Spherulites as Recorders of Multi-batch Felsic Magma Transport and Time Scales: A Case Study from Southeast of China

Abstract Granite, formed by the consolidation of felsic magma, is widely exposed on the surface of the Earth’s continents. However, the physical processes behind the transport of felsic magma from the deep crust to the surface remain poorly understood. Recently, the fracture-dike transfer model has gained widespread acceptance as a comprehensive theory explaining the ascent, transport, and emplacement of felsic magma in the crustal magma plumbing. Observing the magma migration process has been challenging due to the hidden geological evidence. Currently, no geological feature accurately tracks the transport path, composition evolution, and time scale of magma during its ascent. In this paper, we report the discovery of numerous spherulites that developed in two spherulitic rhyolite porphyry dikes. Based on the petrographic and geochemical characteristics of spherulites, we established that the ascent and emplacement processes of the magma were caused by continuous supply from deep magma sources and repeated episodes of ascent. Finally, magma source depletion halted its ascent and stabilization of its location at a depth of about 3 km below the ancient surface. According to the growth rate of the spherulites when the magma resided in the magma chamber, we quantitatively calculated that the magma resided in the three magma chambers for a total residence time of about 90 days. These findings provide valuable insights into the accumulation, ascent, and emplacement of multiple batches of magma into large rock bodies. These insights advance the understanding of the felsic magma plumbing system and provide constrained quantitative data on the dynamics of magma migration in the subsurface magma plumbing systems of volcanoes. Natural spherulites usually occur in rhyolite and obsidian around volcanic institutions. The magma migration path recorded by spherulites in this case study and the established spherulite geochronological method should have general applicability to a larger range of volcanic rock areas with similar geological backgrounds.