existing geological and isotopic data strongly suggests that this orogenic belt includes not only the granulite facies Eastern Ghats Belt but also the Nellore-Khammam Schist Belt and lower grade units at the southern margin of the Singhbhum Craton. The present authors propose its subdivision into four crustal provinces with widely different geological evolutions. The Rengali and Jeypore Provinces formed at the margin of the Bhandara Craton in the Late-Archaean. In the Krishna Province, volcanosedimentary rocks equivalent to the Cuddapah Supergroup accumulated, probably on the Dharwar Craton in the Palaeoproterozoic, and the major tectonometamorphic event took place between 1.67 and 1.55 Ga, subsequent to a short-lived igneous activity. The Eastern Ghats Province, which shows considerable similarities with the Rayner Province of East Antarctica, was strongly affected by pervasive deformation, high-grade metamorphism and crustal-derived magmatism between 1.1 and 0.9 Ga, which extensively modified the crustal structure of present eastern peninsular India. Neoproterozoic and Early Phanerozoic tectonothermal activities were largely restricted to pre-existing shear zones, but the present configuration of the composite orogenic belt may have been achieved only during the Pan-African Orogeny.

Post-Grenvillean transpression in the Chilka Lake area, Eastern Ghats Belt—implications for the geological evolution of peninsular India

Abstract Results of a combined structural and geochronological study of the area around the 0.79 Ga Chilka Lake anorthosite complex in the Eastern Ghats Belt of India place important constraints on the geological history of peninsular India. Th–U–Pb EPMA dating of monazites from the outer margin of the anorthosite complex and from country rocks reveals that the emplacement of the anorthosite occurred at the beginning of a protracted Middle Neoproterozoic tectono-magmatic event, encompassing leucogranite emplacement and/or cooling in the aureole of the anorthosite between 762 and 743 Ma and a fabric-defining transpressional deformation at 690–662 Ma. The progressive deformation was accompanied by granulite facies metamorphism. A thermal event at 520–505 Ma led to local resetting of the Th–U–Pb systematics in some monazites. Ages of 964–921 Ma obtained from metapelitic granulites constituting the oldest lithological component imply that the Chilka Lake area was involved in the global Grenvillean orogenesis, which was the fabric-defining tectonothermal event in most parts of the Eastern Ghats Belt. In combination with additional data from these authors and published data from other domains of the Eastern Ghats Belt, these results indicate that the post-Grenvillean evolution of the Eastern Ghats Belt is characterized by a sequence of Neoproterozoic tectonothermal events between 792 and 662 Ma and a Pan-African thermal event at 520–505 Ma. It further implies slow cooling of the entire granulite terrain. In view of appreciably different metamorphic conditions between the low-grade Proterozoic supracrustal cover of the Archean cratons (East-Dharwar and Bastar) and the neighbouring Proterozoic granulites of the Eastern Ghats Belt, it is suggested that final thrusting of the Eastern Ghats Belt over the Archean forelands took place in Early Phanerozoic time.

Age of emplacement of massif-type anorthosites in the Eastern Ghats Belt, India: constraints from U–Pb zircon dating and structural studies

Abstract Massif-type anorthosites in the Proterozoic Eastern Ghats Belt of India occur as complexes of varying size within a strongly deformed, high to ultra-high grade assemblage of metasedimentary and metaigneous rocks. Recently discovered dikes and sheets of exceptionally HFSE and REE-enriched ferrodiorites at the immediate contacts of three anorthosite complexes (Chilka Lake, Bolangir and Turkel) contain ubiquitous magmatic zircon suitable for U–Pb dating. Due to the coeval and comagmatic nature of the ferrodiorites, the U–Pb zircon data provide reliable constraints on the time of anorthosite emplacement. Zircons from three ferrodiorite samples were analysed applying the conventional multi-grain isotope dilution method. Zircon grains in the Bolangir ferrodiorites (D4 and B2012) are long-prismatic and show the faint fine-scale oscillatory zoning typical for magmatic crystallisation, with some poorly luminescent thin overgrowths of metamorphic origin. The data points obtained for abraded grain fractions are slightly discordant and yield an upper intercept age of 933±32 Ma with a lower intercept at 515±20 Ma, defined by previously published concordant U–Pb titanite data from calc silicate rocks of the border zone of the anorthosite. Zircon grains in the Chilka Lake ferrodiorite (OK 76-2) are prismatic and show zoned igneous interiors that are overgrown by irregular poorly luminescent rims, presumably of metamorphic origin. The U–Pb ages obtained for abraded grain fractions are concordant and define an age of 792±2 Ma. The U–Pb zircon ages for the Chilka Lake and Bolangir ferrodiorites disprove the often quoted Mesoproterozoic age of anorthosite emplacement in the EGB, which was based on a four-point Rb–Sr whole rock ‘isochron’ for the Chilka Lake anorthosite complex (1404±89 Ma; Sarkar et al., 1981 ). Combined with field and geochemical evidence, the new U–Pb zircon ages indicate that the data array in the Rb–Sr evolution diagram should be interpreted as a mixing line resulting from marginal contamination of the ascending hot anorthosite pluton with felsic melts generated in its thermal aureole. The U–Pb zircon ages provide new insights in the deformation history of the Eastern Ghats Belt as structural studies indicate intrusion of the Bolangir anorthosite while its country rocks underwent a thrust shear-dominated deformation. In contrast, emplacement of the considerably younger Chilka Lake anorthosite occurred previous to or at the beginning of a regionally intense transpressive deformation.

