秦岭早中生代壳幔岩浆混合作用——来自东江口花岗岩体闪长质包体的地球化学证据

Early Mesozoic Crust-and Mantle-Derived Magmatic Mixing in the Qinling Orogeny:Evidence From Geochemistry of Mafic Microgranular Enclaves in the Dongjiangkou Pluton

东江口花岗岩及闪长质包体分别获得了218Ma和224Ma的形成年龄,闪长质包体中存在岩浆不平衡结构并发育与寄主花岗岩相同的钾长石斑晶及淬冷形成的针状磷灰石,揭示了花岗岩形成过程中曾发生二元岩浆混合作用。这种混合作用已造成寄主花岗岩和闪长质包体化学组成的趋同,同时使得它们的Sr-Nd-Pb同位素组成发生强烈均一化。但暗色闪长质包体锆石具有较宽的εHf(t)值(-4.58～3.31),保留了二端元岩浆源区的特征。秦岭早中生代同期闪长质包体锆石εHf(t)>10及寄主花岗岩锆石εHf(t)<-10的差异表明,它们分别来自相对亏损地幔源区和中元古代滞留于地壳的幔源基性物质,而两个源区的岩浆,自224Ma以来发生强烈混合作用,形成大规模的壳幔混合花岗岩体。

The zircon U-Pb ages of 218 Ma and 224 Ma have been obtained from the Dongjiangkou granitoids and their mafic enclaves, respectively. The disequilibrium magmatic textures can be fount in the mafic enclaves, which have some K-feldspars similar to those of host granitoids and needle-like apatite formed under a quenching condition, indicating a two-end member magmatic mixing in the early Mesozoic. The two-end member magmatic mixing resulted in similar geochemical compositions and strong homogenization of Sr-Nd-Pb isotopes between host granitoids and their mafic enclaves. However, Hf isotopic composition of the mafic enclaves shows a wide range of aHf（t） that varies from -4.58 to 3.31, registering the source features of two-end member magma. Thus, the positive em（t） values （ 〉 10） of zircons from the contemporary mafic enclaves provide evidence that the mafic magma was derived from the depleted mantle. While, host granitoids with negative t） values （〈10） of zircons were products of partial melting of the Mid-Proterozoic mantle-derived basic rocks. Since then, the distinct crust- and mantle-derived magmas were mixed around 224 Ma, resulting in a large number of granitic intrusions with abundant mafic enclaves during the early Mesozoic in the Qinling orogeny.