南大别蕲春构造混杂岩带中变质沉积岩源区和时代限定

Depositional Age and Provenance of Metasedimentary Rocks in the Qichun Tectonic Mélange,Southern Dabie Orogen

大别造山带南缘蕲春混杂岩带构造属性和地质时限存在争议。本文在1/5万区域地质调查和岩石地层学研究的基础上,通过对实测剖面中四个变质沉积岩基质样品进行了碎屑锆石LA-ICP-MS U-Pb测年和原位Hf同位素分析,为混杂带的形成和演化提供约束。结果表明,碎屑锆石年龄谱主要集中在802-911 Ma,1331-1536 Ma,1776-2224 Ma,2302-2789 Ma和2868-3401 Ma等区间,存在四个明显的特征年龄峰值:~811 Ma、~1385 Ma、~2020 Ma和~2479 Ma;碎屑锆石εHf(t)值介于-16.2~+4.5之间,主要为负值,两阶段模式年龄变化范围在1.4-3.8 Ga之间。最年轻的岩浆成因碎屑锆石年龄约束其最晚形成时代应该不早于~800 Ma。综合对比区域研究结果,认为变质沉积岩的物源可能主要来自于秦岭-大别造山带中早前寒武纪基底物质和近源的新元古代岛弧带,南大别地区可能存在新元古代时期的俯冲增生记录。

The tectonic attributes and geological age of the Qichun Mélange belt on the southern margin of the Dabie orogenic belt are controversial.On the basis of 1:50000 regional geological survey and petrological study,the authors selected four matrix samples of metamorphic sedimentary rocks at the measured section to date their detrital zircon LA-ICP-MS U-Pb ages and in-situ Hf isotopic analysis and probe into the formation and evolution of the Qichun Mélange.The results show that the detrital zircon age spectrum is mainly concentrated in 802-911 Ma,1331-1536 Ma,1776-2224 Ma,2302-2789 Ma and 2868-3401 Ma,with including four obvious characteristic age peaks of~811 Ma,~1385 Ma,~2020 Ma and~2479 Ma;Lu-Hf isotopes of detrital zircons were analyzed on 129 corresponding zircon grains,which yielded a wideεHf(t)values of-16.2~+4.5(mostly<0)and their two-stage model age ranges from 1.4 Ga to 3.8 Ga.The youngest magmatic detrital zircon age limits the latest formation time of the meta-sedimentary rocks to be later than~800 Ma.Based on the results of regional comparison,it is infered that the provenance of the meta-sedimentary rocks may mainly be the early Precambrian basement rocks and the nearby Neoproterozoic island arc rocks in the Qinling-Dabie Orogenic Belt,which indicated the possible Neoproterozoic subduction-accretion in the Southern Dabie Orogen.