云开池垌志留纪辉长岩体的年代学、地球化学特征及构造意义

Geochronological and Geochemical Constraints of Chidong Silurian Gabbroic Pluton in Yunkai Domain and Its Tectonic Implications

华南陆块早古生代的构造属性是华南地质研究争论的焦点,主要有洋-陆俯冲/碰撞和陆内构造两种观点之争.而华南陆块早古生代岩浆岩的报道以花岗质岩石为主,对构造指示意义更为明确的同期镁铁质岩浆岩则研究较少,且对已发现的少量早古生代镁铁质岩浆岩的岩石成因也颇具争议.为了更好地限定华南早古生代的构造属性,对云开地区新近识别出的池垌辉长岩体开展了详细的野外地质、年代学和地球化学研究.其SIMS锆石U-Pb年代学测试得到其^(206)Pb/^(238)U加权平均年龄为433.9±1.5Ma,代表其结晶年龄.样品具较低的SiO_2含量(47.81%～48.83%),高的MgO含量(13.02%～14.65%)、Cr含量(278×10^(-6)～356×10^(-6))和Ni含量(120×10^(-6)～184×10^(-6)),富集大离子亲石元素、亏损高场强元素.(^(87)Sr/^(86)Sr)i=0.708 5～0.710 7,ε_(Nd)(t)=-6.0～-8.2,显示EMⅡ型富集地幔的Sr-Nd同位素特征.结合区域Lu-Hf同位素、古生物、古水流等地质资料,云开地区志留纪时期发育一个受古老俯冲交代作用影响的地幔楔,该地幔楔源区在华南广西运动期间才受到热扰动而部分熔融.认为池垌辉长岩是在早古生代华南陆内造山后的伸展背景下由古老俯冲交代地幔楔熔融而成的产物.

The geodynamic mechanisms of the Early Paleozoic orogeny in the South China block(SCB)remain highly debated,and two major models involving subduction-collision orogeny and intra-continental orogeny have been proposed.The Early Paleozoic granitic rocks are widely developed in the eastern SCB and the synchronous mafic rocks have recently been reported.However,the petrogenesis of these Early Paleozoic mafic rocks are still controversial,thus hindering our understanding for the Early Paleozoic tectonic setting.In this study,it presents new petrographic,geochronological and geochemical data for the Chidong gabbroic pluton in the Yunkai domain.The representative sample for the gabbric pluton yields a SIMS zircon U-Pb age of433.9±1.5 Ma(MSWD=0.18).They have SiO2 of 47.81%-48.83%,MgO of 13.02%-14.65%,Cr of 278×10-6-356×10-6 and Ni of 120×10-6-184×10-6.They are enriched in large ion lithophile elements with significantly negative Nb-Ta and Ti anomalies.These samples show EMⅡ-like geochemical affinities with high initial 87Sr/86Sr ratios(0.708 5 to 0.710 7)and lowεNd(t)values(-6.0 to-8.2),suggestive of derivation from a paleosubduction-modified wedge beneath the Yunkai Domain.The wedge was undisturbed until Silurian and the Kwangsian igneous rocks in the SCB were associated with the intracontinental orogenism rather than the subduction and subsequent collision.