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首页> 外文期刊>Journal of South American earth sciences >Paleoproterozoic granitoids from the northern limit of the Archean Amapa block (Brazil), southeastern Guyana Shield: Pb-Pb evaporation in zircons and Sm-Nd geochronology
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Paleoproterozoic granitoids from the northern limit of the Archean Amapa block (Brazil), southeastern Guyana Shield: Pb-Pb evaporation in zircons and Sm-Nd geochronology

机译：来自圭亚那东南部的太古宙阿马帕区块（巴西）北界的古元古代花岗岩：锆石中的铅-铅蒸发和Sm-Nd年代学

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摘要

Central Amapa, northern Brazil is located at the boundary between: (a) a northern Paleoproterozoic domain, consisting mainly of granite-greenstones terrains and (b) a southern Archean continental block (Amapa block), including an Archean basement reworked during the Transamazonian orogeny (2.26-1.95 Ga). Field investigations, Pb-Pb zircon and Sm-Nd whole rock geochronology supported by geochemical data on granitoids brought further constraints on Paleoproterozoic crustal growth in the southeastern Guyana Shield. A first magmatic episode, dated at 2.26 Ga, is marked by the crystallization of metaluminous low-K tholeiitic tonalites and quartz-diorites, which geochemical affinity with volcanic arc and association with T-MORB amphibolites suggest that they formed in a back-arc basin - island arc system. This event is coeval to the oceanic stage registered in French Guyana during the Eorhyacian (2.26-2.02 Ga). A second magmatic episode is represented by peraluminous, medium- to high-K calc-alkaline tonalite and granodiorite, which revealed some similarities with Mesorhyacian TTG rocks of French Guyana. For gran-itoids of both episodes, T_(DM) and ε_(Nd) values indicate the contribution of some Archean crustal component, probably by assimilation or contamination. This second magmatic episode occurred at 2.10 Ga, indicating that the period of successive calc-alkaline magmatic arcs formation may have extended until the Neo-rhyacian. Meanwhile, during this time, tectonic accretion by collision of the newly formed continental landmass was the prevailing process in French Guyana. The latter magmatic episode, even though poorly constrained, was registered around 2.08-2.02 Ga in central Amapa. It corresponds to the emplacement and solidification of high-K collisional granitoids, produced by partial melting of the Archean continental crust, as testified by the Archean T_(DM). inherited Pb-Pb zircon ages and strongly negative ε_(Nd) values. Our results point toward the existence of a protracted episode of crustal growth during the Neorhyacian in the southeastern Guyana Shield. This episode has been predominantly driven by magmatic arc accretion during, at least, 160 My, along the period of 2.26-2.10 Ga. This cycle ended with diachronic closure of the oceanic basins and arc-continent collision.
机译：巴西北部的阿马帕中部位于以下地区之间的边界：（a）北部的古元古代域，主要由花岗岩-绿岩地形组成；以及（b）南部的太古宙大陆块（Amapa块），包括在Transamazonian造山运动中重造的太古宙基底（2.26-1.95 Ga）。现场调查，Pb-Pb锆石和Sm-Nd整体岩石地质学（由花岗岩类的地球化学数据支持）对东南部圭亚那盾构的古元古代地壳生长带来了进一步的限制。第一次岩浆事件发生在2.26 Ga，其特征是金属性的低K硫代孔雀石和石英闪长岩的结晶，它们与火山弧的地球化学亲和力以及与T-MORB闪石的结合表明它们形成于后弧盆地-岛弧系统。此事件与Eorhyacian（2.26-2.02 Ga）时期在法属圭亚那登记的海洋阶段相近。第二个岩浆事件以高钙，中钾到高钾钙碱性斜长石和花岗闪长岩为代表，揭示了它与法属圭亚那的Mesorhyacian TTG岩石有些相似之处。对于这两个事件的类固醇，T_（DM）和ε_（Nd）值表明某些太古代壳成分的贡献，可能是由于同化或污染。这第二次岩浆事件发生在2.10 Ga，表明钙-碱性岩浆弧连续形成的时期可能已经延长到新流变岩。同时，在此期间，新形成的大陆陆块碰撞形成的构造增生是法属圭亚那的普遍过程。后者的岩浆事件尽管受约束程度不大，但在阿玛帕中部记录在2.08-2.02 Ga附近。它对应于高K碰撞花岗岩的沉积和固化，这是由太古代大陆壳的部分熔融所产生的，正如太古代T_（DM）所证明的那样。继承了Pb-Pb锆石的年龄，并且ε_（Nd）值为负值。我们的结果表明，东南部圭亚那盾构地区的Neorhyacian期存在着长期的地壳生长。这一事件主要是由在至少2.26 Ga.2 Ga期间至少160 My期间的岩浆弧积聚驱动的。该周期以洋盆的历时闭合和弧-陆碰撞而结束。

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