Indosinian tectonic setting, magmatism and metallogenesis in Qinling Orogen, central China

Abstract：The Qinling Orogen in central China has been a long-term geologicalyl studied focus for its unique tectonic location, complicated geological history and abundant mineral resources. In previous studies geologists have clarified its tectonic framework, outlined its geological evolution, and reached a consensus that the final transition from marine basin to intracontinental mountains occurred in Indosinian (Triassic：251~199.6 Ma). However, the details of basin-to-mountain transition process, the age of final oceanic closure, the nature of Triassic tectonic setting, and the associated magmatism and mineralization in the area have been poorly constrained and hotly debated. Based on a comprehensive re-assessment of results from geological, geophysical, geochemical and ore deposit studies, the author compares the Triassic Qinling with present Mediterranean Sea, which contemporaneously accommodates oceanic slab subduction and intercontinental collision as well as gradual transition from oceanic subduction to intercontinental collision. The Qinling paleo-Tethys finally closed in a westward zipper-like way during the period of 230~200 Ma, and instantly followed by intercontinental collision between the Yangtze block and Qinling-North China united continents. Therefore, the Triassic tectonic setting in Qinling was neither a simple intercontinental collision, nor a post-orogenic or post-collisional regime. Indosinian magmatism was intensive in Qinling, forming igneous rocks including at least the adakites, calc-alkaline granitoids, high-K calc-alkaline granitoids, alkaline intrusions, Rapakivi-like granites, and carbonatites. These rocks show zoning spatial distribution, namely, northward from the Mian-Lue suture, the Yangshan-Yanzhiba peraluminous S-type granite belt, southern Qinling high-Mg and adakitic calc-alkaline granite belt, northern Qinling high-K calc-alkaline granite belt, and the carbonatite-alkaline intrusion belt at the southern margin of North China craton. Such Indosinian magmatites with distinctive petrologic complexity, lithologic diversity, spatial zonation and compositional polarity cannot be formed in syn- and/or post-collisional tectonic settings, but could have resulted from a northward paleo-Tethysan slab subduction along the Mian-Lue geosuture. Although the Indosinan mineralization has been ignored for a long time, a great number of Triassic ore deposits of economic significance have been recently discovered, including Molybdenum-containing carbonatite dykes, porphyries and orogenic-type quartz veins; orogenic-type, porphyry-/breccia pipe-type, and Carlin-type and Carlin-like gold deposits; and orogenic-type silver-dominant poly-metal deposits. This shows that mineralization of various genetic types strongly occurred in the transition regime from oceanic slab subduction to intercontinental collision, and that the Indosinian ore deposits are of exploration potential in Qinling Orogen.