Production and Distribution of Melt in Migmatite Deciphered by Micro-Mapping the Local Effective Bulk (LEB) Composition in Domains: Use of XMapTools

Abstract:

Migmatites are fantastic targets to study melt compositions, because they represent levels in the crust where melting has occurred. However, migmatites commonly are products not only of (i) melt producing reactions, but also of (ii) melt loss and (iii) retrograde reactions. Evidence of such events is preserved in the minerals and microstructures of rocks that outcrop in deeply exhumed orogens. The apparent complexity of migmatites is due to their chemical and textural heterogeneity visible in different domains with various mineral assemblages. Partial melting of pelites involves reactions, which are predictable using thermodynamic models. However, a forward modeling approach based on rock-specific equilibrium phase diagrams requires the knowledge of local bulk compositions of each equilibrium assemblage. In this contribution we show how suitable local effective bulk (LEB) compositions can be derived by means of standardized microprobe X-ray images, using the program XMapTools. For chemically heterogeneous samples, such as migmatites, these LEB allow to model the stable mineral assemblages for each domain and to obtain reliable P-T estimates.

A metapelite studied in detail is embedded within a metasedimentary xenolith in the Marcabeli pluton, El Oro Complex, Ecuador. This sample shows complex mineral patterns due to local melt redistribution (at mm to cm-scale), involving major changes that affect the local chemical composition observed today. Four domains are identified: A residuum domain made of cordierite + biotite + plagioclase + spinel, which is predicted to be stable at P-T_max conditions of 750 ± 50°C and 2 ± 1 kbar. In the leucosome (plagioclase + Kspar + quartz + biotite + orthopyroxene) three sub-domains show different mineral assemblages. Domain-specific equilibrium assemblages in the P-T diagrams demonstrate that these three assemblages reflect three interconnected melt-rich domains, each with different melt fractions (from 23 to 90%), yet all coexisted at equilibrium near P-T_max conditions.