Metamorphic Evolution and Zircon U-Pb Ages of the Nanshankou Mafic High Pressure Granulites from the Jiaobei Terrane, North China Craton

Abstract

Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high-pressure mafic granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Peninsula in the north-eastern part of the North China Craton (NCC). The mafic HP granulite is composed of garnet, clinopyroxene, orthopyroxene, amphibole and symplectitic clinopyroxene, orthopyroxene, plagioclase, ilmente and magnetite which were formed after the decomposition of porphyroblastic garnet and clinopyroxene. Four stages of metamorphic mineral assemblages for the mafic HP granulites were constrained by detail petrological and mineralogical investigations. The early prograde assemblage is represented by the mineral inclusions within garnet and clinopyroxene porphyroblasts (Opx1+Pl1+Qtz1), recording the metamorphic conditions at ~754–757 °C, 0.63–0.71 GPa; peak metamorphic conditions were determined at ~874–891 °C, 1.32–1.35 GPa with the mineral assemblage of Grt2+Cpx2+Amp2+Pl2+Qtz2. Retrograde minerals derived from symplectitic assemblage Opx3+Cpx3+Amp3+Pl3+Qtz3+Ilm3±Mag3 were formed at 693–796 °C, 0.60–0.84 GPa. A final greenschist to sub-greenschist facies event was recorded by the exsolution of actinolite and albite within a retrograded clinopyroxene, as well as the occurrence of prehnite, chlorite and calcite minerals. Accordingly, a clockwise P-T path was concluded on the basis of the different stages of mineral asseblage. Cathodoluminescence imaging, trace element and U-Pb dating of zircons from the mafic HP granulites recorded similar charactistics for three episodes of Paleo–Meso Proterozoic metamorphic events. These are the metamorphic events preserved in mafic and pelitic granulites in the Jiao-Liao-Ji belt (JLJB) with 207Pb/206Pb ages of 2.0‒1.9 Ga for peak metamorphism and of 1.86‒1.84 Ga for decomposing process, followed by a retrograde amphibolite facies metamorphic event related to the post-orogenic extension at the age of 1.76‒1.74 Ga, resulting the exhumation of the granulite to the upper crust level.