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

Earliest response of the Neoproterozoic Rodinia break-up in the northeastern Yangtze craton:Constraints from zircon U-Pb geochronology and Nd isotopes of the gneissic alkaline granites in Donghai area

东海片麻状碱性花岗岩是扬子克拉通北东缘最具代表性的含碱性铁镁矿物的过碱性 A 型花岗岩。高精度的LA-ICP-MS 锆石 U-Pb 定年结果表明,这些岩石形成于新元古代中期770Ma 左右,并在约250Ma 时经历超高压变质作用,之后又于210Ma 左右遭受了大规模的角闪岩相退变质作用的改造。Nd 同位素示踪表明东海片麻状碱性花岗岩具有相对偏高的ε_(Nd)(t)值(+1.4～-5.91),指示其应是成熟度较低的下地壳部分熔融的产物,但不排除部分岩体在成岩过程中可能有少量幔源物质卷入。综合 Rodinia 超大陆古地理复原资料,推测扬子板块北东缘自新元古早期以来一直为接受大洋板块俯冲的活动大陆边缘。形成于新元古代中期的东海片麻状碱性花岗岩属 A_2型花岗岩,为弧后拉张背景下岩浆活动的产物,该类岩石的产出代表了 Rodinia 超大陆裂解事件在该地区的最初响应。

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 εnd （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.