Petrogeodynamics of HP-LT rocks : state of the art and application to processes along the subduction
Résumé
Based on examples from the Betics, Oman and the W. Alps, this contribution attempts to (1) critically evaluate the precision with which metamorphic P-T-t histories are retrieved at present and (2) discuss the implications on our understanding of orogenic processes, focusing on those processes taking place along the subduction channel. Thanks to improved interconsistent thermodynamic databases, multiple thermodynamic softwares and analytical tools, numerous quantitative constraints on metamorphic histories are now accessible. We herein assess the merits and shortcomings of our present knowledge. In particular, we stress that, despite impressive improvements, parts of the P-T-time history still remain systematically ill-constrained. Metamorphic histories do not provide a quite continuous record either, and whether mineral reequilibrations are discrete or continuous remains fondamentally ambiguous. We then focus on the early stages of orogenic build-up, namely on processes related with oceanic and continental subduction. After briefly reviewing our current knowledge of metamorphic histories worlwide, we focus on the example of the W. Alps. We present the results of the comparison between the metamorphic evolution of the Schistes Lustrés paleoaccretionary complex and the evolution of some of the major ophiolitic bodies of the W. Alps exhumed from the Alpine subduction (Zermatt-Saas, Avic, Rocciavre, Monviso). We show how this allows placing constraints on the nature and characteristics of the plate interface (eg, the so-called 'subduction channel') and reconstructing the geodynamic processes at work in a subduction zone. We particularly discuss the dimensions of the bodies sliced up and stacked in the subduction channel, the depths and possible mechanisms at which this happens, and the role of fluid transfer. Along strike variations of the exhumation processes within the same subduction zone are also emphasized. In the light of these findings, we finally address other related issues: insights from thermomechanical models combined with petrological techniques, timing of mineral reequilibration with respect to deformation (and implications for rheology), fluid behaviour and kinetic processes.