MANTLE-DERIVED XENOLITHS AND THE TOKEN OF A MODERN BACK-ARC GEOTHERM BENEATH EASTERN CHINA

Xenoliths of garnet and spinel-garnet lherzolite from Mingxi, Eastern China have been studied in terms of comprehensive methods of mineralog y, petrology fabrics, microstructures and infrared spectrum. The tempe rature and pressure equilibration of the xenolith have been calculated using the TEMPEST program with the two-pyroxene geothermometer of Ber trand and Mercier (1985) combined with the geobarometer of Nickel and Green (1985). The temperature and pressure of xenoliths range from sim ilar to 1300 K at 1.9 GPa (60 km) to similar to 1500 K at 2.6 GPa (80 km). The geotherm inferred from xenolith is excellently consistent wit h the measured surface heat flow (70 mW/m(2)) in Mingxi. A character o f geothermal curve and conductive type suggest that these xenoliths ar e from the lithosphere, not from the asthenosphere. The geotherm and m icrostructures and preferred orientations of minerals in the xenolith are consistent with teconic thinning of the lithosphere in the present -day extensional back-arc environment.