滇西北衙富碱斑岩的特征及成因

提供了与滇西北衙大型金矿床密切相关的富碱斑岩的主量元素及稀土和微量元素的分析数据,对富碱斑岩的成因、起源条件和源区类型进行了讨论。研究表明,北衙地区的富碱斑岩具有较高的S iO2(w(S iO2)>68%)和A l2O3(w(A l2O3)=13.45%~15.22%),具有富集Sr(w(Sr)=365×10-6~930×10-6),亏损HREE(w(Yb)=0.64×10-6~1.17×10-6)和Y(w(Y)=7.17×10-6~12.25×10-6),较高的w(Sr)/w(Y)、w(La)N/w(Yb)N比值,无明显的负Eu异常等特征,显示了与埃达克岩的某些相似性。然而,富碱斑岩低的w(MgO)、w(Cr)和w(N i),又与典型的埃达克岩有所不同。富碱斑岩产出的地质背景及岩石学和地球化学特征表明,富碱斑岩并非起源于俯冲的大洋板片,而是源于加厚地壳的底部,源区为榴辉岩相岩石。富碱斑岩显著富集Rb、Ba、Th、K,可能与俯冲大陆板片的脱水作用和流体的交代作用有关。产生富碱斑岩岩浆的动力学机制,与印度大陆板块和扬子大陆板块对“三江”的相向俯冲、金沙江—红河断裂带的走滑拉分及其诱发的软流圈上涌和流体交代作用有关。

Characteristics and Origin of Alkali-rich Porphyries from Beiya in Western Yunnan

This paper presents petrological and geochemical data of Au-mineralization-related alkali-rich porphyries in the Beiya gold deposit and its adjacent regions,western Yunnan.Geochemically,the alkali-rich porphyries from Beiya have relatively high contents of SiO_(2)(>68%) and Al_(2)O_(3)(13.45%-15.22%),high Sr/Y and(La/Yb)_(N) ratios,enrichment in Sr(365×10~(-6)-930×10~(-6)) but depletion in Yb and Y(Yb:0.64×10~(-6)-1.17×10~(-6);Y: 7.17×10~(-6)-12.25×10~(-6)),and weak Eu negative anomalies.All of those characteristics mentioned above indicate that alkali-rich porphyries from Beiya are somewhat similar to adakites in geochemistry.However,their lower contents of MgO,Cr and Ni are different from those of typical adakites.Petrological and geochemical characteristics of alkali-rich porphyries suggest that magmas might not be generated from subducted oceanic slab,but likely from the base of thickened crust,corresponding to eclogite-facies.High(abundance) of trace elements Rb,Ba,Th and K in alkali-rich porphyries may be attributed to fluid metasomatism.Dynamically,magma generation might be related to a face-to-face intra-continental subduction beneath Sanjiang blocks of the India continent versus the Yangtze continent,strike-slip movements of the Jinshajiang-Red River fault and tectonics-induced asthenospheric upwelling and metasomatism by fluids released from subducting slabs.