阿根廷马兹山Grt-St片岩中晚法马世(440-410 Ma)格伦维利亚变质作用的叠加:相平衡模拟、地质年代学和构造意义

IF 3.5 2区 地球科学 Q1 GEOLOGY
Sebastián O. Verdecchia, Carlos D. Ramacciotti, Cesar Casquet, Edgardo G. Baldo, Juan A. Murra, Robert J. Pankhurst
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引用次数: 4

摘要

分带矿物中主微量元素的分析有助于破译多变质岩体的部分构造-热演化过程。我们将这一方法应用于阿根廷西帕米亚山脉的马兹变质沉积系,在famatian造山运动的普遍Rinconada构造阶段,该地区的多变质作用导致了Grenvillian基底(马兹杂岩)的叠加。其中,较老变质作用(M1)为该基底最年轻的Grenvillian变质作用(约1035 Ma),而Rinconada变质作用(M2)为志留纪至早泥盆世,主要在440 ~ 410 Ma之间。后者是相对于晚寒武世早期至奥陶世(490-470 Ma)法马纪造山构造阶段造山锋向海洋迁移的结果。在马兹变质岩系的一颗星辉岩-石榴石片岩中发现了M1和M2变质事件。其中大部分变质矿物形成于M2事件,为巴罗威型(±蓝晶石)。部分变质P-T演化记录在石榴石卟啉母细胞的复杂成分分带中。根据石榴石的结构和化学成分(包括微量元素)鉴定出三种类型的石榴石。为了建立P-T历史,采用了相平衡分析、成分等线和多平衡热压法。M1以Grt1±Kfs±Sil保存为代表,峰值P-T条件为790℃,5.2 kbar,为麻粒岩相。这一早期变质事件与以残余S1片理为代表的D1期变形有关。后者以排列的包裹体的形式保存在橄榄岩卟卟母细胞中,并作为一个更古老的圆纹叶理的遗迹保存在基质的微石中。M2沿顺时针P-T路径生长,有三个矿物生长阶段。最早发生在~585℃,~8.7 kbar,表现为Grt2±St1±Bt1 + Qz。Grt2部分与St1生长同步,St1在~625℃和9.0 kbar下稳定生长。Grt2 + St1与主要的S2叶理(D2片段)同步运动。随后,St2 (+ Bt2 + Ms1 + Qz + Pl)结晶开始减压(D3),石榴石在~612 ~ 620℃和~7.3 ~ 7.7 kbar下被部分消耗。St3 + Grt3在~608℃和~6.8 kbar的温度下在D3末端结晶。M2生长早期P-T条件的增加表明马兹变质沉积系的埋藏,可能与逆冲作用下的构造增厚有关(构造期D2)。在逆冲叠合过程中,变质条件达到高峰。S2片理主要发生在D2构造期,在各个尺度上都具有渗透作用。近等温减压路径与发掘相一致,可能是由于沿离散剪切带(构造期D3)伸展而形成糜棱岩片理S3。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Late Famatinian (440–410 Ma) overprint of Grenvillian metamorphism in Grt-St schists from the Sierra de Maz (Argentina): Phase equilibrium modelling, geochronology, and tectonic significance

The analysis of major and trace elements in zoned minerals is useful for deciphering parts of the tectonothermal evolution of polymetamorphic tarrain. We applied this approach to the Maz Metasedimentary Series in Western Sierras Pampeanas of Argentina, where polymetamorphism resulted in the overprinting of a Grenvillian basement (the Maz Complex) during the pervasive Rinconada tectonic phase of the Famatinian orogeny. The older metamorphism (M1) is assigned to the youngest Grenvillian metamorphic event recognized in this basement at c. 1035 Ma, whereas the Rinconada metamorphism (M2) was Silurian to early Devonian, essentially between 440 and 410 Ma. The latter resulted from oceanward migration of the orogenic front relative to earlier late Cambrian to Ordovician (490–470 Ma) tectonic phases of the Famatinian orogeny. The M1 and M2 metamorphic events have been recognized in a staurolite-garnet schist from the Maz Metasedimentary Series. Most metamorphic minerals from this rock were formed during the M2 event which was of the Barrovian type (±kyanite). Part of the metamorphic P–T evolution is recorded in the complex compositional zoning of garnet porphyroblasts. Three types of garnet were identified based on texture and chemistry, including trace elements (REEs). Phase equilibrium analysis, compositional isopleth, and multi-equilibrium thermobarometry were applied in order to establish the P–T history. M1 is represented by preservation of Grt1 ± Kfs ± Sil, with peak P–T condition of 790°C and 5.2 kbar, that is, granulite facies. This early metamorphic event was related to a deformational D1 episode represented by a relict S1 foliation. The latter is preserved as aligned inclusions in staurolite porphyroblasts and as relics of an older crenulated foliation in microlithons from the matrix. M2 followed a clockwise P–T path with three mineral growth stages. The earliest occurred at ~585°C and ~8.7 kbar and is represented by Grt2 ± St1 ± Bt1 + Qz. Grt2 was partially coeval with growth of St1, which was stable at ~625°C and 9.0 kbar. Grt2 + St1 are syn-kinematic to the main S2 foliation (D2 episode). Subsequently, decompression (D3) started as St2 (+ Bt2 + Ms1 + Qz + Pl) crystallized, and garnet was partially consumed at ~612–620°C and ~7.3–7.7 kbar. St3 + Grt3 crystallized at ~608°C and ~6.8 kbar at the end of D3. Increasing P–T conditions during the earlier M2 growth stage suggest burial of the Maz Metasedimentary Series, probably linked to tectonic thickening by underthrusting (tectonic phase D2). Peak metamorphic conditions were attained during thrust stacking. The tectonic phase (D2) was responsible for the main S2 foliation, which was penetrative at all scales. The nearly isothermal decompression path is compatible with exhumation, probably resulting from extension along discrete shear zones (tectonic phase D3) that produced a mylonitic foliation S3.

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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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