GEOLOGIC MAP OF THE NORTHERN HALF OF THE PINTLER LAKE 7.5' QUADRANGLE AND THE SOUTHERN HALF OF THE WARREN PEAK 7.5' QUADRANGLE, SOUTHWESTERN MONTANA

The Anaconda metamorphic core complex (AMCC) of western Montana is the easternmost in a group of Cordilleran core complexes that formed during Eocene time. Detailed geologic mapping of a 1:24K quadrangle provides insight into the AMCC footwall structure and records previously undocumented cross-cutting relationships. The dominant structure in the map area is the Anaconda detachment. The detachment dips shallowly southeast (~12-15°), separating crystalline and metasedimentary lower plate rocks from mostly unconsolidated Tertiary sediments in the upper plate. A footwall-dominating two-mica granite has a pervasive south-southeast dipping foliation with slip lineations that have a trend and plunge suggesting southeast-directed unroofing. The AMCC hanging wall in the map area is dominated by Quaternary glacial deposits, with limited exposure of the Miocene Big Hole Member of the Six Mile Creek Formation, consisting of mostly well-rounded quartzite clasts. Normal faults in the hanging wall are inferred from topographic lineaments and abrupt changes in float and may sole into the main Anaconda detachment.

Five samples were collected from various granitic dikes and plutons within the AMCC footwall for zircon U-Pb geochronology and Lu-Hf isotopic analysis. Two samples of the two-mica Pintler Granite that dominates the footwall yielded ages of 60.87 ± 0.59 Ma and 61.90 ± 2.6 Ma. A strongly-foliated granodiorite dike and dacite dike cross-cutting the Pintler Granite yielded age of 46.85 ± 0.22 Ma and 52.75 ± 0.61 Ma, respectively. Comparison of documented Ar/Ar thermochronologic ages, previously interpreted to record cooling, with newly obtained U-Pb zircon geochronology ages from the AMCC footwall places constraints on the relationship between magmatism and extension. Igneous activity appears to predate and only briefly overlap with the oldest extension ages, indicating that pluton emplacement may have primarily been the cause of extension rather than a response.