AGE, GEOCHEMISTRY AND SOURCE OF THE PIONEER INTRUSIVE SUITE; PIONEER METAMORPHIC CORE COMPLEX SOUTH-CENTRAL IDAHO

Located in south-central Idaho, the Pioneer metamorphic core complex contains rocks that ~50 million years ago resided at ~15 km depth. The footwall of the core complex contains metamorphic rocks hosting the Eocene Pioneer intrusive suite, a series of syn-extensional compositionally diverse plutonic bodies associated with the Challis volcanics. In contrast to other Eocene intrusive suites in the region, the Pioneer intrusive suite contains a very broad compositional range from highly felsic (up to 76% SiO2) to ultramafics of varied composition (47% SiO2 and up to 25% MgO).We have applied U-Pb geochronology, field mapping, and a variety of geochemical analysis to examine the duration, sources, and tectonic setting of the magmatism. U-Pb geochronology indicates that all of the map-scale magma bodies were emplaced at 50-47 Ma, identical to the ages from this part of the Challis volcanic field. Field and textural observations suggest a complex magma chamber and plutonic system.

While additional analysis is ongoing geochemical results to date suggest the following: (1) despite the varied compositions, all of these Eocene intrusions have similar trace element geochemistry, and isotope signatures, suggesting the bodies formed through the same magmatic processes and events; (2) a combination of magmatic textures combined with trace elements and whole-rock isotopes indicate a rapidly changing melt composition sourced from a specific combination of mantle and crustally derived melt as well as rapid melt mixing and crystallization; (3) the Pioneer intrusive suite formed in a similar tectonic setting and from similar crustal/mantle sources as the geographically/temporally close intrusive and extrusive suites; (4) major element data are similar to the Challis volcanics erupted in the immediate hanging wall; (5) four granite samples contain Neoproterozoic zircons, identical to 695Ma small isolated intrusions in the footwall; other inherited populations suggest an ~1400 Ma source similar to that inferred regionally by others.