Petrology And Metamorphic Evolution Of The Early Precambrian Metamorphic Basement Of Shandong Peninsula

The rock types of the early Precambrian metamorphic basement of Shandong Peninsula are very complicated; they are mainly composed of TTG gneisses, granitic gneisses, khondalite series, mafic granulites, amphibolites and ultramafic rocks. Among them, part of mafic granulites, Al-rich gneisses and ultramafic rocks recorded much important information of the HP granulite facies metamorphism. This thesis mainly deals with the HP granulite facies rocks from the early Precambrian metamorphic basement of Shandong Peninsula. Based on the detailed field investigations, and in-depth studying on the petrography, genetic mineralogy, metamorphism, geochemistry and isotope geochronology, the following new achievements have been obtained:On the basis of previous research, more HP granulites outcrops (about 30 spots) were newly identified in the early Precambrian metamorphic basement of Shandong Peninsula. The (mafic) HP granulites and the concomitant ultramafic rocks are widely distributed in TTG gneisses and granitic gneisses as enclaves or deformed dikes. The HP granulite facies rocks are discontinuously distributed along Laixi-Laiyang-Qixia tectonic belt, trending from northeast to southwest, which constitute a HP granulite-facies metamorphic belt (about 200 km long). The composition of the major elements of the mafic HP granulites is characterized by the tholeiitic basalts. The composition of the rare earth elements of the mafic HP granulites is characterized by the flat and right-inclined on the chondrite-normalized patterns with no Eu anomalies (Eu/Eu= 0.87-1.07). The large ion lithophile elements (K、 Rb、 Ba、 Th、 Sr) of the mafic HP granulites changed in different levels during the metamorphic process. Four stages of the metamorphic evolution of the HP granulites have been identified by the characteristics of metamorphic reactions and mineral evolutions. The core of garnet, together with mineral inclusions of clinopyroxene　+　plagioclase ± quartz, is regarded as a prograde assemblage (M1), which was formed at 740~770 ℃ and 0.90~1.00 GPa. In contrast, the peak HP granulite-facies metamorphism (M2) is defined by high-Ca cores in garnet, high-Al cores in clinopyroxene and high-Na cores in plagioclase as matrix minerals, yielding P-T conditions of 850~880℃ and 1.45~1.65 GPa. Symplectites or coronas of orthopyroxene+clinopyroxene+plagioclase ± quartz ± amphibole ± magnetite around garnet porphyroblasts indicate garnet decompressional reactions (M3) occurring at P-T conditions of 780~830℃ and 0.65~0.85 GPa. Symplectites of amphibole+plagioclase+quartz ± magnetite surrounding garnet formed at a cooling retrograde stage (M4) with P-T conditions of 590-650 ℃ and 0.62~0.82 GPa. The mineral inclusion assemblages composed of Grt+Cpx+PI+Qtz+Rt+Ap have been found in the metamorphic zircon domains from the widely distributed Amp-Two-Px-granulites and amphibolites as well, suggesting that part of the metabasites in the study areas have undergone the HP granulite facies metamorphism. According to SIMS and LA-ICP-MS U-Pb zircon dating, the protolith ages of the mafic HP granulites mainly are~2900 Ma and 2700~2500 Ma, the protolith age of the pelitic HP granulites is 2100~1950 Ma, and the protolith ages of the TTG gneisses and granitic gneisses are~2900 Ma、~2750 Ma、~2550 Ma and~2100 Ma. These age data indicate that three significant mafic and granitic magmatic events occurred at the late Mesorchean (~2900 Ma), Neoarchean (2700~2500 Ma) and Paleoproterozoic (~2100 Ma) era in the early Precambrian metamorphic basement of Shandong Peninsula. Integrated study involving laser Raman analysis of mineral inclusions, CL imaging and in situ U-Pb dating in zircons show that the HP granulite facies metamorphism of the mafic HP granulites, ultramafic rocks, pelitic HP granulites and TTG gneisses took place within 1900-1850 Ma, Whereas, the (medium-to low-pressure) granulite-amphibolite facies retrogression of the HP granulites occurred at 1840~1820 Ma. The P-T-t paths of the HP granulites from Shandong Peninsula are characterized by the clockwise type of the typical collision orgeny belt. During 1950~1900 Ma, The protoliths of the HP granulites and their country rocks were subducted to the depths of 30~35 km and underwent a HP amphibolite-granulite facies prograde metamorphism. During 1900-1850 Ma, metamorphic rocks which underwent the HP amphibolite-granulite facies metamorphism were continuously subducted to the depths of 50~55 km and underwent a HP granulite facies metamorphism. During 1840-1820 Ma, the HP granulites began exhumation and uplift, when the HP granulites were uplifted to the depths of 20-25 km and underwent the medium to low pressure granulite facies retrogressive metamorphism. Eventually, the retrogressive HP granulites were continuously uplifted to the depths of 10~15 km and overprinted by the late amphibolite facies retrogression. The establishments of the clockwise P-T-t paths and the delicate chronological pedigrees of the HP granulite-facies rocks from Shandong Peninsula are significant to understand the genetic mechanism and the subduction-collision-exhumation geodynamic model of the HP granulites from the early Precambrian metamorphic basement in the southeastern margin of the North China Craton.