Geochronological, geochemical and Sr–Nd–Hf isotopic constraints on the origin of the Cretaceous intraplate volcanism in West Qinling, Central China: Implications for asthenosphere–lithosphere interaction

Abstract

This paper reports new zircon LA-ICP-MS UPb age and Lu-Hf isotopes, laser-fusion 40Ar/39Ar age, mineral chemistry, bulk-rock major and trace element, and SrNd isotope data from intraplate volcanic rocks in the Duofuntun Area, West Qinling. It was dated at 105.8±0.9Ma (UPb Zircon) and 103±2Ma (matrix glass laser-fusion 40Ar/39Ar dating). The Duofuntun Volcanic Rocks (DVR) are dominantly composed of alkali basalts with minor basanites and basaltic andesites. Some evolved samples of the DVR underwent a variable fractionation of olivine, clinopyroxene, plagioclase, FeTi oxide, Cr-rich spinel and apatite. Chondrite-normalized rare earth element and primitive mantle-normalized trace element patterns of the DVR remarkably resemble those of oceanic island basalts (OIB). Nearly all the alkali basalts show positive Nb, Ta and Ti anomalies and negative Rb, Th and P anomalies, whereas basanites do not display positive Ti anomalies. Nearly all the samples show marked negative Rb and K anomalies relative to their neighbouring elements on a primitive mantle normalized trace element diagram. 87Sr/86Sr(i) and 143Nd/144Nd(i) ratios for all the mafic samples also show an OIB affinity, covering a range of 0.703264–0.705032 and 0.512718–0.512873, respectively. Several lines of evidence indicate that both garnet and amphibole play a key role for the genesis of the DVR. The melt segregation PT conditions in the source region, around 1371°C at 1.98–3.25GPa, combined with trace element collectively suggest a polybaric melting process during the formation of the DVR. The non-modal batch partial melting modelling result indicates the DVR were generated by small-degree polybaric partial melting of amphibole-bearing spinel peridotites (ca. 4%) and amphibole-bearing garnet peridotites (ca. 0.7%). We suggest the DVR were not formed in a plume. Rather, they originated by the interaction between the upwelling asthenosphere, which might provide both materials and heat flux, and hydrous mineral-bearing (amphibole) lithosphere during lithospheric extension.