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 本文提出巴西南部南卡沙帕瓦地区新元古代帕什费奥变质杂岩（Passo Feio Metamorphic Complex，PFMC）中初始洋盆的形成与闭合，以及岛弧的演化过程。圣加布里埃尔地体的帕什费奥变质杂岩由一套被卡沙帕瓦花岗岩侵入的新元古代表壳变质火山-沉积序列组成。本研究借助岩石学、全岩地球化学及矿物化学方法，识别了帕什费奥变质杂岩中的角闪岩、阿罗伊穆达多尔组玄武岩与枕状熔岩，以及侵入于伊拉里奥组附近火山岩中的岩脉的地球化学亲和性与构造环境。所采用的分析方法可将玄武岩与枕状熔岩的变质相归类为葡萄石-绿纤石相，岩脉为绿片岩相，角闪岩则为低角闪岩相。判别图解显示，伊拉里奥组岩脉具有从拉斑质到碱性的地球化学亲和性。玄武岩、枕状熔岩与角闪岩均具有玄武岩至玄武安山岩的成分组成，且以拉斑质亲和性为主，被认为源自同一原岩。大地构造图解大多指示其属于火山弧玄武岩范畴。本文提出的演化模型始于约760 Ma的博索罗卡弧裂谷作用引发的初始洋盆开启，进而形成岛弧（即帕什费奥变质杂岩相关岩石），并沉积碳酸盐岩与泥灰岩；随后洋盆闭合，发生两期变质事件，并于约562 Ma侵位卡沙帕瓦花岗岩。