Zircon U–Pb Geochronology, Geochemistry and Geological Significance of the Anisian Alkaline Basalts in Gejiu District, Yunnan Province

Abstract

The Gejiu Anisian alkaline basalts (GAAB), distributed in the southern part of the Emeishan large igneous province (ELIP), are crucial to understand the tectonomagmatic activity during the Triassic. Geochronological, geochemical, and Sr-Nd-Pb isotopic analyses were systematically applied to explore the origin, petrogenesis, and tectonic setting of the GAAB, and how they relate to the ELIP. Zircon U-Pb dating set the eruption date at 244 Ma. Most of the samples belonged to alkaline basalts and had high TiO2 (2.14–3.23 wt.%) and MgO (4.43–19.58 wt.%) contents. Large ion lithophile elements (LILEs) were enriched relative to high field strength elements (HFSEs). The rare earth elements (REEs) and trace element signatures in the normalized diagrams were similar to oceanic island basalts (OIB) and Emeishan high-Ti basalts. These samples had consistent Sr-Nd isotope compositions: the initial 87Sr/86Sr values ranged from 0.7044 to 0.7048 and εNd(t) = 3.25–4.92. The Pb isotopes were more complex, the (206Pb/204Pb)t, (207Pb/204Pb)t, (208Pb/204Pb)t ratios were 17.493–18.197, 15.530–15.722, and 37.713–38.853, respectively. Our results indicate that the GAAB originated from the deeper enriched mantle with 5% to 15% partial melting of garnet lherzolite and a segregation depth of 2 to 4 GPa (60–120 km). During the formation of the GAAB, clinopyroxene and Ti-Fe oxides were fractionally crystallized with insignificant crustal contamination. The GAAB were formed in a extensional regime that was related to the Gejiu-Napo rift event in the Triassic.

Petrogenesis and Tectonic Significance of Shenxianshui Alkaline Granite in Gejiu, Yunnan Province, China

“…However, as part of the magmatic complex in this region, Shenxianshui alkaline granite has received relatively little attention, with few studies defining its precise date. Previous studies have determined ages of 81.70 ± 0.60 Ma (Zhang et al, 2011), 83.0 ± 0.35 Ma (Cheng and Mao, 2012) and 81.35 ± 0.22 Ma (Huang et al, 2016), mainly using LA-ICP-MS analysis.…”

Calcioancylite-(La), (La,Ca)2(CO3)2(OH,H2O)2, a new member of the ancylite group from Gejiu nepheline syenite, Yunnan Province, China

“…The intrusion forms an irregular ellipse body that is generally elongated in the S–N direction, covering ~28 km 2 . The Gejiu alkaline complex yielded emplacement ages of 80–82 Ma and intruded into Triassic sandstone and carboniferous rocks (Zhang, 2013; Huang et al , 2018; Wang et al , 2021). The pinkish feldspathoid syenite is a massive, medium- to coarse-grained rock and contains the feldspathoid minerals nepheline and sodalite as well as dark alkaline minerals.…”