Geochemistry of olivine-hosted melt inclusions in the Baekdusan (Changbaishan) basalts: Implications for recycling of oceanic crustal materials into the mantle source
Résumé
We determined the major and trace element concentrations of olivine-hosted melt inclusions in basalts from the active Baekdusan volcano situated on the border between China and North Korea in order to understand better the nature of the mantle source and the geodynamic processes that gave rise to volcanism at this site. Rehomogenized melt inclusions (after ‘Fe-loss’ correction) can be divided into three groups: a low-Si alkaline group, a high-Si alkaline group, and a high-Si sub-alkaline group. The low-Si group is composed of picrobasalt to basanite, and the high-Si group consists of (trachy)basalt to basaltic (trachy)andesite. The low-Si group has generally higher TiO2, CaO, and P2O5, but lower Al2O3, Na2O, and K2O contents at a given MgO concentration compared with those in the high-Si group. The CaO and P2O5 contents are positively correlated, indicating the presence of a calcium phosphate, probably tuite [γ-Ca3(PO4)2], in the source. The melt inclusions are enriched in light rare earth elements [(La/Yb)N = 7.8–30.4]. On a primitive-mantle-normalized incompatible element plot, the low-Si group has positive anomalies in Ba and P compared with typical oceanic island basalt (OIB). On the other hand, the high-Si group exhibits remarkable positive anomalies in Eu, Ba, Rb, K, Pb, P, and Ti, implying that K-hollandite and tuite are essential phases in the source. The high-Si subalkaline group has lower abundances of incompatible elements than the high-Si alkaline group, reflecting different degrees of partial melting from the same source. In contrast to OIB, Baekdusan magmatism is characterized by positively fractionated (Zr/Hf)N ratios, and is best approximated by admixture of partial melts derived from both clinopyroxene-rich eclogite and garnet peridotite. Intraplate volcanism in northeastern Asia is closely associated with deep subduction of the Pacific plate. The subducting Pacific slab flattens and stagnates in the mantle transition zone under northeastern Asia, and this zone may yield a wet plume. Focused mantle upwelling through a gap in the stagnant slab may also be accompanied to the plume responsible for the Baekdusan magmatism. Thermal decomposition of K-hollandite within recycled continent-derived sediments is likely to metasomatize the ambient mantle peridotite above the stagnant slab. As the plume ascends through the upper mantle, the metasomatized mantle and recycled oceanic crustal materials (tuite-bearing eclogite or garnet pyroxenite) entrained by the plume may undergo partial melting, resulting in the volcanism observed at Baekdusan.
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