Anatexis during High-pressure Crustal Metamorphism: Evidence from Garnet–Whole-rock REE Relationships and Zircon–Rutile Ti–Zr Thermometry in Leucogranulites from the Bohemian Massif

Abstract

In situ mineral (garnet, zircon) trace element and garnet–rock REE distribution data obtained on leucogranulites from the Bohemian Massif are consistent with an origin of these now strongly deformed rocks as migmatitic leucogranites formed through dehydration-melting of muscovite-bearing protoliths at high pressures during the Variscan orogeny. Partial melting at P–T conditions of 900–940°C and 1·6–1·8 GPa consumed mica and feldspar to produce peritectic kyanite and high-Ca (20–24 mol % grossular), high-Eu garnet along with leucogranitic melt. DREE(Grt/rock) values calculated from this garnet and its host rock are consistent with garnet–melt equilibrium at these P–T conditions. Ternary feldspar and rutile (2000–3200 ppm Zr) crystallized from the leucogranite melts in the migmatites at minimum temperatures of >880–920°C. These formed along with apatite prior to and accompanying the crystallization of garnet rims and the majority of the garnet that occurs in the leucogranulites. This garnet is typified by strong depletion in Eu and DREE(Grt/rock) values consistent with garnet–melt equilibrium only after the fractionation of apatite (<0·3 wt %) and ternary feldspar (5–15 wt %). Zircons in one leucogranulite are dominated by oscillatory-zoned grains that yield pre-Variscan U–Pb ages (495 and 433 Ma) and crystallized, from Ti-in-zircon thermometry, at 700–810°C in their original host magmas. These xenocrystic grains are potentially preserved because of the short timespan of the high P–T Variscan event at 340 Ma and the moderate amounts of leucogranitic partial melt present in the rocks, estimated as at least 30 wt % melt from Zr mass balance. The production of new zircon associated with the Variscan event in this leucogranulite is limited to weakly zoned zircon rims characterized by flat heavy REE patterns that approach REE equilibrium with garnet rims. This post-peak zircon was formed at minimum P–T conditions of 840°C and 0·8–1·0 GPa along the decompression-cooling path defined by previous work for the Bohemian Massif granulites. The garnet–whole-rock REE relationships, observation of Eu depletion between early and later garnet growth phases, and Zr–Ti thermometric estimates indicating temperatures of >840°C for this evolution cannot be explained by the alternative interpretation of the leucogranulites as reworked and recrystallized pre-Variscan low-P granitoids, but support a model in which the Bohemian leucogranulites were high-P Variscan-age migmatites containing significant amounts of melt.