Geochemical Characteristics And Tectonic Significance Of Kalatage Volcanic Rocks In East Tianshan

The Kalatag volcanic rocks occur in Daheishan area of the Eastern Tianshan Orogenic Belt,It consist of fresh basalts,andesites,dacites,and rhyolites,expirenced slight deformation and alteration.In this thesis,detailed field observation,petrographic studies,major an trace geochemistry and Sr-Nd isotope anylsis on Kalatage volcanic rocks are carried out.Coupled with previous studies,conclusions can be drawn as follow.(1)Kalatag volcanic rocks hold geochemical signature of island-arc,including LREE-enriched or near-flat REE patterns,enriched LILE like Rb,Ba,Th,U,depleted HFSE like Nb,Ta,Ti.Their low sNd(t)and inherited zircon with age apperantly much older than the magmatic age indicate contamination of continental crust during magma ascending.It is suggested that the Kalatag volcanic rocks formed in the active continental margin,is a serie of island-arc volcanics rocks,expirenced crustal contamination.(2)The mafic to intermediate rocks are generated by partial melting of spinel lherzolite in the mantle wedge which metasomated by slab-derived melt,The intermediate-felsic magmas are generated by crystallization differentiation of primary magma.All of the igneous were experienced variable contamination of continential crust during their ascending to the surface.(3)Formation age of the Kalatag volcanic rocks are confined by a zircon U-Pb age at 347.5±5.9Ma.With a synthesis on regional distribution of igneous rocks,we propose that the Paleo-Tianshan ocean is still not closed in the early Carboniferous,The subduction process may continue to the late Carboniferous.The Kalatag volcanic rocks is part of the Paleozoic Tianshan arc.Our geochemical data provide new evidence for the magmatic evolution of the Tianshan Orogenic Belt.