Introduction Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol103.Issue3-4.2000:

Precambrian Research 103 2000 191 – 206 Proterozoic crustal evolution in the NW Himalaya India as recorded by circa 1.80 Ga mafic and 1.84 Ga granitic magmatism Christine Miller a, , Urs Klo¨tzli b , Wolfgang Frank b , Martin Tho¨ni b , Bernhard Grasemann b a Institut fu¨r Mineralogie und Petrographie, Uni6ersity of Innsbruck, Innrain 52 , A- 6020 Innsbruck, Austria b Institut fu¨r Geologie, Uni6ersity of Vienna, Althanstrasse 14 , A- 1090 Vienna, Austria Received 6 July 1999; accepted 19 May 2000 Abstract Single zircon dating of the Rampur metabasalts of the Larji – Kullu – Rampur window in the Lesser Himalayas yielded an evaporation age of 1800 9 13 Ma. The zircon age is considerably younger than the previously published whole rock Sm – Nd age of 2510 9 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 9 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. © 2000 Elsevier Science B.V. All rights reserved. Keywords : NW Himalaya; Proterozoic magmatism; Geochronology; Geochemistry www.elsevier.comlocateprecamres

1. Introduction

The Himalayan orogen is a product of the Cainozoic collision between India and Asia. In the course of this collision, the Higher Himalaya HH was thrusted southwards over the Lesser Himalaya LH along the Main Central Thrust MCT. Both, HH and LH are considered parts of the northern passive margin of India. Many studies have focussed on the Phanerozoic se- quences, but the provenance of the largely unfos- siliferous Proterozoic sediments in both units is less well understood Frank et al., 1995; Parrish and Hodges, 1996. Data on the basement of these Proterozoic supracrustal sequences andor Corresponding author. E-mail address : christine.milleruibk.ac.at C. Miller. 0301-926800 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 0 0 0 0 0 9 1 - 7 magmatic rocks are sparse and contradictory. Pre- vious Rb – Sr and Sm – Nd dating of LH granitic and mafic rocks yielded whole rock ages ranging from 2510 to 1220 Ma Frank et al., 1977; Bhanot et al., 1982; Bhat and LeFort, 1992. Better con- straints on the age and provenance of the Protero- zoic supracrustal sequences, associated igneous suites and their basement, however, are essential for reconstructions of the relationship between HH and LH before the onset of the Himalayan orogeny. We have sought to characterize the age and distribution of the Proterozoic igneous suites in the different units and we have used the Sm – Nd technique to compare their protolith characteris- tics. Another key objective of our study was to determine zircon 207 Pb 206 Pb ages from a metarhy- odacite and a metabasalt that are associated with clastic metasediments in the basal sedimentary succession Rampur formation of the LH. For the metabasalts, our new zircon data do not confirm the Archean Sm – Nd whole rock age in the literature. In addition, we present geochemical data on the Rampur metabasalts and on grani- toids from the LH and the MCT zone which provide constraints on their source characteristics and the crustal evolution of the former passive margin of northern India.

2. Regional geology