Fluid inclusion evidence for progressive folding during decompression in metasediments of the Voltri Group (Western Alps, Italy)

Quartz veins syntectonic to distinct folding events in metasediments from the Voltri Group (Ligurian Alps) were studied in order to compare fluid and structural evolution. Studied veins (VSI, VS2, VS3) pertain to three distinct generations of folds (F-1, F-2, F-3) that formed during the retrograde metamorphic evolution. Two types of fluids characterize the different generations of veins and are represented essentially by aqueo-carbonic mixtures of moderate salinity with decreasing densities (1.01-0.41 g/cm(3)). The chemical evolution is characterised by a progressive decrease of H2O, from early fluids associated with opening of VS1 and VS2 (XCO(2)approximate to 0.08) to fluids related to VS3 formation (XCO(2)approximate to 0.3). The close match between the fluids in VS1 and VS3 suggests that the development of two superimposed systems of folds (F-1 and F-2 folds) occurs under very similar P-T conditions, during a progressive and continuous deformational event at glaucophanic and/or barroisitic metamorphic grade. A different evolution is outlined for the formation of VS3 during low green schist grade. Successive isochores allow us to define a retrograde decompression path for the Voltri Group. Present results indicate that fluid inclusions are powerful markers to constrain the P-T-I conditions of different folding events. (C) 1998 Elsevier Science Ltd. All rights reserved.