Geochemistry Of Late Mesozoic-Cenozoic Basalts From Yitong-Datun And Shuangliao Area, Northeast China: Implication For Lithospheric Evolution And The Involvement Of The Recycled Pacific Oceanic Crust In Basalt Genesis

This research presents new geochemical, isotopic and geochronology data for the basalts from Yitong-Datun and Shuangliao areas, Northeast China. Combined the geochemistry and the lithosphere reversed-model based on major elements, Yitong-Datun region, where volcanic activity persisted for over 80Ma, provides important constraints on the evolution of the lithosphere. On the other hand, the distinct geochemistry characteristics of Shuangliao basalts, compared with other basalts in Northeast China, suggest that the petrogenesis with HIMU signature has relationship with the recycled Pacific oceanic crust.In Yitong-Datun area, from late Cretaceous (92.5±0.5 Ma) Datun basalt in Changchun to Tertiary (31 Ma, 9～15Ma) basalts in Yitong-Shulan graben, alkaline contents and Dy/Yb ratios increase with temporal decrease, and the final melting depths of basalts, estimated according to the mantle dynamic melting model, increase from about 50 km to 110 km. Based on the lithospheric lid effect, the thickness of the lithosphere beneath Yitong-Datun is inferred to have increased since the late Cretaceous. The Yitong Tertiary alkali basalts show a homogeneous composition with trace element spidergram, Nb/U ratio, low (⁸⁷Sr/⁸⁶Sr)_i and positiveε_Nd(t) similar to oceanic island basalts (OIB). All these indicate that Tertiary basalts were derived from an asthenospheric source. In contrast, the Datun tholeiitic basalts have relatively higher Ni, Cr, Sc, Ba/Th, Rb/Nb, Ba/Nb, (⁸⁷Sr/⁸⁶Sr)_i and lower (¹⁴³Nd/¹⁴⁴Nd)_i, compared with Tertiary alkali basalts. This difference is attributed to the reaction between asthenospheric melts and old enriched-lithospheric mantle. The geochemical characteristics of the Yitong-Datun basalts highlight the different roles of the lithosphere in intraplate magmatism. For the Tertiary basalts, the lithosphere acted as a mechanical barrier to upwelling mantle, but has not been involved materially in the magma generation. For the late Cretaceous basalts, the lithosphere played both indirect and direct roles in magmatic genesis.In Shuangliao area, the suit of volcanic rocks erupting during the Eocene (41.6～51.0Ma) by means of the ⁴⁰Ar/³⁹Ar chronology, is only exposed volcanic rocks in the Songliao basin during Paleogene. Compared with other Cenozoic basalts in NE China, Shuangliao volcanic rocks show the characteristics similar to HIMU, with relatively high Fe₂O₃T (13.44～14.56%, excluding the diabase), low large ion lithophile elements concentrations, negative anomaly of potassium in spidergram and low strontium isotopic ratios [(⁸⁷Sr/⁸⁶Sr)_i=0.703254～0.703609]. The metasomatism models described by many geochemists in recent years, cannot simply account for the genesis of Shuangliao volcanic rocks, but recycled oceanic crust seems plausible. From the simple mixing model between primitive mantle and recycled oceanic crust, Shuangliao volcanic rocks can be produced from the source with about 15% recycled oceanic crust. We suggest that the recycled oceanic crust is from the subducted Pacific slab in the mantle transition zone. Beneath the Songliao basin, the diapir and rising mantle flow part of the convection caused by slab subduction can bring the relatively heavy and refractory recycle oceanic crust (or HIMU component) to the asthenosphere. From the Shuangliao basanites to the alkaline olivine basalts, the signatures of HIMU weaken because that the components of recycled oceanic crust decrease with the degrees of partial melting increasing. Shuangliao volcanic rocks maybe are an important cue to prove that the Pacific slab subduction directly gives rise to thinning of the lithosphere beneath East China.Calculated equilibrium pressure and temperature of phenocrysts from Yitong-Datun and Shuangliao basalts indicate that basalts crystallization depths increase with alkaline contents increase. In general, magma chambers of transitional basalt and tholeiite form within the crust (2～11kb) and the calculated magma densities exist within those of crust, in contrast, basanites prefer to stagnate within the lithospheric mantle (12～19kb) and the calculated magma densities exist within those of lithospheric mantle. Consequently, these phenomena demonstrate that the density contrast is the most important controlling factor for crystallization depths. With host basalts stagnation pressure close to equilibrium pressure of xenoliths, we suggest that chamber depths of host basalt have some relationship with depths of xenoliths.