Recognition of melferite – A rock formed in syn-deformational high-strain melt-transfer zones through sub-solidus rocks: A review and synthesis of microstructural criteria

Daczko, Nathan R.; Piazolo, Sandra

doi:10.1016/j.lithos.2022.106850

Cited by 8 publications

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“…; Figures 8h–8l and 12) indicate solid state deformation under green‐schist facies conditions, consistent with a distinct, younger deformation event. There is no evidence that deformation structures formed at amphibolite facies, melt‐present conditions (e.g., low dihedral angles, pseudomorphed melt films, pseudomorphed melt inclusions or pockets; Daczko & Piazolo, 2022; Piazolo et al., 2020).…”

Section: Discussionmentioning

confidence: 99%

Dating Strike‐Slip Ductile Shear Through Combined Zircon‐, Titanite‐ and Apatite U–Pb Geochronology Along the Southern Tan‐Lu Fault Zone, East China

et al. 2023

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Section: Discussionmentioning

confidence: 99%

Dating Strike‐Slip Ductile Shear Through Combined Zircon‐, Titanite‐ and Apatite U–Pb Geochronology Along the Southern Tan‐Lu Fault Zone, East China

et al. 2023

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“…High‐strain zones are important crustal‐scale conduits for melt transfer from regions of melt production (lower to middle crust) to shallower crustal depths, significantly contributing to the observed segregation of highly silicic melt towards the upper crust (Brown, 1994; Brown & Rushmer, 2006; D'lemos et al, 1992; Etheridge et al, 2020; Hollister & Crawford, 1986; Hutton, 1988; Vigneresse, 1995; Weinberg et al, 2004). Furthermore, the former presence of melt in highly strained rocks suggests a high‐strain zone with very weak rheology and by consequence increased strain localization in such zones compared to the surrounding consolidated rocks (Daczko & Piazolo, 2022; Lee et al, 2018; Levine et al, 2013; Rosenberg & Handy, 2005; Searle, 2013). Whereas melt‐present high‐strain zones preserve signatures of the former presence of melt (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…leucosome) arranged in seams, pockets or dykes in the deformed rock (e.g. Daczko & Piazolo, 2022; Stuart et al, 2017, 2018; Weinberg et al, 2013). However, low percentages of preserved crystallized melt volume are expected if deformation is synchronous with melt presence, as deformation will markedly enhance movement of melt (Brown & Solar, 1998; Collins & Sawyer, 1996; Etheridge et al, 2020; Rosenberg & Handy, 2005; Van der Molen & Paterson, 1979).…”

Section: Introductionmentioning

confidence: 99%

“…However, low percentages of preserved crystallized melt volume are expected if deformation is synchronous with melt presence, as deformation will markedly enhance movement of melt (Brown & Solar, 1998; Collins & Sawyer, 1996; Etheridge et al, 2020; Rosenberg & Handy, 2005; Van der Molen & Paterson, 1979). Therefore, the signature of the former presence of melt in high‐strain zones may be cryptic at the outcrop scale and requires in‐depth microstructural analysis to identify it (Daczko & Piazolo, 2022). For example, microstructures may include small mono‐ or multiphase pockets at triple points and/or elongated interstitial grains (Beere, 1975; Holness & Sawyer, 2008; Lee et al, 2018; Stuart et al, 2017, 2018; Von Bargen & Waff, 1986; Závada et al, 2007).…”

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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Mapping the 4D lithospheric architecture of Zealandia using zircon O and Hf isotopes in plutonic rocks

et al. 2023

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Recognition of melferite – A rock formed in syn-deformational high-strain melt-transfer zones through sub-solidus rocks: A review and synthesis of microstructural criteria

Cited by 8 publications

References 157 publications

Dating Strike‐Slip Ductile Shear Through Combined Zircon‐, Titanite‐ and Apatite U–Pb Geochronology Along the Southern Tan‐Lu Fault Zone, East China

Dating Strike‐Slip Ductile Shear Through Combined Zircon‐, Titanite‐ and Apatite U–Pb Geochronology Along the Southern Tan‐Lu Fault Zone, East China

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

Mapping the 4D lithospheric architecture of Zealandia using zircon O and Hf isotopes in plutonic rocks

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