Generalized P–T Path and Fluid Regime of the Exhumation of Metapelites in the Central Zone of the Limpopo Complex, South Africa

IF 1 4区 地球科学 Q3 GEOCHEMISTRY & GEOPHYSICS
O. G. Safonov, V. O. Yapaskurt, D. D. van Reenen, C. A. Smit, S. A. Ushakova, M. A. Golunova
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引用次数: 0

Abstract

The PT paths of the exhumation of Precambrian granulite complexes at craton boundaries usually include two stages: subisothermal decompression and a decompression–cooling stage with a more gently sloped P–T path. Our goal is to understand the possible causes of the change in the slope of the PT exhumation path of the Central Zone (CZ) of the Limpopo granulite complex, South Africa, located between the Kaapvaal and Zimbabwe cratons. For this purpose, rocks (mainly, metapelites) were studied in various structural settings within the Central Zone, i.e., in dome structures, regional cross folds, and in local and regional shear zones. The metapelites are gneisses of similar bulk composition. The rocks contain various amounts of relics of leucosomes composed of quartz–feldspar aggregates with garnet and biotite, and melanocratic domains that are enriched in cordierite and usually mark shear microzones that envelope and/or break garnet porphyroblasts. Study of polymineralic (crystallized melt and fluid) inclusions in the garnet, its zoning with respect to the major (Mg, Fe, and Ca) and some trace (P, Cr, and Sc) elements, fluid inclusions in quartz, as well as phase equilibria modeling (PERPLE_X) showed that the rocks coexisted with granite melts and saline aqueous carbonic fluids (\({a}_{\text{H}_{2}\text{O}}\) = 0.74–0.58) at the peak of metamorphism at 800–850°C and 10–11 kbar. Partial melting of the rocks initiated their subisothermal exhumation to 7.5–8 kbar during diapirism of granitic magmas in the Neoarchean (2.65–2.62 Ga). This is reflected in the specific zoning of the garnet grains in terms of the grossular content. A change in the rheology of the rocks as a result of partial removal and crystallization of melt activated the shear zones during further exhumation to 6–5.5 kbar along a decompression–cooling PT path at 95–100°/kbar, reflecting the slower uplift of the rocks in the middle crust. This process was resumed due to thermal effects and interaction of the rocks with aqueous fluids (\({a}_{\text{H}_{2}\text{O}}\) > 0.85) in the Paleoproterozoic (~2.01 Ga). Such a scenario of metamorphic evolution implies that the Limpopo granulite complex in general and its Central Zone in particular resulted from the evolution of an ultrahot orogen, in which vertical tectonic movements associated with diapirism were coupled to horizontal tectonic processes caused by the convergence of continental blocks.

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Abstract Image

南非林波波岩群中心区辉绿岩出露的广义P-T路径和流体机制
摘要前寒武纪花岗岩群在克拉通边界的P-T掘进路径通常包括两个阶段:亚等温减压和减压-冷却阶段,其P-T路径坡度较缓。我们的目标是了解南非林波波花岗岩群中央区(CZ)(位于卡普瓦尔和津巴布韦陨石坑之间)P-T蜕变路径斜率变化的可能原因。为此,对中央区内各种构造环境下的岩石(主要是玄武岩)进行了研究,这些构造环境包括穹窿构造、区域交叉褶皱以及局部和区域剪切带。玄武岩是块状成分相似的片麻岩。岩石中含有不同数量的由石英-长石集合体与石榴石和黑云母组成的白云母遗迹,以及富含堇青石的黑云母域,通常标志着包裹和/或打破石榴石斑岩的剪切微区。对石榴石中的多矿物(结晶熔体和流体)包裹体、其主要元素(Mg、Fe 和 Ca)和一些微量元素(P、Cr 和 Sc)的分带、石英中的流体包裹体以及相平衡模型(PERPLE_X)的研究表明,岩石与花岗岩熔体和含盐碳酸水流体共存(\({a}_\{H}_{2}\text{O}}) = 0.74-0.58),变质峰值温度为 800-850°C 和 10-11 千巴。在新元古代(2.65-2.62 Ga)花岗岩浆的萃取过程中,岩石的部分熔融引发了它们向7.5-8 kbar的亚低温萃取。这反映在石榴石颗粒的具体分带中的毛玻璃含量上。由于熔体的部分移除和结晶,岩石的流变学发生了变化,在进一步剥蚀至6-5.5千巴期间,沿着95-100°/千巴的减压-冷却P-T路径激活了剪切带,反映了中地壳岩石的缓慢隆起。在古近纪(约2.01 Ga),由于热效应以及岩石与水液的相互作用(({a}_{text{H}_{2}\text{O}}),这一过程得以恢复。)这种变质演化情景意味着林波波花岗岩群,特别是其中央区,是由超热造山运动演化而来的,在这个过程中,与透辉作用有关的垂直构造运动与大陆块汇聚引起的水平构造过程相耦合。
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来源期刊
Petrology
Petrology 地学-地球科学综合
CiteScore
2.40
自引率
20.00%
发文量
27
审稿时长
>12 weeks
期刊介绍: Petrology is a journal of magmatic, metamorphic, and experimental petrology, mineralogy, and geochemistry. The journal offers comprehensive information on all multidisciplinary aspects of theoretical, experimental, and applied petrology. By giving special consideration to studies on the petrography of different regions of the former Soviet Union, Petrology provides readers with a unique opportunity to refine their understanding of the geology of the vast territory of the Eurasian continent. The journal welcomes manuscripts from all countries in the English or Russian language.
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