新元古代Rodinia超大陆裂解事件在扬子北东缘的最初响应: 东海片麻状碱性花岗岩的锆石U-Pb年代学及Nd同位素制约

The Donghai gneissic alkaline granites are the most typical peralkaline A-type granites bearing alkaline ferromagnesian minerals in the northeastern margin of the Yangtze Craton. Highly-precise zircon U-Pb dating by employing LA-ICP-MS technique indicates that these rocks were formed in middle-Neoproterozoic (ca. 770Ma), and then had been experienced both ultrahigh-pressure metamorphism in early-Mesozoic (ca. 250Ma) and subsequently large-scale amphibolite-facies retrograde metamorphism at about 210Ma. Systematic Nd isotopic tracing shows that the Donghai gneissic alkaline granites have higher ε_Nd (t) values (+ 1.4 ∼ -5.91), indicating that they were likely produced by partial melting of immature lower continental crust materials, but minor amounts of mantle materials might be involved in the formation of some of these rocks. By integrating the research results of the paleogeographic reconstruction of the Rodinia supercontinent, it is inferred that since early-Neoproterozoic, the northeastern margin of the Yangtze block might have been under an active continental margin setting which was induced by subduction of oceanic plate. The Donghai gneissic alkaline granites emplaced during middle-Neoproterozoic belong to the A₂ (post-orogenic) subgroup, which were produced in a back-arc extension setting. The formation of these gneissic alkaline granites can represents the earliest response of the Rodinia break-up in this area.