PACROFI VI - Electronic Program

Remnants of subduction-related fluid inclusions in quartz-calcite veins from high pressure/low temperature rocks from the oceanic-derived Diamante-Terranova unit (the Calabrian Arc, Southern Italy)

Chiara Invernizzi*, Maxim O. Vityk**, Robert J. Bodnar**

* Dip. Scienze della Terra, Universita' di Camerino, Camerino, Italy 62032

** Fluids Research Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA USA 24061

The Diamante-Terranova Unit (DIATU), in the Calabrian Arc of southern Italy, is part of an ophiolitic sequence which was involved in a high pressure/low temperature (HP/LT) event (P = 8 kb; T = 400oC) followed by reequilibration at greenschist facies conditions (P = 3 kb; T = 300oC). In this study we present data on microthermometric and textural analysis of fluid inclusions found in quartz-calcite veins from metabasalts in the DIATU. Most of the inclusions are clearly secondary (i.e. related to healed fractures), and a large percentage of them (about 40%) were modified after entrapment. The modified inclusions contain water and display a highly irregular dendritic texture. The dendritic texture is similar to that produced experimentally during laboratory-induced deformation of synthetic aqueous inclusions in quartz. In experimental studies, this texture was obtained only under conditions of internal underpressure, simulating either isobaric cooling or isothermal compression. In both the isobaric cooling and compression experiments, the dendritic texture was generated at high confining P (about 5 kbar) and low T (300 - 400oC). Internal underpressure in inclusions in quartz from the DIATU was generated at high P and low T during nearly isothermal compression associated with subduction-accretion processes.

Quartz samples from the DIATU also contain a large number of low salinity (0 - 5 wt. % NaCl) inclusions which display a rounded and/or negative-crystal habit. Some of these inclusions contain liquid and vapor and homogenize to the liquid (Th 135-180oC), whereas others contain only liquid at room temperature. Both the two and one-phase inclusions occur in the same fractures, and were presumably trapped at greenschist facies conditions during the exhumation of the HP/LT rocks. The one-phase inclusions and the two-phase inclusions are thought to record the same trapping event, with the one-phase inclusions remaining metastable liquid at room temperature.

Microstructural analysis and fluid inclusion studies allowed us to reconstruct the P-T conditions of deformation for the Diamante-Terranova rocks. The results of this study provide convincing evidence that inclusions with a highly irregular dendritic morphology represent early inclusions that have survived prograde conditions in a HP/LT metamorphic environment (but have been modified). The HP/LT "implosion" texture is apparently preserved over geological time, even after being overprinted by internal overpressure conditions generated during retrograde decompression. We suggest that inclusions that have survived prograde metamorphism are common in HP/LT rocks, but often not identified as such due to their morphology which makes their recognition difficult.