Magmatic genesis, hydration, and subduction of the tholeiitic eclogite-facies Allalin gabbro (Western Alps, Switzerland).

IF 1.8 2区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
Swiss Journal of Geosciences Pub Date : 2024-01-01 Epub Date: 2024-06-19 DOI:10.1186/s00015-024-00461-8
Julia Dietrich, Jörg Hermann, Thomas Pettke
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引用次数: 0

Abstract

The Allalin gabbro of the Zermatt-Saas meta-ophiolite consists of variably metamorphosed Mg- to Fe-Ti-gabbros, troctolites, and anorthosites, which are crosscut by basaltic dykes. Field relationships of the various rock types and petrographic studies together with bulk rock and mineral chemical composition data allow the reconstruction of the complete geological history of the Allalin gabbro. With increasing magmatic differentiation, the incompatible element content in clinopyroxene increases (e.g., REEs and Zr by a factor of 5), whereas the Mg# decreases (from 86.4 to 74.6) as do the compatible element contents (e.g., Cr and Ni by factors of 3.5 and 5, respectively). Exhumation to shallower depths led to subsolidus ductile deformation and cooling of the gabbro followed by the intrusion of fine-grained basaltic dykes, which display chilled margins. Bulk rock data of these dykes reveal strong similarities in fluid-immobile trace element patterns to tholeiitic pillow basalts of the Zermatt-Saas and nearby meta-ophiolites. The recalculated REE patterns of the melt in equilibrium with igneous clinopyroxene is very similar to the REE patterns of the mafic dykes, indicating a cogenetic origin of pillow basalts, dykes, and gabbros. Together with the previously determined Jurassic intrusion age of the gabbro, our observations demonstrate that the Allalin gabbro intruded as a tholeiitic magma in a slow spreading MOR environment of the Piemonte-Ligurian ocean of the Alpine Tethys. Subduction of the Allalin gabbro resulted in different eclogitization extents of the Mg-gabbros as a function of variable hydration degrees. Metagabbros with low extents of hydration record incomplete eclogitization; the magmatic mineralogy (olivine + clinopyroxene + plagioclase) is preserved together with disequilibrium textures in the form of reaction coronae surrounding mineral boundaries. Metagabbros with high extents of hydration are completely eclogitized and display pseudomorphic replacement textures of magmatic minerals by eclogite-facies mineral assemblages, which required significant major to trace element transport across mineral domains. The locally variable extents of hydration took place near the sea floor, as recorded by the presence of Cl-apatite (6.28 wt% Cl), and an increase in B concentrations of minerals pseudomorphically replacing olivine (e.g., chlorite with 0.20-0.31 µg/g B and omphacite with 0.22-0.25 µg/g B) compared to magmatic olivine (0.12-0.16 µg/g B). Moreover, the chemical zonation pattern of metamorphic garnet coronae is different in completely eclogitized gabbros and gabbros with relic igneous minerals, in agreement with a main hydration event prior to subduction. The Allalin gabbro therefore represents a classical example of an oceanic gabbro formed in a slow spreading setting in the mid Jurassic that experienced heterogeneous hydration near the sea floor. Paleogene subduction of the gabbro to 70-80 km depth produced variably equilibrated gabbroic eclogites. In eclogite-facies Mg-gabbros, the water-rich minerals chlorite, talc, and chloritoid pseudomorphing magmatic olivine remained stable to these depths, revealing the potential relevance of hydrated Mg-gabbros as a fluid source at subarc depths in subduction zones.

Supplementary information: The online version contains supplementary material available at 10.1186/s00015-024-00461-8.

托勒密黝帘石成因、水合作用和阿拉林辉长岩(瑞士西阿尔卑斯山)的俯冲作用。
泽尔马特-萨斯元辉长岩中的阿拉林辉长岩由不同变质的镁-铁-钛辉长岩、透辉石和正长岩组成,它们被玄武岩堤横切。各种岩石类型的野外关系和岩相学研究以及大块岩石和矿物化学成分数据有助于重建阿拉林辉长岩的完整地质历史。随着岩浆分异的加剧,霞石中的不相容元素含量增加(例如,REEs 和 Zr 增加了 5 倍),而 Mg# 则减少(从 86.4 降至 74.6),相容元素含量也减少(例如,Cr 和 Ni 分别减少了 3.5 倍和 5 倍)。剥蚀到较浅的深度导致辉长岩发生固结下韧性变形和冷却,随后细粒玄武岩堤侵入,显示出冷边。这些岩堤的块状岩石数据显示,其流体流动微量元素模式与泽尔马特-萨斯(Zermatt-Saas)和附近的元玢岩的托勒密枕玄武岩非常相似。重新计算的与火成岩烊辉石平衡的熔体的 REE 模式与岩体的 REE 模式非常相似,这表明枕状玄武岩、岩体和辉长岩是同源的。结合之前确定的辉长岩的侏罗纪侵入年龄,我们的观察结果表明,Allalin辉长岩是在阿尔卑斯特提斯山皮埃蒙特-利古里亚洋的缓慢扩张MOR环境中作为托勒密岩浆侵入的。阿拉林辉长岩的俯冲导致镁辉长岩的蚀变程度不同,这与不同的水化程度有关。水化程度低的辉长岩记录了不完全的蚀变过程;岩浆矿物学(橄榄石+霞石+斜长石)以及矿物边界周围的反应冠状不平衡纹理得以保留。水化程度较高的辉长岩是完全蚀变的,并显示了岩浆矿物被蚀变岩成因矿物组合的假形态置换纹理,这就需要大量的主要微量元素跨矿物域迁移。与岩浆橄榄石(0.12-0.16 µg/g B)相比,Cl-磷灰石(6.28 wt% Cl)的存在以及假象替代橄榄石的矿物(如绿泥石,0.20-0.31 µg/g B;闪长玢岩,0.22-0.25 µg/g B)的硼浓度增加表明,水化程度在海底附近存在局部差异。此外,变质石榴石冠的化学分带模式在完全蚀变辉长岩和含有残余火成岩矿物的辉长岩中是不同的,这与俯冲前的主要水化事件是一致的。因此,Allalin辉长岩是在侏罗纪中期缓慢扩张环境中形成的大洋辉长岩的典型代表,它在海底附近经历了异质水化作用。辉长岩在古近纪俯冲到 70-80 千米深处时,产生了不同平衡的辉长岩蚀变岩。在辉长岩-辉绿岩中,富含水的矿物绿泥石、滑石和绿帘石假象岩浆橄榄石在这些深度保持稳定,揭示了水合辉长岩作为俯冲带弧底深度流体源的潜在相关性:在线版本包含补充材料,可查阅 10.1186/s00015-024-00461-8。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Swiss Journal of Geosciences
Swiss Journal of Geosciences 地学-地质学
CiteScore
4.50
自引率
12.90%
发文量
21
审稿时长
>12 weeks
期刊介绍: The Swiss Journal of Geosciences publishes original research and review articles, with a particular focus on the evolution of the Tethys realm and the Alpine/Himalayan orogen. By consolidating the former Eclogae Geologicae Helvetiae and Swiss Bulletin of Mineralogy and Petrology, this international journal covers all disciplines of the solid Earth Sciences, including their practical applications. The journal gives preference to articles that are of wide interest to the international research community, while at the same time recognising the importance of documenting high-quality geoscientific data in a regional context, including the occasional publication of maps.
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