Transition from oceanic subduction to continental collision: Insights from volcanogenic-sedimentary rocks of the Tannuola terrane (northern Central Asian Orogenic Belt)

Magmatism associated with oceanic subduction plays a dominant role in crustal growth during the Earth’s evolution. The Tannuola terrane, situated in the northern Central Asian Orogenic Belt (CAOB), is a key area to understanding oceanic subduction and initial collision processes in the northern CAOB. In order to investigate the switch from subduction to collision settings, detailed field mapping, zircon SHRIMP U-Pb geochronological and whole-rock geochemical studies of volcanogenic-sedimentary rocks from the Tannuola terrane were carried out. Zircon U-Pb ages indicate multi-stage volcanism lasted at least 30 Ma from ~540 to ~510 Ma, that can be divided into three events: the late Ediacaran (before ~540 Ma), the early Cambrian (~520 Ma) and the middle Cambrian (~510 Ma). These ages are interpreted to the initial, main and final stages of oceanic subduction during the late Proterozoic – early Paleozoic. During the late Ediacaran, tholeiitic basalts with high εNd(t) values (from +7.4 to +8.5) were emplaced. Likely forming by the 10 %–30 % partial melting of spinel – garnet mantle source during slab subduction. During the early Cambrian, transitional from tholeiitic to calc-alkaline basaltic rocks with εNd(t) value (+5.6) and coeval intermediate–felsic volcanic rocks with similar εNd(t) values (+5.9 and +6.5) formed. The early Cambrian basaltic rocks are interpreted to be derived by 10 %–30 % partial melting of a depleted mantle source metasomatized by slab-derived fluids released from the subducting oceanic slab. The middle Cambrian calc-alkaline basaltic rocks with εNd(t) value of +6.2 might be emplaced as a result of low (5 %–10 %) degree partial melting of a metasomatized mantle followed by fractional crystallization of clinopyroxene and plagioclase. Associated intermediate-felsic volcanic rocks with εNd(t) values from +6.0 to +6.8 were formed through fractionation of the juvenile Neoproterozoic sources. The middle Cambrian volcanism is interpreted to be triggered by the slab break-off during the transition to a collisional setting.