The coherent ultrahigh-pressure terrane of the Tianshan meta - ophiolite belt, NW China

Abstract

Blocks of basic rocks metamorphosed at high- and ultrahigh-pressures (HP-UHP) are often embedded in metasedimentary or granitic rocks that do not show evidence for HP metamorphism. The contrasting maximum depth reached by these isolated rock fragments, normally eclogites, and the enveloping rocks is commonly seen as evidence for a form of tectonic mélange. In the Tianshan meta-ophiolite belt of NW China, some reported eclogites and pelitic-felsic schists adjacent to coesite-bearing rocks apparently lack evidence of UHP conditions. Their deduced maximum pressures are 1.5–2.5 GPa. These HP blocks have been regarded as dismembered ophiolite fragments each with its own metamorphic history generated by a forced return flow in a subduction channel. Here we reappraise the P-T estimates, formerly derived by various P-T tools, of these reported HP rocks, including pelitic schists, mafic eclogites and eclogitic rocks of volcaniclastic origin, from the Akeyazi area. P-T pseudosections in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O contoured for garnet core or mantle composition (thermocalc version 3.33) have been used for deducing peak P-T conditions. It was found that all investigated samples were subject to UHP metamorphism (2.8–3.5 GPa at 520–580 °C). The used thermodynamic dataset for these P-T estimates has also been testified valid for typical UHP-LT rocks worldwide. We suggest that garnet isopleth thermobarometry-derived P-T estimates are potentially robust in accordance with UHP mineralogical evidence such as coesite, but are sensitive to garnet solution models and effective bulk compositions. Inconsistent P-T results and related tools involving sodic clinopyroxene and phengite should be dismissed in many cases, if not all. The consequences of the reevaluated P-T data are that eclogites, both interlayered and blocky, and the associated host rocks comprise a coherent Tianshan UHP terrane, not a highly disrupted mélange.