Phase equilibria and oxygen isotopes in the evolution of metapelitic migmatites: a case study from the Pre-Alpine basement of Northern Switzerland

Abstract

Metapelites that have undergone variable degrees of migmatization were encountered in two deep boreholes in Northern Switzerland. Microtextures and calculated phase diagrams yield three successive stages of metamorphism, consistent with one P-T loop: (1) Amphibolite-grade metamorphism (biotite-sillimanite-garnetoligoclase-quartz-ilmenite), P-T conditions are estimated at 4.5–6.5 kbars/550–650 °C. (2) Anatectic migmatization melting reactions. Leucosomes do not represent pure crystallized melt but rather a mixture of melt, cordierite and K-feldspar grown at the expense of biotite and sillimanite. (3) Retrograde hydration, recorded by the growth of large amounts of muscovite at the expense of cordierite and K-feldspar, P-T conditions are estimated at <2.5 kbars/ <600 °C. Oxygen isotope measurements were obtained for whole-rocks and eight mineral species. Isotopic equilibrium among three or even four minerals grown during stage 1 can be demonstrated. Calculated isotopic temperatures are consistent with phase petrology. Migmatization did not re-equilibrate the isotopic composition of the pre-existing quartz grown in stage 1 even over distances as little as 10 cm. High δ18O in migmatitic quartz is best explained by disequilibrium melting in coexistence with an infiltrating, isotopically heavy fluid. Lack of equilibration of oxygen isotopes between different quartz generations suggests that mineralogical and geochemical changes were rapid relative to diffusion rates. A meteoric-hydrothermal alteration at 300–400 °C, probably genetically linked to the intrusion of Variscan granites, strongly affected the rocks. Quartz did not exchange isotopes with hydrothermal fluids except in segregations where it is recrystallized. The δ18O values of micas, feldspars and cordierite are often very low and fractionations with quartz very large, which reflects significant hydrothermal effects that were previously detected in the Black Forest by other workers.