Paleozoic Sanweishan arc in the northern Dunhuang region, NW China: The Dunhuang block is a Phanerozoic orogen, not a Precambrian block

Abstract

The Dunhuang region is located to the south of the Central Asian Orogenic Belt and connecting the Tarim Craton westerly and North China Craton easterly, respectively. It is an important position for understanding the geodynamics and tectonic framework of Central Asia. However, the issue about whether it is a Phanerozoic orogen or a Precambrian block is still controversial. Magmatic arcs are the most prominent and laterally persistent structural elements of orogenic belts and used to reconstruct now-disrupted orogenic belts. This paper concerns the petrogenesis and tectonic setting of andesitic-dacitic-rhyolitic volcaniclastic rocks and dacitic porphyries in the Sanweishan area, northern Dunhuang region. Volcaniclastic rocks overlie the metasedimentary rocks by fault and dacitic porphyries intrude into the metasedimentary rocks. The calc-alkaline signature and relative enrichment of large-ion lithophile elements of volcaniclastic rocks and dacitic porphyries seemingly favor a subduction setting. Zircon U-Pb dating results reveal the deposition at 424–414 Ma for volcaniclastic rocks and emplacement at ca. 364 Ma for dacitic porphyries. The coeval granitoid plutons in the Sanweishan area show arc geochemical signature and are originated from partial melting of the subducted sediments based on the available data. Paleozoic volcanic-subvolcanic rocks and plutons in this area probably represent a magmatic arc developed on the accretionary complex, resembling the arc magmatic rocks formed in the Nankai complex, SW Japanese Island arc. Our results provide constraints on the existence of the Paleozoic magmatic arc in the northern Dunhuang region and suggest the so-called “Dunhuang block” is virtually a Phanerozoic orogen rather than a Precambrian block.