Tectonic mélange records the Silurian–Devonian subduction-metamorphic process of the southern Dunhuang terrane, southernmost Central Asian Orogenic Belt

Abstract

The Hongliuxia tectonic melange of the southern Dunhuang terrane, northwestern China, southernmost Central Asian Orogenic Belt (CAOB), consists of eclogite, mafic granulite, and amphibolite as puddingstones within a matrix of metapelitic gneiss and marble; these rocks are interpreted to be part of an ancient subduction zone setting. Secondary ion mass spectrometry U-Pb dating of metamorphic zircons obtained from the puddingstones and matrix metapelite suggests that the metamorphism occurred at ca. 428–391 Ma. The metamorphic rocks all record similar clockwise metamorphic pressure-temperature-time ( P - T - t ) paths of the western Alpine type. However, remarkable differences between metamorphic peak P - T conditions ranging from 830 °C and 24.2 kbar for the eclogite puddingstone to 700 °C and 10.2 kbar for the metapelite matrix were found in the melange rocks. This indicates the mixing of rocks from significantly different depths to create a tectonic melange in a subduction channel, possibly juxtaposed during the uplift stage. These data suggest that the southernmost CAOB underwent subduction and subsequent exhumation caused by subduction of the Paleozoic Hongliuxia ocean during the middle Silurian to middle Devonian.