Eclogite-facies quartz veins within metabasites of the Dabie Shan (easternChina): pressure-temperature-time-deformation path, composition of the fluid phase and fluid flow during exhumation of high-pressure rocks

Metabasites in the high-pressure unit of the southern Dabie Shan (eastern C hina) contain quartz veins with high-pressure mineral assemblages. Two vein s with the parageneses quartz-paragonite-ankerite-rutile and quartzkyanite- talc-zoisite-rutile-calcite (pseudomorph after aragonite) as well as quartz -kyanite-paragonite-garnet-omphacite were investigated in detail. Host rock s of these veins are garnet amphibolites with eclogitic relies and quartz e clogites, respectively. The oldest phase of deformation (D1) is recorded in the eclogite, which displays a schistosity with shape align ment of high-p ressure minerals such as omphacite, phengite and garnet. An estimate of the metamorphic P-T conditions yields 19-21 kbar at 570-620 degreesC for the e clogites. Fluid inclusion studies reveal a primary, low-salinity aqueous fl uid phase responsible for the formation of the quartz veins. This is in cor respondence with calculations of phase equilibrium curves on minerals of th e vein paragenesis, which yield P-T conditions of 19.4 kbar and 591 degrees C in the presence of an aqueous fluid phase. Geochronology using the U-Pb s ystem of rutile gives evidence for a Triassic minimum age of 207- 221 Ma fo r the vein formation, which fits in the geotectonic framework of the orogen . This rather wide age range arises from the heterogeneous nature of the in itial lead isotopic composition, reflecting the heterogeneous sources of th e lead and the fluid phase. Part of the fluid was probably generated by pro grade dehydration reactions in the country rocks of the veins, whereas stro ng evidence is also given for the derivation from an older basement, which also underwent subduction. The veins may have formed by extensive hydraulic fracturing or by volume reduction during eclogitisation of the crust. Subs equent to the formation of the veins, great parts of the high-pressure rock s experienced an intensive, syn-tectonic (D2) amphibolite facies metamorphi sm at 8.5-10.5 kbar and 590-645 degreesC, indicating isothermal decompressi on during exhumation. In the course of this exhumation, pseudosecondary CO2 inclusions were trapped in quartz of the veins. The last recorded stage of the retrograde overprint took place under static, greenschist facies condi tions (450-480 degreesC) in the stability field of kyanite. The shape of th e metamorphic P-T path for this stage is reflected by the fluid phase, whic h either re-equilibrated during this event or penetrated the rock on fractu res. These findings show that fluid flow during subduction-related high-pre ssure metamorphism and subsequent uplift is rather complex; the fluid phase may originate from different sources and may interact with country rocks i n the form of a chromatographic column transforming mineral assemblages, an d also changing its own character.