Reappraisal of the Sumé Complex: geochemistry and geochronology of metaigneous rocks and implications for Paleoproteorozoic subduction-accretion events in the Borborema Province, NE Brazil

Abstract The Alto Moxotó Terrane represents the largest exposure of Paleoproterozoic rocks in the central Borborema Province. Several medium- to high-grade metamorphic units are described in the terrane, however geochronological data are still scarce, and the main periods of crust build-up and recycling are poorly known. In this study, we investigate the nature and age of metagranitic and metamafic-ultramafic rocks in the type area of the Sumé Complex. U-Pb zircon data from a metasyenogranite yields a concordant age of 1.97 Ga, which is in contrast with the previously published age of the complex (c. 640 Ma). In addition, Sm-Nd isotope signature indicates Paleoproterozoic to Mesoarchean sources that coupled with negative εNd(t) values suggest reworking of a preexisting crust. Whole-rock geochemical data indicate that metamafic rocks are similar to those already described in the terrane, corresponding to island-arc tholeiites, whereas the metagranitic rocks share similarities with magmas that were generated in an evolved stage of an orogenic setting. The obtained results are coherent with a long-lived Paleoproterozoic accretionary-collisional event (c. 2.1 - 1.9 Ga) described in the Borborema Province, which is correlative with Paleoproterozoic (c. 2.2 - 2.0 Ga) basement inliers/terranes from Pan-African Fold belts..

摘要 阿尔托莫克索托地体（Alto Moxotó Terrane）是博尔博雷马构造省（Borborema Province）中部暴露面积最大的古元古代岩石单元。该区域已报道存在多套中高级变质岩系，但年代学数据仍较为匮乏，地壳增生与再循环的主要时期尚不明确。本研究以苏梅杂岩（Sumé Complex）典型研究区为对象，对其中的变花岗岩及变镁铁质-超镁铁质岩石的成因与形成时代开展系统研究。对一件变正长花岗岩开展锆石U-Pb定年分析，获得一致谐和年龄为1.97 Ga，这与该杂岩此前报道的约640 Ma年龄存在显著差异。此外，Sm-Nd同位素地球化学特征显示其源区为古元古代至中太古代，结合负εNd(t)值表明该岩石源自先期地壳的再造作用。全岩地球化学数据显示，变镁铁质岩石与该区域已报道的同类岩石相似，属于岛弧拉斑玄武岩系列；而变花岗岩则与造山环境演化阶段形成的岩浆具有亲缘性。本次研究结果与博尔博雷马构造省已报道的长时限古元古代增生-碰撞事件（约2.1~1.9 Ga）相一致，该事件与泛非褶皱带（Pan-African Fold belts）内的古元古代（约2.2~2.0 Ga）基底残块/地体具有可对比性。