The Late Carboniferous–Early Permian Ocean‐Continent Transition in the West Junggar, Central Asian Orogenic Belt: Constraints from Columnar Jointed Rhyolite

Abstract

The West Junggar of the western Central Asian Orogenic Belt is one of the typical regions in the term of ocean subduction, contraction and continental growth in the Late Paleozoic. However, it is still controversial on the exact time of ocean‐continent transition so far. This study investigates rhyolites with columnar joint in the West Junggar for the first time. Based on zircon U‐Pb dating, we determined that the ages of the newly‐discovered rhyolites are between 303.6 and 294.5 Ma, belonging to Late Carboniferous–Early Permian, which is the oldest rhyolite with columnar joint preserved in the world at present. Geochemical results show that the characteristics of the major element compositions include a high content of SiO2 (75.78–79.20 wt%) and a moderate content of Al2O3 (12.21–13.19 wt%). The total alkali content (K2O + Na2O) is 6.14–8.05 wt%, among which K2O is 2.09–4.72 wt% and the rate of K2O/Na2O is 0.38–3.05. Over‐based minerals such as Ne, Lc, and Ac do not appear. The contents of TiO2 (0.09–0.24 wt%), CaO (0.15–0.99 wt%) and MgO (0.06–0.18 wt%) are low. A/CNK=0.91–1.68, A/NK=1.06–1.76, and as such, these are associated with the quasi‐aluminum‐weak peraluminous high potassium calc‐alkaline and some calc‐alkaline magma series. These rhyolites show a significant negative Eu anomaly with relative enrichment of LREE and LILE (Rb, Ba, Th, U, K) and depletion of Sr, HREE and HFSE (Nb, Ta, Ti, P). These rhyolites also have the characteristics of an A2‐type granite, similar to the Miaoergou batholith, which indicates they both were affected by post‐orogenic extension. Combining petrological, zircon U‐Pb dating and geochemical characteristics of the rhyolites, we conclude that the specific time of ocean‐continent transition of the West Junggar is the Late Carboniferous–Early Permian.