Glimmerite: A product of melt-rock interaction within a crustal-scale high-strain zone

Silva, David; Daczko, Nathan R.; Piazolo, Sandra; Raimondo, Tom

doi:10.1016/j.gr.2021.09.005

Cited by 13 publications

(13 citation statements)

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“…Based on shear band orientations and sigma clast kinematic indicators, the GDSZ represents an S‐directed reverse high‐strain zone that juxtaposed the northern SMC with the Harts Range Metamorphic Complex (HRMC) (Bendall, 2000; Collins & Shaw, 1995; Figure 1). This high‐strain zone is characterized by metasomatic hydration of the bimodal interlayered anhydrous mafic‐felsic granulites, quartzo‐feldspathic gneisses, minor calcsilicate and amphibolite rocks constituting the Palaeoproterozoic SMC basement of the Arunta Region (Piazolo et al, 2020; Silva et al, 2022). The formation of glimmerite schist in the GDSZ and glimmerite selvedges observed around granitic lenses was recently proposed by Silva et al (2022) as involving multiple periods of migration of extensive volumes of hydrous peraluminous melt through the high‐strain zone during the ASO.…”

Section: General Geological Backgroundmentioning

confidence: 99%

“…This high‐strain zone is characterized by metasomatic hydration of the bimodal interlayered anhydrous mafic‐felsic granulites, quartzo‐feldspathic gneisses, minor calcsilicate and amphibolite rocks constituting the Palaeoproterozoic SMC basement of the Arunta Region (Piazolo et al, 2020; Silva et al, 2022). The formation of glimmerite schist in the GDSZ and glimmerite selvedges observed around granitic lenses was recently proposed by Silva et al (2022) as involving multiple periods of migration of extensive volumes of hydrous peraluminous melt through the high‐strain zone during the ASO. Reaction of the migrating melts with the precursor quartzo‐feldspathic granulite rocks was inferred within mainly channelized pathways parallel to the high‐strain zone foliation.…”

Section: General Geological Backgroundmentioning

confidence: 99%

“…Reaction of the migrating melts with the precursor quartzo‐feldspathic granulite rocks was inferred within mainly channelized pathways parallel to the high‐strain zone foliation. The magnitude of melt needed to drive the melt‐mediated reaction forming glimmerite and the size and longitudinal continuity of field indicators of former melt flux increase towards the centre of the GDSZ (Silva et al, 2022). Recent research on the nearby Cattle Water Pass shear zone also demonstrated that melt‐rock interaction and migration of significant volumes of melt through high‐strain zones was important in enriching the rocks in oxide minerals such as ilmenite (Ghatak et al, 2022).…”

Section: General Geological Backgroundmentioning

confidence: 99%

“…To establish what field signatures can be used to recognize former syntectonic melt presence, it is necessary to develop an understanding of how igneous components may be preserved despite syntectonic melt flux and expulsion. In this contribution, we present a detailed microstructural study of rocks present in the 2‐ to 4‐km‐wide Gough Dam shear zone (GDSZ) that was active during the Alice Springs Orogeny (ASO) in central Australia, which has been shown to have deformed in the presence of melt (Piazolo et al, 2020; Silva et al, 2022). This high‐strain zone is characterized by highly strained, biotite‐dominated schist (glimmerite schist) formed by melt‐mediated metasomatic reaction with a granulite precursor rock (Silva et al, 2022).…”

Section: Introductionmentioning

confidence: 99%

“…In this contribution, we present a detailed microstructural study of rocks present in the 2‐ to 4‐km‐wide Gough Dam shear zone (GDSZ) that was active during the Alice Springs Orogeny (ASO) in central Australia, which has been shown to have deformed in the presence of melt (Piazolo et al, 2020; Silva et al, 2022). This high‐strain zone is characterized by highly strained, biotite‐dominated schist (glimmerite schist) formed by melt‐mediated metasomatic reaction with a granulite precursor rock (Silva et al, 2022). To advance our ability to recognize formerly melt‐present high‐strain zones, we investigate the microstructural signature of samples with obvious igneous components and samples lacking these clear field signatures.…”

Section: Introductionmentioning

confidence: 99%

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

Silva

Piazolo

Daczko

2022

Journal Metamorphic Geology

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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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