Hydrous shear zones are sites of melt transfer in the lower arc crust: A case study from Fiordland, New Zealand

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2024-06-19 DOI:10.1111/jmg.12788
Aditi Chatterjee, Nathan R. Daczko, Joyjit Dey, Sandra Piazolo
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Abstract

Recent studies of the Cretaceous lower arc crust exposed in Fiordland, New Zealand, conclude that shear zones are sites of melt migration and mass transfer through the deep crust. Here, we investigate the 4–10 km-wide George Sound Shear Zone, which cuts the Western Fiordland Orthogneiss, comprising two main rock types: two-pyroxene gneiss and hornblende gneiss. Previous studies infer a predominantly igneous origin for the two types of gneiss. However, this study finds that melt-rock interaction within the George Sound Shear Zone formed the hornblende gneiss from the precursor two-pyroxene gneiss. Petrographic analyses of samples collected in transects across the shear zone show hydration reaction textures ranging from rims of hornblende + quartz around pyroxene grains to complete replacement of pyroxene grains. Plagioclase is recrystallized and partially replaced by clinozoisite. Additionally, biotite mode increases towards the higher strain rocks in the shear zone. Backscatter images and polarized light microscopy show microstructures indicative of former melt-present deformation, including (a) interconnected mineral films of quartz and K-feldspar along grain boundaries, (b) grains that terminate with low apparent dihedral angles, (c) interstitial grains, with some (d) undulose extinction in plagioclase and (e) serrated grain boundaries. In addition, zircon microstructures are consistent with Zr mobility, further supporting the former presence of melt; geochemical data show enrichment of Zr in the hornblende gneiss as compared to the two-pyroxene gneiss. From the above observations, it is inferred that a felsic to intermediate hydrous melt migrated through the George Sound Shear Zone reacting with the host two-pyroxene gneiss of the Western Fiordland Orthogneiss. Melt migration along grain boundaries was deformation assisted, (i) causing hydration of pyroxene to hornblende + quartz, and plagioclase to clinozoisite, (ii) increasing proportions of biotite within the shear zone and (iii) causing depletion of Cr + Ni and Zr enrichment in the hydrated rock. Our interpretation is supported by published observations of pegmatite dyke swarms that intruded into the George Sound Shear Zone, the P-T conditions of deformation and characterization of microstructures that contrast sharply with those typically found in mylonitic rocks formed under solid-state metamorphic conditions. Our results confirm that hydrous shear zones within otherwise anhydrous country rock are retrogressive and may represent evidence of melt migration through zones of deformation.

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含水剪切带是下弧地壳熔体转移的场所:新西兰峡湾地区的案例研究
最近对新西兰峡湾地区出露的白垩纪下弧地壳进行的研究得出结论,剪切带是地壳深处熔体迁移和质量转移的场所。在这里,我们研究了 4-10 公里宽的乔治湾剪切带,该剪切带切割了西峡湾正长片麻岩,由两种主要岩石类型组成:双辉石片麻岩和角闪石片麻岩。之前的研究推断这两种片麻岩主要起源于火成岩。然而,本研究发现,乔治湾剪切带内的熔岩相互作用形成了角闪石片麻岩,而角闪石片麻岩的前身是双辉石片麻岩。在剪切带横断面上采集的样品的岩相分析表明,水化反应纹理从辉石晶粒周围的角闪石+石英边缘到辉石晶粒的完全置换不等。斜长石被重结晶,部分被黝帘石取代。此外,在剪切带的高应变岩石中,斜长石模式有所增加。背散射图像和偏振光显微镜显示了表明以前熔体存在变形的微观结构,包括(a)沿晶界的石英和钾长石相互连接的矿物膜;(b)以低表观二面角终止的晶粒;(c)间隙晶粒,以及(d)斜长石中的一些波状消光;(e)锯齿状晶界。此外,锆石的微观结构与锆元素的流动性相一致,进一步证明了熔体的存在;地球化学数据显示,角闪石片麻岩中的锆元素比二辉石片麻岩中的富集。根据上述观察结果,可以推断出长英质至中等含水熔体穿过乔治湾剪切带,与西峡湾正长片麻岩的主质双辉石片麻岩发生反应。熔体沿晶界的迁移是在变形的辅助下进行的,(i) 使辉石水合为角闪石+石英,斜长石水合为黝帘石,(ii) 增加了剪切带中生物橄榄石的比例,(iii) 使水合岩石中的铬+镍和锆富集。我们的解释得到了对侵入乔治湾剪切带的伟晶岩岩体群、变形的 P-T 条件和微观结构特征的公开观察结果的支持,这些微观结构特征与在固态变质条件下形成的岩浆岩中通常发现的微观结构形成了鲜明对比。我们的研究结果证实,原本无水的乡村岩石中的水化剪切带是逆行的,可能是熔体通过变形区迁移的证据。
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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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