Pressure-Temperature and Fluid Evolution of Quartzo-Feldspathic Metamorphic Rocks with a Relic High-Pressure, Granulite-Facies History from the Central Erzgebirge (Saxony, Germany)

Abstract

The Gneiss-Eclogite Unit is a composite tectonometamorphic unit within the Variscan Erzgebirge mega-antiform. It comprises migmatitic para- and orthogneisses, high-temperature (HT) mylonites, kyanite-bearing granulites, eclogites and garnet peridotites. Four different quartzo-feldspathic assemblages are recognized, in which maximum conditions of up to 830 degrees C and 21 kbar were determined. The assemblages are characterized by the nearly complete prograde breakdown of biotite, by high grossular content (23-47 mol %) of garnet in the presence of albite, and high Si contents of phengite [3.3-3.4 per formula unit (p.f.u.)]. Water activities at this stage are variable and range from <0.15 to >0.4. The maximum pressures indicated for individual rock volumes may vary considerably between 12 and 24 kbar at 700-800 degrees C, so that non-coherency of the entire Gneiss-Eclogite Unit appears likely during the high-pressure event itself. After decompression, concomitant with penetrative HT mylonitization, hydration led to overprinting of the rocks to variable degrees, owing to channelized fluid influx. Partial equilibration at medium-pressure conditions of about 7-10 kbar and 600-700 degrees C occurred, involving abundant retrograde migmatization. The water activity increased to 0.5-1.0. During later exhumation, deformation and re-equilibration at 2-3 kbar and 400-500 degrees C were concentrated in local, discrete, ductile normal fault zones. The kinked geometry of the PT path is thus characterized by (1) high-pressure (HP) equilibration, followed by near-isothermal decompression at high temperatures, during which rocks from different depths were amalgamated, and (2) extensive hydration and reequilibration at medium pressures, followed by rapid cooling during continued uplift, when the entire unit came into contact with cooler, now over- and underlying units. This scenario is attributed to continent collision, orogenic collapse and disintegration of the HP unit during continuing collision, crustal stacking and uplift controlled by extension.