MINERAL DIVERSITY THROUGH TECTONIC CYCLES: THE ROLE OF ALKALINE INTRUSIONS

Investigation of changes in mineralogical diversity on Earth over time has highlighted fascinating correlations between the number of observed mineral species throughout Earth’s history and key events in the geosphere and biosphere such as biological transitions, environmental changes, and redox states of elements. Rarely considered is the role of tectonic setting with respect to mineralogical diversity. On average, tectonic cyclicity preserved in young orogens suggest that a continental area close to a plate boundary experiences a cycle from continental extension, passive ocean margin proximity, subduction and collision within about 200-400 million years. We highlight one of the lesser known aspects of plate tectonics that produces rocks with the greatest mineralogical diversity in the lithosphere: the relatively rare alkaline igneous intrusions, or agpaites. Often generalized as peralkaline nepheline syenites, these rocks are characterized by extreme amounts of Na, Zr, Cl and F, along with an abundance of rare elements including Li, Be, Nb, Ta, and rare earth elements (REEs), which give rise to a diverse and unique mineralogy. This distinct mineralogy and concentration of rare elements often lead to interest in economic development of these deposits. Agpaites form as a result of extensive partial melting of their parental mafic magmas under conditions of low oxygen fugacity, thereby preventing typically water soluble components such as Na, Cl and F from being removed from the melt. Some of the most well-known examples include the Complexes of the Kola Peninsula, the Ditrau Alkaline Massif in the Carpathians, and Mont Saint-Hilaire in Quebec. Some of these locations have documented over 500 minerals species, with some pegmatites containing over 100 species alone. However, their spatial distribution is less predictable from a tectonic/petrologic perspective than other magmatic rocks, but these alkaline intrusions are associated with the early stages Wilson cycles.