板片俯冲过程中Co、Ni的分布及迁移特征——以西南天山为例

在板片俯冲过程中, 基性洋壳、下伏蛇纹石化岩石圈地幔和覆盖在俯冲洋壳上的大洋沉积物在不同深度会依次发生板片脱水作用和沉积物熔融, 形成的俯冲带熔/流体可携带某些元素交代地幔楔, 进而通过弧岩浆作用返回地壳。同时, 俯冲板片内不同组分之间也会发生化学成分的迁移和交换, 并最终进入深部地幔, 造成地幔不均一性和不同程度壳幔相互作用。本文报道了我国西南天山典型高压-超高压变质带中蛇纹岩、辉石岩、绿片岩、蓝片岩和榴辉岩内各矿物相的主量元素及Cu、Co、Ni含量数据。结果表明, 除辉石岩中透辉石的Cu含量较高(1.26×10^-6~76.9×10^-6)外, 其它硅酸盐矿物的Cu含量均在1.0×10^-6~10.0×10^-6左右; 而Co和Ni在不同岩性及不同矿物之间显示较大的含量差异: 蛇纹岩中蛇纹石的Co和Ni含量分别为22.6×10^-6~49.6×10^-6和482×10^-6~1097×10^-6, 榴辉岩中绿辉石的Co和Ni含量分别为6.0×10^-6~66.2×10^-6和21.6×10^-6~506×10^-6, 辉石岩中透辉石的Co和Ni含量分别为20.8×10^-6~289×10^-6和69.5×10^-6~351×10^-6, 蓝片岩中蓝闪石的Co和Ni含量分别为9.0×10^-6~94.3×10^-6和75.5×10^-6~495×10^-6。绿片岩、蓝片岩和榴辉岩中均含石榴子石, Co、Ni含量变化范围分别为1.2×10^-6~134×10^-6和7.7×10^-6~26.9×10^-6。相较于绿片岩, 蓝片岩和榴辉岩中石榴子石的Co、Ni含量较高。矿物成分剖面分析表明, 蓝片岩和榴辉岩的石榴子石中由核至边Co含量明显增加, 而Ni含量无太大变化。这些矿物中Cu、Co和Ni的分布特征揭示, 随着板片持续俯冲深度的增加, 变质作用程度从绿片岩至榴辉岩, 硅酸盐矿物中Cu含量几乎不发生变化, 而Co和Ni的含量显示升高的趋势, 这可能与蛇纹岩和辉石岩的参与程度有关, 也显示Co具有较强的熔流体活动性和可迁移性。三种元素的迁移特征和变化规律表明俯冲板片对产于俯冲带的富Cu矿床中金属Cu的贡献可能主要来自于其中的硫化物而非硅酸盐矿物, 残余板片中硅酸盐矿物的熔融可能对俯冲结束后形成的岩浆Cu-Ni硫化物矿床提供Ni和Co元素的贡献。结合西南天山高压-超高压变质带所揭示的俯冲熔/流体的还原性特征, 上述推论可为解释中亚造山带南缘古生代斑岩铜矿, 尤其是还原性斑岩铜矿的发现, 并对岩浆铜镍硫化物矿床的时空分布和成矿特征提供重要制约。

Distribution and transfer of Co and Ni during slab subduction: A case study of Southwest Tianshan

During the slab subduction, the basic oceanic crust, underlying serpentinized lithospheric mantle and sediments covering the subducted oceanic crust will undergo slab dehydration and sediment melting at different depths in turn. The melts/fluids released from subducting slabs could carry some elements to metasomatize the mantle wedge, which might return to the crust during arc magmatism. Meanwhile, migration and exchange of chemical compositions between different components occurred, and eventually travelled into deep mantle, resulting in mantle heterogeneity and deep crust-mantle interaction. This paper reports new data of major elements and contents of Cu, Co and Ni of minerals in serpentinite, pyroxenite, greenschist, blueschist and eclogite in the typical HP-UHP metamorphic belt from the Southwest Tianshan, China. The results show that except for high Cu contents (1.26×10-6~76.9×10-6) of diopside in pyroxenite, other silicate minerals have low Cu contents of 1.0×10-6~10.0×10-6; while Co and Ni show significant variations among different rocks and different minerals. The contents of Co and Ni are 22.6×10-6~49.6×10-6 and 482×10-6~1097×10-6 in serpentine, 6.0×10-6~66.2×10-6 and 21.6×10-6~506×10-6 in omphacite, 20.8×10-6~289×10-6 and 69.5×10-6~351×10-6 in diopside, 9.0×10-6~94.3×10-6 and 75.5×10-6~495×10-6 in glaucophane, respectively. Garnet grains are present in greenschist, blueschist and eclogite, respectively. Their Co and Ni contents are in ranges of 1.2×10-6~134×10-6 and 7.7×10-6~26.9×10-6, respectively. In comparison, garnet in both blueschist and eclogite has higher Co and Ni contents than in greenschist. Profile analyses demonstrate that the Co content in garnet of blueschist and eclogite increases significantly from core to rim, while Ni content is relatively invariable. The distribution characteristics of Cu, Co and Ni in those minerals reveal that the metamorphic grade changes from greenschist to eclogite with the increases of the subduction depth of the slab, Cu in silicate minerals rarely changes, while their Co and Ni contents display an increasing trend, which may be related to the involvement of serpentinite and pyroxenite, and more mobile behavior of Co than Ni during melt-fluid activities. The distribution and transfer characteristics of the three elements indicate that the Cu contribution of the subduction plate to Cu-rich deposits formed in subduction zones may mainly come from sulfides rather than silicate minerals, and the melting of the silicate minerals in the residual slab may contribute Ni and Co to magmatic Cu-Ni sulfide deposits formed at post-subduction stages. Combined with the reductive characteristics of subduction melts/fluids revealed from HP-UHP metamorphic belt in the Southwest Tianshan, the above inferences provide important constraints for explaining the discovery of Paleozoic porphyry copper deposits in the southern margin of the Central Asian Orogenic Belt, especially the discovery of reduced porphyry copper deposits, as well as the temporal and spatial distribution and metallogenic characteristics of magmatic Cu-Ni sulfide deposits.