西藏甲玛铜多金属矿床暗色包体岩石成因:对岩浆混合和成矿的启示

The origin of the mafic microgranular enclaves from Jiama porphyry Cu polymetallic deposit,Tibet: Implications for magma mixing/mingling and mineralization

岩浆混合作用的研究对揭示壳幔相互作用,探讨成岩成矿过程具有重要意义。甲玛矿区位于冈底斯成矿带东段,为超大型斑岩-矽卡岩型铜多金属矿床,矿区内的中酸性岩浆岩中普遍发育暗色包体,对其中的暗色包体中的闪长质包体开展详细的岩相学、岩石地球化学、Hf同位素地球化学及U-Pb同位素地质年代学等方面研究以期查明岩石成因,为岩浆混合作用和成矿作出启示,完善甲玛成岩成矿模型。岩相学观察表明,闪长质包体及寄主岩浆岩中存在多种反映岩浆混合作用的典型组构,如长石-石英熔蚀结构、石英镶边结构、长石交代筛状结构、长石反环带结构、磷灰石针柱状结构等,锆石LA-ICP-MS UPb同位素定年结果显示,包体形成时代(15. 3±0. 3Ma)与中酸性寄主岩石在误差范围内一致,也符合了岩浆混合作用的存在。闪长质包体化学成分上类似高Mg埃达克岩(MgO=3. 53%～6. 62%,Sr/Y=20～57,(La/Yb)N=51～64),具有低SiO_2(52. 44%～59. 45%),高K_2O(3. 19%～5. 62%),高相容元素(Ni=86×10^(-6)～146×10^(-6); Cr=102×10^(-6)～228×10^(-6))的特征,∑REE高于中酸性寄主岩浆岩,且轻重稀土分异明显((LREE/HREE)N=21～23),富集LILE(Rb=189×10^(-6)～284×10^(-6),Sr=498×10^(-6)～658×10^(-6),Ba=1247×10^(-6)～1378×10^(-6)),相对亏损HFSE(Nb、Ta、Ti),在稀土元素配分图及微量元素蛛网图中闪长质包体介于冈底斯带碰撞后时期的超钾镁铁质岩(来源于富集的岩石圈地幔)与甲玛中酸性寄主岩浆岩(主要来源于加厚新生下地壳)之间,Hf同位素(ε_(Hf)(t)=-0. 9～4. 6)同样也介于超钾镁铁质岩与花岗闪长斑岩(代表中酸性寄主岩浆)之间。这些特征说明闪长质包体是富集的岩石圈地幔部分熔融形成的镁铁质岩浆与加厚新生下地壳部分熔融形成的中酸性岩浆发生混合的产物,同时指示了东冈底斯带中新世时期也存在岩石圈地幔伸展对流减薄事件,以及证实了南拉萨地体广泛分布的高钾埃达克质岩在形成过程中,伴随着与富集岩石圈地幔来源的超钾镁铁质岩浆发生不同程度混合。此外,富集的岩石圈地幔部分熔融形成的镁铁质岩浆的混入,将会为中酸性岩浆系统加入大量的水和金属物质,这也是控制甲玛超大型斑岩-矽卡岩型矿床形成的关键因素。

The research on magma mixing/mingling is instructive to unravel the interaction of crust-mantle, and discuss the geodynamic setting of magma and ore-forming process. The Jiama deposit, located in eastern part of the well-known Gangdese Metallogenic Belt on the Tibetan Plateau, is the largest porphyry-skarn Cu polymetallic system in this region. We studied the dioritic mafic microgranular enclaves (MMEs) in the host felsic porphyries from Jiama, at the aim of ascertaining the origin of rocks, improving the magmatic rock diagenetic model, and providing implication for magmatic mixing and mineralization. Petrographic observation shows that there are many typical textures in the dioritic MMEs and the host porphyries whose origin can be explained in terms of magmatic mixing and mingling, such as resorption of feldspar-quartz, quartz edging texture, feldspar sieve structure, feldspar reverse zoned, and acicular apatite morphology. The results of zircon LA-ICP-MS U-Pb isotopic dating show that the age (15.3±0.3Ma) of MMEs is consistent with those of the felsic host porphyries within the error range, further indicating the existence of magma mixing. The dioritic MMEs are similar in chemical composition to high-Mg diorite. These MMEs are characterized by low content of SiO2 (52.44%~59.45%), high contents of K2O (3.19%~5.62%), MgO (3.53%~6.62%) and compatible trace elements (e.g., Ni:86×10^-6~146×10^-6;Cr:102×10^-6~228×10^-6), as well as by high Sr/Y and La/Yb ratios. As for the characteristics of REE and other trace elements, the MMEs have higher ∑REE values than those in host felsic porphyries with ratios of (LREE/HREE)N=21~23, and they are enriched in LILE (Rb=189×10^-6~284×10^-6, Sr=498×10^-6~658×10^-6, Ba=1247×10^-6~1378×10^-6), while relatively depleted in HFSE (Nb, Ta and Ti). On chondrite-normalized REE and primitive-mantle-normalized multielement plots, data for the dioritic MMEs fall in between domains for the ultrapotassic mafic rocks and the felsic host porphyries, which are generated by partial melting of metasomatized continental lithospheric mantle and subduction-modified juvenile lower crust, respectively. The Hf isotope data (εHf(t)=-0.9~4.6) also fall in between the fields for the ultrapotassic mafic rocks and the granodiorite porphyries (representing the host felsic porphyries). These features demonstrate that the dioritic MMEs were formed by the mixing between ultrapotassic and adakite-like melts, derived from metasomatized Tibetan lithospheric mantle and juvenile lower crust, respectively, and also indicate that the removal of the lower part of the thickened lithosphere also took place in the eastern Gandese belt. This also allows us to propose the varied contribution of ultrapotassic mafic melts derived from metasomatized Tibetan lithospheric mantle in the generation of high-potassium adakitic rocks which are widely distributed in the southern Lhasa terranes. In addition, the incorporation of ultrapotassic mafic rocks will add a large amount of water and metal materials to the felsic magmatic system, and this is the fundamental factor which controlled the formation of the Jiama superlarge porphyry-skarn type deposit.