LATE CRETACEOUS MAGMATISM IN SOUTHWEST MONTANA: THE BOULDER BATHOLITH AND ELKHORN MOUNTAINS VOLCANICS

Magmatism associated with the Boulder batholith and Elkhorn Mountains volcanics (EMV) of southwestern Montana is characterized using a combination of geochronology, geochemistry, and field mapping. Integrated studies of this system provide insights into coeval magmatic and structural features associated with the initiation of the Laramide fold and thrust belt, and allow for comparisons between coeval intrusive and extrusive magmatism, between coeval large batholithic and smaller satellite intrusions, and between calc-alkaline magmatism located >1000 km inward of the ocean-continent margin and magmas at the more common setting, located <300 km from the margin. Sixteen new U-Pb analyses using HR-TIMS and laser ablation ICPMS confirm earlier studies and demonstrate the initiation of the volcanic activity (>82 Ma) preceded the main phase of the intrusions by several million years but the bulk of the extrusive rocks erupted contemporaneously with the intrusive phases of the batholith (79-74 Ma). The EMV are geochemically related to the Boulder batholith and associated satellites, but some important differences are observed. In particular, some EMV and satellites have more mafic compositions than those observed in batholithic rocks. Additionally, the EMV display a significantly higher and wider range in K₂O and PbO content and have a lower Na₂O content compared to both the batholith and satellite samples. We suggest that the large batholith intrusions represent more evolved melt segregations derived from deeper mafic bodies, whereas the extrusive units and the smaller satellite bodies include magmas that experienced smaller degrees of internal melt segregation. Our geochemical data do not support the earlier contention that the Boulder magmatic system comprises two ‘distinct magma series’, rather the magmas reflect a continuum with variable degrees of mixing between two end-member magma types, either in their source and/or within crustal reservoirs. When compared to the geochemistry of earlier North American arc suites (located considerably closer to the continental margin), the Boulder/Elkhorn magmas are enriched in K₂O, Rb, Ba, Pb, Ce, Zr, La, and Nb, but are depleted in Na₂O, thus signifying a more prominent role for hydrous fluids and/or phlogopite in their source region.