FLUID INCLUSIONS IN HIGH-PRESSURE LOW-TEMPERATURE ROCKS FROM THE CALABRIAN ARC (SOUTHERN ITALY) - THE BURIAL AND EXHUMATION HISTORY OF THE SUBDUCTION-RELATED DIAMANTE-TERRANOVA UNIT

The Diamante-Terranova Unit (DIATU), in the Calabrian Arc of southern Italy, is Dart of an ophiolitic sequence involved in a high pressure/l ow temperature event (P=8 kbar; T=400 degrees C) followed by re-equili bration at greenschist facies conditions (P=3 kbar; T=300 degrees C). The rocks contain two types of quartz-calcite veins - an earlier gener ation of deformed, folded and faulted veins formed during or before su bduction, and a later set of planar, undeformed veins formed during ex humation of the DIATU. The earlier folded quartz-calcite veins contain regularly shaped aqueous inclusions as well as inclusions with a high ly irregular dendritic texture. The later planar veins contain only re gularly shaped aqueous inclusions similar to those in the earlier vein s. In both vein types, all inclusions are demonstrably secondary in or igin. Regularly shaped inclusions from both vein types are low salinit y (0-5 wt% NaCl). Most contain liquid and vapour and homogenize to the liquid (Th 135-180 degrees C), whereas others contain only liquid at room temperature. Both the two-phase and monophase inclusions occur in the same fractures and are thought to record the same trapping event, with the monophase inclusions remaining metastable liquid at room tem perature. No microthermometric data could be obtained from the dendrit ic inclusions in the earlier folded veins. Inclusions with the highly irregular dendritic texture found in the earlier veins are similar to those produced experimentally during laboratory-induced deformation of synthetic inclusions in quartz under conditions of internal underpres sure, simulating either isobaric cooling or isothermal compression. Th e occurrence of inclusions with the dendritic texture in the earlier f olded veins, and their absence from the later planar veins, suggests t hat the earlier veins formed before or during subduction and were fold ed and faulted in a compressional environment and their contained flui d inclusions were modified to produce the dendritic texture. During la ter uplift of the DIATU, planar veins containing regularly shaped aque ous inclusions formed and some of the fluids forming these veins were also trapped as secondary inclusions in the earlier folded veins. The results of this study provide convincing evidence that inclusions with a highly irregular dendritic morphology represent early inclusions th at have survived prograde conditions in a high pressure/low temperatur e metamorphic environment (but have been texturally modified). The hig h pressure/low temperature 'implosion' texture is preserved over geolo gical time, even after being overprinted by internal overpressure cond itions generated during retrograde decompression. We suggest that incl usions that have survived prograde metamorphism are common in high pre ssure/low temperature rocks, but are often not identified as such due to their morphology which makes their recognition difficult.