四川木里混杂带海山玄武岩辉石斑晶中的熔体包裹体:甘孜-理塘古特提斯洋内热点与洋中脊相互作用的记录

Clinopyroxene-hosted melt inclusions in seamount basalts in the Muli mélange in Sichuan, SW China: Tracks of hotspot-ridge interaction in the Ganzi-Litang Paloetethys Ocean

川西木里混杂带位于扬子板块西缘,向西与甘孜-理塘弧前混杂带相接,位于一个大地构造上十分重要的部位。我们详细野外地质调查揭示,木里混杂带由不同类型的大洋板片地层组成,其中海山岩石组合保存相对完整,枕状熔岩与上覆碳酸盐岩帽接触关系以及海山斜坡滑塌堆积组合完整清晰。海山枕状玄武岩具斑状结构,主要斑晶矿物相为单斜辉石,少量基性斜长石,基质为微晶斜长石和辉石。原生熔体包裹体主要寄存于单斜辉石斑晶中,形状不规则,大小20~50μm。熔体包裹体内部组成和结构简单清晰,主要为不透明玻璃质,有的熔体包裹体含圆形-椭圆形收缩气泡,个别熔体包裹体壁可见子矿物结晶析出。对单斜辉石斑晶及其中熔体包裹体的地球化学分析结果揭示,木里混杂带中保留了OIB和E-MORB两类海山玄武岩,其原始岩浆源区为石榴子石二辉橄榄岩低程度熔融。其中,OIB型海山玄武岩(样品HS5)是地幔柱轴部岩浆活动的产物,而E-MORB型海山玄武岩(样品HS2)是某种程度的热点(地幔柱)与洋中脊相互作用的产物。磷灰石U-Pb测年结果和古生物化石证据表明,木里混杂带中的海山形成于石炭纪末-二叠纪初(302±11Ma)。这表明甘孜-理塘(松潘-甘孜)古特提斯分支洋在石炭纪末或更早时期就已经发育。在该分支洋盆内发育地幔柱以及地幔柱(热点)与洋中脊的相互作用。

The Muli mélange in Southwest Sichuan is located on the western margin of the Yangtze plate and plays an important role in understanding the relationship between the Yangtze plate and Ganzi-Litang Paleotethys Ocean.Our careful field investigation shows that the Muli mélange is composed of different types of oceanic plate strata,in which two seamount associations(basalt-limestone type of mélange)are relatively intact and composed of the basalt base and overlying carbonate cap with olistostromes occurred on the seamount slopes.The seamount basalts display the typical porphyritic texture that consists of clinopyroxene phenocrysts and subordinate plagioclase phenocrysts in a matrix of acicular plagioclase.The primary melt inclusions(MIs),20~50μm in size,are hosted in clinopyroxene phenocrysts.The internal component and texture of the MIs are simple and clear,and composed of opaque glass with circular or oval contraction bubbles.A few of MIs have crystallized the daughter minerals along the rims.Using LA-ICP-MS and electron microprobe techniques,the major and trace elements of the MIs and chemical compositions of their hosted clinopyroxene phenocrysts were analyzed.The results reveal that the seamount basalts in the Muli mélange gave the geochemical features similar to OIB and E-MORB.The OIB-like geochemistry indicates that the seamount basalts in the Muli mélange were derived from magma source of low degree melting of garnet lherzolite within the oceanic plate.While the E-MORB-like feature indicates some degree of hotspot-ridge interaction,suggesting the mantle plume occurred in the Ganzi-Litang Paleotethys Ocean.Apatite U-Pb analyses indicate that Ganzi-Litang Paleotethys Ocean has been developed in the duration of the end of the Carboniferous to Early Permian(302±11Ma).