Variation of olivine composition in the volcanic rocks in the Songliao basin, NE China: lithosphere control on the origin of the K-rich intraplate mafic lavas

Abstract

Lithospheric thickness and the heterogeneity of the mantle lithosphere are two major parameters that play a role in determining the final composition of the mafic melts and their minerals. The Songliao basin in northeast China represents an ideal natural laboratory to study the effect of these two parameters on early Pliocene to Holocene K-rich mafic lavas (K2O>4wt.%; K2O/Na2O>1). A series of Cenozoic volcanic edifices (Erkeshan, Wudalianchi, Keluo and Xiaogulihe) are tentatively divided into three groups (Group 1 — thin, Group 2 — middle, and Group 3 — thick) according to the lithosphere thickness. They are located in the northern region of the Songliao basin extending in a near north–south direction along a broad zone where the lithosphere thickness increases gradually. We present a detailed petrographical and geochemical study on olivine macrocrysts in combination with new geochemical data on their host lavas, including major and trace element abundances as well as Sr, Nd, and Pb isotopic signatures. Our ultimate aim is to quantitatively and qualitatively determine the role of lithospheric mantle thickness (named as “lid effect”) and composition in the variation of mafic lavas and olivine composition.When corrected to Mg#=0.72, a number of major elements in the lavas correlate with increasing lithospheric thickness (L): Si72 and Al72 decrease, whereas Mg72, Fe72, Ti72 and P72 increase. Sm/Yb ratios in the lavas increase, implying that lithospheric thickness exerts an important control. Group 3 mafic lavas are ultrapotassic (showing lamproite affinity) with K2O/Na2O>4: their La/Sm and Pb isotope ratios deviate from the above correlations, indicating that the lavas from the thickest part of the basin exhibit the highest extent of metasomatic enrichment of the mantle source.Several parameters (e.g. [Ni], Ni/Mg, Ni/(Mg/Fe), Mn/Fe and Ca/Fe) in melt-related olivine from Group 1 and Group 2 lavas are controlled by variable lithosphere thickness. Olivine from Group 3, however, is heavily influenced by the composition of the lithosphere, which disturbs the lid effect. Trace elements (e.g. Al, V, Zn and Li) in olivine are mostly invariable across all three groups. The low-Ni olivine and olivine crystal fractionation model indicates magma mixing in the source region of Group 3 lavas, with melts derived from a phlogopite-bearing wehrlitic metasomatic component and surrounding peridotitic mantle. Al and Cr concentrations in olivines indicate that the mantle sources are highly refractory harzburgite instead of lherzolite, and that the SCLM under the basin is stratified.Our new olivine data offer additional support for the lithospheric mantle origin of the Songliao K-rich mafic lavas, with a clear affinity towards an EM1 mantle component. The identification of phlogopite in the source strongly implies a lithospheric mantle source, and high P (200–600μg/g) in olivine suggests that metasomatism by volatile-rich melts occurred prior to melting.