PARAGONITE-HORNBLENDE ASSEMBLAGES AND THEIR PETROLOGICAL SIGNIFICANCE- AN EXAMPLE FROM THE AUSTROALPINE SCHNEEBERG COMPLEX, SOUTHERN TYROL, ITALY

Paragonite-bearing amphibolites occur interbedded with a garbenschist- micaschist sequence in the Austroalpine Schneeberg Complex, southern T yrol. The mineral assemblage mainly comprises paragonite + Mg-hornblen de/tschermakite + quartz + plagioclase + biotite + ankerite + Ti-phase +/- garnet +/- muscovite. Equilibrium P-T conditions for this assembl age are 550-600 degrees C and 8-10 kbar estimated from garnet-amphibol e-plagioclase-ilmenite-rutile and Si contents of phengitic muscovites. In the vicinity of amphibole, paragonite is replaced by symplectitic chlorite + plagioclase + margarite +/- biotite assemblages. Muscovite in the vicinity of amphibole reacts to form plagioclase + biotite + ma rgarite symplectites. The reaction of white mica + hornblende is the r esult of decompression during uplift of the Schneeberg Complex. The br eakdown of paragonite + hornblende is a water-consuming reaction and t herefore it is controlled by the availability of fluid on the retrogre ssive P-T path. Paragonite + hornblende is a high-temperature equivale nt of the common blueschist-assemblage paragonite + glaucophane in Ca- bearing systems and represents restricted P-T conditions just below om phacite stability in a mafic bulk system. While paragonite + glaucopha ne breakdown to chlorite + albite marks the blueschist/greenschist tra nsition, the paragonite + hornblende breakdown observed in Schneeberg Complex rocks is indicative of a transition from epidote-amphibolite f acies to greenschist facies conditions at a Batter P-T gradient of the metamorphic path compared to subduction-zone environments. Ar/Ar dati ng of paragonite yields an age of 84.5 +/- 1Ma, corroborating an Eoalp ine high-pressure metamorphic event within the Austroalpine unit west of the Tauern Window. Eclogites that occur in the Otztal Crystalline B asement south of the Schneeberg Complex are thought to be associated w ith this Eoalpine metamorphic event.