Geochemical characteristics of the Late Carboniferous bimodal volcanic rocks in Jijitaizi area, eastern Bogda orogenic belt, and their geological significance

Abstract: The genesis of the Late Carboniferous bimodal volcanic rocks in Jijitaizi area of eastern Bogda orogenic belt was revealed recently, which comprise mainly sub-alkaline basalts and subordinately rhyolite rocks, belonging mainly to the tholeiitic series. The basalts have high Al (Al2O3=19.81%-20.60%), low Mg (MgO=2.95%-3.25%, Mg#=32.33-34.50), Na2O/K2O ratios of 2.84-3.01 and TiO2 values of 1.32%-1.43%, slightly higher than values of N-MORB, indicating obvious fractional crystallization of olivene and pyroxene during basaltic magmatism and an evolution product of original magma’s fractional crystallization. They have the following characteristics: Zr/Nb=41.13-41.38, Zr/Y=4.26-4.52 (all higher than 4.00), Rb/Sr=0.02-0.04 (all less than 0.05), Dy/Yb=1.92-2.00, La/Nb=2.42-2.49, Ba/Nb=113.58-126.05, Ba/La=47.02-50.62, slight enrichment of trace elements Rb, Ba and P, slight positive Eu anomalies (δEu=1.02-1.10), and relative depletion of Nb, Ta, Th, Sr and Ti. In addition, the Zr-Nb, La/Yb-Dy/Yb and La/Nb-La/Ba discrimination diagrams indicate that the basalts had geochemical features indicative of intraplate basalts formed within the continent, originated from high degree partial melting of the transition product from depleted spinel phase mantle peridotite to garnet phase mantle peridotite, and suffered a certain degree of crustal contamination during the rising process. The rhyolites of bimodal volcanic rocks have high SiO2 (76.54%-77.74%) and ALK (Na2O+K2O=6.70%-7.70%), but low TiO2 (0.15%-0.16%), Al2O3 (10.70%-10.90%) and MgO (0.17%-0.18%, Mg#=13.93-14.52). Their trace elements generally have features of enrichment of Rb, Th and U, obvious depletion of Sr, P and Ti, and slight depletion of Ta and Nb; the REE distribution patterns are characterized by LREE enrichment, right-oblique form and obvious depletion of Eu (δEu=0.32-0.33), suggesting that the rocks originated from the melting of crustal material with plagioclase as an important relic mineral in the source. Geochemical characteristics of the bimodal volcanic rocks in Jijitaizi area indicate that the volcanic rocks were formed in a typical tectonic setting of continental rift. The rhyolites with LA-ICP-MS zircon U-Pb age of (312±1) Ma suggest that this series of volcanic rocks were formed at the beginning of Late Carboniferous. According to these characteristics in combination with the Rb-Sr and Zr-Zr/Y diagrams, the crust thickness (20.0-30.0 kilometers) and extension speed (2.0 cm/a-5.0 cm/a) might suggest the fast development of Bogda rift, which stared in the Early Carboniferous and reached its culmination at the beginning of Late Carboniferous, and also represent the transitional period of geodynamic environment from collision and compression to stretching and extension. The results achieved by the authors further confirm the view that the Carboniferous Bogda orogenic belt belongs to continental rift and it could provide evidence for understanding the tectonic evolution and plate system of the Bogda orogenic belt of the Late Paleozoic.