从源到汇:大别造山带物源区与合肥盆地南缘中生代沉积耦合关系——来自碎屑锆石U-Pb年龄证据

From source to sink: Coupling relation between the Dabie Shan Orogen provenance and Mesozoic sediments of the south-margin of Hefei Basin,based on the evidence of detrital zircon ages

形成于印支期的大别造山带和周缘中生代盆地构成了一级源汇系统,其中位于造山带北缘的合肥盆地中生代地层发育,且以盆地南缘出露最好,这为盆山源汇系统研究提供了理想的沉积记录。笔者从合肥盆地南缘采集了10个砂岩样品和1个砾岩样品,进行锆石U/Pb (LA-ICP-MS)定年分析,获得了742个有效年龄(置信度不小于85%),范围为113±3. 6-2983 Ma。这些碎屑锆石年龄谱可以被分为5个年龄段:113-137 Ma,峰值131 Ma; 184-273 Ma,峰值226 Ma; 274-517. 3 Ma,具有2个峰值280 Ma和474 Ma; 532-856. 6 Ma,具有3个峰值572 Ma、649 Ma和772 Ma; 1786-2600 Ma,具有2个峰值2035 Ma和2506 Ma。同时,总结了物源区大别造山带不同单元锆石U-Pb年龄特征。根据锆石U/Pb年龄和Th/U值,发现这5个年龄段比较准确地记录了物源区地质体,分别是早白垩世的岩浆岩、大别山高压—超高压变质岩、北淮阳的浅变质岩、北大别的正片麻岩和卢镇关群变质岩。根据锆石最小年龄,修正了合肥盆地南缘中生代地层格架,为源汇系统研究确立了时间框架。合肥盆地南缘中生代沉积可以分为4个演化阶段:晚三叠世瑞替期—早侏罗世辛涅缪斯期、中—晚侏罗世、早白垩世早期和早白垩世晚期,并据此确定了每个阶段主要物源区特征及其时空变化。碎屑锆石U/Pb年龄和Th/U值限定了大别造山带仅存在三叠纪的超高压变质作用,且超高压变质岩折返到地表的最早时间是晚三叠世瑞替期,大别造山带大陆岛弧发育的时间是新元古代。上述研究结果不仅为恢复大别造山带构造古地理做出了新的贡献,而且更为盆山源汇系统研究提供了一个实例。

The Dabie Orogen formed in Triassic and its peripheral Mesozoic basins form a source to sink system.The Hefei Basin with well-developed Mesozoic in the north and well-exposed outcrops in the south provides ideal sedimentary records for the investigation of the basin-ridge source to sink system.Ten sandstone and one conglomerate samples were collected for zircon U-Pb(LA-ICP-MS)dating analysis which result in 742 effective ages(confidence level larger or equal to 85%)ranging from 113 Ma to 2983 Ma.The detrital zircon ages can be divided into five age groups,including 113-137 Ma and with a peak of 131 Ma,184-273 Ma and with a peak of 226 Ma,274-517.3 Ma and with two peaks of 280 Ma and 474 Ma,532-856.6 Ma and with three peaks of 572 Ma,649 Ma and 772 Ma,1786-2600 Ma and with two peaks of 2035 Ma and 2506 Ma.Moreover,we summarized the zircon U-Pb age data of various geological terrains from the Dabie Orogen and comparised the data with the aforementioned five age groups.Based on the zircon U/Pb ages and their Th/U ratios,the five age groups correctly correspond to the provenance geological terrains that are respectively Early Cretaceous magma rocks,Dabie Shan HP-UHP metamorphic rocks,north Huaiyang lower-grade metamorphic rocks,north Dabie orthogneiss and Luzhengguan group metamorphic rocks.According to the youngest zircon U-Pb ages,the Mesozoic stratigraphic framework of the south margin of the Hefei Basin is newly amended,which supports a time frame for source to sink system research.Mesozoic sedimentation in the south margin of the Hefei Basin can be divided into four evolutive stages(i.e.,Later Triassic Rhaetian-Early Jurassic Sinemurian,Middle-Later Jurassic,early and Later stages of Early Cretaceous),and major characters and space-time changes of source rocks in each stage also are determined.Detrital zircon U/Pb ages and Th/U ratios suggest that the UHP process happened in the Dabie Orogen to Triassic,not Caledonian or Neoproterozoic.It was in Late Triassic Rhaetian that UHP rocks exhumated and outcropped in surface,and it was Neoproterozoic that the paleo-inland arc of the Dabie Orogen developed.Results from the present study contribute to rebuild tectonic palaeogeography of the Dabie Orogen and provide a case for study of basin-ridge source to sink system.