Trapped K-feldspar phenocrysts as a signature of melt migration pathways within active high-strain zones

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2022-11-02 DOI:10.1111/jmg.12698
David Silva, Sandra Piazolo, Nathan R. Daczko
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引用次数: 1

Abstract

Melt migration through high-strain zones in the crust fundamentally influences their rheological behaviour and is important for the transfer of fluids to upper crustal regions. The inference of former melt-present deformation, based on field observations, may be hampered if the high-strain zone experience a low time-integrated melt flux or high melt volume expulsion during deformation. In these cases, typical macro-scale field evidence of former melt presence limits interpretations. In this contribution, we investigate igneous field evidence ranging from obvious to cryptic in the Gough Dam shear zone (central Australia), a 2- to 4-km-wide high-strain zone shown to have acted as a significant melt pathway during the Alice Springs Orogeny. Within bands of the high-strain zone, granitic lenses are easily discernible in the field and are inferred to have formed during melt present deformation. Related coarse K-feldspar is observed in biotite-rich (>75 vol%) schist (glimmerite) as either isolated grains, forming trails (sub)parallel to the main foliation, or in aggregates with subordinate quartz. Detailed characterization of the granitic lenses shows that pockets of phenocrysts may be entrained in the shear zone. If melt expulsion and melt-rock interaction is severe, isolated K-feldspar grains in glimmerite may form. These grains exhibit (i) partially preserved crystal faces; (ii) a lack of internal grain deformation; (iii) reaction textures preferentially formed along the main crystallographic axes showing dissolution of K-feldspar and precipitation of dominantly biotite; (iv) low-strain domains between multiple K-feldspar grains being inferred to enclose crystallized melt pockets, with some apparently isolated grains showing connectivity in three dimensions; and (v) a weak quartz and K-feldspar crystallographic preferred orientation. These observations suggest an igneous phenocrystic origin for the isolated K-feldspar grains hosted in glimmerite, which is consistent with the observed REE concentration patterns with positive Eu anomaly. We propose that the K-feldspar phenocrysts are early-formed crystals that were entrained into the glimmerite rocks as reactive melt migrated through the actively deformatting high-strain zone. Previously entrained K-feldspar phenocrysts were trapped during the collapse of the melt pathway when melt flux-related fluid pressure waned while confining pressure and tectonic stress were still significant. The active deformation facilitated expulsion or loss of the melt phase but retainment and trapping of phenocrysts. Hence, the presence of isolated or ‘trains’ of K-feldspar phenocrysts is a cryptic signature of syndeformational melt transfer. If melt transfer occurs in an open chemical system, phenocrysts will be entrained within the reaction product of melt-rock interaction. We suggest that these so-called trapped phenocrysts are a viable indicator of former syntectonic melt passage through rocks.

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作为活动高应变带内熔体迁移路径标志的捕获钾长石斑晶
熔体通过地壳高应变带的迁移从根本上影响其流变行为,并对流体向上地壳区域的转移至关重要。如果高应变区在变形过程中经历低时间积分熔体通量或高熔体体积排出,则基于现场观测的先前熔体存在变形的推断可能会受到阻碍。在这些情况下,先前熔体存在的典型宏观尺度现场证据限制了解释。在这篇文章中,我们研究了Gough Dam剪切带(澳大利亚中部)的火成岩场证据,从明显到隐蔽,这是一个2至4公里宽的高应变带,在Alice Springs造山运动中被证明是一个重要的熔体通道。在高应变带的带内,花岗岩透镜体在野外很容易辨认,并被推断是在熔体变形过程中形成的。在富含黑云母(>75 vol%)片岩(格列美脲),其为孤立颗粒,形成平行于主要叶理的轨迹(亚),或与次要石英聚集在一起。花岗岩透镜体的详细特征表明,剪切带中可能夹带斑晶。如果熔体排出和熔体-岩石相互作用严重,则可能会在格列美脲中形成孤立的钾长石颗粒。这些颗粒表现出(i)部分保留的晶面;(ii)内部晶粒没有变形;(iii)反应织构优先沿主结晶轴形成,显示钾长石的溶解和主要黑云母的沉淀;(iv)多个钾长石颗粒之间的低应变域被推断为包围结晶熔体袋,一些明显孤立的颗粒显示出三维连通性;和(v)弱石英和钾长石晶体的优选取向。这些观察结果表明,存在于格列美脲中的孤立钾长石颗粒是火成斑晶成因,这与观察到的具有正Eu异常的REE浓度模式一致。我们提出,钾长石斑晶是早期形成的晶体,当反应性熔体迁移通过主动变形的高应变带时,这些晶体被夹带到格列美脲岩石中。先前夹带的钾长石斑晶在熔体通道坍塌期间被捕获,此时熔体通量相关的流体压力减弱,而围压和构造应力仍然很大。主动变形促进了熔体相的排出或损失,但保留和捕获了斑晶。因此,孤立的或“一连串”的钾长石斑晶的存在是同变形熔体转移的一个隐蔽特征。如果熔体转移发生在开放的化学系统中,斑晶将被夹带在熔体-岩石相互作用的反应产物中。我们认为,这些所谓的捕获斑晶是前同构造熔体穿过岩石的可行指标。
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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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