Dehydration of metapelites during high-P metamorphism: Thecoupling between fluid sources and fluid sinks

Polymetamorphic metapelites and embedded eclogites share a complex, episodic interplay of dehydration and fluid infiltration at the eclogite type-locality (Saualpe-Koralpe, Eastern Alps, Austria). The metapelites inherited a fluid content (i.e. mineral-bound OH expressed in terms of mol.% H2O) of approximate to 6-7mol.% H2O from high-T-low-P metamorphism experienced during the Permian. At or near P-max of the subsequent Eoalpine event (approximate to 20kbar and 680(degrees)C), the breakdown of paragonite to Na-rich clinopyroxene and kyanite in metapelites released a discrete pulse of hydrous fluid. Prior to the dehydration event, the rocks were largely fluid absent, allowing only limited re-equilibration during the prograde Eoalpine evolution. Similarly, Permian-aged gabbros have persisted metastably due to the absence of a catalyst prior to fluid-induced re-equilibration. The fluid triggered partial to complete eclogitization along a fluid infiltration front partially preserved in metagabbro. Near-isothermal decompression to approximate to 7.5-10kbar and 670-690 degrees C took place under fluid-absent conditions. After decompression, a second breakdown of phengitic white mica and garnet produced muscovite, biotite, plagioclase and approximate to 0.1-0.7mol.% H2O that enhanced extensive fluid-aided re-equilibration of the metapelites. Potential relicts of high-P assemblages werelargely obliterated and replaced by the recurrent amphibolite facies assemblage garnet+biotite+staurolite+kyanite+muscovite+plagioclase+ilmenite+quartz. The hydrous fluid originating from the metapelites infiltrated the embedded eclogites at these P-T conditions and induced the local breakdown of the peak assemblage omphacite and garnet to fine-grained symplectites of diopside and plagioclase. Further fluid infiltration led to the formation of hornblende-quartz poikiloblasts at the expense of the symplectites. The metapelites re-equilibrated until the growth of retrograde staurolite consumed any remaining free fluid, thereby terminating the process. Further re-equilibration is inhibited by both the lack of a catalytic fluid and H2O as a reactant essential for rehydration reactions. The interplay between fluid sources and fluid sinks describes a closed cycle for the rocks at the eclogite type-locality. Final, near-isobaric cooling is indicated by a slight increase of X-Fe in garnet rims. Post-decompression dehydration and fluid-aided re-equilibration arrested by the introduction of staurolite might explain the apparently homogeneous retrogression conditions as well as the notorious absence of diagnostic high-P assemblages in metapelites at the eclogite type-locality.