云开地块北缘构造带中变质基性火山岩的地球化学特征及其大地构造意义

Geochemical characteristics and tectonic signification of metabasic volcanic rocks in tectonic belt of northern margin of Yunkai block,southeastern Guangxi

对云开地块北缘构造带中变质基性火山岩的地质、地球化学综合研究结果证明,该区存在洋中脊型(MORB)和岛弧型(IAT)两类玄武岩。MORB型具有大体类似的地球化学性质,它们均含较低的TiO2(0.80%～1.62%,平均值1.50%)和P2O5(0.05%～0.15%,平均值0.13%),轻稀土元素呈亏损—平坦型,具洋中脊玄武岩特征;而IAT型含更低的TiO2(0.47%～1.26%,平均值0.80%)和P2O5(0.04%～0.07%,平均值0.06%),轻稀土元素呈富集型,具岛弧型玄武岩特征。MORB和IAT组合的出现说明位于华南板块西南端的广西云开地区在Grenville期(四堡期)可能存在过洋盆,它们很可能代表了Grenville期(四堡期)造山作用形成统一的Rodinia超级大陆以前的一个“多岛洋盆”系统,华夏古陆和扬子板块可能在该古洋盆于新元古代早期发生的全球性Grenville期(四堡期)造山运动而发生消减消失后才开始碰撞和造山,形成统一的Rodinia超级大陆。

Based on geology and geochemical researches of the metabasic volcanic rocks, two types of basahs are recognized in the tectonic belt of the northern margin of the Yunkai block, southeastern Guangxi, namely, midoceanic ridge basalt （MORB） and island arc basalt （IAT）. The mid-oceanic ridge basahs have similar geochemical characteristics： low contents of Ti （TiO2 = 0.80% - 1.62%, with an average of 1.50% ） and P （P2O5 = 0.05% - 0. 15%, with an average of 0. 13% ） and depleted or fiat LREE patterns, which are similar to those of MORB. The island arc basahs are characterized by low Ti （TiO2 = 0.47% - 1.26%, with an average of 0.80% ）, P （P2O5 = 0.04% - 0.07%, with an average of 0.06% ） and enrichment in LREE, which are similar to those of IAT. The age of MORB-type metabasic volcanic rocks is （1462±28） Ma using the SHRIMP zircon U-Pb method. The presence of MORB and IAT assemblages suggests the existence of an ocean basin in Yunkai area, Guangxi, southwestern South China Block during Grenvillian （Sibaoian）. They probably defined as a part of archipelagic ocean system before South China has formed unified Rodinia during the Grenvillian （Sibaoian） orogeny. Cathaysia block and Yangtze block have experienced continent-continent collision subducted, and have formed unified Rodinia. behind this ocean basinsubducted, and have formed unified Rodinia.