The thermal history of the Eastern Ghats Belt (India) as revealed by U-Pb and Ar-40/Ar-39 dating of metamorphic and magmatic minerals: implications for the SWEAT correlation

The Eastern Ghats belt is a large high-grade terrane exposed along the east coast of India. The late Proterozoic orogen consists of N-S trending charn ockite and metasedimentary belts. The younger E-W trending Godavari Rift di vides the orogen into a northern and a southern segment. Textural and struc tural relationships indicate a complex thermo-tectonic evolution involving several episodes of metamorphism and deformation. This orogenic belt repres ents an important part of the reconstructed global Southwest-United-States- East-Antarctica (SWEAT) orogen. The exact timing of metamorphism and tecton ism in this large orogenic belt plays a key role in this global reconstruct ion. Textural and structural relationships indicate a complex thermo-tecton ic evolution involving several episodes of metamorphism and deformation. In order to reconstruct parts of the regional thermal history of the Eastern Ghats belt, U-Pb ages were determined on monazite, allanite, zircon and sph ene and Ar-40/Ar-39 ages on hornblende. Monazite, allanite and some sphenes provide evidence that in the central and eastern tectonic units north of t he Godavari Rift the last high-grade metamorphism occurred at ca. 960 Ma. I n the most western unit, the Western charnockite zone, allanite and monazit e from late pegmatites indicate a major thermal event around 1.6 Ga. This i s the first indication that there may be pronounced temporal differences in the thermal evolution of different tectonic or ltihological units in the E astern Ghats belt. The majority of the sphenes from the Eastern Ghats belt are discordant and lie along a reference line extending from ca. 935 Ma to 504 Ma. This discordance is interpreted to be due to a thermal disturbance during a Pan-African deformation phase, which is dated by the emplacement o f apatite-magnetite veins that contain zircon with a concordant U-Pb age of 516 +/- 1 Ma. All hornblende Ar-40/Ar-39 ages from the central units of th e belt provide Pan-African plateau ages. The partially reset sphene and com pletely reset hornblende ages indicate that the Pan-African metamorphism re ached at least middle-amphibolite facies conditions in parts of the Eastern Ghats belt. This thermal overprint was not strong enough to cause retrogre ssion of the high-grade mineral assemblages, but has so far only been recog nized in the reset mineral ages and some late magmatic rocks. A hornblende Ar-40/Ar-39 age of ca. 1110 Ma from an amphibolite south of the Godavari ri ft indicates that the Pan-African thermal overprint was weaker in the Weste rn charnockite zone, if it affected this area at all. The new ages presente d for the Eastern Ghats belt are similar to published ages for the Rayner C omplex and the Prydz Bay region of Eastern Antarctica. This similarity in a ges is strong support for these areas in East Antarctica and the Eastern Gh ats belt being complementary parts of an extensive orogenic belt formed dur ing the Grenvillian Orogeny around 1 Ga. Both continental fragments also sh ow evidence for a Pan-African overprint, however, this event is much more p ronounced in Antarctica than in the Eastern Ghats belt. (C) 1999 Elsevier S cience B.V. All rights reserved.