UNRAVELLING THE INTERACTIONS OF SYNPLUTONIC MAGMAS DURING GRANITOID CRYSTALLIZATION

The differentiation of chemically evolved, felsic magmas and their emplacement at shallow structural levels is one of the key mechanisms for producing Earth’s upper continental crust. Investigating the chemical and physical evolution of these magmas, therefore, has the potential to improve our understanding of the processes associated with granitoid magma petrogenesis.

The research presented here focuses on a sample from the Quartz Monzosyenite unit of the lithologically diverse Finnemarka batholith (336km³) in the central region of the Permo-Carboniferous Oslo Rift, Norway. This sample preserves clear mineralogical and chemical evidence of magma mingling, offering a window into the processes inherent to the formation of granitoid rocks. From petrographic analysis, two distinct lithologies (Zone 1 and Zone 2), separated by a diffuse boundary, can be seen interacting. Both zones’ major mineralogies are characterized by variable proportions of quartz, alkali feldspar, plagioclase, biotite, amphibole and titanite with the coarser grained Zone 1 characterized by large alkali feldspars (up to 1.5 cm) and the relatively finer grained Zone 2 predominantly characterized by higher proportions of biotite and titanite.

Bulk rock elemental analyses classify both Zone 1 (wt. % SiO₂ of 62.4% at wt. % NaO₂+K₂O of 9.1%) and Zone 2 (wt. % SiO₂ of 58.3% at wt. % NaO₂+K₂O of 8.9 %) as syenodiorites in a classical TAS plot. Both zones are also clearly metaluminous (ASI <1, AI < 1) which is worth noting due to the generally peraluminous nature (ASI > 1, AI > 1) of the majority of the other Finnemarka batholith units (n=6).

Future work will include detailed in-situ chemical analysis of the mineral populations of both zones. This will be done through microprobe analysis of two major phases: amphibole and biotite, in addition to analyses of the ubiquitous accessory phase: titanite. The goal is to focus primarily on core to rim changes in chemistry to track changes in chemistry during crystallization. In addition, these three phases, alongside accessory apatite and zircon, will be characterized for their trace element compositions via LA-ICP-MS in order to further unravel the interaction of these synplutonic magmas.