Abstract:

Highpressure(HP)granulites as enclaves and irregular veinlike blocks are widely distributed in TTG gneisses of the Early Precambrian metamorphic basement of Shandong Peninsula, in the southeast of the North China Craton. Zircon grains separated from HP granulites are subdivided into three main types based on cathodoluminescence(CL), Laser Raman spectroscopy and insitu UPb dating； they are inherited(magmatic)zircon, HP metamorphic zircon and retrogressive zircon. The obviously or weakly zoned, dark luminescent inherited(magmatic)zircon with mineral inclusions of apatite(Ap) records two group 207Pb/206Pb ages of 2890-2915 Ma and 2510-2763 Ma, indicating two magmatic events occurring at Mesoarchean and Neoarchean epoches. In contrast, the homogeneous, HP metamorphic zircon with index HP mineral inclusions of garnet(Grt)+clinopyroxene(Cpx)+plagioclase(Pl)+quartz(Qtz)+rutile(Rt)+apatite(Ap) records younger 207Pb/206Pb ages ranging from 1850 Ma to 1900 Ma, which represent peak HP granulitefacies metamorphic times. The retrogressive zircon with mineral inclusions of orthopyroxene(Opx)+plagioclase(Pl)+quartz(Qtz)+apatite(Ap)±amphibole(Amp), however, records the youngest 207Pb/206Pb ages of 1820-1840 Ma, which represents late medium and lowgranulitesfacies retrograde metamorphic times. This result indicates that Mesoarchean and Neoarchean igneous protoliths of mafic HP granulites, together with their TTG country rocks and metasedimentary rocks are subducted to coeval Paleoproterozoic HP metamorphism(1850-1900 Ma), followed by late medium and lowgranulitesfacies retrogression(1820-1840 Ma). This new result is of very important significance for deeply discussing the genetic mechanism and geodynamic model of the HP granulites distributed in the southeast part of the North China Craton.

Key words:  highpressure granulites, zircon, UPb dating, metamorphic ages, Shandong Peninsula