RETHINKING THE EVOLUTION OF THE BOULDER BATHOLITH, WESTERN MONTANA

The late Cretaceous Boulder Batholith was emplaced into Proterozoic rift clastic sedimentary rocks of the Belt Supergroup overlain by a 3-km-thick Paleozoic-Cretaceous carbonate-clastic sequence.

Arc magmatism migrated eastward from the Idaho batholith likely in response to flattening of the subducting slab and initiated by eruption of the ~3-km-thick Elkhorn Mountain Volcanics (EMV, ~86 to 83 Ma). Andesites of the lower member (Ar/Ar hbl: 85.39 ± 0.29 Ma, 84.65 ± 0.35 Ma) are overlain by rheomorphic dacite to rhyolite ignimbrites of the Middle Member (upper unit Ar/Ar hbl: 83.72 ± 0.31 Ma), and thin rhyolite pyroclastic rocks of the upper member. In the Elkhorn Mountains these units are gently folded in the upper plate of the east-vergent Lombard thrust.

Early small volume plutons of the Boulder Batholith are mostly peripheral to the Butte granite and part of the Tilling’s sodic magmatic series. Diorite to monzonite and granite have U/Pb zircon ages from ~84 Ma to 78 Ma. The central Butte hornblende-biotite granite is 100 by 50 km elongated to the north-northeast and has an U/Pb age of 76.3 Ma near Butte. The granite has variable mafic content (15-20 vol%), and is associated leucocratic granites and pegmatite-aplite sheets of similar age. Three small granodiorite plutons and associated porphyry dikes of similar age intrude the EMV in the Elkhorn Mountains. Intermediate volcanics erupted synchronously to the north (Two Medicine Fm & Adel Mt Volc, 80-74 Ma). Zircon in granites, except the Butte intrusion, contain populations with Proterozoic ages.

Along its 100 km length, the batholith uniformly yields ~2 kb (~7 km) by hornblende barometry where intruding the overlying middle and upper members EMV. Plutons of the Elkhorn Mountains are shallower. The ~1 km roof thickness of the EMV is insufficient for the observed pressures of the Boulder Batholith. Locally, thrust faults preserve an upper plate of Paleozoic rocks, and suggests that the high pressures are due to tectonic thickening between ~83 Ma and 76 Ma. Rapid exhumation, likely also a result of shortening, post-dates the Butte porphyry deposits (66-64 Ma) and was complete before eruption of the Lowland Creek Volcanics (55-48 Ma). Porphyries with high Eu/Eu* and Yb/Gd zircon are associated with older porphyry Cu and polymetallic veins containing Cu-Au(Mo)-Ag-Zn-Pb-As from 83 – 76 Ma, but Au-poor ores at the younger Butte deposit.