P-T PATHS OF THE LULIANG SHAN UHP LOCALITY, WESTERN CHINA

Ultrahigh-pressure (UHP) metamorphic rocks record the subduction of continental crust to mantle depths (~100 km) and their subsequent exhumation. A key feature of these rocks is their possible very rapid return to the crust. The exhumation mechanism of deeply buried rocks remains uncertain and therefore the rate of this process provides a crucial constraint for any proposed exhumation model. Exhumation rates for UHP terranes can be inferred from P-T paths. The North Qaidam UHP terrane is located in North Western China, north of the Qaidam Basin. Detailed petrologic studies have been conducted in Luliang Shan locality of the North Qaidam terrane in order to constrain more precisely the P-T paths of high-pressure rocks during exhumation. The Luliang Shan is comprised of a homogeneous leucocratic epidote-amphibolite grade orthogneiss containing discrete blocks of partially retrograded mafic eclogite. The eclogites contain the peak assemblage garnet, omphacite, rutile, quartz (coesite) and phengite. The retrograde assemblage is taramitic and pargasitic amphibole, diopside/augite, albite, titanite, ilmenite and epidote. P-T paths has been constructed for several eclogites in the Luliang Shan in conjunction with a detailed study of mineral chemistry and zoning in the participating phases. Preliminary results indicate an open clockwise P-T path where maximum temperature was reached at much shallower levels along the exhumation path; it suggests heating during decompression from UHP depths. The post-UHP heating appears to increase significantly to the SW, suggesting non-uniform exhumation through the middle crust.