ABSTRACT

Late Mesozoic lithospheric extension in NE China resulted in the development of a large extensional province and widespread formation of volcanic-sedimentary basins. The Songliao Basin is the largest one and contains voluminous volcanic rocks composing the majority of the basin fill. This study focuses on two adjacent eruption cycles, the Huoshiling Formation (K₁h) and the first member of the Yingcheng Formation (K₁yc¹), in the Changling fault depression of the Songliao basin, constrains their time ranges and compares the origins of volcanic rocks. Zircon populations from K₁h yield U-Pb ages of 124–118 Ma; those from K₁yc¹ yield younger ones (115–106 Ma). Different volcanic rocks are identified in K₁h (alkaline basic rocks, calc-alkaline intermediate-acid rocks) and K₁yc¹ (alkaline basic rocks, A-type rhyolites), and their geochemistry and isotope characteristics confirm their different origins. The alkaline basic rocks from K₁h have the similar magma source as those from K₁yc¹; both formed under within-plate conditions from an ocean island basalt (OIB)-type mantle source, assumed to be the heterogeneous shallow asthenosphere. The calc-alkaline intermediate-acid volcanic rocks from K₁h formed by fractional crystallization of magma, related to the enriched lithospheric mantle, whereas the A-type rhyolites from K₁yc¹ directly originated from the partial melting of crust newly formed from the depleted mantle. The development of alkaline basic rocks and A-type acid rocks, a typical bimodal volcanic assemblage in K₁yc¹, requires a relatively thin lithosphere and thus implies that lithospheric thinning reached its maximum during this time (115–106 Ma). However, the rare expression of alkaline basic rocks and the formation of acid rocks mainly by fractional crystallization in K₁h indicate that the lithosphere was relatively thick; during this time (124–118 Ma), the lithosphere below the Songliao Basin had just begun thinning. These different stages of lithospheric extension may be due to the intermittent rollback of the Palaeo-Pacific Plate.