Textural evolution in the transition from subsolidus annealing to melting process, Velay Dome, French Massif Central

Petrographic analysis is a useful, but underused tool to aid in distinguish ing between subsolidus and anatetic-related textures in migmatites. This st udy focuses on assessing the relative contributions of these two processes in the development of migmatitic orthogneiss textures in the Velay Massif, French Massif Central. The results of this study show that subsolidus proce sses are more important in the development of migmatitic textures in the or thogneiss than anatectic leucosome development. Four textural stages are id entified from the mylonitic non-anatectic orthogneiss, annealed, migmatitic orthogneiss to diatexite. The monomineralic K-feldspar and plagioclase-mus covite banding was transformed with increasing temperature to polymineralic plagioclase-quartz-muscovite and K-feldspar-quartz-muscovite layers by the wetting of feldspar boundaries during heterogeneous nucleation of quartz f rom a fluid phase at high surface energy triple points. A further increase of temperature led to the growth of K-feldspar probably related to producti on of small amounts of melt in plagioclase rich aggregates, controlled by m uscovite abundance. Solid state annealing processes in conjunction with inc ipient anatexis resulted in the formation of apparent granitic-like texture s in plagioclase dominated aggregates. By contrast, in K-feldspar dominated aggregates exclusively subsolidus processes prevail, leading to the develo pment of coarse grained leucosome. With the onset of biotite dehydration me lting the plagioclase-dominated aggregates are destroyed by the melt wherea s the K-feldspar aggregates may be preserved.