Thermal History of the Eastern Ghats Belt (India) Deduced from U-Pb and 40Ar-39Ar Dating of Metamorphic Minerals

K. Mezger Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany

M. A. Cosca Université Lausanne, Institut de Minéralogie et Pétrographie, 1015 Lausanne, Switzerland

M. Raith Mineralogisch-Petrologisches Institut, Poppelsdorfer Schloß, D-53115 Bonn, Germany

The Eastern Ghats Belt forms an extended high grade terrane along the east coast of India. The orogen is separated into a northern and southern segment by the E-W trending Godavari Rift. The rocks in the belt show upper amphibolite facies to granulite facies metamorphism including ultra high temperature granulites. For a reconstruction of parts of the thermal history U-Pb ages were determined on monazite, allanite and sphene and 40Ar-39Ar ages on hornblendes extending over the whole exposed part of the Eastern Ghats Belt. Monazite, allanite and some sphenes provide evidence for the last high-grade metamorphism at ca. 960 Ma north of the Godavari Rift. South of the rift U-Pb ages of allanites indicate a major metamorphism around 1.6 Ga. This is the first indication that there may be a difference in the thermal evolution of the two segments of the Eastern Ghats Belt.

The majority of the sphenes from the northern segment of the Eastern Ghats Belt are discordant and lie along a reference line from 940 Ma to 500 Ma. The degree of discordance ranges from 0 to 98%. There is no correlation between the degree of discordance and the origin of the sphenes within the belt. There is also no evidence for a mixture of two or more generations of sphene within a single mineral separate. This is evidence for real discordance of these sphenes caused by a thermal overprint at ca. 500 Ma. Hornblende 40Ar-39Ar ages from the northern segment of the belt all provide pan-African plateau ages. These 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 in the Eastern Ghats Belt and is only noticeable in the reset mineral ages. A hornblende 40Ar-39Ar age of ca. 1100 Ma from an amphibolite south of the Godavari rift indicates that the thermal overprint was weaker in the southern Eastern Ghats Belt than in the northern segment. Further to the south the pan-African thermal episode is also responsible for the granulite facies metamorphism in S-India and Sri Lanka.

The ages obtained from the Eastern Ghats Belt are similar to published ages for the segment of Eastern Antarctica that may form the counterpart to the Eastern Ghats Belt during the major metamorphic episode around 1 Ga. The new mineral ages indicate that the two segments stayed together until at least pan-African times, when the rocks of Eastern Antarctica underwent a last thermal overprint that reached upper amphibolite facies conditions.