大别造山带天柱山中酸性岩起源及地质意义

Genesis of the Tianzhushan Intermediate-Felsic Rocks from the Dabie Orogen and Its Geological Significance

天柱山岩体是大别造山带具代表性的中生代中酸性岩体,为两阶段岩浆侵入形成。早阶段角闪石英二长岩具有较大的SiO2含量变化范围(59.83%~70.12%),全碱含量为7.02%~9.56%,晚阶段黑云母二长花岗岩SiO2含量为72.46%~78.65%,具较高的全碱含量(8.05%~10.22%),两者可归为钙碱性-碱钙性花岗岩类;它们明显富集Rb,Th,K等大离子亲石元素和Pb,U元素,显著亏损Nb,Ta,Ti等高场强元素;与早阶段相比,晚阶段具显著的Eu负异常,表明其经历了斜长石的分离结晶过程。Sr,Nd同位素特征显示,天柱山中酸性岩可能来源于化学组成与北大别变质杂岩类似的古老的下地壳物质。LA-ICP-MS定年结果显示天柱山岩体的侵位年龄为(127±1)Ma^(131±1)Ma,与大别山大规模分布的中酸性岩及基性-超基性岩的侵位年龄(120~135Ma)一致。天柱山花岗岩是在陆内伸展的构造环境中形成的,早白垩世大别造山带大规模岩浆热事件的产生与岩石圈拆沉和软流圈上涌有关,古太平洋板块俯冲远程效应加剧了大别造山带在早白垩世的拉伸,从而导致岩石圈拆沉。

The Tianzhushan intrusive complex is one of the Late Yanshanian intermediate-felsic complexes in the Dabie orogenic belt, which was emplaced in two stages. The amphibole quartz monzonite of the earlier stage has larger SiO 2 content variation ranging from 59.83% to 70.12%, with K 2 O＋Na 2 O from 7.02% to 9.56%. The biotite monzogranite of the later stage has SiO 2 content variation ranging from 72.46% to 78.65%, with K 2 O＋Na 2 O from 8.05% to 10.22%. They both belong to the calc-alkaline to alkali-calcic series. The intermediate-felsic rocks are enriched in Rb, Th, K, Pb and U, and depleted in Nb, Ta and Ti. The biotite monzogranite also has remarkable Eu depletion which can be ascribed to separation of plagioclase. Sr and Nd isotope geochemical characteristics suggest that the intermediate-felsic rocks were derived from the old lower crust which is similar in chemical composition to the metamorphic assemblage in North Dabie. New LA-ICP-MS zircon U-Pb dating suggests that the crystallization age of the Tianzhushan complex is （127±1） Ma^（131±1） Ma, in agreement with the ages （120~135 Ma） of the large-scale intrusive intermediate-felsic and mafic-ultramafic rocks in the region. The Tianzhushan granitoids were formed under extensional tectonic setting, providing an evidence that the Early Cretaceous large scale magmatism was generated by lithosphere delamination and asthenosphere upwelling. Subduction of the paleo-Pacific plate intensified Cretaceous extention of the Dabieorogen which triggered delamination.