Geochronology, geochemistry, and petrogenesis of the Mashan basalt in southeast Guangxi Province: Constraints on the Indosinian tectonic setting of South China

This paper is the result of geological survey engineering.

Objective Indosinian basalt, exposed in the northern part of the Mashan Complex in southeast Guangxi province, is a key rock probe for the study of Indosinian movement in South China. It has been seldom researched so far, which restricts the recognition of evolution of tectonic setting in South China.

Methods We carried out zircon U-Pb geochronology, geochemistry, Sr-Nd isotopic geochemistry and petrogenesis of the Mashan basalt.

Results The results show that the LA-ICPMS zircon U-Pb age of the Mashan basalt is (246.7±1.5) Ma, (MSWD=0.16). The basalt belongs to potassium trachybasalt with high alkali (K₂O+Na₂O=5.21%-8.02%), especially potassium (K₂O=2.59%-4.96%), and is enriched in large ion lithophile elements (Rb, Th, U, K, Pb, LREE) but depleted in-high field strength elements (Nb, Ta, P, Ti, HREE). The Sr-Nd isotopes of the Mashan basalt have affinity with enriched mantle (EMⅡ).

Conclusions The geochemical characteristics of the Mashan basalt exhibit shoshonitic features, and is mainly a product through fractional crystallization without obvious crustal contamination. It was probably derived from partial melting of phlogopite- and garnet-bearing lithospheric mantle (>80 km), which was metasomatized by subducted crustal materials. The basalt is developed in an intraplate setting, likely to be shaped by magmatic eruption and intrusion upwards through favorable space created by the extension in the later stage of the Indosinian thrust-nappe structure.