Partial melting and melt segregation in migmatites from the Southern margin of Velay dome (French Massif Central, Variscan belt).

Partial melting and melt-solid segregation play a key role in crustal differentiation. However, quantifying the melt proportion and its distribution remains challenging. In this study, we document these processes by a structural-petrological analysis of migmatite samples from the southern margin of the Velay dome (French Massif Central), a region that exhibits a progressive transition from micaschists to migmatites formed during the Variscan orogeny.

We first estimate the melt fraction based on the identification of leucosome and mesosome at the outcrop to sample scale within a variety of migmatites across this metamorphic transition. The melt fraction and its chemical composition are further evaluated based on classical petrological analysis using optical microscopy, and geochemical tools (XRF (X-ray fluorescence) and EPMA (Electron Probe Microanalyser)). These data have been used for thermodynamic modeling of migmatite formation and evolution throughout P-T changes using PERPLE_X. We compare these models with textural-micro structural analysis based on SEM (Scanning Electron Microscope) and EBSD (Electron Back Scatter Diffraction) in order to decipher the former residual and peritectic minerals from the minerals crystallized from the melt.

Field observations show migmatites transitioning from metatexites to diatexites. Within metatexites, a network of texturally continuous leucosome veins concordant and discordant relative to the syn-migmatitic foliation points to grain scale melt segregation and melt migration beyond the grain scale. The mineral paragenesis of metatexite migmatite is Mus + Pl + Qz → Melt + Fdk, which is consistent with the modeled mineral reactions identified by thermodynamic modeling. Textural analysis indicates that part of the leucosome consist of a proportion of residual plagioclase, peritectic K-feldspar and quartz crystallized from the melt. Thermodynamic modeling suggests an estimation of melt fraction ranging from ~14% up to ~29% in the metatexites at temperatures ranging from 570 to 650°C and 6 kbar pressure, which is consistent with the estimate derived from textural analysis and is close to the transition between a partially molten rock and a crystal mush (Vanderhaeghe 2009).

This research provides new insights into the mechanisms driving crustal differentiation across scales by quantifying melt fraction and identifying melt-derived textures in partially molten rocks.