Geochemistry, metamorphic evolution and tectonic significance of metabasites from Caçapava do Sul, southern Brazil

Abstract This paper proposes the generation and closure of a proto-ocean and formation of an island arc in the Neoproterozoic Passo Feio Metamorphic Complex (PFMC), Caçapava do Sul region, southern Brazil. The PFMC, São Gabriel Terrane, consists of a Neoproterozoic supracrustal metavolcano-sedimentary sequence intruded by the Caçapava Granite. The study uses petrography, whole-rock geochemistry and mineral chemistry to identify the geochemical affinity and tectonic environment of the amphibolites contained in PFMC, the basalts and pillow lavas of Arroio Mudador Formation and dikes intruded in nearby volcanogenic rocks of Hilário Formation. The applied methods allow the metamorphic facies classification of prehnite-pumpellyite for the basalts and pillow lavas, greenschist for the dikes and lower amphibolite for the amphibolites. The discriminant diagrams indicate from tholeiitic to alkaline affinities for the Hilário Formation dikes. The basalts, pillow lavas and amphibolites have basaltic to basaltic-andesite composition and predominant tholeiitic affinity, interpreted as derived of the same protolith. The geotectonic diagrams mostly indicate volcanic arc basalts. The proposed evolution model starts with a proto-ocean opening through the Bossoroca Arc rifting around 760Ma, where an island arc is formed (PFMC rocks), precipitation of carbonates and marls, subsequent closure, two metamorphism events and the Caçapava Granite intrusion around 562 Ma.

摘要 本文提出巴西南部卡察帕瓦杜苏尔（Caçapava do Sul）地区新元古代帕什费奥变质杂岩（Neoproterozoic Passo Feio Metamorphic Complex，PFMC）中原大洋的形成与闭合及岛弧的发育过程。圣加布里埃尔地体（São Gabriel Terrane）的帕什费奥变质杂岩由一套被卡察帕瓦花岗岩（Caçapava Granite）侵入的新元古代表壳变火山-沉积岩系组成。本研究借助岩相学、全岩地球化学与矿物化学手段，识别帕什费奥变质杂岩中角闪岩、阿罗伊奥穆达多尔组（Arroio Mudador Formation）的玄武岩与枕状熔岩，以及侵入于邻近希拉里奥组（Hilário Formation）火山岩中的岩脉的地球化学属性与构造环境。所采用的分析方法可将玄武岩与枕状熔岩的变质相归为葡萄石-绿纤石相，岩脉归为绿片岩相，角闪岩则归为低角闪岩相。判别图解显示，希拉里奥组岩脉的地球化学属性从拉斑系列过渡至碱性系列。玄武岩、枕状熔岩与角闪岩均具有玄武岩至玄武安山岩的化学成分，且以拉斑系列为主，被认为源自同一原岩。大地构造图解大多指示其为火山弧玄武岩。本文提出的演化模型以约7.6亿年前博索罗卡弧（Bossoroca Arc）张裂引发原大洋开启为起点，随后形成岛弧（即帕什费奥变质杂岩的相关岩石），沉积碳酸盐岩与泥灰岩；之后原大洋闭合，经历两期变质作用，并于约5.62亿年前发生卡察帕瓦花岗岩侵位。