南秦岭东河群碎屑锆石U-Pb年龄及其板块构造意义

U-Pb ages of detrital zircon grains from the Donghe Group in the Southern Qinling Microcontinent: Implications for tectonic evolution

南秦岭微陆块是秦岭造山带的重要构造单元,其早白垩世沉积物是研究物源区及南秦岭微陆块构造演化的理想对象。南秦岭微陆块南缘观音坝盆地早白垩世砂砾岩中的碎屑锆石LA-ICP-MSU-Pb年龄给出了5个年龄峰,范围分别是2600～2300Ma、2050～1800Ma、1200～750Ma、650～400Ma和350～200Ma,对应于Kenor、Columbia、Rodinia、Gondwana和Pangaea等5次超大陆事件。碎屑锆石源区复杂,但主要源自华北克拉通和北秦岭增生带,表明晚古生代南秦岭微陆块是秦岭-华北联合大陆板块的一部分,而非独立的微陆块。最年轻的锆石年龄峰给出了勉略洋向秦岭-华北大陆俯冲的时限,即350～200Ma;扬子与秦岭-华北联合大陆板块的碰撞造山作用始于三叠纪-侏罗纪之交,强烈的挤压造山作用发生在侏罗纪,而非三叠纪或更早。

The Southern Qinling Microcontinent is an important tectonic unit of the Qinling orogenic belt. The Early Cretaceous sedimentary sequence is useful to trace the tectonic evolution of the Southern Qinling Microcontinent and sedimentary provenances. Detrital zircon grains from the Early Cretaceous sediments in Southern Qinling Microcontinent yield five U-Pb age clusters of 2600-2300Ma, 2050-1800Ma, 1200-750Ma, 650-400Ma and 350-200Ma, corresponding five supercontinental events, namely Kenor, Columbia, Rodinia, Gondwana and Pangaea, respectively. The zircon grains have very complicated sources, but mainly from the Northern Qinling Accretionary Belt and North China Craton, implying that the Southern Qinling Microcontinent was a part of the Qinling-North China united continent, never being an isolated continent, during Late Paleozoic. The youngest age peak constrained that the subduction duration of the Mian-Lue ocean plate northwardly beneath the Qinling-North China united continent was 350-200Ma; and that the continental collision between the Yangtze Craton and the Qinling-North China united continent began at the Triassic-Jurassic transition, and culminated in Jurassic, instead of in Triassic or earlier suggested in previous studies.