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U–Pb zircon ages from the Spontang Ophiolite, Ladakh Himalaya

¹ Department of Geology, University of Bergen, Allégaten 41, 5007 Bergen, Norway (e-mail: rolf.pedersen{at}geol.uib.no)
² Department of Earth Sciences, Oxford University, Parks Road, Oxford, OX1 3PR, UK
³ Present address: BP Amoco Exploration, Farburn Industrial Estate, Dyce, Aberdeen AB21 7PB, UK

Scientific editing by Randall Parrish.

The Spontang Ophiolite in the Ladakh Himalaya consist of a thrust sheet of upper mantle and oceanic crustal rocks obducted onto the northern passive continental margin of India during the late Cretaceous. Precise isotopic ages of Himalayan ophiolites are presently unavailable. U–Pb dating of zircons from a dioritic segregation in the high-level gabbros of the Spontang ophiolite shows that the complex formed at 177±1 Ma (mid-Jurassic) and we suggest that it represents a fragment of Neo-Tethyan oceanic crust. This Jurassic age contrasts with late Cretaceous ages (91–95 Ma) previously reported for the formation of the Troodos and Oman ophiolite complexes further west along the Neo-Tethyan suture zone and also predates the early to mid-Cretaceous Masirah ophiolite (c. 150 Ma) and its alkaline volcanic seamounts (115–125 Ma). An andesitic rock from an arc sequence overlying the Spontang ophiolite was also dated using as 88±5 Ma. This Late Cretaceous age constrains the minimum age of initiation of subduction beneath the Spontang Ophiolite. The complex was emplaced during the Turonian–Maastrictian, some 80 Ma after its formation when the Indian continental margin collided with a Cretaceous arc about 25 Ma before the India–Asia collision. The lack of a sub-ophiolite metamorphic sheet beneath the Spontang mantle sequence may be explained by the fact that the ophiolite was old and cold at the time of obduction.

Keywords: Himalaya, U–Pb, ophiolites, absolute age.

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