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

Zhi Shang,Yongqing Chen

doi:10.3390/min10111030

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.

Highlights

Basaltic magmatism is a fundamental geological process, and of the various kinds of basalts, alkaline basalts are thought to have unique geological implications [1,2]
Massive stretches of basaltic magma are distributed along the western margin of the South China Block in SW China, making up the widely known Emeishan large igneous province (ELIP), which covers an area of over 5 × 105 km2 in SW China and northern Vietnam (Figure 1a) [3]
Several formation mechanisms have been proposed for the Gejiu Anisian alkaline basalts (GAAB), these have included an orogenic environment [13], a within-plate extensional environment [14,15], a back-arc rift basin [16,17,18,19], and a decompressive melting of the pre-existing Emeishan plume head induced by a rift

Summary

Introduction

Basaltic magmatism is a fundamental geological process, and of the various kinds of basalts, alkaline basalts are thought to have unique geological implications [1,2]. Massive stretches of basaltic magma are distributed along the western margin of the South China Block in SW China, making up the widely known Emeishan large igneous province (ELIP), which covers an area of over 5 × 105 km in SW China and northern Vietnam (Figure 1a) [3]. The Gejiu Anisian alkaline basalts (GAAB), formed later than the ELIP, are important in understanding the tectonic evolution of the western South China Block during post-ELIP tectonomagmatism. Several formation mechanisms have been proposed for the GAAB, these have included an orogenic environment [13], a within-plate extensional environment [14,15], a back-arc rift basin [16,17,18,19], and a decompressive melting of the pre-existing Emeishan plume head induced by a rift

Results

Discussion

Conclusion