西南三江普朗铜矿岩浆混合作用:矿物学和地球化学证据

Mineral and geochemical evidences of magma-mixing from Pulang porphyry copper deposit in SW Sanjiang,China

岩浆混合作用是近年来岩石学研究中的热点与难点,其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。普朗铜矿位于西南三江义敦岛弧南段,是我国近年来发现的超大型斑岩铜矿床。矿体产于复式岩体中,复式岩体内发育大量暗色微粒包体。包体粒度较细,多呈椭圆形,其内可见具暗色矿物镶边的眼球状石英、针状磷灰石以及具有结构与成分不平衡现象的斜长石斑晶;与寄主岩相比,包体富集黑云母、角闪石等暗色矿物。主量元素组成上,包体相对于寄主岩Si O2（53.67%~61.50%）含量较低,Mg O（3.12%~5.40%）和Fe2OT3（3.38%~9.00%）含量较高。微量元素结果显示,包体与寄主岩具有相似的微量元素组成与稀土元素配分模式,均表现为富集大离子亲石元素（K、Rb、Ba、Sr）,亏损高场强元素（Nb、Hf、P、Ti）,无明显的δCe异常,轻稀土元素富集,重稀土元素亏损;但与寄主岩相比,包体具有较高的稀土元素总量（119.0×10-6~308.9×10-6）与较明显的Eu负异常（δEu=0.56~0.95）。以上结果表明,包体为岩浆混合作用的产物。文章进一步采用多重分形理论解析了包体中Mg、Fe、Ca、Al、K、Ti、P及Ba等元素的分布特征,定量刻画了岩浆混合作用的程度。结果表明,元素分布的多重分形谱越宽,关联维越小,则岩浆混合程度越高。

Magma mixing is the topic issue of petrology,although it is still under discussion,and it is instructive to unravel the interaction of crust-mantle,and discuss the geodynamic setting of magma and ore-forming process. Pulang ore deposit,located in the south end of Yidun continental arc of SW Sanjiang,is one of the largest porphyry copper deposits in China. The orebodies were developed in Pulang porphyries,in which many mafic microgranular enclaves（ MMEs） developed. The MMEs are fine-grained in texture and generally ellipse in shape,and characterized by resorbed quartz,needle-like apatites,and plagioclase phenocrysts with compositional and textural disequilibrium. Compared to the host rock,the MMEs are rich in biotite and amphibole. Geochemically,the MMEs have lower Si O2（ 53. 67% ~ 61. 50%） content and higher Mg O（ 3. 12% ~ 5. 40%） and Fe2OT3（ 3. 38% ~ 9. 00%） contents than the host rock. Furthermore,the MMEs have similar trace element signatures and REE pattern with the host rock,characterized by enrichment in LILEs（ e. g.,K,Rb,Ba,Sr） and LREEs,depletion in HFSE（ e. g.,Nb,Hf,P,Ti） and HREEs,and non-evident Ce depletions. However,the MMEs are different from the host rock in having relatively high ∑REE（ 119. 0 × 10- 6~ 308. 9 × 10- 6）and obvious Eu depletions（ δEu = 0. 56 ~ 0. 95）. These results reveal that the MMEs are derived from magma mixing. Moreover,we applied the multifractal model to describe the distribution of elements Mg,Fe,Ca,Al,K,Ti,P and Ba in MMEs,with the aim to quantify the intensity of magma mixing. It shows that the wider multifractal spectrum and lower correlation dimension suggesting a higher degree of magma mixing.