Partial melting in crustal xenoliths and anatectic migmatites: a comparison

Anatectic xenoliths from the volcanic fields of the Eifel (Germany) and the Chaine des Puys (Massif Central, France) contain a glass phase (quenched in-situ melt) along mineral grain boundaries. Microanalytical studies show that on a mm- to cm-scale melt chemistry is strongly controlled by the mineral phases involved in anatexis. Domains rich in mafic phases (biotite, orthopyroxene, spinel, garnet) contain brown glass with much higher abundances of FeO, MgO and TiO₂ than colourless glass in quartzofeldspathic layers. By contrast, K₂O abundances are similar. Differences in melt composition in turn strongly control the crystallization of minerals from the melt as well as minor reactions between the melt and the xenolith phase assemblage.

In comparison, textural features of anatectic HT-LP migmatites from SW Finland provide evidence of both prograde and retrograde reactions between minerals and melt. They reveal, that the modal and also the chemical composition of migmatites is more strongly affected by retrograde processes than in xenoliths. Hence, microanalytical and textural studies of partially melted crustal xenoliths can provide important insights in prograde processes operating in migmatitic terranes.