The Petrographic Characteristics and U-Pb Zircon Isotope Age of Felsic Extrusive Rocks in the Western Thanh Hoa Area, Song Ma Suture Zone

Abstract

Felsic extrusive rocks are widely distributed in the western region of Thanh Hoa, primarily comprising rhyolite and rhyolitic tuff formations. They show porphyritic textures, and are composed of 15–25% phenocrysts and 75–85% groundmass. Phenocrysts are mainly euhedral–subhedral plagioclase (5-10 vol.%), K-feldspar (5-10 vol.%) and quartz (5-10 vol.%). The groundmass consists of plagioclase (30-45 vol.%), K-feldspar (25-40 vol.%), quartz (30-40 vol.%), and minor biotite with random distribution. In terms of geological context, these extrusive rocks overlie Silurian-Devonian sequences of sandstone and siltstone interbedded with shale and limestone lenses, and were intruded by the Early Triassic granite of the Song Ma complex. Zircons selected from rhyolite were LA-ICP-MS U-Pb dated, yielding an age of 253 Ma. This age is the timing of the magma crystallization, corresponding to the Late Permian - Early Triassic Indosinian tectonomagmatic event. These findings together with historical study results, prove that the Indosinian magmatic event widely occurred in the Truong Son belt, the Song Ma domain and the Kon Tum area. The eruption of the rhyolite is linked to the amalgamation between the South China and Indochina blocks.