MAGMA MINGLING: UNRAVELLING GRANITE CRYSTALLIZATION ONE CRYSTAL AT A TIME

The crystallization of magma to produce granites or granitoid rocks is one of the fundamental processes by which Earth differentiation occurred and its continental crust was produced. Therefore, constraining the processes that contribute to the production of granitic magmas is essential for advancing our understanding of continental crustal growth. When granites, which crystallize within the subsurface, often at several kilometers depth, are exposed at the Earth’s surface due to erosion, they offer an opportunity to study processes associated with their crystallization.

The research presented here focuses on a granite sample from the Oslo Rift, Norway. This sample is of particular interest as it preserves evidence of magma mingling, offering a window into the processes inherent to the formation of these granitoid rocks. From petrographic analysis, two distinct magmas (Zone 1 and Zone 2), separated by a diffuse boundary, can be seen interacting. A preliminary inference is that, while the two rock types in the sample formed from distinct crystal mushes, they were comagmatic. This interpretation is based, primarily, on the absence of unambiguous reaction boundaries which would otherwise be indicative of a hot mush intruding into a relatively cold mush or crystalline solid.

This project aims to characterize, in detail, the chemistry of two of the sample’s major phases (amphibole and biotite) as well as two accessory phases (titanite & apatite) in order to unravel the extent to which magmas mingle and interact during granite crystallization. This characterization will be based on both major element analysis, acquired through microprobe analysis, and trace element analysis, acquired through LA-ICP-MS.