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Dating specific pressure–temperature–time–deformation–fluid (P–T–t–d–f) events is a major petrological issue, particularly for polymetamorphic assemblages. In order to better assess such events using mica populations, this study uses a combined in situ Rb–Sr and Ar–Ar dating approach coupled with chemical mapping, with application to an exhumed subduction complex (Schistes Lustrés, Western Alps). Geochronological investigation of the most Si-rich (Tschermak substitution) mica population allows us to investigate the (near-)peak burial ages of the various tectonometamorphic units of this high-pressure/low-temperature subduction complex. In the blueschist-facies units, a wide range of ages between ∼36 (younger than previously obtained peak ages) and ~ 52 Ma suggests a diachronous slicing of units with different initial paleogeography and/or a long residence time at near-peak conditions. The combined use of Ar–Ar and Rb–Sr isochron ages reveals the existence of some excess argon in the studied metamorphic rocks, interpreted as the result of (i) heterogeneous contamination from an argon-rich fluid in the higher grade eclogite-facies units and (ii) partial removal, during pseudomorphic recrystallization, of argon inherited from earlier stages of metamorphism in the lower grade blueschist-facies units. [ABSTRACT FROM AUTHOR]

Abstract: Since the Mesozoic, Africa has been under extension with shorter periods of compression associated with obduction of ophiolites on its northern margin. Less frequent than 'normal' subduction, obduction is [...]

The spatiotemporal scales at which fluid flow events occur along the subduction interface remain poorly constrained. This study illustrates the benefit of using in‐situ Rb/Sr dating on metasomatic white mica to constrain the timing of metasomatic events, taking advantage of the well‐studied reaction zones of the Kampos‐Lia unit of Syros, Greece. This unit is a subducted block‐in‐matrix structure corresponding to a preserved fragment of a discontinuous, slow‐spreading oceanic domain. Results reveal that the main metasomatic event took place at ~36 Ma, coincident with the transition from blueschist to greenschist facies and syn‐ to post‐orogenic exhumation in the Cyclades marking the end of a first exhumation stage along the subduction interface. Results highlight that fluid–rock interaction along the subduction interface cannot be treated as the result of continuous fluid influx, but rather reflect punctuated, heterogeneously distributed events (both in time and space) tied to specific tectonometamorphic episodes. [ABSTRACT FROM AUTHOR]

The northeastern continental margin of Oman in the Saih Hatat region is characterized by high-pressure (HP) chloritoid- or carpholite-bearing metasediments and highly deformed mafic eclogites and blueschists in a series of tectonic units bounded by high-strain ductile shear zones. New data on the upper cover units of this HP nappe stack indicate that all of them underwent similar P conditions to the underlying Hulw structural unit (with a cooler exhumation pressure-temperature path). Early SSW directed crustal thickening during ophiolite emplacement created recumbent folds and strong schistose fabrics in these Permian-Mesozoic shelf carbonates and was followed by later NNE dipping normal sense shear zones and normal faults. The Mayh unit shows high strain in a 15-25 km long sheath fold that likely formed at carpholite grade pressures of 8-10 kbar. We show that there are no significant P differences across the Hulw shear zone (upper plate--lower plate discontinuity) or between the overlying Mayh, Yenkit-Yiti, and Ruwi units. Postpeak metamorphic exhumation of the HP rocks was therefore accomplished by bottom-to-SSW (rather than top-to-NNE) active footwall extrusion beneath a fixed, static, passive hanging wall. Footwall uplift beneath these passive roof faults resulted in progressive expulsion of the HP rocks from depths of ~80-90 km (eclogites) and mainly 30-35 km (blueschists and chloritoid-/ carpholite-bearing units) during the Campanian-Early Maastrichtian. Oman thus provides a detailed record of how continental material (thick platform shelf carbonates) progressively jammed a subduction zone and emphasizes the contrasting behavior between cover units and their underlying basement. doi: 10.1029/2010TC002669.