The geochemical nature of mantle sources for two types of Cretaceous basaltic rocks from Luxi and Jiaodong in east-central China

Abstract

The composition of mantle-derived basalts reflects the geochemical nature of their source regions in the mantle. Olivine is generally the first mineral to crystallize from basaltic melts, with the composition of this mineral providing important constraints on primary magma compositions. This study presents new whole-rock major- and trace-element, Sr-Nd-Hf isotopic, olivine compositional, and mineral O isotopic data for Cretaceous basalts within the Luxi and Jiaodong areas of east-central China. These data provide insights into the processes that caused the compositional differences between the two types of basaltic rocks. The first type was erupted during the Early Cretaceous with high SiO2 contents and Mg# values [100 Mg/(Mg + Fe2+), molar ratio], and low (Fe2O3)T and TiO2 contents. These basalts have island-arc basalts (IAB)-like trace-element patterns and enriched Sr-Nd-Hf isotopic compositions with high (87Sr/86Sr)i ratios and negative εNd(t) and εHf(t) values. They also contain phenocrysts of olivine and clinopyroxene that have consistently high δ18O values. The most primitive olivine phenocrysts within these basalts have Ni and Mn contents and Fe/Mn ratios similar to those of olivine from mid-ocean-ridge basalts (MORB). The second type generally has basanite compositions and was erupted during the Late Cretaceous, with low SiO2 contents and Mg# values, and high (Fe2O3)T and TiO2 contents. These basanites have ocean-island basalt (OIB)-like trace-element patterns and depleted Sr-Nd-Hf isotopic compositions with low (87Sr/86Sr)i ratios and positive εNd(t) and εHf(t) values. Olivine and orthopyroxene phenocrysts within these basalts have low δ18O values whereas the clinopyroxene phenocrysts have variable δ18O values. The most primitive olivine phenocrysts contain more Ni but less Mn and have higher Fe/Mn ratios than olivine formed in MORB settings. The contrasting geochemistry of these two types of basalts is indicative of derivation from mantle source regions with different compositions, reflecting different types of subduction-related crust-mantle interaction. The Early Cretaceous basalts were derived from an arc-like, isotopically enriched mantle domain that was formed at subarc depths by the metasomatic reaction of mantle wedge peridotite with felsic melts derived from the subducted continental crust during a Triassic continental collision event. In contrast, the Late Cretaceous basanites were derived from an OIB-like isotopically depleted mantle domain generated at postarc depths by the metasomatic reaction of mantle wedge peridotite with felsic melts derived from subducting oceanic crust of the Paleo-Pacific Plate.