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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]

The relationship between the Shangdan and Erlangping suture zone (ESZ) and the initiation stretch time of the Early Paleozoic back‐arc system in the Qinling Orogen still remains unclear. This paper reports the chronological and geochemical data of the Miaogou mafic intrusion along the southern margin of the Shangdan suture. The pyroxenite in the mafic intrusion has a zircon U–Pb age of 485 ± 5 Ma (Mean Standard Weighted Deviation (MSWD) = 0.08, n = 18). It has low SiO2 (45.41–46.02 wt%), K2O (0.42–0.44 wt%), and TiO2 (0.73–0.83 wt%) contents, high MgO (13.9–15.13 wt%) contents with high Mg# (74–77). In combination with its high clinopyroxene portion, the pyroxenite represented the accumulates from the mafic magma. Some sieve rims of the clinopyroxene indicate that the primitive pyroxenite was metasomatized by subduction‐related fluids. However, their consistent trace element and rare earth element (REE) patterns suggest that the metasomatism process has insignificant effects on their geochemical features. Zircons from the pyroxenite have positive εHf(t) (+7.18 to +11.75) values, reveal a depleted source region. In combination with its high V (322–1,093 ppm) contents, low Ti/V (10–15) ratios and smooth REE patterns, we propose that the pyroxenite was derived from metasomatized depleted mantle wedge in a back‐arc basin. In considering the duration limit (<475 Ma) of the back‐arc basin along the ESZ, we argued that the ~485 Ma Miaogou pyroxenite represented the initiation back‐arc basin in the Shangdan suture, which might represent the initial rifting of the North Qinling microcontinent induced by the northward subduction of the Shangdan Ocean. [ABSTRACT FROM AUTHOR]