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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°C, corresponding to a mantle potential temperature of 1279 ± 47°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. [ABSTRACT FROM AUTHOR]

Early Palaeozoic alkaline magmatism is widely preserved in the North Daba Mountains, South Qinling Belt (SQB), predominately composed of basalt–diabase and trachyte–syenite. The petrogenesis of the felsic rocks and their genetic connections with the mafic members remain controversial. Here, an integrated investigation, combining geochronology and whole-rock and Sr–Nd isotopic geochemistry, is conducted to further constrain the origin and tectono-magmatic evolution of the trachytes in Pingli–Zhuxi area, North Daba Mountains. Zircon U–Pb dating for the Pingli–Zhuxi trachytes yield ages of 406.0 to 427.9 Ma, which are close to those of the mafic rocks (420–455 Ma) in research area. The trachytes show relatively high SiO2 (60.88–63.87 wt.%) and total alkali (Na2O + K2O = 10.19–12.24 wt.%) contents, and are characterized by pronounced enrichment in LREEs and HFSEs, with insignificantly negative Eu anomalies and significantly negative Sr anomalies. All of the samples display low and variable initial 87Sr/86Sr ratios of 0.6959 to 0.7083 and narrow range initial 143Nd/144Nd ratios of 0.5123 to 0.5125 with positive εNd(t) values of +3.0 to +7.3, suggesting a depleted source. Together with the published data from coeval SQB diabases, our geochemistry evidences show regular and linear variations between mafic and felsic end-members, suggesting their closely genetic link. A plausible petrogenetic hypothesis for the genesis of the Pingli–Zhuxi trachytes implies a protracted process of fractional crystallization driven by separation of K-feldspar, plagioclase, biotite, apatite, and Ti-magnetite from a basaltic melt. Enrichment in REEs and HFSEs and similar geochemical characteristics point to that the Pingli–Zhuxi trachytes and the coeval mafic rocks originate from a cogenetic metasomatized lithosphere source in rift setting. Asthenospheric upwelling is a key factor for continental break up and lithosphere metasomatism. Collectively, melting of the metasomatized source, followed by protracted K-feldspar-dominated fractional crystallization, leading to the occurrence of the Pingli–Zhuxi trachytes in the North Daba Mountains. [ABSTRACT FROM AUTHOR]