THE MINERALOGY AND PETROLOGY OF THE MESOPROTEROZOIC MOUNT ROSA PERALKALINE GRANITE COMPLEX, EL PASO COUNTY, COLORADO (USA)

The ~1.04 Ga Mount Rosa Complex (MRC) is located in the central Front Range of Colorado, West of the city of Colorado Springs, and consists of peraluminous to peralkaline granitic rocks. This complex is the youngest intrusive center in the ~1.08 Ga A-type (anorogenic) Pikes Peak Batholith. The MRC is enriched in rare earth (REE) and other high field strength elements (HFSE, e.g. Th, Ti, Zr, Nb and Ta) and hosts numerous Niobium-Yttrium-Fluorine (NYF)-type pegmatite bodies. Despite many years of research on the MRC, its diverse mineralogy has not been related through quantitative chemical data, nor has a mechanism been proposed for the enrichment in REE and HFSE across the complex. By combining field work, petrography and mineral chemistry, we will attempt to trace the evolution of the MRC and the formation of simple- and complex-NYF-type pegmatite bodies hosted therein. The MRC is hosted in peraluminous Pikes Peak biotite granite, and comprises, from oldest to youngest, fayalite-bearing syenite, dikes and aplites of biotite syeno- to monzogranite, peralkaline Na-amphibole granite (Mount Rosa granite), mafic dikes ranging from diabase to diorite, and simple- to complex-type pegmatites. Simple-type pegmatites typically contain alkali feldspar, quartz, Na-amphibole/pyroxene, biotite, zircon, thorite, and fluorite, whereas complex-type pegmatites also contain aluminofluoride minerals, REE minerals (typically fluorocarbonates and Na-F-REE phases), columbite-tantalite, and sulfides. Simple-type pegmatite bodies occur close to and within the Mount Rosa granite, whereas complex-type pegmatite bodies occur in biotite granite and fayalite-bearing syenite 3 to 10 km from the Mount Rosa granite. Petrography and SEM analysis show evidence for an early subsolidus alteration stage of the Mount Rosa granite characterized by albitization of alkali feldspar and hematization and aegirinization of Na-amphiboles, followed by later Ca-F-CO₂-metasomatism in which mafic minerals are replaced by fluorite and associated REE fluorocarbonate minerals. This study will shed light on the magmatic-hydrothermal evolution of the MRC and provide insights into the formation of NYF-type pegmatite bodies in peralkaline systems.