Lava-Seawater Interaction of the Mesoproterozoic Xiong’er Group Volcanic Rocks in the Southwestern Ordos Basin: Insights from Alteration Mineralogy and Geochemistry

Abstract

The volcanic rocks of the Mesoproterozoic Xiong’er Group (Changcheng System) in the Ordos Basin are pervasively albitized, a phenomenon mainly attributed to lava-seawater interaction. Understanding the mineralogical and textural imprints of this alteration is therefore essential.

This study focuses on deep core samples of the Xiong’er Group from the southwestern Ordos Basin. An integrated methodology was applied, combining petrographic observation with geochemical analyses (major and trace elements, TIMA automated mineralogy, C-O isotopes, and zircon U-Pb dating). Our results confirm intense lava-seawater alteration in the Xiong’er Group magmas and define the diagnostic mineral assemblages and textures produced by this process. Additionally, a comparative analysis with coeval volcanic rocks from the eastern Ordos Basin was conducted to reconstruct the tectonic environment during magma genesis.

Key findings are summarized as follows:

(1) Core and thin-section observations reveal volcanic rocks with vesicular-amygdaloidal textures, plagioclase-phyric porphyritic structures, and interbedded sedimentary layers. Distinct dark-reddish alteration zones occur along lithological contacts. Microscopically, the rocks show porphyritic texture with feldspar phenocrysts in a cryptocrystalline groundmass. Vesicles are commonly filled with calcite, quartz and chlorite.

(2) Geochemical data indicate that the Xiong’er Group volcanic rocks in the southwest basin are predominantly basaltic. They exhibit high alkalinity (σ = 4.6~10.6) alongside anomalous silica contents, classifying them as basic to intermediate igneous rocks. Rare earth element patterns are consistent with an intracontinental rift setting linked to mantle plume activity, with evidence of crustal contamination.

(3) TIMA automated mineralogical mapping shows that feldspar phenocrysts in the basalts are exclusively albite. The groundmass is pervasively altered by chloritization and argillization. Slilceous sediments occur widely, filling vesicles in basalts and appearing within sedimentary rocks at basaltic contacts.

(4) Marked petrological and geochemical differences exist between the Xiong’er Group volcanic rocks in the southwest and eastern Ordos Basin, reflecting contrasting tectono-magmatic environments-intracontinental rift versus continental arc settings.

The results advance the understanding of mineral alteration and element exchange during such interactions at the micro-scale and provide key mineralogical constraints on lava-seawater alteration processes.

Keywords

Xiong’er Group, Volcanic Rocks, Lava-Seawater Interaction, Alteration Mineralogy, Geochemistry, Ordos Basin