Insights Into Magma Ascent During Shallow-Level Crustal Shortening From Magnetic Fabrics of the Philipsburg Batholith, SW Montana

Latest Cretaceous development of the Sevier fold and thrust belt in SW Montana overlapped spatially with silicic magmatism. In the fold thrust belt, large volumes of magma were emplaced well east of the main magmatic arc, now exposed as the Idaho Batholith. Hypothesized mechanisms for emplacement of magma within the overthrust belt often involve magma ascent along shallow, west-dipping faults. The ~ 74 Ma (K-Ar method) Philipsburg Batholith is a 122 km2 tabular granodiorite emplaced into deformed Precambrian Belt Supergroup through Cretaceous strata. The Philipsburg Batholith lies in the upper plate of the Georgetown- Princeton Thrust, NW of Anaconda, Montana and cross-cuts two other previously mapped faults. Anisotropy of magnetic susceptibility (AMS) measurements of 122 sites from the Philipsburg Batholith define magnetic foliations and/or lineations to test magma ascent along the Georgetown-Princeton Thrust. AMS fabrics in the Philipsburg Batholith, dominantly defined by magnetite, are generally oblate or triaxial and are typically very consistent at the site level. Preliminary fabric data show subhorizontal foliations across most of the batholith, with steeply dipping foliations near the margins and a minor increase in foliation dip near the inferred fault trace. The hypothesis of magma ascent along fault surfaces will be supported if further data confirm the concentration of relatively steep foliation orientations across the trace of the Georgetown-Princeton thrust.