Linking composition, deformation, and P-T conditions using peridotite xenoliths from the subcratonic mantle lithosphere, Wyoming craton, Montana, USA

Abstract:

The Archean Wyoming craton, USA receives close attention as a craton with a thick lithosphere that underwent deformation during the Laramide. We present a coupled study of mineral major and trace element compositions and deformation microstructures of Wyoming craton peridotites. The peridotite xenoliths were sampled from the Homestead and Williams diatremes in the northwest margin of the Wyoming. Many peridotites bear coexisting garnet and spinel, and range in crystal texture from coarse grained to porphyroclastic and recrystallized. Samples have predominantly harzburgitic mineral abundances while some of the porphyroclastic samples are lherzolitic. Among the samples analyzed, most are depleted with respect to major elements (e.g. olivine Mg# = ~92). However, trace element compositions show significant systematic variations in enrichment, indicating varying degrees and styles of metasomatism among the analyzed sample suite. Primitive-mantle normalized REE patterns in cpx range from hump-shaped to negatively sloped; the former suggests metasomatism at high melt/rock ratios and the latter represents unperturbed subsolidus equilibration with garnet. Compositions indicative of metasomatism generally correspond to samples bearing porphyroclastic, recrystallized textures. Garnet-pyroxene major element thermobarometry constrain final equilibration of the xenoliths to ~4 GPa and 1140-1330ºC, conditions that are significantly above the presumed shield geotherm and agree with previous estimates of lithospheric P-T conditions in this region of the craton. Preliminary REE-in-two-pyroxene temperatures overlap with major element-based temperatures. Crystal-preferred orientations of olivine and pyroxenes will be presented at the meeting, and will be interpreted in conjunction with major and trace element compositional data.