Title	Neoarchean emplacement and metamorphism of Karimnagar granite and harnockite: zircon U-Pb age and Hf isotopic results

Work Description:

The NW-SE trending Godavari Graben that separates the Archaean Dharwar and Bastar cratons of Peninsular India marks a zone of recurrent rifting. Along the southern shoulder of the graben (the northern limit of the Eastern Dharwar Craton), a belt of high-grade rocks (~ 150 x 30 km) - the Karimnagar granulites terrane (KGT), was described earlier (Rajesham, Bhaskar Rao and Murthy., 1993). The KGT comprises a fascinating lithological association of orthopyroxene-bearing quartofeldspathic gneiss, amphibolite-granulite facies supracrustal belts that include metamorphosed quartz- arenites, pelites, carbonates, iron formation and mafic rocks as well as several zones of migmatite and younger granitoid intrusives. Xenoliths within the charnockites show a wide-range of bulk compositions and multiple sapphirine-bearing mineral paragenesis. Previously, geochronologic studies of Karimnagar charnockites indicated a minimum age of metamorphism ca. 2500 Ma from convergence of i) Rb-Sr isochron ages (Rajesham et al., 1993; Sarbajna et al., 2012) and ii) EPMA CHIME ages showing zircon cores up to 3100 Ma mantled by rims of 2600 Ma, monazites and uraninites between 2420 and 2470 Ma (Santosh et al., 2004).

We determined zircon U-Pb ages and Hf-isotopic compositions (at the CSIR-NGRI LAM-MC-ICPMS National Facility) for two samples representing the widespread Karimnagar granite (older granite, KRN-90-30) and charnockite (KRN-90-24) (Fig. 20.1). Populations of zircons with < 10% discordance yield weighted mean 207Pb/206Pb ages of 2573±28 Ma and 2538±8 Ma (2s) respectively. These ages define crystallization of magmatic protoliths and regional high-grade metamorphism respectively. Both samples define lower intercept ages ~ 450 Ma. A similar range of eHf(T) between +1 to -10 and model TCDM Lu-Hf ages (~3.60 Ga) for both the suites indicate the involvement of Mesoarchean crustal components in the genesis of these Neoarchean magmas. The present data reinforce the Neoarchean (ca. 2.55 Ga) age for the KGT charnockite-granite suites, which is quite analogous to those in the transition zone and the SGT at the southern margin of the Dharwar craton, it would be fascinating to explore the age of their xenoliths.

Fig.20.1:
Geological sketch of the Karimnagar granulite terrain in the northeastern part of the Eastern Dharwar Craton

(Y.J. Bhaskar Rao, EVSSK Babu, T. Vijaya Kumar, B. Sreenivas, B. Vijaya Gopal, T. Rajesham)