Geodynamic evolution of the south Tianshan orogen: Geochronological and geochemical constraints from granitic and mafic dikes

Abstract

Giant mafic and granitic dike swarms can provide invaluable insight into the paleo-geographic reconstruction of supercontinents and the identification of mantle plumes. Although some case studies of orogeny-related dikes have been undertaken, rare systematic information is available concerning the relationship between the formation mechanism of the dikes and orogenesis. In this study, we conducted a detailed geological, geochronological and geochemical study on recently identified granitic and mafic dikes close to the Wuwamen ophiolite complex in the Chinese South Tianshan, aiming to reveal the link between the formation of the dikes and the orogenic evolution. A dike swarm comprising 456 Ma biotite granite dikes, 423 Ma alkali feldspar granite dikes and a 301 Ma diabase dike crosscuts the Xieliwancaikai gneissic diorite with a zircon UPb age of 475 Ma. In addition, a 301 Ma granite porphyry dike extends across the middle Devonian meta-sedimentary strata in the Tianqiao area. The geochemical characteristics of the biotite granite and the alkali feldspar granite dikes and their gneissic diorite host suggest that the granitic magma was produced by lower crustal melting. However, the granite porphyry dike has chemical characteristics resembling those of S-type granites commonly produced by partial melting of overthickened crust in collisional orogenic belts. Furthermore, the mafic magma source of the diabase dike resulted from low-degree partial melting of a fertilized sub-continental lithospheric mantle. In conjunction with previously published data on the Wuwamen ophiolite Complex, we propose that the 456 Ma metaluminous granite dike reflects the subduction onset of the South Tianshan Ocean, while the 423 Ma metaluminous granite dike represents the felsic arc melts generated during the subsequent subduction. The 321–313 Ma peraluminous mica granite dikes are thought to have formed during the collision between the Tarim and Kazakhstan–Yili–Central Tianshan blocks. In addition, the peraluminous granite, the strongly peraluminous granite porphyry and the diabase dikes that formed between 307 and 301 Ma were generated in a post-collisional extensional environment. Therefore, bimodal dike swarms provide crucial constraints on the geodynamic evolution of orogens.