New lead isotope data on Cretaceous volcanic rocks of Mongolia: the sources and the origin of the magmatic melts

The Eastern Mongolia Volcanic Area (EMVA) and the Gobi-Altai Volcanic Area (GAVA) are large parts of the Late Mesozoic volcanic-plutonic belt which is located in northeast Asia. The main value of the EMVA and the GAVA was formed during the Cretaceous. Previous research devoted to Cretaceous volcanic rocks of both volcanic areas has focused mainly on its geochemical features of main and trace components, and Nd – Sr isotope composition (Bars et al., 2018; Dash et al., 2015; Sheldrick et al., 2018; Sheldrick et al., 2020). At the same time, the published data on the Pb isotope composition of volcanic rocks of the EMVA and the GAVA is too scarce (Sheldrick et al., 2018; Sheldrick et al., 2020). However, the Pb isotope characteristic can be a key to the understanding of parent melts sources of the EMVA and the GAVA rocks.  Therefore, the goal of the presented work is a more extensive study of the Pb isotope systematics of the Cretaceous volcanic complexes within the EMVA and the GAVA.

Obtained data on Pb isotope characteristics of the EMVA volcanic rocks demonstrate the role of the upper crust terrigenous component (UCC) in magma generation. The role of the UCC in the EMVA formation is consistent with the Nd – Sr isotope composition and elevated LILE contents in rock samples. In contrast to the EMVA the Pb isotope features of the same aged GAVA rocks (135 – 120 Ma) with the enriched Nd – Sr composition point to the role of the lower crust component in their formation. Thus, there is a difference between the sources of the coeval rocks of two volcanic areas reflecting the difference in the melts source composition between the two areas.

The Late Cretaceous rocks of the GAVA (about 90 Ma), as well as the Early Cretaceous rocks of the EMVA, lie nearby a field of lithospheric mantle xenoliths on the Pb isotope ratios diagram. In turn, the obtained Pb isotope data on the lherzolite xenoliths as well as that on paleooceanic complexes of Mongolia reveal the obvious difference of Pb isotope composition of the lithospheric mantle of the region from that of the Paleo-Asian ocean mantle. The observed difference can be explained by the metasomatic alteration of the suboceanic mantle during accretion and subduction processes before the EMVA and the GAVA formation. Thus, the conclusion about the key role of the metasomatized lithospheric mantle in the GAVA Late Cretaceous rocks formation can be made.

The study was supported by the RFBR (20-05-00401).

KEYWORDS: Eastern Mongolia, Gobi-Altai, Cretaceous volcanic rocks, lead isotope composition.