贵州梵净山白云母花岗岩锆石年代、铪同位素及对华南地壳生长的制约

In-situ zircon geochronology and Hf isotope of muscovite-bearing leucogranites from Fanjingshan,Guizhou Province,and constraints on continental growth of the Southern China block

贵州梵净山地区位于"江南造山带"西南段,出露地层为中新元古界梵净山群、青白口系板溪群、南华系及古生界地层。梵净山群已强烈褶皱,被青白口系板溪群角度不整合覆盖。浅色白云母花岗岩侵位于褶皱的梵净山群中;锆石原位LA-ICP-MS分析结果显示,其形成时代为(838.5±1.5)Ma。地球化学分析结果表明,其A/CNK值>1.1,达1.27、1.30,为典型强过铝质花岗岩,具有低—中等CaO/Na2O比值(0.09～0.52),形成于富粘土质砂岩的高温(>875℃)局部熔融,可能是始于840Ma的地幔柱活动为地壳的高温局部熔融提供了额外热源。锆石原位Hf同位素分析结果显示,新元古代之前华南地块具有两次明显的地壳生长过程。

The Fanjingshan Mountain in northeast Guizhou Province is located in the southwestern segment of the Jiangnan orogenic belt,and consists of the early Neoproterozoic,late Neoproterozoic and Paleozoic strata.The early Neoproterozoic strata were strongly folded and then intruded by numerous muscovite-bearing leucogranites.Both the folded strata and granites were unconformably covered by the Nanhua to Paleozoic strata.In-situ zircon U/Pb analyses reveal the emplacement age of 838.5±1.5 Ma for the leucogranite.Geochemical studies demonstrate that the leucogranite is strongly peraluminous with low to medium CaO/Na2O values（0.09-0.52） and high A/CNK value 1.1（1.27-1.30）,suggesting that they likely have resulted from a high temperature（875 ℃） partial melting of clay-rich psammitic rocks.The supposed super mantle plume started at ca.840 Ma may have provided extra heat for the high-temperature partial melting.In-situ zircon Hf isotope suggests two stages of continent growth before the Neoproterozoic for the South China block.