Geochemistry, Sm-Nd isotopes and SHRIMP U-Pb geochronology of the Morro do Coco Granite (RJ, Brazil): another piece of the post-collisional magmatism of the Ribeira Belt

Abstract The Morro do Coco granite (MC) is a post-collisional intrusive body that was emplaced at Ribeira Belt, in the Cambro-Ordovician, succeeding the final stages of the Gondwana supercontinent amalgamation. This unit requires more studies, contrasting with similar best investigated occurrences of the central Ribeira Belt. This work presents a detailed investigation of such unit using petrography, Inductively Coupled Plasma (ICP), X-Ray Fluorescence (XRF) whole-rock geochemistry, U-Pb (SHRIMP) geochronology and Sm-Nd isotopes. The rock is a nearly undeformed syenogranite, geochemically classified as a high-K calc-alkaline magnesian, with metaluminous to slightly peraluminous affinity. Strongly light rare earth elements (LREE)-enriched patterns with significant negative Eu anomalies, typical of post-collisional granites, are identified. Similarly to other post-collisional granites of the central Ribeira Belt, U-Pb zircon data provide crystallization age of 496 ± 3 Ma . Sm-Nd isotopic data suggest crustal magma source as indicated by εNd(496) of -9, 143Nd/144Nd ratios between 0.511747 and 0.511752 and Mesoproterozoic TDM ages of 1.3 Ga. Based on its geochemical characteristics, morphology and geochronology, this unit is considered here as a member of Nova Friburgo Suite. Geochronological data defined three pulses of magmatism in this part of the orogen during the Cambro-Ordovician (515, 490 and 460 Ma).

摘要 莫罗杜科科花岗岩（Morro do Coco granite，简称MC）是形成于寒武纪-奥陶纪的碰撞后侵入体，侵位于里贝拉造山带，其形成晚于冈瓦纳超大陆聚合的最终阶段。与里贝拉带中部研究程度最高的同类岩体相比，该岩体的相关研究仍有待深化。本研究采用岩石学、电感耦合等离子体（Inductively Coupled Plasma，简称ICP）、X射线荧光光谱法（X-Ray Fluorescence，简称XRF）全岩地球化学分析、U-Pb（SHRIMP）地质年代学以及Sm-Nd同位素分析方法，对该岩体开展了详细研究。该岩体为近未变形的正长花岗岩，地球化学归类为高钾钙碱性镁质岩石，兼具准铝质至弱过铝质的属性。研究识别出典型碰撞后花岗岩所特有的轻稀土元素（Light Rare Earth Elements，简称LREE）强烈富集配分模式，以及显著的铕负异常。与里贝拉带中部其他碰撞后花岗岩一致，锆石U-Pb定年数据给出其结晶年龄为496 ± 3 百万年。Sm-Nd同位素数据显示其岩浆源区为地壳成因：εNd(496)值为-9，143Nd/144Nd比值介于0.511747至0.511752之间，中元古代地壳模式年龄（TDM）为1.3 十亿年。综合其地球化学特征、形貌特征与地质年代学数据，本研究将该岩体归为新弗里堡岩套（Nova Friburgo Suite）的成员。地质年代学数据表明，该造山带段在寒武纪-奥陶纪期间存在三期岩浆活动脉冲，分别为515、490和460 百万年。