Petrogenesis and Tectonic Implications for High Ba-Sr Porphyries from South Qinling Oroganic Belt, China
Abstract:The Qinling orgenic belt resulted from collision between the North China plate and the Yangtze blocks during Triassic. In the South Qinling orogenic belt, there are lots of small porphyry bodies with area <0.5 km2. These porphyry bodies consist mainly of granodiorite porphyries in petrography. They are closely related to Cu, Mo, Au and Fe mineralization. In this presentation, we carry out an integrated study of LA-ICP-MS zircon U-Pb dating, geochemical and Sr-Nd-Hf isotopic compositions for the porphyry bodies. U-Pb zircon dating shows that they have magma crystallization ages of 145~150 Ma.They are high-potassium calc-alkaline, characterized by high Sr (up to 1300 ppm) and Ba (up to 5000 ppm). Rear earth element data for the porphyries display moderately fractionated REE patterns with (La/Yb)N=9~26 and Eu/Eu*=0.8~1.0. Geochemical characteristics of the granodiorite porphyries are good consistent with high Ba-Sr granitoids [1,2]. These granodiorite porphyries have whole-rock initial 87Sr/86Sr ratios ranging from 0.7046 to 0.7075, εNd (t) values ranging from - 4.6 to - 2.5, and zircon εHf(t) values ranging from - 2.2 to +0.8. We suggest that their magma was derived from partial melting of enriched mantle sources. The strong enrichment of Sr and Ba imply that the mantle sources could be metasomatized by fluid or melt released from subducting slab (including sediments) due to previous subduction of the Ma-Lue ocean slab at the south of the South Qinling orogenic belt. Lithospheric delamination at ~150 Ma can account for their magma generation for the porphyries.
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