甘肃省南祁连化石沟地区发现中二叠世埃达克质岩:大地构造及成矿意义

南祁连地区化石沟花岗闪长岩位于化石沟铜矿附近,东距阿克塞县城约120 km。花岗闪长岩LA-ICP-MS锆石U-Pb年龄为261.1±3.8 Ma,形成于中二叠世晚期。岩石属钙碱性系列,具有埃达克岩的地球化学特征,SiO_2含量在68.35%~69.14%之间,高铝(Al_2O_3=15.83%~16.06%)、Sr(367×10~(–6)~381×10~(–6))、低Y(15.12×10~(–6)~19×10~(–6))和Yb(1.34×10~(–6)~1.85×10~(–6)),富Na贫K(Na_2O=4.3%~4.47%,K_2O=2.22%~2.46%,Na_2O/K2O=1.75~2.01),MgO介于0.79%~0.89%之间,Mg~#为0.35左右。岩石富集强不相容元素Ba、Rb、Sr、Th、U和LREE,强烈亏损高场强元素Nb、Ta、Ti以及HREE,(La/Yb)_N=8.3~11.77,具轻微的Eu负异常(δEu=0.76~0.86),高Rb/Sr(≈0.2)值。化石沟花岗闪长岩的(~(87)Sr/~(86)Sr)i、ε_(Nd)(t)、(~(176)Hf/~(177)Hf)_i、δ~(18)O_(V-SMOW)分别为0.7065~0.7068、–1.45~–0.78、0.282765、12.1‰~12.5‰。综合以上特征,认为化石沟花岗闪长岩源于新生地壳物质在玄武岩浆底侵作用下发生的部分熔融,在岩浆作用过程中可能发生过轻微的分离结晶作用,形成于板内背景,处于造山挤压向后造山伸展的构造体制转变阶段。花岗闪长岩的Nd和Hf模式年龄(分别为1.1 Ga和0.9 Ga)指示玄武质岩浆上侵的时间应为中元古代晚期。化石沟埃达克质岩显示了良好的斑岩型Cu、Au成矿潜力,寻找与埃达克岩有关的斑岩型Cu、Au矿应是下一步找矿的重点方向。

Discovery of Middle Permian Adakitic Rocks in South Qilian Area,Gansu and Implications for Tectonics and Cu (Au) Mineralization

The Huashigou granodiorites,located about 120 km south of Akesai county,tectonically in the South Qilian area,were formed in the Middle Permian epoch with U-Pb age of 261.1±3.8 Ma (LA-ICP-MS zircon).The rocks have geochemical features similar to those of typical adakites.The granodiorites are calc-alkalic with SiO2 contents of 68.35％-69.14％,high A1 (15.83％-16.06％),Sr (367×10 6-381×10-6) contents,and Sr/Y (19.7-25) ratio,coupled with low Y (15.12×10-6-19×10-6),Yb (1.34×10 6-1.851×10 6),as well as MgO (0.79％-0.89％) and Mg# (≈0.35).Furthermore,they are obviously enriched in strongly incompatible elements (Ba,Rb,St,Th and U) and LREE with (La/Yb)N=8.3-11.77,Rb/Sr ratios of 0.2,but depleted in HFSEs (Nb,Ta and Ti),and HREE with Y/Yb ratio of 10.3-11.3,slightly negative Eu anomalies (δEu=0.76-0.86).However,the high Rb/Sr ratios and low Ni,Cr and Mg# of the rocks are different from those of the typical adakites originated from partial melting of subducted slab,indicative of partial melting of lower crust without percolation of mantle melt.Sr-Nd-Hf-O isotope compositions showed that the Huashigou adakitic rocks were formed in intraplate environment during the tectonic transition from orogenic crumpling to post-orogenic extension,and stemmed from partial melting of medium-basic lower crust heated by the underplating basaltic magma.Nd and Hf model ages (1.1 Ga and 0.9 Ga,respectively) suggest that the lower crust source rocks were likely emplaced in late Mesoproterozoic.The potential Cu-Au mineralization associated with the adakitic rocks in south Qilian area should be taken seriously in the future geological reconnaissance.