A prolonged subduction-accretion in the southern Central Asian Orogenic Belt: Insights from anatomy and tectonic affinity for the Beishan complex

Abstract

The Beishan complex, located along the Liuyuan suture in the southern Beishan, separates the Dunhuang terrane from the southern Central Asian Orogenic Belt. The ages and origins of the Beishan complex are still controversial. Based on integrated field investigation and petrological, geochronological, and geochemical work, the Beishan complex is interpreted to consist of four diachronous lithologic units with different tectonic origins. Unit I is mainly composed of meta-clastic and minor carbonate rocks; these metaclastic rocks were derived from a proximal source with dominantly intermediate–acidic magmatic materials, and were deposited in a continental arc setting during 430–280 Ma. Unit II exhibits a block-in-matrix structure with variably sized blocks of ultramafic rock, meta-gabbro, Nb-enriched pillow meta-basalt, amphibolite, calc-alkaline mylonitized diorite, marble, argillaceous slate, and quartzitic meta-chert embedded in a matrix of sheared fine-grained sandstone; this tectonic mélange was emplaced during 284–267 Ma. Unit III consists of coherent quartzite, paragneiss, and biotite schist with maximum depositional ages of 1214 Ma, 817 Ma, and 1210 Ma, respectively. Unit IV was formed in a continental arc setting, and predominantly contains early Paleozoic diorite (430 Ma), TTG (455–437 Ma), and tonalite (430–429 Ma), as well as biotite–quartz schist (507–420 Ma). Consequently, we propose that the Beishan complex represents an assemblage of a continental arc and a fore-arc accretionary complex resulting from subduction of the Paleo-Asian oceanic slab. Our results indicate long-lived subduction–accretion of the Paleo-Asian Ocean and the final closure may have occurred posterior to the middle Permian.