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

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 micro- structural- 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 unde- formed, 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 geolo- gists 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 per- centages 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 rec- ognising this important subset of melt-migration pathways by assisting in clarity of description and interpretation of high-strain rocks.