粤西福湖岭早新元古代片麻状花岗岩的岩石成因及构造意义

Petrogenesis and Tectonic Implications of the Early Neoproterozoic Gneissic Granite in the Fuhuling Area, Western Guangdong Province

华南陆块由扬子地块与华夏地块在新元古代碰撞拼合形成,其形成与构造演化历史特别是扬子地块前寒武纪研究已取得较大进展,但华夏地块在早新元古代时期的构造属性尚存在不同认识。本文选取华夏地块云开地区东南缘的粤西福湖岭片麻状花岗岩,通过岩相学、锆石U-Pb年代学、锆石原位Lu-Hf同位素和全岩元素‒同位素研究,为华夏地块新元古代早期演化提供信息。粤西福湖岭片麻状花岗岩样品经后期变形作用,其边缘发育微弱的片麻状构造,局部可见拉长的黑云母定向排列。年代学结果表明,片麻状花岗岩的结晶年龄为986~982 Ma。地球化学特征显示,片麻状花岗岩样品SiO_(2)含量为67.70%~77.38%,K_(2)O含量为1.59%~4.50%,A/CNK为1.13~1.76,具较高Rb/Sr值和较低Al_(2)O_(3)/TiO_(2)值,为一套钙碱性过铝质S型花岗岩;轻、重稀土元素分异明显,富集LILE,亏损HFSE(Nb、Ta、Ti),具Eu负异常;全岩ε_(Nd)(t)值和锆石ε_(Hf)(t)值分别为−7.4~−5.4和−8.7~−1.5,二阶模式年龄(t_(DM2))分别为2.2~2.1 Ga和2.4~1.9 Ga。结合前人研究,本文认为粤西福湖岭片麻状花岗岩为华夏地块云开地区东南缘早新元古代俯冲背景下古‒中元古代变沉积岩和基性变火成岩部分熔融的产物。

The South China Block is one of the major continental blocks in China.The formation and tectonic evolution history of the South China Block,especially the Early Neoproterozoic tectonic attributes of the Cathaysia Block are still controversial.Therefore,the Fuhuling gneissic granite in the western Guangdong province,the southeastern margin of the Yunkai Domain of the Cathaysia Block,is selected to investigate the Early Neoproterozoic evolution of the Cathaysia Block through detailed fieldwork,petrographic,zircon U-Pb geochronological and Lu-Hf isotopic,and whole-rock geochemical analyses.The gneissic granite samples exhibit weak schistosity texture developed at the edge after late deformation,and elongated biotite is orientated locally.The zircon U-Pb ages of the samples are 986–982 Ma.Their SiO_(2) and K_(2)O contents are 67.70%–77.38%and 1.59%–4.50%,respectively,with A/CNK of 1.13–1.76.They have high Rb/Sr and low Al_(2)O_(3)/TiO_(2) ratios,showing features of calc-alkaline series and peraluminized S-type granites.The gneissic granties are enriched in LILEs,but depleted in HFSEs(Nb,Ta,Ti),exhibiting obvious REE fractionated patterns with significant negative Eu anomalies.Theε_(Nd)(t)andε_(Hf)(t)values of the rock samples range from−7.4 to−5.4 and−8.7 to−1.5,respectively,and corresponding two stage model ages of 2.2–2.1 Ga and 2.4–1.9 Ga,respectively.These characteristics indicate that the gneissic granites were derived from partial melting of the Paleo-Mesoproterozoic metaigneous and metasedimentary rocks in the Cathaysian basement.Compared with the contemporary magmatic activities in the Wuyi-Nanling-Yunkai region,it is further presumed that the gneissic granites were formed by partial melting of the basic metaigneous and metasedimentary rocks triggered by the subduction in the southeastern margin of Yunkai Domain of the Cathaysia Block during the Early Neoproterozoic.