Subduction-controlled temporal and spatial variations in early Palaeozoic sedimentary and volcanic record of the Mongol-Altai Domain

Abstract

The early Palaeozoic volcano–sedimentary evolution of the Khovd and Gobi-Altai zones of the Mongol–Altai Domain was investigated by quantitative petrographic analysis of sediments, whole-rock geochemistry of volcanic rocks, and U–Pb zircon dating. Three evolutionary stages were identified in the Khovd Zone: (1) Cambrian–mid Ordovician basaltic (Nb/Yb = 0.09–1.25) and rhyodacitic (Nb/Yb = 4.36) volcanism and sedimentation of immature greywackes in the fore-arc basin in front of the juvenile continental arc, (2) late Ordovician flysch-type sedimentation accompanied by a volcanic gap coincided with Barrovian metamorphism in footwall sequences reflecting a compressional stage, and (3) Silurian–Devonian within-plate bimodal volcanism (with a strong asthenospheric imprint; Nb/Yb = 4.45–20.34) followed by mature siliciclastic deposition interpreted as transformation of the former fore-arc region into a back-arc domain associated with HT metamorphism. In the Gobi-Altai Zone the three stages were: (1) Cambro–Ordovician marginal greywacke–sandstone succession adjacent to a mature continental arc (mafic rocks Nb/Yb = 0.71–4.36), (2) Silurian siliciclastic sedimentation associated with migration of the volcanic gap and Barrovian metamorphism, and (3) Lower Devonian deep-marine sedimentation followed by shallow-marine carbonate sedimentation, initially accompanied by calc-alkaline basalts (Nb/Yb = 0.74–4.57) and andesites to rhyolites (Nb/Yb = 0.69–4.87), evolving in the mid–late Devonian to within-plate alkali basalts (Nb/Yb = 11.05) thus reflecting back-arc basin development. These temporal and spatial variations in the Mongol-Altai Domain are interpreted as a diachronous sequence of retreating and advancing subduction modes instead of juxtaposition of suspect terranes as proposed in previous studies.