The thermal history of the Eastern Ghats Belt (India) as revealed by U–Pb and [formula omitted] dating of metamorphic and magmatic minerals: implications for the SWEAT correlation

Abstract

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 charnockite and metasedimentary belts. The younger E–W trending Godavari Rift divides the orogen into a northern and a southern segment. Textural and structural relationships indicate a complex thermo-tectonic evolution involving several episodes of metamorphism and deformation. This orogenic belt represents an important part of the reconstructed global Southwest-United-States–East-Antarctica (SWEAT) orogen. The exact timing of metamorphism and tectonism in this large orogenic belt plays a key role in this global reconstruction. Textural and structural relationships indicate a complex thermo-tectonic 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 sphene and 40Ar/ 39Ar ages on hornblende. Monazite, allanite and some sphenes provide evidence that in the central and eastern tectonic units north of the Godavari Rift the last high-grade metamorphism occurred at ca. 960 Ma. In the most western unit, the Western charnockite zone, allanite and monazite from late pegmatites indicate a major thermal event around 1.6 Ga. This is the first indication that there may be pronounced temporal differences in the thermal evolution of different tectonic or ltihological units in the Eastern 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 of apatite–magnetite veins that contain zircon with a concordant U–Pb age of 516±1 Ma. All hornblende 40Ar/ 39Ar ages from the central units of the belt provide Pan-African plateau ages. The partially reset sphene and completely reset hornblende ages indicate that the Pan-African metamorphism reached at least middle-amphibolite facies conditions in parts of the Eastern Ghats belt. This thermal overprint was not strong enough to cause retrogression of the high-grade mineral assemblages, but has so far only been recognized in the reset mineral ages and some late magmatic rocks. A hornblende 40Ar/ 39Ar age of ca. 1110 Ma from an amphibolite south of the Godavari rift indicates that the Pan-African thermal overprint was weaker in the Western charnockite zone, if it affected this area at all. The new ages presented for the Eastern Ghats belt are similar to published ages for the Rayner Complex and the Prydz Bay region of Eastern Antarctica. This similarity in ages is strong support for these areas in East Antarctica and the Eastern Ghats belt being complementary parts of an extensive orogenic belt formed during the Grenvillian Orogeny around 1 Ga. Both continental fragments also show evidence for a Pan-African overprint, however, this event is much more pronounced in Antarctica than in the Eastern Ghats belt.