Petrogenesis of the Cenozoic volcanic rocks in Baekdu volcano in northeastern Asia and the expected depth of the magma chamber based on geochemistry, mineral chemistry, and Sr-Nd-Pb isotope chemistry

摘要

Baekdu volcano is a Cenozoic composite volcano in northeastern Asia that consists of a basaltic plateau, a basaltic shield, and a trachytic to comenditic stratovolcano. In this study, the petrogenesis and tectonic setting of Baekdu volcano are studied based on the whole-rock geochemistry and Sr-Nd-Pb isotope compositions of the Baekdu volcanic rocks. The K?Ar age and mineral chemistry are also investigated to estimate the evolution of the magma chamber depth beneath Baekdu volcano. The Toudao and Baishan basalts formed ca. 2.5 Ma and 1.1 Ma and erupted from magma chambers located at depths of 11.5 +/- 2.2 km and 24.9 +/- 3.8 km, respectively. The Xiaobaishan and Middle trachytes and the comendites formed during the Pleistocene from magma chambers located at depths of 2.9 +/- 0.2 km, 5.4 +/- 0.45 km, and 4.4 +/- 0.3 km, respectively. These data suggest that the trachytes and comendites erupted from magma chambers located at depths of approximately 3?5 km. The Baishan basalts have geochemical characteristics comparable with those of ocean island basalt (OIB), whereas the Toudao basalts show enriched mid-ocean ridge basalt (E-MORB) or OIB characteristics. The trends of several components, such as the CaO/Al2O3 ratio and the TiO2 and Sc concentrations, are different between older basalts (>2 Ma, including the Toudao basalts) and younger basalts (<1.5 Ma, including the Baishan basalts), indicating that the magma-forming process changed ca. 2-1.5 Ma. The enrichment in Th/Yb ratios in the Toudao basalts indicates that they experienced crustal contamination. Trachytes and comendites are considered to be products of the fractional crystallization of the Baishan basalt based on the similar geochemical trends of the two rocks. Crustal assimilation (up to 20%) also occurred with fractional crystallization (more than 70%) during the formation of peralkaline silicic volcanic rocks. All volcanic rocks in Baekdu volcano have Sr-Nd-Pb isotope compositions comparable with the enriched mantle (EM1)-like signature. This EM1-like signature may have originated from the contamination of asthenospheric mantle with Paleoproterozoic lithosphere that sank into the asthenospheric mantle due to delamination caused by regional extensional events in northeastern Asia between the Cretaceous and the Cenozoic. The characteristics of trace element compositions indicate a within-plate tectonic setting for Baekdu volcano. These data imply that Baekdu volcano origin may have been related to a plume of EM1-type upper mantle under the North China Craton. (C) 2021 Elsevier B.V. All rights reserved.