Sequential trace element analysis of zoned skarn garnet: Implications for multi-stage fluxing and flow of magmatic fluid into a skarn system

Abstract

A high-resolution fluid flux–flow model for a shallow crustal system related to skarn formation was established using oscillatory zoning in garnet. In situ analytical methods were used to determine major and trace element contents of andradite-rich and lower-andradite-content garnet zones. Continuous analysis of pure andradite (And87–98) shows these garnets record first- and second-order fluid fluxes. The first-order fluid flux exhibits a stepwise increase in Sn contents and a decrease in the contents of other elements (Ti, V, W, As, Mo, Y, and rare earth elements), indicating that pure andradite records a large range changes in fluid flux from the magma. The second-order flux is evident from oscillatory variations in the contents of Sn and other elements, reflecting small-scale and pulsed changes in the fluid flux from a degassing magma reservoir. Based on the garnet major and trace element variations and mineral textures, these fluid fluxes in the skarn system were controlled by pulsed degassing of a cooling magma. Continuous analysis of oscillatory zoning in garnet with a lower andradite content (And72–81) showed that Sn concentrations increase gradually and other element concentrations decrease gradually. This garnet crystallized from a fluid that was locally equilibrated with a small-scale, stagnant fluid. Thick and low-andradite garnet bands (And62–76) are observed to have grown between the first-order fluid fluxing events. These bands are Al-rich and have retrograde textures, indicating prolonged magma residence after the first-order fluid flux, suggesting that circulating fluids persisted for a relatively long time.