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

Abstract

Melt transfer and migration occurs through both supra- and sub-solidus rocks. Mechanisms of melt transfer include dyking, mobile hydrofracturing and diffuse porous melt flow where melt flow may or may not be channelized via instabilities or into high-strain zones of active deformation. Here, we highlight the microstructural- and outcrop-scale signatures of syn -deformational melt-migration pathways through high-strain zones that cut sub-solidus rocks. High-strain zones with high proportions (>10%) of macroscopic, internally undeformed, felsic or leucocratic material are readily interpreted as important melt-migration pathways and are most common in supra-solidus host rocks. However, it is challenging to recognise high-strain melt-migration pathways through sub-solidus rocks; these pathways may lack noticeable felsic or leucocratic components at the outcrop scale and share many macroscopic features in common with ‘classic' sub-solidus mylonite, such that the two are generally conflated. We contrast field and microstructural characteristics of ‘classic' mylonite originating from solid-state deformation with those of high-strain zones that also cut sub-solidus rocks yet have microstructural indicators of the former presence of melt. We compile several features allowing one to distinguish solid-state from melt-present deformation in high-strain zones that cut sub-solidus rocks. Our aim is to encourage geologists to assess such high-strain zones on a case-by-case basis, in view of sub-solidus (i.e., mylonitic) versus melt-present deformation. Such assessment is crucial as (1) rocks deformed in the presence of melt, even small percentages of melt, are orders of magnitude weaker than their solid-state equivalents, (2) melt-rock interaction in such zones may result in metasomatism, and (3) such zones may sustain long-lived melt migration and ascent enabling chemical differentiation at a crustal scale. With this contribution we aim to increase the ease of recognising this important subset of melt-migration pathways by assisting in clarity of description and interpretation of high-strain rocks. • Briefly review mylonite, and the field and solid-state deformation characteristics considered typical • Review high-strain zones recognized as syn-deformational melt-transfer zones through sub-solidus rocks • Focus on microstructural features of high-strain zones atypical for mylonites but typical for former melt-presence