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

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 epsilon(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 Ar-40/Ar-39 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 Ar-40/Ar-39 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. (C) 2002 Elsevier Science B.V All rights reserved.