赣南清溪黑云母花岗岩的成因及演化特征

Magmatic evolution and petrogenesis of the Qingxi biotite granite in Southern Jiangxi Province

花岗岩是大陆地壳的主要岩石组成并与众多稀有金属成矿有关,因而花岗岩的成因和演化一直是地质学家关注的焦点问题。本研究对华南赣南地区清溪黑云母花岗岩进行了详细的年代学、地球化学、矿物学以及电气石B同位素分析。锆石LA-ICP-MS U-Pb定年结果显示,清溪黑云母花岗岩侵位时代为晚三叠世(226.4±0.5 Ma)。元素地球化学分析结果显示,清溪黑云母花岗岩具有较高的铝饱和指数(A/CNK=1.04~1.73,平均1.21)、较低的电气石B同位素值(δ11B=−13.4‰~−10.4‰)和锆石Hf同位素值(εHf(t)=−9.8~−2.2,平均−6.6),结合岩石中存在不同年龄的继承锆石,认为该岩石是沉积物部分熔融的产物,属于S型花岗岩,其源区可能是变质杂砂岩或变质泥岩。黑云母斑晶和富黑云母析离体的出现指示清溪黑云母花岗岩经历了黑云母的分离结晶。花岗岩和析离体中黑云母均具有较高的Nb/Ta值(6.6~15.3),黑云母的分离结晶可能是导致清溪黑云母花岗岩较低Nb/Ta值(3.8~9.0)的主要因素,也是导致岩石地球化学不均一性的关键。由于黑云母本身具有较高的Li含量和F含量,岩浆演化过程中形成的黑云母堆晶体可能对后续形成Li-F花岗岩具有重要意义。

Granite is one of the main components of the continental crust and is frequently associated with rare metal mineralization,but its petrogenesis and evolution processes are still mysterious.In this study,the geochronology,geochemistry,mineralogy,and tourmaline B isotope studies are presented for the Qingxi biotite granite in Ganzhou,Jiangxi Province,South China.Zircon LA-ICP-MS U-Pb dating indicates the Qingxi biotite granite emplaced in the Late Triassic(226.4±0.5 Ma).The granite belongs to S-type granite because of its high Aluminum Saturation Index(A/CNK=1.04–1.73,average 1.21),low tourmaline B isotopic composition(δ11B:−13.4‰to−10.4‰),low zircon Hf isotopic composition(εHf(t):−9.8 to−2.2,average−6.6),and various inherited zircon ages.The source of the granite could be composited by metagreywacke or metapelite.The Qingxi biotite granite has experienced biotite fractional crystallization,which is indicated by the presence of biotite phenocrysts and biotite-rich schlieren.Both the biotite in the granite and schlieren have high Nb/Ta ratios(6.6–15.3),so the biotite fractional crystallization could account for the low various and Nb/Ta ratio(3.8–9.0)of the Qingxi biotite.Because the biotite has high Li and F contents,their cumulation during magma evolution could be beneficial for generating the Li-F granite.