Geochemical Characteristics and its Significance of the Early Permian Bimodal Volcanic Rocks in Jijitaizi Area, East of the Bogda Orogenic Belt

The early Permian Bogda orogenic basalts and acidic volcanic rocks, developed in the Jijitaizi area of the eastern section of Bogda orogenic belt, are bimodal volcanic rocks, which are composed by sub-alkaline basalt and rhyolite, mainly belonging to the calc-alkaline series. The basalts have high values of Al(Al₂O₃=14.06%-20.93%), Na₂O/K₂O(2.34-28.36) and TiO₂(1.26%-1.52%), which are slightly higher than the ones of N-MORB, and they have low contents of Mg(MgO=2.99%-8.62%, Mg^#=27-52). These major element characteristics indicate that these basalts had experienced an obvious fractional crystallization of olive and pyroxene during basaltic magmatism, which were the evolution products of original magma's fractional crystallization. The basalts are slightly enriched in trace elements(such as Rb, Ba and P), with no Eu anormalies(δEu=0.85-1.02), but are relatively depleted in Nb, Ta, Sr and Ti. These trace element characteristics of volcanic rocks suggest that,these basalts were originated from the partial melting of the depleted lithospheric peridotite mantle and had suffered a certain degree of crustal contamination, showing the geochemical features of intraplate basalts formed within the intracontinental extensional environment. The rhyolites of bimodal volcanic rocks have high values of SiO₂(73.58%-75.45%) and ALK(Na₂O+K₂O=8.56%-8.79%), but low contents of TiO₂(0.12%-0.18%), Al₂O₃(12.77%-13.24%) and MgO(average about 0.21%).Their trace elements generally have the enrichment features of Rb, Th and Ba, obvious depletion of Nb, Ta, Sr, P and Ti, and the REE distribution patterns are characterized by LREE enrichment, right-deviation type and obvious depletion of Eu(δEu=0.52-0.70). The REE characteristics indicate that, the rhyolites have similar geochemical characteristics of A-type granite, suggesting that the rhyolites were originated from the fractionation of underplating basaltic magma. The geochemical characteristic of the bimodal volcanic rocks in Jijitaizi area of the eastern section of Bogda orogenic belt indicate that,these volcanic rocks were formed in a post-collisional extensional environment.The LA-ICP-MS zircon U-Pb ages of rhyolites are(294.2±1.3) Ma, suggesting that this set of volcanic rocks was formed in the beginning of early Permian. Our study of the bimodal volcanic rocks in Jijitaizi area indicate that, the research area had witnessed the transitional period of geodynamic environment from Carboniferous collision and compression to Permian stretching and extension, and it could provide an evidence for understanding the tectonic evolution and plate system of the Bogda orogenic belt of the late Paleozoic.