广州市白云山片麻状花岗岩成因及构造意义

Genesis and tectonic significance of the Baiyunshan gneissic granites in Guangzhou City,South China

华南板块内部广泛分布的早古生代片麻状花岗岩的形成究竟是与大洋板块俯冲有关,还是与陆内挤压碰撞有关,存在激烈争议。本文对广州市白云山风景名胜区内的片麻状花岗岩进行了LA-ICP-MS锆石U-Pb年代学、全岩主量元素和微量元素、Sr-Nd同位素和锆石Hf-O同位素地球化学研究,结果表明:白云山片麻状花岗岩形成于晚奥陶-早志留世(444±6 Ma)。白云山片麻状花岗岩含有原生白云母和黑云母,并且具有高的SiO2,Al2O3和低的MgO含量(SiO2=74.8%~80.4%,Al2O3=9.64%~12.7%,MgO=0.46%~0.61%)。白云山片麻状花岗岩的稀土总量相对较低,变化于44~173μg/g之间,具有轻稀土相对富集的右倾分布模式,同时具有中-强的Eu负异常(Eu*=0.27~0.63)。白云山片麻状花岗岩富集Rb、Th、U和Pb,亏损Ba、Sr、Nb、Ta、Zr、Hf和Ti。白云山片麻状花岗岩具有富集的全岩Nd和锆石Hf同位素组成(εNd(t)=-12.1~-7.88;εHf(t)=-10.8~-2.63)以及高的锆石O同位素组成(δ18O=8.06‰~11.4‰)。岩相学和地球化学特征表明白云山片麻状花岗岩主要起源于地壳变质沉积岩的部分熔融。结合区域地质和白云山片麻状花岗岩的地球化学特征,提出白云山片麻状花岗岩最有可能形成于早古生代陆内挤压造山的构造背景。

Two formation mechanisms, intracontinental orogeny and oceanic lithospheric subduction, have been proposed to explain the Early Paleozoic gneissic granites widely distributed in the South China Block. This study presents LA-ICP-MS zircon U-Pb geochronological, whole-rock major and trace elements, and Sr-Nd and zircon Hf-O isotopic geochemical data of Baiyunshan gneissic granites in Guangzhou, South China. The Baiyunshan gneissic granites formed from the Late Ordovician to Early Silurian(444±6 Ma). They contain primary muscovite and biotite and have high SiO2 and Al2O3 and low MgO contents(SiO2=74.8%–80.4%, Al2O3=9.64%–12.7%, MgO=0.46%–0.61%). They are enriched in Rb, Th, U, and Pb and depleted in Ba, Sr, Nb, Ta, Zr, Hf, and Ti. In addition,they have relatively low rare earth element(REE) abundances(44–173 μg/g) with enriched light REE patterns and pronounced negative Eu anomalies(Eu*=0.27–0.63). They exhibit enriched whole-rock Nd and zircon Hf isotopic compositions(εNd(t)=-12.1 to-7.88;εHf(t)=-10.8 to-2.63) as well as high zircon δ18O values(8.06‰–11.4‰).Petrographic and geochemical characteristics suggest that the Baiyunshan gneissic granites were derived from metasedimentary rock–dominated crustal sources. Owing to the regional geological and geochemical results, we suggest that the Baiyunshan gneissic granites most likely formed in an intracontinental orogenic setting.