金湖凹陷基性岩捕获锆石年龄及其区域构造意义

Trapped zircon U-Pb geochronology of mafic rocks from Jinhu Sag and its geotectonic significance

郯庐断裂带的形成演化是中国东部中生代关键大地构造事件,该断裂带起源机制问题是了解华南与华北板块汇聚过程与方式的基本前提。本文利用郯庐断裂带东侧金湖凹陷内7个新生代基性岩样品394个点的LA-ICP-MS锆石U-Pb年代学数据,在分析岩浆活动时代基础上,利用捕获锆石年龄信息探讨了大别造山带的古生代演化及郯庐断裂带起源机制问题,为认识上述问题提供了一个全新的视角。研究表明,金湖凹陷的基性岩浆活动时代为古近纪早期(66.1~56.5 Ma),与围岩的沉积时代吻合;基性岩中的捕获锆石来自大别造山带,是印支期板块碰撞造山中楔入郯庐断裂带东侧华南板块的造山带物质,具有华南板块、北淮阳和华北板块的混杂岩石圈信息;捕获锆石中445 Ma(变质锆石)、290 Ma(岩浆锆石)及214 Ma(变质锆石)等峰值年龄信息指示古生代以来北淮阳构造带经历了与北秦岭构造带类似的弧—陆俯冲、碰撞历程,暗示秦岭造山带的二郎坪洋、商丹洋和勉略洋可延伸至大别造山带东端;年代学数据及构造分析支持郯庐断裂带起源于嵌入碰撞背景下的华南板片撕裂边界,其起源形式为陆—陆碰撞下的板块斜向汇聚边界。

Origination and evolution of the Tan-Lu Fault Zone(TLFZ)is a key geotectonic event in eastern China during Mesozoic,and the origin mechanism of this fault zone is the basic premise for understanding the process and type of convergence between the South China Block(SCB)and North China Block(NCB).In this paper,LA-ICP-MS zircon U-Pb geochronology data from 394 grains of 7 Cenozoic basic rock samples in Jinhu Sag on the east of the TLFZ are used to infer the magmatic age,the Paleozoic evolution of the Dabie Orogenic Belt(DOB)and the origination of the TLFZ,which provides a new perspective for understanding the above issues.The results show that the mafic magmatic age is the Early Paleogene(66.1~56.5 Ma),which is consistent with the depositional age of country rocks.The trapped zircons in the mafic magma are from the North Huaiyang Belt(NHB),which is the orogenic material wedged into the SCB on the east of the TLFZ during the collision of the NCB and SCB,having the hybrid crustal information of the SCB,NHB and NCB.The peak ages of 445 Ma(metamorphic zircons),290 Ma(magmatic zircons)and 214 Ma(metamorphic zircons)of the age spectrum for the captured zircons indicate that the NHB has experienced arc-continental subduction and collision processes similar to the North Qinling belt during Paleozoic,suggesting that the Erlangping Ocean,Shangdan Ocean and Mianlue Ocean once extended to the eastern DOB.The geochronological data and structural analysis support that the TLFZ originated from the indentation-induced tearing boundary of the SCB during collision between the SCB and NCB,and its origination is an oblique convergence boundary under the continental-continental collision.