Petrogenesis of Basalts from Changbai Mts, NE China, and its Geodynamic Implications

M. Zhang, S.Y. O'Reilly, GEMOC Macquarie and M. Thirlwall (Geology, Royal Holloway University of London, UK)

Basaltic volcanism in the Changbai Mts region, at the northeastern margin of the Sino-Korean Craton, can be divided into 2 major episodes: 21-15 Ma and 3.0-1.5 Ma. MgO contents of the basalts range 10.4-1.4 wt\% (Mg\#=0.72-0.25), accompanied by wide variations in SiO2 (48.85-55.3 wt\%) and alkaline elements (Na2O+ K2O=3.9-9.3 wt\%). The oldest Changbai basalts belong to the SiO2-undersaturated alkaline basalts, whereas the younger ones are SiO2-saturated tholeiites. Most of the alkaline basalts are enriched in Rb, K and/or Sr, but depleted in Th and U in their incompatible element signatures. The tholeiites, in contrast, exhibit remarkable depletion of Rb relative to Ba, Th, U, and Nb depletion relative to K, La and Ce depletion relative to Sr, with moderate depletion of Zr and Hf relative to Sm. In terms of radiogenic isotopic ratios, the former have lower 87Sr/86Sr (0.70436-0.70472) but higher 143Nd/144Nd (0.51273-0.51276) than the latter (0.70475-0.70514 and 0.51252-0.51264 respectively). Despite the wide variations in elemental chemistry (eg 87Rb/86Sr=0.025-0.31 for the alkaline basalts), Nd isotope ratios are spectacularly homogeneous, especially for the alkaline basalts. 206Pb/204Pb ratios are 17.68-18.06 for the alkaline basalts and 17.28-17.48 for the tholeiites. All the samples bear a strong Dupal signature (delta 7/4Pb=5.5-15.2, delta 8/4Pb=73-118). In addition to possible crustal contamination, geochemical data require at least two mantle sources: an EM1-type lithospheric mantle and a sublithospheric mantle (more likely an Indian-type asthenosphere, related to the opening of the Japan Sea, than a mantle plume). Temporal chemical variations of the basalts show a gradual increase with time in the EM1-type lithospheric source although EM2-type geochemical signatures may have been added to the lithospheric mantle via either accretion processes at the craton margin since Mesoproterozoic or subduction processes of the Pacific plate shortly before the Japan Sea opening.

Back to the GEMOC Abstract Titles Page