Cooperative Research: Petrogenesis of HP and UHT Granulites from the Brasilia Fold Belt in Minas Gerais and Goias, Brazil

BrownEAR-0227553这个项目与巴西的同行一起，研究了高压(HP；P 1.0 Gpa，T 700EC)和超高温(UHT；T 900EC)麻粒岩和巴西利亚褶皱带核心的“普通”麻粒岩之间的关系。巴西利亚褶皱带造山核由一个以麻粒岩相岩石为特征的变质杂岩组成，而最近在该带南部(米纳斯吉拉斯；蓝晶石-钾长石麻粒岩)和中部(戈伊亚斯；蓝宝石-石英和尖晶石-石英麻粒岩)扇区内发现的高压和超高温麻粒岩为这一研究提供了主要动力。我们将绘制HP(Minas Gerais)和UHT(Goias)麻粒岩类型在巴西利亚褶皱带核心变质杂岩中的分布图，并建立它们与普通麻粒岩的空间、时间、构造和变质关系。我们将研究熔体损失和熔体保留在极端地壳变质条件下形成的矿物组合的保存和/或退化中的作用。熔体损失将使用多元素模型系统中的亮色体、大块岩石化学和假截面的分布来评估，该多元素模型系统允许研究固相线下和超固相线相平衡。我们将表征HP和UHT麻粒岩中一系列主体成分的显微结构关系和矿物组合，特别是在保留不平衡反应微结构的样品中，如共晶石和反应冠状结构。利用这些数据和具有适当低方差的岩石中的详细矿物化学，将建立P-T路径的峰段和峰后段。人们普遍关心的是高压麻粒岩和超高温麻粒岩的发育、保存和折返方式与会聚(俯冲-碰撞)造山带构造的关系。巴西利亚褶皱带显示了由于洋盆演化和俯冲而形成的构造要素，最终在新元古代大陆-大陆碰撞中达到顶峰。我们将在当前板块构造理论的框架内评估我们的研究结果，并提出一个与数据一致的模型，该模型解释了巴西利亚褶皱带的演化。

BrownEAR-0227553This project, with counterparts in Brazil, investigates the relationship between high-pressure (HP; P 1.0 GPa, T 700EC) and ultrahigh-temperature (UHT; T 900EC) granulites and 'common' granulites in the core of the Brasilia Fold Belt. The orogenic core of the Brasilia Fold Belt is composed of a metamorphic complex characterized by granulite facies rocks, and the recent discovery of HP and UHT granulites within the southern (Minas Gerais; kyanite-K-feldspar bearing granulites) and central (Goias; sapphirine-quartz and spinel-quartz bearing granulites) sectors of the belt provides the principal motivation for this research. We will map the distribution of HP (Minas Gerais) and UHT (Goias) granulite types within the metamorphic complex of the core of the Brasilia Fold Belt, and establish their spatial, temporal, structural and metamorphic relationship with 'common' granulites. We will investigate the role of melt loss and melt retention in the preservation and/or retrogression of mineral assemblages formed under conditions of extreme crustal metamorphism. Melt loss will be evaluated using distribution of leucosome, bulk rock chemistry, and pseudosections in multi-element model systems that allow the investigation of subsolidus and suprasolidus phase equilibria. We will characterize the microstructural relationships and mineral assemblages within HP and UHT granulites for a range of bulk compositions, particularly in samples that preserve disequilibrium reaction microstructures such as symplectites and reaction coronae. Peak and post-peak segments of P-T paths will be established using these data and detailed mineral chemistry in rocks with suitably low variance. Of general interest is the relationship between the development, preservation and mode of exhumation of HP and UHT granulites to the tectonics of convergent (subduction - collision) orogens. The Brasilia Fold Belt displays tectonic elements that developed due to ocean basin evolution and subduction that culminated with continent-continent collision in Neoproterozoic time. We will evaluate the results of our research within the framework of current plate tectonics theory, and propose a model that is consistent with the data and which accounts for the evolution of the Brasilia Fold Belt.