华南早古生代花岗岩中暗色包体的成因：岩石学、地球化学和锆石年代学证据

摘要

The early Paleozoic is an important period for intracontinental orogeny of South China Block. In the eastern Yangtze Block, massive mafic enclaves occurred in the granitic plutons. The Hongxiaqiao pluton (434 Ma), located at the eastern Yangtze Block, is a typical I-type granodioritic pluton. Mafic enclaves from this pluton are mainly ellipsoid and stilliform with fine-grained magmatic texture. In contrast to the host rock, the mafic enclaves have lower SiO2 (52.19%-58.72%), higher MgO (2.98%-9.28%), Fe2O3T (6.17%-8.35%) and CaO (5.08%-6.91%), and homogeneous total alkali contents (K2O+Na2O=3.79%-5.92%). This indicates that the enclaves are typical mafic microgranular enclaves (MME). The relationship of mutually enwrapped texture between the MMEs and host rocks suggests that these rocks formed contemporaneously, which is also supported by their similar zircon U-Pb ages (~430 Ma). In the enclaves, acicular apatites and plagioclases with reverse zoning bestriding the interface between MMEs and host rocks can be observed. These features indicate that the parental high-temperature mafic magma must have injected into a low-temperature acid magma. High Ni (12-171 μg/g) and Cr (69-424 μg/g), high Mg# (>60), and low εHf(t) (−2- −15) of zircons suggest that the MMEs may derived from a metasomatized lithospheric mantle, similar to the source of contemporaneous gabbros and basalts in Cathaysia Block. Therefore, it is possible that metasomatized lithospheric mantle beneath the eastern South China Block (eastern Yangtze Block and whole Cathaysia Block), and the metasomatized lithospheric mantle melted at ~430 Ma. We propose a scenario to interpreter the formation of massive early Paleozoic granites in South China Block. Partial melting of enriched mantle was triggered by the post-orogenic extensional collapse, and then the high-temperature enriched melts underplated into the middle and lower crust and caused extensive crustal partial melting. Thereafter, the early Paleozoic granites were generated in an extension setting with planar-distribution. During the formation of these granites, the mantle has provided not only heat but also materials.%早古生代是华南构造演化和陆内造山过程的关键时期。在扬子板块的东缘，早古生代花岗岩发育大量暗色包体。该地区的宏夏桥岩体(434 Ma)为 I-型花岗闪长岩，其中的暗色包体具有细粒岩浆结构，形态以椭圆形和水滴状为主，与寄主岩的接触界线清晰。与寄主岩相比，暗色包体具有较低的 SiO2(52.19%~58.72%)含量，较高的 MgO(2.98%~9.28%)、Fe2O3T(6.17%~8.35%)和 CaO(5.08%~6.91%)含量，相似的全碱含量(K2O+Na2O=3.79%~5.92%)。岩相学和地球化学特征显示这些暗色包体为典型的镁铁质微粒包体(MME)。寄主岩与暗色包体之间的互相包裹关系说明它们可能是近同时间形成的，暗色包体的锆石 U-Pb 年龄(~430 Ma)与寄主岩年龄(434 Ma)的相接近也支持这一解释。部分斜长石贯穿包体与寄主岩之间的边界，寄主岩中部分斜长石被细粒的黑云母环绕，暗色包体中含有大量的针状磷灰石。电子探针数据显示暗色包体中的斜长石斑晶具有反环带特征。这些现象均呈现出高温的基性岩浆注入到低温的酸性岩浆并发生岩浆混合作用的特点。高的 Ni(12~171μg/g)、Cr(69~424μg/g)含量，高 Mg#(>60)值，以及低的锆石εHf(t)值(–2~–15)表明暗色包体可能来源于富集的岩石圈地幔，与华夏板块同时期辉长岩和玄武岩的源区特征一致。因此，我们认为早古生代华南板块东部地区(扬子板块东缘+华夏板块)存在一个广泛的受古俯冲板块交代过的岩石圈地幔，并于430 Ma 左右发生了部分熔融。通过对宏夏桥岩体中 MME 的成因研究，我们认为华南早古生代大面积花岗岩的形成可能经历了如下过程：早古生代造山后的伸展垮塌导致了被古俯冲板片交代的岩石圈地幔发生部分熔融，这些高温的地幔熔体底侵于中下地壳，使中下地壳发生大规模的部分熔融，在伸展背景下形成了面状分布的华南早古生代花岗岩。在花岗岩形成过程中，地幔物质不仅有热量的供应，而且有物质的直接参与。