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

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 Sr-87/Sr-86 values ranged from 0.7044 to 0.7048 and epsilon Nd(t) = 3.25-4.92. The Pb isotopes were more complex, the (Pb-206/Pb-204)t, (Pb-207/Pb-204)t, (Pb-208/Pb-204)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.