江西印支期蔡江岩体中暗色微粒包体的发现及其地质意义

Discovery of Mafic Microgranular Enclaves in the Indosinian Caijiang Pluton,Jiangxi Province,and Its Geological Implications

文章报道了江西蔡江花岗质岩体中发现暗色微粒包体,以及这些包体的地质、岩相学、LA-ICP-MS锆石U-Pb年代学和元素地球化学特征。包体多呈椭圆状,显示淬冷边和反向脉,具有典型的岩浆结构并含有针状磷灰石,有的包体含有长石捕虏晶。包体具有相对较低的SiO_(2)(低至57.05 wt%)和较高的MgO+Fe_(2)O_(3)(高达14.21 wt%)含量。LA-ICP-MS锆石U-Pb定年数据表明,包体形成于晚三叠世(224 Ma),与寄主花岗岩(230~228 Ma)在误差范围内基本一致。上述特征表明,包体是离散的幔源偏基性岩浆团或者是幔源与寄主岩浆混合的产物。原始包体岩浆属于超钾质岩浆,可能是通过岩石圈地幔中交代成因的金云母辉石岩脉发生部分熔融而形成的。暗色微粒包体的发现为幔源岩浆底侵提供了直接证据,从而为蔡江花岗质岩石形成于较高温度提供佐证。该研究对于进一步探讨华南印支期花岗岩形成的热源机制具有意义。

In this work,mafic microgranular enclaves have been found in the Caijiang granitic pluton,Jiangxi Province.This paper reports the geological and petrographic,as well as LA-ICP-MS zircon U-Pb geochronology and element geochemistry characteristics of these enclaves.Most of the enclaves are elliptical,showing quenched margin and back veins.They have typical magmatic texture and acicular apatite.Some enclaves contain feldspar xenocrysts.These enclaves have relatively lower SiO_(2)(as low as 57.05 wt%)and higher MgO+Fe_(2)O_(3)(up to 14.21 wt%).LA-ICP-MS zircon U-Pb dating shows that these enclaves were formed at 224 Ma,and there is a time gap of about 4-6 Ma with the host granite(230-228 Ma).Characteristics above indicate these enclaves are the masses of discrete mantle-derived basic magma or mixing products of mantle-derived magma and host magma.It is likely that the enclave magma was injected when the host granitic magma was in a state of crystal mush.The discovery of MMEs provide direct evidence for the underplating of mantle derived magma,and the formation of Caijiang granitic rocks at a high temperature.This work is of great significance to further study the heat source mechanism of Indosinian granites in South China.