科研之友
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摘要
The diorite and hornblendite associated with TTG gneisses constitute the important components of Precambrian metamorphic terranes in the Taishan area and their petrogenesis can provide significant insights into understanding the crustal growth and reworking events involving in the evolution of the North China Craton. The lithological, geochronological and geochemical analyses were carried out for the study of the representative diorite and hornblendite in the Taishan area.Zircon U-Pb dating of 1 diorite and 2 hornblendite samples yields 2615 ± 8 Ma, 2627 ± 14 M and 2616 ± 10 Ma respectively, which are consistent with the emplacement age of 2621 ± 7 Ma yielded from bulk rock-mineral Rb-Sr isochron dating. They are all magmatic products of the Neoarchean ~2.6 Ga. The similarity of the formation ages and mineral geochemical characteristics of the two intrusives indicates that they are the products of different evolution stages of the homologous magma. The parent magma experienced the influence of separation crystallization in the process of uplift and emplacement in the late stage, while the first hornblende was less affected by separation crystallization.Based on the composition of amphibole and the known amphibole melt partition coefficient, the melt in equilibrium with amphibole in diorite and amphibole is characterized by low Mg# value(mean value is 31.9 and 43.4, respectively), enrichment of large ion lithophile elements such as Ba and Pb, and depletion of high field strength elements such as Nb, Ta, Zr and Ti, and a flat distribution pattern of heavy rare earth elements.In addition, the rocks have a low initial 87Sr/86Sr value close to the εNd(t)value(-0.01~2.87)and εHf(t)value(-0.76~4.89)of the depleted mantle, and the two-stage Nd model age TDM2 ranges from 2.97 Ga to 2.73 Ga.On the basis of these data and in the context of the regional geology, it is inferred that the diorite and hornblendite were probably derived from the partial melting of juvenile mafic lower crust under the amphibolite facies conditions. The initial magma underwent crystallization differentiation of minerals such as plagioclase and hornblende. The west Shandong underwent extensive continental crustal growth at 2.9~2.7 Ga and extensive continental crustal remelting at 2.6~2.5 Ga in the late Neoarchean. The episodic diapirism of mantle derived magma might be one of the main dynamic causes for the growth and differentiation of the continental crust in the Early Neoarchean(2.8~2.6 Ga)of the North China Craton.
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