PRESSURE-TEMPERATURE AND FLUID EVOLUTION OF QUARTZO-FELDSPATHIC METAMORPHIC ROCKS WITH A RELIC HIGH-PRESSURE, GRANULITE-FACIES HISTORY FROMTHE CENTRAL ERZGEBIRGE (SAXONY, GERMANY)

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 gr anulites, eclogites and garnet peridotites. Four different quartzo-fel dspathic assemblages are recognized, in which maximum conditions of up to 830 degrees C and 21 kbar were determined. The assemblages are cha racterized by the nearly complete prograde breakdown of biotite, by hi gh 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 durin g the high-pressure event itself. After decompression, concomitant wit h 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, defo rmation and re-equilibration at 2-3 kbar and 400-500 degrees C were co ncentrated 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 tem peratures, during which rocks from different depths were amalgamated, and (2) extensive hydration and reequilibration at medium pressures, f ollowed by rapid cooling during continued uplift, when the entire unit came into contact with cooler, now over- and underlying units. This s cenario is attributed to continent collision, orogenic collapse and di sintegration of the HP unit during continuing collision, crustal stack ing and uplift controlled by extension.