大别造山带宿松地体中—基性脉岩锆石年龄及其地质意义

Zircon Ages and Geological Significances of Intermediate-mafic Dykes in Susong Terrene of Dabie Orogenic Belt

大别造山带宿松地体大量早白垩世中—基性脉岩侵位于宿松群高压变质岩石中,它们是认识大陆板块深俯冲过程的壳-幔相互作用和折返过程中岩石圈构造响应的重要载体之一。报道了这些中—基性岩脉LA-ICP-MS锆石U-Pb年龄,并讨论了其隐含的地质意义。单颗粒锆石的年龄范围分布较广,主要可分为太古代(>2 500 Ma)、古元古代(1 850~2 500、1 650~1 800 Ma)、新元古代(720~850Ma)、早古生代(420~450Ma)、早中生代(190~230 Ma)和早白垩世(120~140 Ma)。具有岩浆成因的早白垩世锆石U-Pb加权平均年龄约为132 Ma,应代表中—基性岩浆的侵位时代,证明其为大别地区碰撞后岩浆作用的重要组成部分。综合锆石形貌、内部结构和微量元素特征,这些年龄群落与扬子板块记录的典型构造-岩浆事件具有对应关系。早中生代之前的继承性锆石年龄分布与扬子板块的锆石年龄特征有明显的相似性,暗示着宿松地体可能具有扬子板块属性。因此,推测宿松地体中—基性脉岩成因与早白垩世中国东部强烈岩石圈伸展背景下大别造山带增厚的岩石圈拆沉作用密切相关。

Numerous Early Cretaceous intermediate-mafic dykes intrude into the high-pressure metamorphic rocks of Susong Group in Susong terrene of Dabie orogenic belt.They are one of the important objects for understanding the process of mantle-crustal interaction during the deep subduction of the continental plate and the tectonic response in the uplift.Zircon U-Pb ages of these intermediate-mafic dykes obtained by the LA-ICP-MS technique were reported,and the geological implications were discussed.The single zircon gives a wide age range from ~2 600 Ma to^127Ma.Apparent U-Pb ages of more than one hundred zircon grains cluster around Archean（2 500 Ma）,Paleoproterozoic（1 850-2 500,1 650-1 800 Ma）,Neoproterozoic（720-850 Ma）,Early Paleozoic（420-450 Ma）,Early Mesozoic（190-230 Ma）,and Early Cretaceous（120-140 Ma）.Early Cretaceous zircon grains are typically of magmatic origin and yield a weighted mean U-Pbage of about 132 Ma,representing the emplacement time of the dykes.This magmatic event should be related to intense post-collisional magmatism in Dabie orogenic belt.Combined with morphology,internal structures,and trace element contents of zircons,it can be observed that their age patterns are well correlated with the tectonothermal events in Yangtze Block.The inherited zircons,which are older than Early Mesozoic and contained in intermediate-mafic dykes,are comparable with the age distribution of zircons from Yangtze Block,likely suggesting the Yangtze Block affinity of Susong terrene.From the evidence of the inherited zircons,it is speculated that the intermediate-mafic dykes in Susong terrene originate from the delamination of thickened lithosphere beneath Dabie orogenic belt on the background of Early Cretaceous strong lithospheric extension in the eastern of China.