Formation and evolution of high-pressure leucogranulites: Experimental constraints and unresolved issues

Abstract

High-pressure (HP) leucogranulites of the Bohemian Massif are interpreted as the metamorphosed equivalents of HP leucogranites produced by deep crustal melting. This is supported by their preserved mineral assemblages (Grt-Ky-mesoperthite), bulk rock chemistry, P-T estimates, and garnet and accessory phase trace element abundances. Following melting and peak metamorphism, the leucogranulites have been exhumed from lower crustal depths to their present position at the highest structural level of the Gföhl Nappe. The nearisothermal decompression (ITD) P-T path and available geochronological data imply high exhumation rates.The dry character of the leucogranulites reflects the water-undersaturated conditions that prevailed during formation of the precursor leucogranitic melts and their subsequent recrystallization in the middle and lower crust. Compositions of the leucogranulites are displaced towards the Qz-Or join in the Qz-Ab-Or ternary diagram, which corresponds to experimental results for water undersaturated melting. Trace element and REE abundances in whole rocks, garnets and accessory phases are consistent with muscovite and biotite dehydration melting coupled with K-feldspar fractionation or separation as the principal controls on the chemical evolution of the rocks. The melting reactions and protoliths potentially involved in the generation of these HP leucogranite melts are evaluated in the light of available experimental data for water-saturated and dry melting of crustal rocks.