Petrogenesis of Early Cretaceous alkaline basalts in the West Qinling: Constraints from olivine chemistry

Olivine crystals in alkaline basalts preserve important information about the Oxygen fugacity and thermal condition of the mantle-derived magma. This paper presents new olivine mineral chemistry and whole-rock geochemistry for Early-Cretaceous alkali basalts in the Ganjia area, northeastern margin of the Tibetan Plateau. The olivine phenocrysts of Ganjia basalts have low Ni (74-2261 ppm) and high Ca contents (1217-4443 ppm), similar to the olivines in MORB, suggesting that they were crystallized from peridotite-derived basaltic magma. Al-in-olivine/spinel thermometer results indicate crystallization temperatures of olivine around 1202 +/- 46 degrees C, corresponding to a mantle potential temperature of 1279 +/- 47 degrees C, with low oxygen fugacity of FMQ -1 to FMQ 0, suggesting a reduced mantle source. Whole-rock geochemical compositions of the Ganjia basalts were characterized by low SiO2 contents (45.39%-48.70 wt%), high alkali contents (3.88-5.11 wt%), positive Nb and Ta anomalies, and enrichment in LREEs, HFSEs and LILEs, similar to those of OIB rocks, with high ratios of (Dy/Yb)(N) (1.38-1.54) and (La/Yb)(N) (5.37-7.42) ratios, indicating garnet residue. The above results suggest that the Early Cretaceous intraplate alkaline basalts in West Qinling were formed by a low degree of partial melting of peridotite and that no high-temperature mantle plume existed. The Qinling Orogenic Belt may have been affected by the subduction of the Palaeo-Pacific. We propose that the Ganjia basalts originated from the interaction between the upwelling asthenosphere and a lithospheric source.