Architecture and P-T-deformation-time evolution of the Chinese SW-Tianshan HP/UHP complex: Implications for subduction dynamics

Abstract

We present the first comprehensive P-T-deformation-time and kinematic constraints for HP/UHP eclogite, blueschist and greenschist-facies metavolcano-sedimentary rocks cropping out in the Chinese SW-Tianshan metamorphic complex (within the ~30\u202fkm wide N-S Akeyazi-Kebuerte area). We reappraise this HP/UHP “melange”, which should be divided into three main tectonic units, from north to south, according to their discrepant lithologies and P-T-time-deformation histories. These three units crop out in a tectonic window beneath greenschist-facies metavolcanics. P-T estimates point to 1) UHP-LT conditions around 2.6–2.9\u202fGPa at ~520\u202f°C for the metavolcano-sedimentary rocks of the northern HP/UHP unit, 2) HP-LT conditions of 1.8–2.1\u202fGPa and ~500\u202f°C for the central blueschist horizon and 3) lower-blueschist facies conditions of 1.0–1.5\u202fGPa at ~485\u202f°C for the southern ultramafic-mafic unit. In situ laser probe Ar-Ar age constraints (with textural control) on recrystallized phengites from the HP/UHP unit cluster within 315 to 325\u202fMa. Phengite ages from the central blueschist horizon are 10–15\u202fMa older, at ~325–345\u202fMa. Laser-ICP-MS U-Pb dating on zircon from the south ultramafic-mafic unit yield ages around ~360\u202fMa for metamorphic overgrowths. In contrast, step-heating Ar-Ar phengite age constraints for the greenschist-facies metavolcano-sediments fall within 280–300\u202fMa. These new field and P-T-time data disclose an episodic exhumation of three main tectonic slices, respectively from ~85\u202fkm, ~65\u202fkm and ~45\u202fkm depths, and a progressive change of metamorphic gradients from ~12 to ~6–7\u202f°C/km, which could reflect a cooling of the subduction system with time. Final juxtaposition at ~20\u202fkm was probably achieved around 300\u202fMa, prior to collision.