Petrogenesis and age of skarns associated with felsic and metamafic dykes from the Paraíba do Sul Complex, southern Espírito Santo State

ABSTRACT: This paper concerns the study of petrography, mineral chemistry and geochronology of skarns generated at the contact of marbles of the Paraíba do Sul Complex with felsic and metamafic dykes in the southern Espírito Santo State. The marbles were metamorphosed under P-T granulite facies conditions during the syn-collisional stage of the Neoproterozoic Araçuaí orogen. Metamafic bodies are composed of amphibolite and hornblende granofels, while felsic dykes consist of alkali-feldspar granite, monzogranite or syenogranite. From marble towards the dyke, skarns related to the metamafic bodies are composed of carbonate + olivine and diopside + hornblende zones. Skarn associated to the granitic dykes are composed of three different zones: carbonate + tremolite, diopside, scapolite + diopside. Variations in mineral chemical compositions along the metasomatic zones suggest introduction of Mg and Ca from the marbles, Fe from the metamafic dykes and Na from the granitoids. The presence of spinel in the metamafic dykes and their skarns indicates that both were metamorphosed under granulite facies conditions during the 580-560 Ma syn-collisional stage. U-Pb zircon geochronology (LA-ICP-MS) of an alkali-feldspar granite dyke resulted in a crystallization age of ca.540 Ma, which suggests that its skarns are therefore younger than skarns associated with the syn-collisional metamafic dykes.

ABSTRACT: 本文针对巴西圣埃斯皮里图州南部帕拉伊巴苏复合体（Paraíba do Sul Complex）大理岩与长英质、变基性岩脉接触带产出的夕卡岩开展岩石学、矿物化学及地质年代学研究。该套大理岩于新元古代阿拉苏瓦伊造山带同碰撞阶段，在麻粒岩相温压条件下发生变质作用。变基岩体由角闪岩与角闪石粒变岩组成，而长英质岩脉则包括碱长花岗岩、二长花岗岩或正长花岗岩。从大理岩向岩脉方向延伸，与变基岩体相关的夕卡岩依次发育碳酸盐+橄榄石带与透辉石+角闪石带；与花岗质岩脉伴生的夕卡岩则分为三个不同带域：碳酸盐+透闪石带、透辉石带以及方柱石+透辉石带。交代带内的矿物化学成分变化特征显示，Mg、Ca元素来自大理岩，Fe元素源自变基性岩脉，Na元素则来自花岗质岩体。变基性岩脉及其伴生夕卡岩中均产出尖晶石，指示二者均在580~560 Ma的同碰撞阶段经历了麻粒岩相变质作用。对一条碱长花岗岩岩脉开展锆石U-Pb激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）地质年代学测试，获得其结晶年龄约为540 Ma，这表明该岩脉伴生的夕卡岩时代晚于同碰撞阶段变基性岩脉相关的夕卡岩。