Paired Magmatic Belts and Multiple Orogenesis: a Case Study of the Central China Orogen

Many orogens on Earth, which are developed in long-lived (>300 Ma) mobile belts, have undergone multiple orogenesis. Hyndman et al. (2005) proposed that these mobile belts had been evolved from backarcs, where extensional rifting had resulted in the thinning of lithosphere and thermal convection, which in turn led to the heating of backarcs. When continents collided, deformation took place exactly in the hot and softened backarc regions. Orogenic heat responsible for magmatic intrusion, high grade metamorphism and ductile deformation was largely derived from the early-formed hot backarc lithosphere rather than the deformation of the orogens themselves. The searching for the ¡°paired magmatic belts¡± is critical for the understanding of the mechanism and processes of multiple orogenesis. The so-called paired magmatic belts are termed as two parallelly distributed and similarly aged magmatic belts that are formed in association with island arc-continental arc and backarc extension. While there is common occurrence of ¡°paired magmatic belts¡± in orogens around the world, the Paleozoic ¡°paired magmatic belts¡± of the Central China Orogen including the Qinling-Dabie Orogen has provided us with a typical case. Within the northern portion of the Central China Orogen, i.e., the Beihuaiyang- North Qinling-Qilian ranges, Early Paleozoic intrusions are widely distributed. They are largely dated as 460- 430Ma. The rock types range from mafic to acidic intrusions, in which gabbros, (quartze) diorite and granite dominates. Petrographic and geochemical features suggest that the magmatic belt is related to subduction of the Paleo-Qinling Ocean Plate. In the southern margin of the Central China Orogen, Mid-Late Paleozoic mafic dyke swarms and bimodal alkaline dyke swarms are well developed. These rocks are mostly dated as 440-390 Ma. This magmatic belt stretches westward from Suizhou in the southern piedmont of Dabieshan to the southern margin of the Qaidam block and corresponds to the back-arc extension resulted from the subduction in the north. The bimodal alkaline rocks are enriched in large ion lithophile elements and high field strength elements, sharing features with rifting-related bimodal suit. The Suizhou alkaline granite in the southern piedmont of Dabieshan is a representative of these extension-related plutons. The two Paleozoic magmatic belts of the Central China Orogen are of distinct nature but similarly aged and parallelly distributed, suggesting that they were formed in the same dynamic system. Here we envisage a model to explain the occurrence of the Paleozoic paired magmatic belts. In the Ordovician-Silurian period, the subduciton of the Paleo-Qinling Oceanic plate underneath the Yangtze craton have resulted in roughly simultaneous back-arc extension in the southern margin of the Dabie-Qinling. These processes led to the formation of the paired magmatic belts in association with subduciton and backarc extension, respectively. As an Early Paleozoic backarc, the Mianlue-Suizhou belt in South Qinling-South Dabie continued extension in the Late Paleozoic, leading to the heating, thinning and softening of lithosphere and even the local occurrence of oceanic crust and subduction. These facilitate the collisional orogenesis, crustal deformaiton and deep subduction along the Mianlue-Suizhou belt in the Early Mesozoic, and are critical to the composite orogenesis of the Central China Orogen.