The Pioneer metamorphic complex in south-central Idaho formed by extreme extension of the crust during Eocene times. The extensional faults the bound the structure brought rocks that resided at ~15 km depth some 50 m.y. ago to the surface. These rocks provide an opportunity to study deformation and magmatism that occurred at depth during extension and also to gain insight into the earlier geologic history by studying the older rocks within the core complex.
Previous maps of the Pioneer metamorphic core complex show that there are several sheet or lens-shaped igneous intrusions within two map-scale structural domes. While these intrusions are mapped as Paleoproterozoic, they have been bimodally dated as Neoproterozoic (~695 Ma), and Eocene (~49.5 Ma). Both bodies are compositionally and texturally similar, making it difficult to distinguish them by hand. Thus my research methods will involve:
(1) Mapping the intrusive bodies using modern GPS-based mapping tools and applications in order to identify the map scale geometries and observe and measure deformation fabrics.
(2) Uranium-lead dating, which will involve use of the scanning electron microscope to detect zoning within minerals, and laser ablation ICP mass spectrometer analyses to collect isotope data.
(3) Thin section and hand sample petrography to examine the mineralogy, textures, and shapes of minerals. From this information I will assess the general composition of the rock, and if Eocene bodies were deformed while in a magmatic state and/or whether they were strained after crystallization and at high or low temperatures.
(4) Major and Trace element geochemical analysis, performed on the XRF and ICPMS
From this work I will address important questions regarding both Eocene magmatism and tectonism, as well as Proterozoic magmatism. By studying the Eocene intrusions, I will better document the full range of timing of magma emplacement and how the magma volume input changed over the time period between ~50 and 44 Ma. I will also improve understanding of the character and distribution of strain during Eocene extension.
The Neoproterozoic intrusions represent magmatism during early rifting of western Laurentia. Since the ~695 Ma bodies in the core complex are the only exposures of this age suite in the area, studying these Neoproterozoic intrusions we identify the full range of ages, compositions, chemistry of magmas associated with this rifting event.