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°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°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 myulonitization, 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°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°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 re-equilibration 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.