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    Metamorphism, anatexis, zircon ages and tectonic evolution of the Gongshan block in the northern Indochina continent-an eastern extension of the Lhasa Block [查看] ShuguangSong、YaolingNiu、ChunjingWei、JianqingJi、LiSu
    The Gongshan block near the Eastern Himalayan Syntaxis is a fault-bounded block at the northern tip of the triangle-shaped Indochina continent (NIC). Exposed in this block are late Paleozoic (Carboniferous to Permian) strata and a north–south belt of intermediate to felsic batholiths (i.e., Gaoligongshan magmatic belt). The contact between the Gaoligongshan batholiths and Carboniferous/Permian strata is characterized by a series of high-grade metamorphic gneisses with leucosome granite veins (i.e., the so-called “Gaoligong Group”). U-Pb SHRIMP and LA-ICP-MS dating of zircons indicate that these gneisses are actually metamorphosed Paleogene sediments containing inherited Archean to Cretaceous detrital zircons (from 2690 to 64 Ma) and have undergone medium- to high-pressure granulite-facies metamorphism at ~22 Ma.Leucosome and S-type granite of 22–53 Ma by anatexis are ubiquitous within high-grade metamorphic rocks in the southern part of the Gongshan block. An Early Paleozoic gneissic granite and granitoid intrusions of Jurassic, Cretaceous and Oligocene-Miocene ages are also recognized in NIC blocks. These ages suggest that the NIC differs distinctly from the Indian continent, the Greater and Lesser Himalaya zones, and the Yangtze Craton, but resembles the Lhasa Block in terms of Paleozoic to Mesozoic magmatism and detrital zircon ages.This offers an entirely new perspective on the tectonic evolution of the Gongshan block in particular and of the history of the Lhasa Block in the context of the India-Asia continental collision in general. Furthermore,the high-grade metamorphism in the NIC indicates a strong crustal thickening (vs. strike-slip shearing) event during much of the Eocene to the Oligocene (~53–22 Ma) that has brought the Paleogene sediments to depths of greater than 25 km. Continuous northward convergence/compression of the Indian Plate at the Eastern Himalayan Syntaxis may have led to the clockwise rotation, southeastward extrusion and extension of the southeastern part of the Indochina continent.
    EPMA U-TH-Pb MONAZITE AND SHRIMP U-Pb ZIRCON GEOCHRONOLOGY OF HIGH-PRESSURE PELITIC GRANULITES IN THE JIAOBEI MASSIF OF THE NORTH CHINA CRATON [查看] XIWENZHOU、GUOCHUNZHAO、CHUNJINGWEI、YUANSHENGGENG、MINSUN
    ABSTRACT. The Jiaobei massif constitutes the southern segment of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton, and contains high-pressure (HP) mafic and pelitic granulites. The HP pelitic granulites contain four distinct mineral assemblages: garnet kyanite biotite muscovite rutile (M1),garnet kyanite ternary-feldspar biotite muscovite rutile (M2), garnet sillimanite biotite perthite anti-perthite (M3), and biotite muscovite sillimanite plagioclase (M4), of which M2 is a typical HP pelitic assemblage. The origin of these HP granulites is important for understanding the tectonic evolution of the Jiao-Liao-Ji Belt, but the timing of the HP event still remains unknown. This study applied the EPMA U-Th-Pb monazite and SHRIMP U-Pb zircon dating techniques to determine the metamorphic ages of the HP pelitic granulites in the Jiaobei massif. CL images and Th/U ratios show that all monazite and zircon grains in the analyzed samples are of metamorphic origin. Eighty-two EPMA analyses on 16 monazite grains from two samples yielded three age groups at 1895 to 1882 Ma, 1814 to 1813 Ma and 1706 to 1692 Ma, and the ages of 16 SHRIMP analyses on 14 zircon grains from one sample range from 1864 to 1803 Ma. The oldest ages of 1895 to 1882 Ma were yielded from high-Y monazite grains enclosed in garnet and sillimanite pseudomorph after kyanite, and thus interpreted as the time of the HP (M2) granulite-facies metamorphism.The EPMA monazite ages of 1814 to 1813 Ma and SHRIMP zircon ages of 1864 to 1803 Ma are interpreted as the approximate ages of the post-HP granulite-facies metamorphism (M3), most probably resulting from the exhumation of the HP granulites to the medium-pressure granulite-facies levels. The youngest age group of 1706 to 1692 Ma obtained from low-Y monazite grains in the matrix can be interpreted as the age of late cooling and retrograde metamorphism (M4) that occurred when the HP pelitic granulites were exhumed to the upper crust. The presence of Paleoproterozoic HP mafic and pelitic granulites in the southern segment of the Jiao-Liao-Ji Belt suggests that the evolution of the Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton must have been involved in subduction- or collision-related tectonic processes.
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