GRANULITES IN THE TONGBAI AREA, QINLING BELT, CHINA - GEOCHEMISTRY, PETROLOGY, SINGLE ZIRCON GEOCHRONOLOGY, AND IMPLICATIONS FOR THE TECTONIC EVOLUTION OF EASTERN ASIA

The Tongbai area of the eastern Qinling belt in China includes granulite-grade metamorphic assemblages (Qinling Complex) which were previously regarded as Archean to early Proterozoic in age and belonging to the southern margin of the North China plate (craton). Our petrological and geochemical data characterize these rocks as two-pyroxene granulites and garnet granulites which formed at temperatures of 757-degrees-840-degrees-C and pressures of about 9.5 kbar and are now found as xenoliths in granodioritic gneisses. The protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro. The Pb-207/Pb-206 ratios derived from evaporation of single zircons yield ages of 470+/-20 and 470+/-14 Ma, respectively, for the basic granulites which we interpret to reflect the time of protolith emplacement. These are intruded by a 435+/-14 Ma granodioritic gneiss post-dating granulite formation. A metaquartzite sample contains detrital zircons as old as 2555+/-8 Ma. Two samples of granitoid gneiss from the Tongbai Complex S of the Qinling granulites have single-zircon Pb-207/Pb-206 evaporation ages of 776+/-8 and 746+/-10 Ma, respectively, and document late Proterozoic igneous activity. We suggest that the Qinling granulites document an important and hitherto unknown phase of early Silurian crustal thickening following subduction and continental collision and that both the Qinling and Tongbai Complexes were part of the southern margin of the North China craton prior to this event and record late Proterozoic igneous activity.