A New Case of Late Mesozoic Crust-mantle Interaction and Magma Mingling in Eastern Zhejiang-U-Pb Geochronology and Geochemistry of the Xiaojiang Granitic Complex in Xinchang

摘要

Some granitic complexe intrusions have been found recently in southeast coast of China. They have many important clues for crust-mantle interaction and magma mingling. A new case of granitic complex has been found in Xinchang Xiaojiang area, eastern Zhejiang. It is mainly composed of host-granites and meladiorite enclaves. The crystallization age of the host-granite determined by zircon LA-ICP-MS U-Pb method is 121.7±1.9Ma, the minimum age is 116.9±1.7Ma, and the meladiorite enclave is 116±1.6Ma,indicating that solidified time of the host-granite is earlier than meladiorite enclave, they all belong to lower Cretaceous,Late Mesozoic. Geochemically,they share same calc alkaline series. The host granites are characterized by high silica、 rich in total alkalis and relatively high potassic, and show metaluminous to weakly peraluminous attribute. Both of host-granites and their dioritic enclaves are enriched in LREE and LILE (e. g. K, Rb and Th), having similar REE patterns and trace element spide-diagrams..But the host granites display strong negative europium anomalies (δEu=0.23~0.30) and uncommonly depleted in Sr、 Ba、 P and Ti, implying that the host rocks are highly fractionated I-type granites. The elemental characteristics of Xiaojiang granitic complex are similar to that of the late Mesozoic composite igneous rocks in eastern Zhejiang and SE-Fujian. It's implying that the mafic magma was derived from partial melting of sub-continental lithospheric mantle which could have been induced by subduction process. And its underplating and interaction with deep crust produced the granite magma, and mixed together. Therefore, the finding and researching for the Xiaojiang granitic complex in Xinchang area provide a new case of crust-mantle interaction and magma mingling in the southeastern coast of China.