Residence time of igneous garnet in Si-rich magmatic systems: Insights from diffusion modeling of major and trace elements

Abstract

Although garnet is an important accessory phase in many granitic rocks, the petrogenetic history of such garnets is often debated. Explanations range from crystallization from (mostly) peraluminous melts to entrainment of peritectic or xenocrystic garnets originating from country rocks. Here we present a detailed microchemical study of a mid-crustal granodiorite from the Ivrea-Zone (N-Italy), which contains metapelitic enclaves and composite metamorphic-magmatic xeno-phenocryst garnet. Garnets from this locality exhibit high-phosphorus (∼800 ppm) metamorphic cores and multiple igneous overgrowth rims. The high-phosphorus cores appear to originate from granulite facies country-rock, while low-phosphorus rims indicate magmatic overgrowth in two main episodes. Zircon-bearing nanogranitoid inclusions are trapped along embayments between garnet cores and the first magmatic growth zones. Zirconium saturation thermometry indicates a temperature of 820 ± 5°C during the first magmatic overgrowth, and a temperature of 778 ± 10°C for the second overgrowth. The latter temperature agrees with garnet-biotite thermometry using the garnet rim composition, which yields a temperature of 773 ± 29 °C. In order to quantify the duration of the overgrowth episodes, we have numerically modeled Cr, Y, REEs and Hf trace element diffusion, as well as multicomponent major divalent cation diffusion within garnet using available experimental diffusion data and Cr diffusion data retrieved from natural garnets. All modeled diffusants conform to a single temperature-time path, in which the temperatures associated with the first and second magmatic overgrowths persisted for 5.4 and 6.3 kyr, respectively. Composite garnets in Si-rich magmatic systems have the potential to constrain magma transit rates in the continental crust.