The dynamothermal aureole of the Donqiao ophiolite (northern Tibet)

Abstract

Metamorphic rocks found at the base of the Jurassic Donqiao ophiolite of northern Tibet are interpreted as a basal dynamothermal aureole produced during obduction of the massif. The rocks form a sequence some 8 m thick, varying from high-grade amphibolites at the contact with overlying harzburgites to greenschist facies metasedimentary rocks lower down. The mineral paragenesis is similar to other such aureoles, and indicates that temperatures in excess of 750°C may have been reached during metamorphism. The lack of high-pressure minerals suggests that the rocks were produced by subcretion in a relatively shallow dipping subduction zone. Ar-Ar geochronology on amphibole separates provides dates of 175-180 Ma for the displacement of the ophiolite, significantly older than the age of emplacement estimated from stratigraphic relationships. The ophiolite was clearly obducted very soon after its formation in a suprasubduction zone environment.