Lithospheric control on late Cenozoic magmatism at the boundary of the Tuva-Mongolian Massif, Khubsugul area, northern Mongolia

Abstract

Late Cenozoic lavas from the western wall of the Khubsugul rift trough were erupted within the Tuva-Mongolian Massif with a pre-Vendian basement, and the lavas in the eastern wall of the trough were erupted within Early Caledonian terranes. The composition of the lavas was determined to vary across the strike of the boundary of the Tuva-Mongolian Massif. The western wall of the trough is dominated by hawaiites and contains subordinate volumes of basanites and much lower amounts of olivine tholeiites and basaltic tra- chyandesites. The eastern wall contains, in addition to hawaiites, widespread olivine tholeiites and basaltic andesites with subordinate amounts of basaltic trachyandesites. The boundary zone contains practically all rock types (except basaltic andesites) in roughly equal proportions. The trace-element simulations of the partial melt- ing processes demonstrates that the basaltic magmas were produced mainly by 0.5-5% partial melting of garnet lherzolite, with the probable mixing with partial melts derived from spinel lherzolite. The main factor control- ling the compositional variations of the lavas was likely the variable depths of their derivation due to variations in the lithosphere thickness at the boundary of the Tuva-Mongolian Massif. Based on the assumption that the source of the magmas was relatively homogeneous and on the results of simulations with the use of experimen- tal data on peridotite melting, we concluded that the asthenospheric sources of the basaltic magmas occurred at depths of 75 ± 10 km ( 24.6 ± 3.2 kbar) beneath the Tuva-Mongolian Massif and at 60 ± 12 km (20.1 ± 3.8 kbar) beneath the Early Caledonian terranes.