皖南屯溪盆地火山岩的地球化学、年代学及其构造意义

Geochemistry and geochronology of the volcanic rocks from Tunxi basin in southern Anhui and their tectonic significance

安徽南部侏罗至白垩纪屯溪盆地位于NE向中下扬子火山岩带和NNE向中国东南部火山岩带的交接部位,其火山岩的年代学和成因研究可以提供华南晚中生代构造演化的信息。4个火山岩样品的LA-ICP-MS锆石U-Pb年龄及其岩石地球化学数据指示屯溪盆地发育两期火山岩：早期火山岩形成于156～152 Ma,为中、高钾钙碱性系列,稀土元素分布型式为轻稀土元素富集的右倾型,铕负异常不明显,富集轻稀土元素和大离子亲石元素,亏损高场强元素Nb、Ti、P;晚期火山岩形成于136～130 Ma,具有双峰式火山岩特征,其酸性组分属中钾钙碱性系列,稀土元素分布型式为右倾斜的＂V＂型,铕负异常明显,同样具有富集轻稀土元素、大离子亲石元素及亏损高场强元素Nb、Ti、P的特征。皖南屯溪盆地晚侏罗世-早白垩世火山活动类似于中国东南部火山岩带。结合前人资料,认为屯溪盆地晚侏罗世火山岩形成于挤压构造环境,而早白垩世火山岩形成于伸展环境,其成因与晚侏罗至早白垩世古太平洋板块俯冲角度变化和俯冲板片后撤作用有关。

The Jurassic-Cretaceous Tunxi basin in southern Anhui is located at the junction between the NE-trending Middle-Lower Yangtze volcanic belt and the NNE-trending Southeast China volcanic belt. Geochronological and petrogenetic study of the volcanic rocks can provide information regarding the Late Mesozoic tectonic evolution of South China. LA-ICP-MS zircon U-Pb ages and geochemical data of four volcanic rock samples indicate that two volcanic stages are developed in Tunxi basin. The earlier volcanic rocks were formed at 156 ～ 152 Ma. These rocks belong to the calc-alkaline series with middle-to-high K content,and are characterized by the REE right-inclined patterns with weak negative Eu anomalies,LILE and LREE enrichment,and HFSE（such as Nb,Ti,P） depletion. In contrast,the late volcanic rocks were formed at 136 ～ 130 Ma and are characterized by bimodal volcanic rocks. Acidic components of these rocks belong to the calc-alkaline series with intermediate K content. These rocks exhibit the right-inclined ＂V＂ type REE patterns with strong negative Eu anomalies,LILE and LREE enrichment,and HFSE（ such as Nb,Ti,P） depletion. Late Jurassic- Early Cretaceous volcanism features of Tunxi basin resemble those of Southeast China volcanic belt. Combined with previous studies,Late Jurassic volcanic rocks in Tunxi basin were generated in a compressional tectonic environment,whereas Early Cretaceous ones were generated in an extensional tectonic environment. Both of them were attributed to the subductional angular variation and the rollback of the Paleo-Pacific plate from Late Jurassic to Early Cretaceous.