Proterozoic crustal evolution in the NW Himalaya (India) as recorded by circa 1.80 Ga mafic and 1.84 Ga granitic magmatism

Single zircon dating of the Rampur metabasalts of the Larji-Kullu-Rampur window in the Lesser Himalayas yielded an evaporation age of 1800 +/- 13 Ma. The zircon age is considerably younger than the previously published whole rock Sm-Nd age of 2510 +/- 90 Ma, suggesting that the Sm-Nd age may be geologically meaningless and that the Sm-Nd whole rock array may have resulted from mixing. In the NW Himalaya, there is also evidence for extensive silicic melt generation in the Paleoproterozoic. Zircons from a metarhyodacite in the Larji-Kullu-Rampur window yielded an evaporation age of 1840 +/- 16 Ma, which we interpret as the minimum age of magmatism. The Main Central Thrust granitic mylonites are interpreted as the basement of the Neoproterozoic Haimanta Group metasediments. Together with the granitic rocks from the Lesser Himalaya, they were derived from pre-existing continental crust prior to 1.84 Ga. The Nd depleted mantle model ages are in the range of 2.6-2.4 Ga, suggesting a contribution of Archean crust. A recycled Archean component is also documented by a 2.9 Ga domain in one of the zircons. (C) 2000 Elsevier Science B.V. All rights reserved.