苏鲁地体折返与郯庐断裂活动:莱阳盆地中生界碎屑锆石年代学的制约

胶莱盆地位于苏鲁造山带的北缘,其莱阳群沉积对反演郯庐断裂和苏鲁造山带中生代的折返过程具有重要的制约作用。本文利用LA-ICP-MS方法对莱阳盆地的莱阳群碎屑岩和青山群火山岩进行锆石U-Pb年龄测定,分析其年龄谱特征,探讨其沉积物源区,进而为苏鲁造山带的折返机制提供依据。(1)莱阳盆地发育莱阳群和青山群为代表的中生代沉积岩,锆石年龄测试得到莱阳群形成时代为125±0.6Ma;青山群形成时代为119±1Ma,表明两者都是早白垩世中-晚期的沉积产物;(2)超高压带北缘莱阳盆地与合肥盆地的碎屑锆石年龄谱对比表明,莱阳盆地的沉积物无论沉积时代还是物源区均明显不同与郯庐断裂西侧的合肥盆地,表明郯庐断裂应该形成于两个盆地形成之前,可能为三叠纪-早侏罗世之间;(3)莱阳盆地内发育大量的华北型碎屑物质,进一步表明在扬子大陆板块俯冲过程中华北板块曾经仰冲到扬子板块之上;(4)莱阳盆地发育少量的新元古代岩浆和印支期变质锆石的年龄,表明早白垩世苏鲁超高压变质岩已经折返到地表;(5)超高压变质岩与中生代岩浆岩同时作为莱阳盆地的物源,结合五莲拆离断层的同期活动和莱阳盆地的同时代沉积,说明苏鲁超高压带中生代的折返具有与变质核杂岩类似的大型伸展构造背景。

The exhumation of the Sulu Terrane and the forming of the Tancheng-Lujiang Fault: Evidence from detrital zircon U-Pb dating of the Mesozoic sediments of the Laiyang Basin, Central China

Laiyang Basin located in the northern margin of the Sulu UHP belt. The sediments of Laiyang Group in this basin are important unit for understanding the forming mechanism of the Tancheng-Lujiang Fault and the exhumation mechanism of the Sulu Orogen. In this paper, we present LAICP-MS U-Pb zircon dating for both Mesozoic Laiyang and Qingshan groups that will enable us to establish the provenance of the Laiyang Basin and its relationship to exhumation of the Sulu Orogen. (1) Laiyang Basin consists of both Laiyang and Qingshan groups. The formation age for the Laiyang Group is about 125±0.6Ma, whereas the Qingshan Group is 119±1Ma. (2) The age populations are very different between the Laiyang and Heifei basins, thus the Tancheng-Lujiang Faul was formed earlier than two basins, probably between the Late Triassic to Early Jurassic. (3) Laiyang Group consists of much detrital material with NCB affinity, which is further evidence for overthrusting of the North China Craton onto the South China Craton during continental subduction. (4) Laiyang Basin also consists of both Neoproterozoic and Triassic ages, suggesting that the Sulu UHP rocks should be exhumation to the surface during the Early Cretaceous, and (5) Mesozoic magmatite and extensional tectonics is the exhumation mechanism for the Sulu UHP rocks. The Sulu Orogen is the metamorphic core complexes (MCC) in that time.