科研之友
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摘要
Granites and diabases in the Mesozoic are two very important rock types in South China. We newly found granitic porphyry and two episodes of diabases within the Mugua Neoproterozoic gneissic granitic pluton in Chun' an area of the Northwestern Zhejiang Province ( ZXB) of SE China. The age of 142. 2 +/- 1. 2Ma was obtained from the granitic porphyry by zircon U-Pb dating, belonging to the Early Cretaceous. Geochemically, the granitic porphyry is characterized by high SiO2, K2O + Na2O contents and 1(20/ Na2O, and being a peraluminous granite series. It was enriched Rb and light rare earth elements as well as high field strength elements ( Ta, Ce, Zr, Hf), and having moderate Nb depletion in the primitive mantle normalized diagram, indicating a source derived from subduction-related materials and/or crustal contamination. The early-stage diabase has similar chemical characters compared to the Mugua granitic porphyry. The late-stage diabase with no Eu anomalies has distinct composition compared to that of the early-stage diabase and granite porphyry with negative Eu anomalies. Granitic porphyry has Y/Nb ratios >1. 2, and depleted in Nb, Ti and Sr, indicating crustal origin with subduction zone signatures. Sr-Nd-Pb isotopic components show that the granitic porphyry has (Sr-87/Sr-86) values of 0.71134 similar to 0.71748 and epsilon(Nd) ( t) values of 3. 3 similar to 6.4, (Pb-206/(204) Pb), values of 18.49 18. 56, (Pb-207/Pb-204) values of 15. 62 similar to 15. 65 and zos Pb-208/Pb-204), values of 38. 57 similar to 38. 68; and the early-stage diabase has (Sr-87/Sr-86), values of 0. 71408 0. 73010 and epsilon(Nd)(t) values of 0.05 similar to +0. 85. Based on the above study combined with the previous study, we suggest that the ZXB granitic porphyry may be derived from the Mesoproterozoic metamorphic rocks by partial melting induced by the heat of the deep mantle-derived hot magma upwelling. The diabases may be derived from mantle source. A geological comparison between ZXB and adjacent areas, we propose that the formation mechanism of the Early Cretaceous granitic porphyry and diabases in the ZXB as follows. A subduction and collision process between the paleo-Pacific plate and Eurasian continent accompanied by inland remobilization of the deep big faults in South China was started since ca. 170Ma. The ZXB granitic porphyry and A-type granites may be formed under an inland extensional setting during the Early Jurassic ( 123 similar to 145Ma). It entered a strong subduction and collision process from the Pacific plate with the Eurasian plate during the age of 117 similar to 121 Ma. After the age of ca. 117Ma, there are large amounts of felsic volcanic rocks and A-type granites and diabase dykes in the southeastern coastal area of China formed under the extensional setting.
- 出版日期2013-10
- 单位浙江大学; 南京地质矿产研究所
