Intra-continental back-arc basin inversion and Late Carboniferous magmatism in Eastern Tianshan,NW China:Constraints from the Shaquanzi magmatic suite

摘要

The Yamansu belt,an important tectonic component of Eastern Tianshan Mountains,of the Central Asian Orogenic Belt,NW China hosts many Fe-(Cu)deposit.In this study,we present new zircon U-Pb geochronology and geochemical data of the volcanic rocks of Shaquanzi Formation and diorite intrusions in the Yamansu belt.The Shaquanzi Formation comprises mainly basalt,andesite/andesitic tuff,rhyolite and sub-volcanic diabase with local diorite intrusions.The volcanic rocks and diorites contain ca.315-305 Ma and ca.298 Ma zircons respectively.These rocks show calc-alkaline affinity with enrichment in large-ion lithophile elements(LILEs),light rare-earth elements(LREEs),and depletion in high field strength elements(HFSEs)in primitive mantle normalized multi-element diagrams,which resemble typical back-arc basin rocks.They show depleted mantle signature with ε_(Nd)(t)ranging from+3.1 to +5.6 for basa+2.1 to+4.7 for andesite;-0.2 to+1.5 for rhyolite and the ε_(Hf)(t)ranges from-0.1 to +13.0 for andesites;+5.8 to +10.7 for andesitic tuffs.We suggest that the Shaquanzi Formation basalt might have originated from a depleted,metasomatized lithospheric mantle source mixed with minor(3-5%)subduction-derived materials,whereas the andesite and rhyolite could be fractional crystallization products of the basaltic magma.The Shaquanzi Formation volcanic rocks could have formed in an intracontinental back-arc basin setting,probably via the southward subduction of the Kangguer Ocean beneath the Middle Tianshan Massif.The Yamansu mineralization belt might have undergone a continental arc to back-arc basin transition during the Late Carboniferous and the intra-continental back-arc basin might have closed in the Early Permian,marked by the emplacement of dioritic magma in the Shaquanzi belt.