Ocean-continent transition process in the West Junggar tectonic belt:Constraints from the basalts of the Baiyanghe Formation
West Junggar structural zone is located in the southwestern part of the Central Asian Orogenic Belt and is considered a key area for studying the evolution of the Paleo-Asian Ocean.However,there are currently controversies regarding the tectonic evolution of the West Junggar tectonic zone during the late Carboniferous period.The recently discovered volcanic rocks from the Lower Permian Baiyanghe Formation have the potential to address these controversies.These volcanic rocks mainly consist of basalts and associated subvolcanic rocks.They display an angular unconformity relationship with the Lower Permian Jiamuhe Formation,which represents a continental molasse sequence.Geochemically,the basalts exhibit relatively high concentrations of Al2O3 and low concentrations of TiO2,placing them within the middle-low K calc-alkaline series.The ratio of ∑LREE/∑HREE ranges from 2.31 to 5.00.All the basalts from the Baiyanghe Formation are enriched in LREE without obvious Eu anomaly(δEu=0.99～1.29).Furthermore,they are enriched in light LILE,such as Ba,K,and Sr.In addition,they are depleted in HFSE such as Nb,Ta,Zr,and Hf,exhibiting a negative"TNT"anomaly.Isotopic composition shows(208Pb/204Pb)s=17.924～18.000,(20-Pb/204Pb)i=15.469～15.509,(208 Pb/204 Pb)i=37.737～37.879.The eNd(t)values range from+5.93 to+6.76,and the 87Sr/86Sr ratios range from 0.703974 to 0.704299.These characteristics suggest that the source of these basalts is probably the dehydration of a subducted plate to produce the fluid that results in a metasomatically depleted mantle.These geochemical characteristics are consistent with those of Late Carboniferous marine island arc basalts,indicating temporal and spatial correlation and suggesting excellent inheritance.The Baiyanghe Formation is later in age than the Jiamuhe Formation molasse,which confirms that the Baiyanghe Formation basalts were erupted after the closure of the ocean basin,so it belongs to the"lagged arc"formation.Considering previous research and the regional geological background,it can be inferred that the ocean basin closed in the Late Carboniferous while deep subduction continued,eventually leading to the formation of the Baiyanghe Formation.This unique tectonic evolution process of the"lagged arc"occurred during the ocean-continent transition.The volcanic rocks associated with the"lagged arc"formation provide crucial constraints for determining the suture location of the Paleo-Asian Ocean and understanding the tectonic evolution process in West Junggar tectonic zone.
万方数据
维普
