南祁连化石沟花岗岩年代学、地球化学特征及其构造意义

化石沟二长花岗岩呈长条状侵入于泥盆系-石炭系阿木尼克组。岩石LA-ICP-MS锆石U-Pb加权平均年龄为252.0± 2.1 Ma，形成于晚二叠世。岩石高SiO2（69.8%~72.2%）、Al2O3 (14.2%~15.4%)，低的TiO2、MgO 含量（分别为0.28%~ 0.36%、0.60%~0.77%），A/CNK=1.08~1.14，富碱（Na2O+K2O=6.57%~8.00%）；相对富集Th、K、Pb，亏损Nb、Ta、P、 Ti，低Sr（201×10-6~248×10-6），高Y（19.4×10-6~24.0×10-6），富集LREE，（La/Yb）N=10.8~18.4，中等负Eu异常（δEu=0.55~ 0.68）；全岩（87Sr/86Sr）i 为0.7060~0.7061，εNd（t）为1.63~1.84，εHf（t）=8.79。二长花岗岩的全岩Nd和Hf模式年龄分别为 780~794 Ma和694 Ma。综合研究表明二长花岗岩是新元古代中期形成的玄武质下地壳在晚古生代晚期玄武质岩浆底侵加热 作用下发生部分熔融的产物，形成于造山挤压向后造山伸展转变的构造环境。

Geochronology and Geochemistry Characteristics of the Granites from the Huashigou Area,South Qilian and Their Tectonic Significance

The monzonitic granites intruded into Devonian-Carboniferous Amunike formation in a long strip fashion in the Huashigou area. According to the zircon U-Pb weighted mean age of 252.0 ± 2.1Ma determined using LA-ICP-MS, it was suggested that the rock was formed in late Permian. Monzonitic granites contain rich SiO2 (69.8%~72.2%) and Al2O3 (14.2%~15.4%), and low TiO2, MgO (0.28%~0.36%, 0.60%~0.77%, respectively), with A/CNK=1.08~1.14, rich alkali (Na2O+K2O = 6.57%~8.00%), relatively enriched Th, K, Pb, depleted Nb, Ta, P and Ti, low content of Sr (201×10-6~248×10-6), high content of Y (19.4×10-6~24.0×10-6), enriched LREE, (La/ Yb)N=10.8~18.4, moderate anomaly negative Eu (δEu=0.55~0.68); whole-rock (87Sr/86Sr)I=0.7060~0.7061, εNd(t)=1.63~1.84, and εHf(t)= 8.79. The Nd and Hf model ages of whole-rock of monzonitic granites are 780~794 Ma and 694 Ma, respectively. Overall, our study shows that the Huashigou monzonitic granites is the product of partial melting of Mid-Neoproterozoic basaltic lower crust by the underplating heating of late Paleozoic basaltic magma under the tectonic setting of orogenic compression transforming into post orogenic extension.