在实施武汉市多要素城市地质调查过程中,于长江新城北部的钻孔中发现了隐伏的浅肉红色中细粒黑云二长花岗岩侵位于南华系双台组变质岩中,通过研究其同位素年代学和元素地球化学特征,对长江新城以北地区早白垩世构造背景进行探讨.该岩体具有高SiO2、Sr、Sr/Y比值,低CaO、MgO、Fe2O3、Y、Yb和正Eu异常特征,是典型的埃达克质岩.野外调查与周边钻孔均未见到有同时期大规模基性岩浆岩与其共生,其富集轻稀土元素和大离子亲石元素,亏损高场强元素(Ta、Nb、P、Ti),可能为加厚下地壳物质部分熔融的产物.该岩体锆石U-Pb年龄为(128.3±0.6)Ma,指示其形成于早白垩世.该岩体与鄂北七尖峰岩体具有相似的地球化学性质,表明大别造山带南缘的长江新城以北地区在早白垩世(约 128 Ma)经历了从碰撞挤压构造体制向碰撞后伸展构造体制的转换.
During the implementation of the multi-element urban geological survey of Wuhan City,a shallow flesh red fine-grained biotite monzogranite was found to have intruded into the metamorphic rocks of Shuangtai Formation of Nan-hua System in a borehole in the northern part of Changjiang New Town.By studying its isotopic chronology and elemental balloon chemistry,the early Cretaceous tectonic background in the area north of the Changjiang New Town was explored.The rock mass has high SiO2,Sr and Sr/Y ratios,low CaO,MgO,Fe2O3,Y and Yb,and Eu positive anomalies,making it a typical adakite rock.No large-scale basic magmatic rocks coexisting with them during the same period were found in field investigations and surrounding boreholes,enriched in light rare earth elements and large ion lithophile elements,and depleted in high field strength elements(Nb,Ta,P,Ti),which may have been formed by partial melting of thickened lower crustal materials.The zircon U-Pb age of the rock mass is(128.3±0.6)Ma,indicating that it was formed in the Early Cretaceous.The rock mass has similar geochemical properties to the Qijianfeng rock mass in northern Hubei,indi-cating that the area north of the Changjiang New Town on the southern edge of the Dabie orogenic belt underwent a tran-sition from a collision compression tectonic system to a post collision extension tectonic system during the Early Creta-ceous(approximately 128 Ma).