南天山川乌鲁碱性杂岩体的岩石学和地球化学特征及其岩石成因

位于中国南天山西侧阔克萨彦岭一带的川乌鲁碱性杂岩体,与该区川乌鲁铜金多金属矿床有着直接的成因联系,该杂岩体由早期的辉长岩—闪长岩岩、主期的二长岩—正长岩和晚期的正长花岗斑岩脉组成,各期岩石在矿物组成和化学成分上有明显的变化。从早到晚,SiO2含量增加,变化范围是50.52%～70.64%;全碱含量先增后减,在SiO2含量小于61.69%时,随SiO2含量增加而增加,而当SiO2含量大于61.69%时,与SiO2含量负相关。在AR-SiO2图解上,大多样品落入碱性区间,在A/CNK-A/NK图解上表现出由准铝质向过碱性演化的趋势。微量元素表现为大离子亲石元素相对高场强元素富集,Rb、Ba、Th、Sr等元素的相对富集和Nb、Ta、P、Ti等元素的负异常。稀土元素表现为轻稀土相对富集的特征,其(La/Yb)N为14.13～25.09,具有Eu的正异常或极微弱的Eu负异常。一些元素比值的线性关系暗示了该杂岩体为岩浆混合成因,基性岩浆的源区为富水的岩石圈地幔,而酸性岩浆是中下地壳中性火成岩在含饱和水条件下部分熔融的产物。这些性质指示川乌鲁杂岩体是在后碰撞拉张环境中由岩石圈地幔熔融的基性岩浆的底侵作用导致地壳的熔融以及后期的岩浆混合作用有关。

Petrology and geochemistry of the Chuanwulu alkaline complex in South Tianshan: Constraints on petrogenesis and tectonic setting

The Chuanwulu alkaline complex is located in the Kuokesayanling region in the west sector of the South Tianshan orogenic belt. The complex is spatially and genetically associated with the Cu, Au polymetallic ore deposits. It consists of gabbro-diorite at early stage, monzonite-syenite at main stage and alkaline granite porphyry dyke at late stage. They have distinct mineral and chemical compositions. From early to late stage, SiO₂changes from 50.52% to 70.64%, where total alkali contents increase at SiO₂<61.69% but decreases at SiO₂ >61.69%. Most samples are of alkaline affinities based on the SiO₂-AR diagram, and display a trend from metaluminous to peralikine. The primitive mantle normalized trace element diagrams are characterized by the enrichment of the LILE relative to HFSE, and Rb, Ba, Th and Sr enrichment coupled with negative Nb, Ta, P and Ti anomolies. They exhibit a relative enrichment of LREE with (La/Yb)_N ratios of 14.13~25.09 as well as very slightly positive or negative Eu anomalies. The petrological observations and geochemical variations suggest that the Chuanwulu alkaline complex was produced by magma mixing. Mafic magma was derived from hydrous lithospheric mantle while felsic magma is produced by partial melting of water-saturated intermediate rocks in middle and lower continental crust. The complex was likely formed under an extension setting in response to a post-collision tectonic environment.