Geochronological constraints on Meso- and Neoarchean regional metamorphism and magmatism in the Dharwar craton, southern India
Introduction
Precise isotopic age data are essential to quantify the time-framework as well as the dynamics of the lithospheric processes like magmatism, metamorphism, deformation, and exhumation rates. Metamorphic P–T–t paths together with the time scales on which successive P–T conditions were recorded are crucial for understanding the geodynamic evolution of orogens. In recent years tremendous progress has been made in texturally controlled in situ dating of minerals, which provide direct link between textures, P–T conditions and ages, which in turn revolutionized our understanding of the time scales of orogenic processes (Catlos et al., 2002, Muller, 2003, Paquette et al., 2004, Turkina et al., 2012, Shi et al., 2012). Texturally controlled in situ dating of monazite provides precise information on timing of thermal events in polymetamorphic terrains (Cocherie et al., 1998, Stern and Berman, 2000). Garnet is the main target of REE based Sm–Nd and Lu–Hf dating, as its lattice preferentially incorporates heavy REE and therefore show elevated Sm/Nd and Lu/Hf ratios (Duchene et al., 1997, Scherer et al., 2000, Cheng et al., 2013). Garnet as one of the major constituents of metamorphic rocks is widely used in geothermobarometry, thus providing direct link between P–T and time.
Metamorphic history of the Dharwar craton is poorly understood. A continuous increase of metamorphic grade is documented from greenschist facies in the north through amphibolite to granulite facies to the south (Pichamuthu, 1965, Raith et al., 1982, Raase et al., 1986). However, it is not known whether this north to south increase in metamorphic grade is related to one single thermal event or multiple thermal events. X-ray diffraction study of K-feldspars from younger (2.56–2.51 Ga) granite plutons also reveal metamorphic recrystallization (Kano et al., 2004) indicating that the whole preserved Archean continental crust was affected by metamorphism. Published geochronologic and structural data indicate a major tectonothermal event that affect the whole Archean crust from 2.55 to 2.51 Ga (Bidyananda et al., 2011, Peucat et al., 1993, Peucat et al., 2013, Chardon et al., 2008) whilst few studies consider the possibility of an earlier metamorphic event close to 3.0 Ga (Mahabaleswar and Peucat, 1988, Mahabaleswar et al., 1995a). More recently Jayananda et al. (2011b) documented a 2.62 Ga granulite assemblages close to ultra-high temperature (UHT) conditions in the central part of eastern Dharwar craton. All these studies point to the occurrence of more than one event of regional high-grade metamorphism in the Dharwar craton. However, the age, number and duration of these events as well as their spatial link(s) to crustal accretion, regional deformation, cratonization is not well constrained, particularly for pre-2.5 Ga tectonometamorphic events. The main purpose of this contribution is to present a first overview of the regional metamorphism including time framework of pre-2.5 Ga metamorphic events and their spatial link to crustal accretion patterns in the Dharwar craton. In order to address this issue, we have performed texturally controlled in situ dating of monazite together with dating of monazite separates in combination with SHRIMP U–Pb zircon, Sm–Nd whole rock–garnet dating of mafic rocks, metasediments, TTG and granites along well defined crustal corridors from north to south covering large parts of the Dharwar craton at various crustal levels.
Section snippets
Regional geological and tectonic framework of the Dharwar craton
The southern Indian shield is divided into two major crustal provinces viz. northern Archean domain and southern Proterozoic domain (Fig. 1) with large remnants of Archean crust (Harris et al., 1994, Bhaskar Rao et al., 2003). The northern Archean domain is classically termed as ‘Dharwar craton’ that forms a wide time window (3.7–2.5 Ga) onto the preserved Archean continental crust (Bhaskar Rao et al., 2008, Nutman et al., 1992, Peucat et al., 1993, Peucat et al., 2013, Jayananda et al., 2008,
Geological setting and metamorphic pressure–temperature conditions in the studied corridors
In the central part of the WDC, the Bababudan greenstone belt (Fig. 1a) unconformably overlies a basement made of Peninsular gneisses and Sargur Group slivers (Chadwick et al., 1985). A detailed kinematic analysis showed that Bababudan greenstones have undergone radial converging downward flow with respect to their basement in a context of regional strike-slip shearing (Chardon et al., 1998). Mineral assemblages in the Bababudan greenstone basin such as chlorite–actinolite–garnet together with
Analytical techniques
Texturally controlled in situ dating of monazite as well as dating of monazite separates were performed at Department of Geology, National Science Museum, Tokyo. Sm–Nd isotope analysis of whole rock as well as minerals was carried out at Geosciences Rennes Universite de Rennes1 (France) and Institute for Earth Evolution, JAMSTEC (Japan). The theoretical concepts of EPMA dating technique including texturally controlled in situ monazite dating and monazite separates are followed in this study and
Discussion
The ages presented in this study together with the published isotopic age data reveal that the three crustal blocks have independent crustal histories. The oldest crustal nuclei is located in the central part of WDC which accreted during 3.4–3.2 Ga (Meen et al., 1992, Nutman et al., 1992, Peucat et al., 1993, Jayananda et al., 2008) whilst CDC contain large remnants of 3.36–3.2 Ga within dominant 2.7–2.52 Ga crust (Nutman et al., 1996, Balakrishnan et al., 1999, Jayananda et al., 2000, Jayananda
Conclusion
EPMA chemical dating of monazites, Sm–Nd garnet–whole rock isochrons together with SHRIMP U–Pb zircon ages document multiple regional thermal events and a contrasted metamorphic record within the three sub-provinces constituting the Dharwar craton. The Western Dharwar craton kept record of thermal pulses at 2.5–2.42 Ga and 3.1–3.0 Ga, whereas the Central Dharwar craton was affected by thermal pulses at 2.55–2.42 Ga, 2.62 Ga, and 3.2 Ga. The Eastern Dharwar craton keeps the record high-grade
Acknowledgements
This work was financially supported by the DST transect Project ESS//16/337/2007, DU Project (R&D/2011-12) IFCPAR Project 1111-1 and JSPS Projects (T. Miyazaki and T. Kano). M. Santosh and Bor-ming Jahn are thanked for inviting to contribute this synthesis. We thank three anonymous reviewers for their useful comments which helped to improve the quality of this contribution. M. J gratefully acknowledge D. Chardon and J.-J. Peucat for their constant support during the course of this work as well
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