Oceanic crust components in continental basalts from Shuangliao, Northeast China: Derived from the mantle transition zone?

Abstract

Basaltic rocks from Shuangliao, northeast China include basanite, alkali olivine basalt, transitional basalt and sub-alkaline diabase. Ar–Ar dating shows that the basanites and alkali olivine basalts formed earlier (48.5–51Ma) than the transitional basalts and diabases (43–41.6Ma). These rocks have the highest Fe2O3 contents (13.4–14.6wt.%) and lowest (87Sr/86Sr)i ratios (<0.703) among the Cenozoic basalts from eastern China. On a primitive-mantle normalized variation diagram, they show positive Eu, Sr, Nb and Ta anomalies, and depletion in very incompatible elements (Rb, Ba, Th, U), reminiscent of HIMU-type oceanic island basalts. In particular, the basanites possess noticeable negative K anomalies. Nevertheless, their Pb isotopic compositions (206Pb/204Pb=18.13–18.34) do not show the high time-integrated 238U/204Pb mantle component expected for a HIMU basalt. On a 206Pb/204Pb versus 207Pb/204Pb diagram, most samples straddle the Northern Hemisphere Reference Line (NHRL), in salient contrast to the majority of Chinese Cenozoic basalts, which plot above the NHRL. These data, as well as a comparison with high-pressure experimental melts, are consistent with the presence of young subducted oceanic crust (SOC) in the source of Shuangliao basalts. Varying (87Sr/86Sr)i, La/Nb and Eu/Eu* with rock-type suggests that the upper oceanic crust (with variable amount of lower oceanic crust) was preferentially sampled by earlier (51–48Ma), highly alkaline rocks, whereas the lower oceanic crust was predominantly sampled in later (41–43Ma) transitional basalts and diabases. This temporal trend is attributed to the differential melting of a heterogeneous source in association with lithospheric thinning, during which fusible upper oceanic crust melted earlier than lower oceanic crust and peridotites. We postulate that the SOC components may have been derived from the seismically detected stagnant Pacific slab within the mantle transition zone. This hypothesis is supported by the same Indian MORB-like isotopic composition being found in the Shuangliao basalts and in the extinct Izanaghi–Pacific plate of NW Pacific. The latter has been subducting underneath the eastern Asian continent since the early Cretaceous.