Geochemistry and Mineralogy of Two Contrasting Cretaceous Lavas: Implications for Lithospheric Mantle Evolution beneath the Northeastern North China Craton

A combined geochemical and mineralogical study has been conducted on a volcanic sequence at Jianguo, northeastern North China craton (NCC). This sequence contains two contrasting rock types, the lower andesitic lava and the upper basaltic lava. The lower lava is comprised of Cr-spinel (chromite mostly)-bearing andesites, whereas the upper lava consists of titanomagnetite (up to 5965 mol% Usp)-bearing basalts. Ar-40/Ar-39 dating shows that the Jianguo andesites and basalts are 116.8 +/- 3.0 Ma and 105.5 +/- 0.5 Ma, respectively. The basalts have initial Sr-Nd isotopic ratios characteristic of mid-ocean ridge basalt (MORB) and have been interpreted as derived from MORB-type asthenosphere. The andesites, with evolved Sr-Nd isotope features and negative Ta and Nb anomalies, are interpreted to have been ultimately derived from MORB-type asthenosphere accompanied by combined assimilation and fractional crystallization. This suggestion is applicable to the genesis of two other coeval andesites nearby. These geochemical contrasts imply that the lithospheric mantle beneath the eastern part of NCC was considerably thinned by ca. 117 Ma.