大别山碰撞造山带俯冲盘陆壳基底组成:白垩纪脉岩捕获/继承锆石的证据

The components of the subducted continental basement within the Dabieshan orogenic belt as evidenced by xenocrystic/inherited zircons from Cretaceous dykes

蕴藏在年轻造山带中的古老陆壳基底是揭示陆块早期构造事件和演化的良好载体。碰撞造山带中俯冲盘大陆基底一般位于多层次地壳结构的下部而难以在地表广泛出露,而浅表火成岩中的捕获/继承锆石可以用来示踪大陆地壳深部古老物质的组成和演化。本文报道了在大别山南缘8件中生代脉岩(140~127 Ma)中新发现捕获/继承锆石的U-Pb年龄和Hf同位素组成特征,用以探讨鲜有关注的大别山碰撞造山带俯冲盘陆壳基底的组成和演化等关键问题。捕获/继承锆石U-Pb年龄显示出显著的前寒武纪峰值年龄,记录了丰富的3.44~3.23 Ga、2.99~2.82 Ga、2.79~2.60 Ga、2.47 Ga、2.00 Ga、1.82 Ga、0.89 Ga、0.82~0.78 Ga和0.71~0.59 Ga等多阶段岩浆事件,以及3.28 Ga、2.87 Ga、2.73 Ga、2.51 Ga、1.98 Ga和0.80 Ga等六期变质事件。捕获/继承锆石的Hf同位素显示大别山南缘在太古宙发生过显著的陆壳生长,主要集中在古太古代(3.6~3.3 Ga)和新太古代(2.7~2.6 Ga),并经历了多期次的陆壳再造(3.4~3.2 Ga、3.0~2.8 Ga、2.7~2.5 Ga、2.0~1.8 Ga、0.9~0.6 Ga和0.1 Ga)。综合分析认为大别山南缘应代表了未卷入深俯冲的俯冲盘早前寒武纪陆壳基底,并至少经历了古元古代、新元古代等多期次岩浆构造热事件的叠加改造,其形成分别与Columbia超大陆和Rodinia超大陆的聚合与裂解有关。

Ancient continental basement of young orogenic belts may reveal the early evolutionary history of continental blocks.Continental basement of subduction plate in a collision orogen is generally located in the Earth’s lower crust and unlikely to be widely exposed on the Earth’s surface,however,xenocrystic/inherited zircons from shallow igneous rocks can be used to trace the content and evolution of continental crust at depth.Here we present in situ U-Pb and Hf isotope data for the newly discovered xenocrystic/inherited zircons from Late-Mesozoic dykes(140127 Ma)in the southern Dabieshan orogen to address the critical issue about the nature,age and evolution of geologic basement of the Dabieshan orogenic belt.The xenocrystic/inherited zircons are dated to the Precambrian and show an episodic age distribution.They record a large number of multi-stage magmatic events(at 3.443.23 Ga,2.992.82 Ga,2.792.60 Ga,2.47 Ga,2.00 Ga,1.82 Ga,0.89 Ga,0.820.78 Ga and 0.710.59 Ga)as well as six metamorphic events(at 3.28 Ga,2.87 Ga,2.73 Ga,2.51 Ga,1.98 Ga and 0.80 Ga).The Hf isotopic composition of the xenocrystic/inherited zircons shows that the southern Dabieshan orogen experienced significant continental growth in the Archean,mainly concentrated in the Paleoarchean(3.63.3 Ga)and Neoarchean(2.72.6 Ga),and underwent multi-stage reconstruction(3.43.2 Ga,3.0~2.8 Ga,2.72.5 Ga,2.01.8 Ga,0.90.6 Ga and 0.1 Ga).According to comprehensive analysis,the southern Dabie orogen should represent basement of subduction plate that was not involved in deep subduction and at least modified by multi-stage tectono-thermal events in the Paleoproterozoic and Neoproterozoic relating to the formation of the Columbia and breakup of the Rodinia supercontinents,respectively.