Formation of HP–LT rocks and their tectonic implications in the western Tianshan Orogen, NW China: geochemical and age constraints

Abstract

The Western Tianshan high-pressure/low-temperature metamorphic belt in northwestern China formed within a Paleozoic accretionary wedge on the south side of the Yili–Central Tianshan plate. It comprises various lithologies, including mafic metavolcanic rocks, metavolcaniclastic rocks, metagreywacke, marble and serpentinite. The metavolcanics comprise eclogite, omphacitite, epidosite and blueschist. The trace and rare-earth element characteristics of these rocks indicate MORB to OIB affinities suggesting a seamount-like setting for the protoliths of these rocks. The ε Nd value of the eclogite (+8.9) indicates that the protolith was derived from a long-term depleted mantle, i.e. an N-MORB source, supporting the trace and rare-earth element characteristics of these samples. Sm–Nd isochron dating of 343±44 Ma (Omp–Gln–Grt–WR) and an age of 346±3 Ma (Grt–Gln) were determined using an eclogite boudin within blueschist. A well-defined 40Ar/ 39Ar plateau age of 344±1 Ma was obtained for crossite from an omphacite–phengite-bearing blueschist and is concordant with the Sm–Nd isochron age for the eclogite, suggesting that neither argon loss nor excess argon affected the crossite. A well-defined 40Ar/ 39Ar plateau age of 331±2 Ma was determined for phengite from the same sample. The high-pressure metamorphic rocks were formed by the B-type subduction of a Paleozoic ‘South Tianshan Ocean’ about 344 Ma ago. They were, however, exhumed to higher crustal levels during the collision of the Tarim and Yili–Central Tianshan plates about 331 Ma ago, similar to an ‘Alpine-type’ tectonic regime.