Age, provenance and tectonic setting of the high-grade Jequitinhonha Complex, Araçuaí Orogen, eastern Brazil

ABSTRACT: The Jequitinhonha Complex of the northeastern Araçuaí orogen is an extensive sedimentary unit metamorphosed in the amphibolite-granulite facies transition around 580-545 Ma. The unit consists of Al-rich (kinzigitic) paragneisses with decametric intercalations of graphite gneisses and quartzites, and centimetric to metric lenses of calcsilicate rocks. A new detrital zircon U-Pb age spectrum is reported for a sample of quartzite, and whole-rock geochemical (major and trace elements, 9 samples) and Sm-Nd isotope data (10 samples) for Jequitinhonha Complex paragneiss. Together with published data these show that: (1) the geochemistry of paragneiss samples of the Jequitinhonha Complex are similar to those of passive margin sedimentary protoliths; (2) detrital zircon data yield U-Pb age populations between ca. 0.9 and 2.5 Ga; and (3) Sm-Nd TDM model ages range from 1.6 to 1.8 Ga and εNd(575 Ma) around -7.5. The data reveal a mixture of Cryogenian to Mesoproterozoic rift-related igneous rocks with the Palaeoproterozoic-Archaean basement rocks of the São Francisco-Congo palaeocontinent as the main source areas, and also support the correlation between the Jequitinhonha Complex and the passive margin units of the upper Macaúbas Group, constituting the precursor basin of the orogen. Our results, with the absence of ophiolites in the Jequitinhonha Complex, reinforce the interpretation that the São Francisco-Congo palaeocontinent was not divided to the north of the focused region, suggesting an ensialic termination of a gulf during the Neoproterozoic.

摘要：阿拉苏阿伊造山带（Araçuaí orogen）东北部的热基通哈杂岩（Jequitinhonha Complex）是一套大范围沉积岩系，于约5.8亿至5.45亿年前在角闪岩相-麻粒岩相过渡环境中发生变质作用。该岩系由富铝长英质副片麻岩（kinzigitic paragneisses）构成，伴生数十米级的石墨片麻岩与石英岩互层，以及厘米级至米级的钙硅酸盐岩透镜体。本研究针对一件石英岩样品报道了全新的碎屑锆石（detrital zircon）U-Pb年龄谱，同时针对热基通哈杂岩副片麻岩提供了全岩地球化学数据（主量与微量元素，共9件样品）以及Sm-Nd同位素（Sm-Nd isotope）数据（共10件样品）。结合已发表数据，本次研究结果显示：①热基通哈杂岩副片麻岩的地球化学特征与被动大陆边缘（passive margin）沉积原岩相似；②碎屑锆石U-Pb年龄分布区间约为9亿年至25亿年；③Sm-Nd亏损模式年龄（Sm-Nd TDM model age）区间为16亿年至18亿年，ε_(Nd)(5.75亿年)约为-7.5。数据表明，热基通哈杂岩的物源主要为成冰纪（Cryogenian）至中元古代（Mesoproterozoic）的裂谷相关火成岩，以及圣弗朗西斯科-刚果古陆（São Francisco-Congo palaeocontinent）的古元古代（Palaeoproterozoic）-太古代（Archaean）基底岩石；同时本研究支持热基通哈杂岩与上马卡乌巴斯群（upper Macaúbas Group）的被动大陆边缘岩系具有成因关联，二者共同构成该造山带的前陆盆地原型。结合热基通哈杂岩中未发现蛇绿岩（ophiolite）的事实，本次研究结果进一步佐证：圣弗朗西斯科-刚果古陆在研究区域以北并未发生裂解，暗示新元古代时期该区域存在以陆壳为基底的海湾终止演化环境。