The ⁴⁰Ar/³⁹Ar dating of muscovite quartz schist in Sumdo area, central Lhasa terrane and its constraints on the exhumation time of the Sumdo eclogite

Objective The high−pressure metamorphic belt, represented by the Sumdo eclogite, is of great constrained significance in exploring the evolution hisotry of the Sumdo Paleo−Tethys Ocean in the Lhasa terrane, Qinghai−Xizang Plateau. Although detailed studies on the Sumdo eclogite has been carried out, the metamorphic age of the wall rock and whether the deep subduction process occurs are still unclear, which restricted further understanding of the exhumation process of the Sumdo eclogite.

Methods Based on the detailed field geological research, we present new ⁴⁰Ar/³⁹Ar age of muscovite from the muscovite quartz schist, as the surrounding rock of the Sumdo eclogite, and present electron microprobe analyses of the muscovite.

Results The regional schistosity and microstructure characteristics of muscovite quartz schist reveal that the protolith has undergone intense regional metamorphism and deformation. Muscovite is filled between quartz grains in an orientation arrangement. The structure of mica fishes is well developed and uniformly sized particle, indicating that it is derived from metamorphism of muddy minerals. Results of the EPMA show that the muscovite in muscovite quartz schist is mainly phengite. The Ar−Ar plateau age of phengite obtained by ⁴⁰Ar/³⁹Ar dating is 247 ± 5 Ma, revealing that the metamorphism of the protolith of muscovite quartz schist occurred at the end of Early Triassic.

Conclusions Combined with the field geological characteristics and previous studies of the Sumdo eclogite, it is shown that the muscovite quartz schist have experienced the deep subduction process of the Sumdo Paleo−Tethys Ocean as the wall rock with the Sumdo eclogite and began the rapid exhumation process at the end of the Early Triassic.