Convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel. Studies of high-pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to mid-crustal levels, but the details of such dynamics are controversial. To understand the dynamics of a subduction channel we report the results of a petrochronological study from the central Sesia Zone, a key element of the internal Western Alps. This comprises two polymetamorphic basement complexes (Eclogitic Micaschist Complex and Gneiss Minuti Complex) and a thin, dismembered cover sequence (Scalaro Unit) associated with pre-Alpine metagabbros and metasediments (Bonze Unit). Structurally controlled samples from three of these units (Eclogitic Micaschist Complex and Scalaro-Bonze Units) yield unequivocal petrological and geochronological evidence of two distinct high-pressure stages. Ages (U-Th-Pb) of growth zones in accessory allanite and zircon, combined with inclusion and textural relationships, can be tied to the multi-stage evolution of single samples. Two independent tectono-metamorphic 'slices' showing a coherent metamorphic evolution during a given time interval have been recognized: the Fondo slice (which includes Scalaro and Bonze rocks) and the Druer slice (belonging to the Eclogitic Micaschist Complex). The new data indicate separate stages of deformation at eclogite-facies conditions for each recognized independent kilometer-sized tectono-metamorphic slice, between ~85 and 60 Ma, with evidence of intermittent decompression (ΔP ~ 0.5 GPa) within only the Fondo slice. The evolution path of the Druer slice indicates a different P-T-time evolution with prolonged eclogite-facies metamorphism between ~85 and 75 Ma. Our approach, combining structural, petrological and geochronological techniques, yields field-based constraints on the duration and rates of dynamics within a subduction channel.
|Titolo:||Multiple Metamorphic Stages within an Eclogite-facies Terrane (Sesia Zone, Western Alps) Revealed by Th–U–Pb Petrochronology|
|Parole Chiave:||Pressure cycles; Sesia zone; Subduction channel; Tectonometamorphic slice; Yo-yo subduction|
|Settore Scientifico Disciplinare:||Settore GEO/03 - Geologia Strutturale|
Settore GEO/07 - Petrologia e Petrografia
|Data di pubblicazione:||2014|
|Digital Object Identifier (DOI):||10.1093/petrology/egu029|
|Appare nelle tipologie:||01 - Articolo su periodico|