39Ar/40 Ar dating of the trans-Himalayan calc-alkaline magmatism of southern Tibet

Abstract

The age of the trans-Himalayan calk-alkaline magmatism has not been clearly established: on the basis of geological control, it is possible to infer a pre-late Cretaceous age for the plutonites at the southern edge of the batholith belt, and a late Cretaceous or younger age for the lava series on the top and/or to the north of the plutons. K–Ar radiometric data previously obtained by Chinese groups agree with these geological ages. The 39Ar–40Ar method has been used to analyse more accurately some of the rocks which have been sampled during the joint French–Chinese field work in southern Tibet in the summer of 1980:the incremental degassing of argon gives generally more information about the thermal history of the sample. Using this method we now show that the earlier basic plutons of the southern edge of the trans-Himalayan belt have ages of 90–110 Myr (Albian to Cenomanian), whereas the lavas of the upper unconformable series are clearly younger (60 Myr, Palaeocene). The Ladakh and Kohistan magmatic belts further west are probable equivalents of the trans-Himalaya, north of the Indus suture. Their precise ages are also poorly known: Orbitolinas of middle Cretaceous (Aptian) age have been found in the Drosh–Yasin series which lie unconformably on dioritic plutons in the northern edge of the belt. The few radiometric data that are available give ages in fairly good agreement with those we have obtained in the trans-Himalaya. So we conclude that a subduction zone has been active during the Cretaceous between India and Asia. The only outstanding question is whether this active margin was, at that time, close to India, or to Asia, or somewhere in between.