Transforming mylonitic metagranite by open-system interactions during melt flow

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Abstract

Gneisses and migmatites of the Gföhl unit (Moldanubian Zone, Bohemian Massif) range from banded mylonitic orthogneiss with recrystallized monomineralic bands, through stromatic (metatexite) and schlieren (inhomogeneous diatexite) migmatite, to isotropic nebulite (homogeneous diatexite). This sequence was classically attributed to increasing degree of anatexis. Under the microscope, the evolution is characterized by progressive destruction of the monomineralic banding that characterizes the original mylonitic orthogneiss. Throughout, the mineral assemblage is biotite–K-feldspar–plagioclase–quartz ± garnet ± sillimanite, but the mineral compositions exhibit systematic changes with progressive disintegration of the layering. From banded orthogneiss to nebulite, the garnet composition changes systematically, Alm75→94Prp17→0.8Grs2.5→1.2Sps2→11 and XFe = 0.45→0.99 and for biotite, XFe = 0.80→1. This is consistent with a decrease in equilibration temperature and pressure of 790 °C and 8.5–6 kbar, to 690 °C and 5–4 kbar respectively. There is also a systematic change of whole-rock composition, marked by an increase in SiO2 (71→77 wt%) and XFe (0.62→0.85) and by a decrease in Al2O3 (16→13 wt%) and CaO (1.50→0.43 wt%). Assuming that the rocks started with the same composition, these systematic changes indicate open-system behaviour. The predicted consequences of various open-system processes are assessed using thermodynamic modelling. The observed variations are interpreted as being a consequence of melt flow through, and interaction with the rocks, and, to change the rock composition sufficiently, a large volume of melt must have been involved.

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