三江地区俯冲增生-碰撞造山过程中含矿斑岩的地球化学特征对比:对成矿物质源区差异的指示

Geochemical comparison of the Mesozoic and Cenozoic ore-bearing porphyries related to subduction accretion and collision orogenesis in the Sanjiang region:Implication for source variety

青藏高原东南部三江地区在中-新生代时期的大洋俯冲增生和陆-陆碰撞造山过程中,为不同时期的成矿过程提供了有利的构造环境,其中最为典型的是与洋壳俯冲相关的晚三叠世中甸弧斑岩成矿区和与陆内汇聚作用相关的新生代玉龙-马厂箐斑岩成矿带。虽然上述含矿斑岩均为具有高Sr/Y比值的埃达克质岩组成特征;但两者在其他地球化学组成上还是存在一定的差异。同玉龙-马厂箐成矿带相比,中甸弧成矿区含矿斑岩有着相对较高的TiO2含量、MgO含量、Mg#值以及高的CaO/Na2O比值、低的Al2O3/TiO2和La/Yb比值。上述地球化学组成的差异,结合研究区晚三叠世和新生代早期的构造演化,指示中甸弧成矿区含矿斑岩的母岩浆很可能为俯冲的大洋板片熔体与其上覆的地幔楔相互作用的结果,同时这些含矿熔体在随后的上升过程中并没有经历广泛的MASH(即熔融作用、同化作用、存储和均一化)过程;而玉龙-马厂箐成矿带含矿斑岩母岩浆很可能是含石榴子石角闪岩相的加厚下地壳与少量的源于富集地幔的钾质熔体相互作用并在地壳深部经历了一定程度分异的结果。

Since the Mesozoic, the Sanjiang region in SE Tibet has transitioned from an oceanic subduction accretion to intra-continental collision orogeny, which has formed numerous porphyry Cu deposits, such as the Late Triassic Zhongdian arc and the Cenozoic Yulong-Machangqing porphyry Cu belts. Although these ore-bearing porphyries have some adakitic affinities (high Sr/Y ratios), there still are some differences between them in other geochemical compositions. Compared with ore-bearing porphyries developed in the Yulong- Machangqing belt, those in the Zhongdian arc have high TiO2, MgO, Mg#, CaO/Na2O, and low Al2O3/TiO2 and La/Yb values. These differences, when combined with tectonic evolution during the Meso-Cenozoic in the study region, indicate that the ore-bearing parent magmas of the Zhongdian arc were probably produced by the interaction between melts derived from subducted oceanic slab and overlying mantle wedges. Furthermore, these ore-bearing melts did not experience extensive melting-assimilation-storage-homogenization processes during ascent. The Yulong-Machangqing ore-bearing porphyries probably resulted from the mixing of melts originating from a thickened lower crust and the enriched mantle (potassic melts), which subsequently experienced some differentiation in the deep crust (≥ 30 km).