巴西圣弗朗西斯科克拉通Ibitiara岩体的锆石U-Pb年代学、地球化学特征及地质意义

Ibitiara岩体出露在南美巴西中东部巴伊亚州Sao Francisco克拉通内,岩性为花岗闪长岩,属于壳幔岩浆混合作用形成的准铝质-弱过铝质钙碱性Ⅰ型花岗岩。地球化学研究表明该花岗闪长岩富集Ba等大离子亲石元素(LILE)和不相容元素U、Th等,强烈亏损高场强元素(HFSE) Nb、P、Ti等,为富硅、高钾钙碱性岩系列,稀土元素显示轻微负Eu异常,轻重稀土分馏相对明显,富集轻稀土,配分曲线呈明显右倾稀土配分模式。锆石SHRIMP U-Pb测试Ibitiara岩体形成年代为2 103±8 Ma,为古元古代层侵纪晚期-造山纪早期岩浆活动的产物。该岩体形成年龄与亚马逊构造旋回(2.1 Ga±)岩浆活动区域构造背景相呼应,可能为哥伦比亚超大陆聚合增生大西洋陆块群岩浆活动的产物。

Zircon UPb geochronology,geochemical characteristics and geological implication of Ibitiara pluton from Sao Francisco Craton in Brazil

Ibitiara pluton is outcropped in S o Francisco Craton in Bahia,central and eastern Brazil,South America.The intrusive rocks are mainly composed of granodiorite,which falls into the metaluminous to weakly peraluminous calcium-alkaline I-type granites formed by the mixing of crustal and mantle magma.Geochemical studies indicate that the granodiorite is characterized by the enrichment in large ion lithophile elements(LILE)(e.g.,Ba)and incompatible elements(e.g.,U and Th).But depletion in high field strength elements(HFSE)(e.g.,Nb,P and Ti).It has the features of high SiO 2 and high potassium calc-alkaline series.It also shows high fraction of LREE and HREE,and enrichment of LREE,with weak Eu anomaly.The REE distribution curve is obviously right-inclined.SHRIMP U-Pb age suggests that Ibitiara pluton granodiorite is defined as 2103±8 Ma,which is the product of magmatic activity from Late Rhyacian to Early Orosirian of Paleoproterozoic.The age of the rock mass is consistent with the magmatic tectonic setting of the Amazonian structural cycle(around 2.1 Ga)in the S o Francisco Craton.Ibitiara pluton also belonged to the component of the Columbia supercontinent and was possible to be the product of magmatic activity of Atlantic continental blocks.