古特提斯演化的岩浆作用记录:来自桂东南印支期火山岩证据

Magmatism Records of Palaeo-Tethyan Evolution:Evidence from Indosinian Volcanic Rocks in Southeastern Guangxi

为了探讨古特提斯构造演化是否在华南大陆西南缘保存有岩浆作用记录,本文对桂东南印支期火山岩开展了年代学和元素地球化学分析.研究结果表明,两个代表性英安斑岩样品分别给出了(245±2)Ma和(247±4)Ma的形成时代,表明桂东南发育早‒中三叠世火山作用.英安斑岩具有高钾、富铝、贫钛、贫镁的特征,表现出准铝质高钾钙碱性岩石的特征.样品总体富集Rb、Th、U,亏损Ti、Ba、P、Sr,Nb、Ta负异常明显.轻、重稀土分馏较明显,Eu具中等负异常.综合分析表明,英安斑岩主要来源于地壳物质的熔融,并经历了一定程度的分离结晶作用;桂东南与海南岛、金沙江、哀牢山一带的印支早期岩浆岩在岩石类型、地球化学特征、沉积序列和古生物化石方面均具有可对比性,暗示这些地区在印支早期可能经历了相似的构造演化事件,主要受控于古特提斯构造域,推测桂东南于早‒中三叠世进入同碰撞阶段.

To determine the availability of the magmatism records of Palaeo-Tethyan evolution in the southeastern margin of the South China continent,detailed LA-ICP-MS zircon U-Pb geochronological and elemental geochemical analyses of the Indosinian volcanic rocks in the southeastern Guangxi have been carried out in this paper.Geochronological results show that two representative dacite porphyry samples yield zircon U-Pb ages of(245±2)Ma and(247±4)Ma,respectively,indicating that Early to Middle Triassic volcanism occurred in the southeastern Guangxi.Elemental geochemical results show that the dacite porphyry is characterized by high K_(2)O and Al_(2)O_(3) contents,and low TiO_(2) and MgO contents,belonging to the metaluminous high-K calc-alkaline series rocks.The samples show enrichment in Rb,Th,and U,and are characterized by negative Ba,Ti,Sr,P,and Nb-Ta anomalies.They also show obvious LREE and HREE fractionation,and have moderate negative Eu anomalies.The above data indicate that the dacite porphyry mainly originated from the melting of crustal materials,and fractional crystallization occurred during the process of magmatic evolution.Combined with regional geological data,it is suggested that the Indosinian magmatic rock assemblages,geochemical characteristics,sedimentary sequences,and paleontological fossils in the southeastern Guangxi are comparable to those in the Hainan Island and Jinshajiang-Ailaoshan regions,suggesting that these regions may have experienced similar tectonic events during the Indosinian Period,which were mainly controlled by the Palaeo-Tethyan tectonic domain during the Indosinian.The formation of the Indosinian dacite porphyry in southeastern Guangxi possibly represents a syn-collisional stage during the Early to Middle Triassic.