On the origin of ‘arrested’ charnockitization in the Chilka Lake area, Eastern Ghats Belt, India: a reappraisal

Arrested-type charnockite formation occurs in an assemblage of high-grade gneisses at several localities of the Chilka Lake area that belongs to the Proterozoic Eastern Ghats Belt of India. The isolated ellipsoidal domains are found exclusively in leucogranite (leptynite) bands that intruded lit-par-lit interbanded granulite-grade supracrustal and intermediate igneous rocks (khondalite–enderbite). Macrostructures and microfabrics document a multiple deformation of the rock assemblage under high-grade conditions. The intrusion of the leucogranitic melts separates a first episode of deformation, D 1 , from a younger progressive deformation, D 2 –D 4 . A transpressive regime and inhomogeneous deformation is indicated for D 2 –D 4 by the associated structures and fabrics. But quartz c -axis patterns show that pure shear prevailed during the closing stages of deformation. The spatial distribution and orientation of the ellipsoidal charnockite domains within the host leptynite and the orientation pattern of orthopyroxene c -axes inside the domains provide evidence for a synkinematic in situ formation of the domains during D 3 , through partial breakdown of the leptynite assemblage (Bt + Grt + Qtz + Fl 1 ⇌ Opx + Fsp + Ilm + Fl 2 /L). Local fluid migration along steep foliation planes associated with large-scale D 3 folds triggered the reaction. Orthopyroxene blastesis was confined to the centre of the domains, and an envelope formed in which the residing fluid caused secondary intergranular formation of chlorite, ore and carbonate, imparting the domains’ typical greenish-brown charnockite colour. The shape of the envelope, which varies from prolate in limbs to oblate in hinges of D 3 folds, is responsive to the local stress field. Comparison of chemical rock compositions supports the in situ formation of charnockite in leptynite. Subtle compositional differences are controlled by the changing mineralogy. Compared to the host leptynite, the charnockite domains are enriched in K 2 O, Ba, Rb and Sr, but depleted in FeO*, MnO, Y and Zr. The data obtained in this study provide conclusive evidence that the ellipsoidal charnockite domains do not represent remnants of stretched enderbite layers as proposed by Bhattacharya, Sen & Acharyya, but formed in situ in the leptynite as a result of localized synkinematic fluid migration late in the deformation history.

Origin and evolution of FeAl-granulite in the thermal aureole of the Chilka Lake anorthosite, Eastern Ghats Province, India

The Chilka Lake anorthosite (792 ± 2 Ma) in the northeastern part of the Eastern Ghats Province provides an excellent example of deep-crustal diapiric emplacement and ultra-high temperature (UHT) contact metamorphism. Within a narrow contact zone, supracrustal granulates were heated to temperatures in excess of 1000°C, causing renewed partial anatexis and melt segregation under dynamic conditions, with hybridization of the marginal leuconorite. The thermal history of the innermost aureole is well manifested by reaction textures in a FeAl-granulite occurring at the immediate contact SE of Balugaon. The tightly folded rock shows a finely banded structure. Typically, spinel+ilmenite layers are separated from quartz+garnet 1 ± perthite layers by successive coronitic layers of sillimanite and garnet 2 . The sillimanite fabric indicates synkinematic development of the corona. The reaction texture, mineral and bulk chemical characteristics, and geothermobarometric modelling suggest the following thermal evolution: (1) near-isobaric heating of a banded metapelitic granulite (khondalite: grt 1 +sil+qtz+kfsp) to c . 1100°C (7–8 kbar). Coeval with partial melting and extraction of the felsic melt, the divariant reaction (FeMg)grt+2sil → 3(FeMg)spl+6qtz is crossed at c . 1050°C, leading to the formation of a restitic FeAl-granulite made up of alternating spl+ilm and qtz+grt 1 layers. The segregated felsic melt causes hybridization of the adjacent leuconorite. (2) Subsequent near-isobaric cooling of the FeAl-granulite (spl+qtz+ilm+grt 1 ) from c . 1100°C to the stable geotherm. Reversal of the divariant reaction [3(FeMg)spl+6qtz → (FeMg)grt+2sil] produces the multi-layered coronitic texture. During this stage, coronal garnet forms also in the closely associated ferrodiorite via the reaction opx+pl → grt+qtz. The P-T data indicate that the Chilka Lake anorthosite intruded thickened crust at a depth of c . 28 km.