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

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.