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《Acta Geologica Sinica》 1997-04
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CHRONOLOGICAL FRAMEWORK OF THE EARLY PRECAMBRIAN IMPORTANT EVENTS OF THE NORTHWESTERN HEBEI GRANULITE TERRAIN

Geng Yuansheng,Liu dunyi and Song biao(Institute of Geology, Chinese Academy of Geological Sciences, Beijing)  
Based on the chronological data and other geological information (contact relationships of geological bodies, geochemical features and so on), the chronological framework of the main geological events in the northwestern Hebei granulite terrain has been established in this paper.The Sm-Nd isochron age and model-Nd age (tpM) of 2868-2932 Ma of basic granulite and amphibolite are interpreted as the formation age of the earliest volcanics. It means that the primitive crust in the terrain had already been formed 3. 0 Ga ago. The grey gneiss which contains a number of basic granulite in the Huai'an area has a zircon U-Pb SHRIMP (sensitive high-resolution ion-microprobe) age of 2761 Ma. Although a zircon concordia age of 2346 Ma and zircon SHRIMP ages of 2536 Ma and 2539 Ma have been obtained in the grey gneiss, these zircons are mainly of metamorphic origin. So the event of emplacement of TTG magma took place at about 2761 Ma. Subsequently, basic magma intruded into supracrustal rocks at 2650 Ma. The gran-odioritic gneiss widely exposed in the Xuanhua-Chongli area has zircon concordia ages of 2561 Ma and 2503 Ma. The ages can be explained as the time of emplacement of granodioritic magma. Emplacement of TTG and granodioritic magma resulted in crustal thickening in the study region. The intrusion of charnockite and regional granulite-facies metamorphism took place almost simultaneously in 2477-2461 Ma, when all rocks formed in the Archaean were transformed into granulite s and gneisses. Then, basic granulite and a part of gneiss underwent second stage of granulite-facies metamorphism and deformation at about 2. 3 Ga, when fine-grained hypersthene occurred at rims of coarse-grained pyroxene, and symplectites composed of fine-grained hypersthene, plagioclase and honblende surrounded garnet porphyroblasts. Pink granite was intruded in a WSW-ENE direction into the upper crust from Tianzhen to Xigeyu in 2144-2087 Ma. Finally, ductile shear deformation caused superimposition of different metamorphic terranes in 2. 0-1. 8 Ga.
【Fund】: 国家自然科学基金(编号49070125)
【CateGory Index】: P534.41
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