Shear Zone-hosted Migmatites (Eastern India): the Role of Dynamic Melting in the Generation of REE-depleted Felsic Melts, and Implications for Disequilibrium Melting

Abstract

In the Ranmal migmatite complex, non-anatectic foliated granite protoliths can be traced to polyphase migmatites. Structural^microtextural relations and thermobarometry indicate that syn-deformational segregation^crystallization of in situ stromatic and diatexite leucosomes occurred at 8008C and 8 kbar. The protolith, the neosome, and the mesosome comprise quartz, K-feldspar, plagioclase, hornblende, biotite, sphene, apatite, zircon, and ilmenite, but the modal mineralogy differs widely. The protolith composition is straddled by element abundances in the leucosome and the mesosome. The leucosomes are characterized by lower CaO, FeOþMgO, mg-number, TiO2, P2O5, Rb, Zr and total rare earth elements (REE), and higher SiO2, K2O, Ba and Sr than the protolith and the mesosome, whereas Na2O and Al2O3 abundances are similar. The protolith and the mesosome have negative Eu anomalies, but protolith-normalized abundances of REE-depleted leucosomes show positive Eu anomalies.The congruent melting reaction for leucosome production is inferred to be 0 325 quartzþ 0 288 K-feldsparþ 0 32 plagioclaseþ 0 05 biotiteþ 0 014 hornblendeþ 0 001 apatiteþ 0 001 zirconþ 0 002 sphene1⁄4melt. Based on the reaction, large ion lithophile element, REE and Zr abundances in model melts computed using dynamic melting approached the measured element abundances in leucosomes for40 5 mass fraction of unsegregated melts within the mesosome. Disequilibrium-accommodated dynamic melting and equilibrium crystallization of melts led to uniform plagioclase composition in migmatites and REE depletion in leucosome.