Middle Neoproterozoic (similar to 845 Ma) continental arc magmatism along the northwest side of the Jiangshan-Shaoxing suture, South China: Geochronology, geochemistry, petrogenesis and tectonic implications

摘要

This paper presents the first detailed zircon U-Pb chronology, major and trace element, and Nd-Hf isotope geochemistry of two Neoproterozoic plutons (Shanhou and Jiangshan) along the northwest side of the Jiangshan-Shaoxing suture, Zhejiang province, South China. The Shanhou pluton consists of gabbros, dioritic rocks and monzogranites. SHRIMP and LA-ICP-MS zircon U-Pb dating indicates that the gabbros (similar to 846 Ma), dioritic rocks (similar to 845 Ma) and monzogranites (similar to 847 Ma) were emplaced simultaneously. The Jiangshan pluton consists mainly of dioritic rocks that were also emplaced in the middle Neoproterozoic with a SHRIMP zircon U-Pb age of ca. 842 Ma. The Shanhou gabbros are mainly high-K calc-alkaline, and are enriched in LREE and LILE and depleted in HFSE with marked negative Ta-Nb anomalies. They have positive epsilon(Nd) (T) (2.0-3.5) and epsilon(Hf) (T) (in-situ zircon) (3.3). The Shanhou and Jiangshan dioritic rocks as well as the Shanhou granites are also high-K calc-alkaline, enriched in LREE and LILE and depleted in HFSE with marked negative Ta-Nb anomalies. They also show positive epsilon(Nd) (T) (0.1-4.7) and epsilon(Hf) (T) (in-situ zircon) (0.8-5.0). Detailed elemental and isotopic data suggest that both plutons were formed in a continental arc setting with gabbros derived from partial melting of subduction-modified mantle wedge. Such gabbroic magmas underwent fractionation crystallization of clinopyroxene + amphibole, forming the less felsic dioritic rocks. Progressive fractionation crystallization of amphibole + plagioclase from the evolved melts with a little crustal assimilation produced the more felsic dioritic rocks and granites. Our new data suggest that final amalgamation between the Yangtze and Cathaysia blocks postdated 842 Ma. (C) 2015 Elsevier B.V. All rights reserved.