班公湖-怒江成矿带中段早白垩世含铜岩体地球化学特征:以雄梅地区为例

Geochemical characteristics of Early Cretaceous Cu-rich rocks in middle segment of Bangong-Nujiang metallogenic belt: A case study of Xiongmei area

班公湖-怒江成矿带是青藏高原上第三条世界级铜矿带,已发现的中大型铜矿床多集中于西段,中东段却鲜有发现。为了使找矿工作由点向带展开,文章选取该带中段早白垩世富铜岩浆岩较为发育的雄梅地区,分析了与成矿相关的岩体的元素地球化学特征,讨论该带中段早白垩世富铜岩浆的属性,并得到以下认识:与矽卡岩型铜矿相关的舍索岩体表现为富w(SiO_2),弱过铝质二长花岗岩,明显的Eu负异常;与斑岩型铜矿相关的雄梅岩体则为强过铝质花岗闪长斑岩,无明显的Eu负异常;舍索二长花岗岩以壳源物质熔融为主并伴有地幔物质的参与,雄梅花岗闪长斑岩则为壳幔混合源。2个岩体总体上都表现富集LREEs,LILEs,亏损HFSEs等类似岛弧花岗岩的地球化学特征,同时表现出Ba、Sr元素负异常,同属于I型花岗岩。低的Y含量,Sr/Y比值及相对高的HREE含量,暗示两者的熔融源区压力较低,不同的Eu异常及地壳组分表明2种富铜岩浆的熔融深度略有不同。综合已有资料认为雄梅地区富铜岩浆岩的形成与班公湖-怒江洋壳岩石圈南向俯冲结束后的板片断离过程有关。

The Banggong-Nujiang metallogenic belt is the third world-class copper belt found in Tibet. The medium and large size copper deposits so far found are mainly distributed in the western part, and only a few copper deposits have been reported in other parts of this belt. For extending the prospecting work, the authors chose the Xiongmei area in the middle segment of the belt as the study area where the Shesuo skarn-type and the Xiongmei porphyry-type copper deposits were recently discovered, and carried out geochemical analysis of copper mineralization-related pluton to investigate the characteristics of the early Cretaceous copper-bearing magma. According to the results obtained, Shesuo pluton is enriched in w(SiO2) and is of weakly peraluminous monzogranite;the content of ΣREE is relatively low (46.7×10-6~180.6×10-6), it is relatively enriched in LREE and shows a obvious negative Eu anomaly. The Xiongmei pluton is a strongly peraluminous granodiorite porphyry with a low ΣREE content (51.0×10-6~90.4×10-6) and relatively high LREE with no obvious Eu anomaly. The two plutons are both generally enriched in LILEs and depleted in HFSEs (Nb, Ta, Zr, Hf), shows negative anomalies of Ba, Sr, have features of island arc granite, and thus belong to I-type magma. The Shesuo monzonitic granite shows obvious crustal features while the Xiongmei granodiorite porphyry shows the crust-mantle mixing features. The low Y content and Sr/Y ratio imply that the melting pressures of the magma sources are relatively low. The difference of Eu anomaly and crustal components between Shesuo monzonitic granite and Xiongmei granodiorite porphyry resulted from the difference of melting depth. In combination with information available, the authors hold that the Bangong-Nujiang oceanic lithosphere still subducted southward until 115 Ma. At the end of the subduction, the slab broke off. During this process, the heat materials in the asthenospheric flowed up along the slab window, heated the overlying mantle lithosphere and the ancient lower crust, and resulted in depressurization and melting. At the same time, the addition of the oceanic crust material and mantle material contributed to the extensive distribution of copper-bearing magma.