Cretaceous isochron ages of K-Ar system in the UHP metamorphic rocks of the Tso Morari dome, western Himalaya, India

Abstract

Ultrahigh-pressure metamorphic (UHPM) rocks occur commonly in continent-continent collision type orogenic belts and their mineral assemblages indicate the formation at the depth of 100 km and more. Since Chopin (1984), the geology and petrology of the UHPM rocks have been studied well to estimate their P-T histories at ca. 20 localities in the world. However, the geochronological approaches to estimate time of events, especially exhumation, are still not well successful. In UHPM rocks and the associated high-pressure metamorphic (HPM) rocks of many metamorphic sequences, discordant K- Ar and Ar/Ar age relations have been reported, probably caused by the excess 40 Ar. To reveal exhumation history of UHPM rocks, phengite and paragonite, separated from UHPM rocks from the Tso Morari dome (TMD), western Himalaya, India, are analyzed with K-Ar and Ar/Ar methods. The authors applied an isochron method to the UHPM rocks of which the host lithologies were continental materials having potential excess 40 Ar. Then petrology and geochronology of UHPM rocks from Tso Morari dome (50 km x 100 km), western Himalaya, India were examined. The lithology, which is thought to be margin of Indian continent, consists of eclogites, and basic and pelitic schists, which are closely associated with each other. Recently, Sachan et al. (2001) discovered coesite from an eclogite block. The authors collected the three types of rocks from an area (ca. 20 km x 15 km) of the Tso Morari dome where coesite has been found, and calcschist and basic schist from the adjacent Indus suture zone, which is the boundary between Eurasian and Indian continents. Eclogite having Grt+Omp+Ca- amp+Czo+Phn+Rtl±Pg±Chl±Cc occurs only in the core of meter- scale mafic rock lenses, which were intercalated with the pelitic schists. This suggests that the coexisted mafic and pelitic rocks have suffered the UHPM and the same P-T-t history. The smaller mafic lenses and pelitic rocks were heavily retrograded and have mineral assemblages of Ca-Amp+Czo+Bt+Chl+Ab+Cc+Tnt ±Phn±Grt (relict)±Rtl and Qtz+Phn+Ab+Tnt+Zrn±Pg±Chl± Grt±Bt±Ap±Czo±Kfs±Cc, respectively.