福建太姥山地区和鼓山地区A型花岗岩对比及其地球动力学意义

Comparison of A-type Granites from Taimushan and Gushan Regions in Fujian Province and Their Geodynamic Significance

太姥山和鼓山位于福建东南沿海地带。岩石学和岩相学研究表明太姥山地区和鼓山地区花岗岩分属铝质A型花岗岩和碱性A型花岗岩。锆石LA-ICP-MS U-Pb同位素定年结果表明,两地区花岗岩成岩年龄分别为(96.6±1.6)Ma(MSWD=0.65)和(99.4±2.3)Ma(MSWD=0.49),成岩时代均属晚白垩世早期。结合中国东部沿海两类A型花岗岩,讨论了它们在岩石学、岩相学、地球化学及其判别图解上的区别。总体上认为,碱性A型花岗岩在AKI值、TFeO/MgO比值、104×Ga/Al以及(Zr+Nb+Ce+Y)值上均大于同区域的铝质A型花岗岩,但上述化学指标的数值也存在一定范围的重叠;且传统A型花岗岩判别图解不完全适用于强分异的铝质A型花岗岩。地球化学特征表明,两地区A型花岗岩应具有相似的源区,即岩浆起源于地壳物质熔融,同时成岩过程有一定的地幔物质参与。但鼓山地区碱性A型花岗岩较太姥山地区铝质A型花岗岩有更多地幔物质的加入,导致前者显示出部分幔源岩浆起源的特征。结合地球化学、野外地质、区域背景及年代学综合判定,两地区花岗岩成因与古太平洋板块对欧亚板块的俯冲角度密切相关,均属于弧后拉伸体制之下的构造环境。

Taimushan and Gushan are located in the southeast coastal areas of Fujian.Petrology and petrography studies have shown that Taimushan and Gushan granites belong to aluminous A-type granitoids and alkaline A-type granitoids respectively.Zircon LA-ICP-MS U-Pb dating results indicate that the granites in two regions,whose diagenetic ages were（96.6±1.6） Ma（MSWD=0.65） and（99.4±2.3） Ma（MSWD=0.49） respectively,formed in the earlier of Late Cretaceous.Based on granites of the two regions,we focused on the difference of two types of A-type granites in the eastern coast of China in terms of the petrology,petrography,geochemistry and discrimination diagrams.On the whole,the AKI values,TFeO/MgO ratio,104×Ga/Al and（Zr＋Nb＋Ce＋Y） values of the alkaline A-type granites are higher than those of aluminous A-type granites of the same region,but also there is a range of overlap in above chemical indicators,and the traditional A-type granite discrimination diagrams are not thoroughly applicable to the strong differentiation aluminous A-type granite.Geochemistry shows that the granites in two regions should have a similar source,namely magma originated from the partial melting of crustal material and the process of diagenesis have some mantle material involved in.However,alkaline A-type granites in Gushan region may have more mantle material involved in,showing some characteristics of mantle-derived magmatic origin.Integrated with geochemistry,field geology,regional background and geochronology reveal that petrogenesis of the granites of two regions is closely related to the angle of the subduction of Paleo-Pacific Plate to the Eurasian Plate,and their tectonic environment belongs to a back-arc tensional tectonic environment.