Subcretion Versus Relamination: Testing Processes of Lower Crustal Modification in the Klamath Mountain Accretionary Province

Texas Tech University, Lubbock TX

Investigators

Abstract

Tectonic collision zones represent locations in which continental crust is formed and/or physically and chemically modified. It is therefore important to determine how large masses of rock (terranes) are added to or removed from the crust during collision events. It is also important to understand the thermal history of these environments in terms of timing, temperature extremes, sources of heat, and geologic consequences, including potential for economic mineral deposits. Traditional theories for terrane addition involves emplacement of cold oceanic terranes beneath existing continental rocks. However, recent theories suggest a process (relamination) in which hot, partially melted rocks are emplaced at the base of the crust. This research will test these theories in the Klamath Mountain geologic province (CA and OR), where the potential for ancient relamination is strong. The research will also develop best practices for future studies of relamination elsewhere and will develop a detailed thermal history of the Klamath province that can be used to better understand economic resources, regional geologic history, and landscape development. Lastly, because the economy of the area is becoming increasingly dependent on tourism, additional products designed to promote eco-tourism include field guides and web-based information resources for the general public. Because relamination involves emplacement of hot metasedimentary rocks at the base of the crust, a number of distinctive geologic features should result, in particular: rapid heating and metamorphism of the overlying terranes and lower-crustal modification of arc magmas due to contamination by relaminated metasedimentary rocks in the lower crust. The research will tightly constrain the timing of deformation and high-grade metamorphism relative to magmatism through detailed field mapping and sampling. High-precision (CA-ID-TIMS) U-Pb ages on zircon and rutile from high-grade migmatites and key pinning plutons will provide age constraints of orogenic events. These ages will be combined with field and microstructural data and P-T estimates (mineral equilibria and pseudosections) to develop a complete Pressure-Temperature-time-Deformation (P-T-t-D) history. Regional sampling of plutons that span accretionary events in both time and space will yield zircons that will be analyzed for oxygen isotopes (SIMS) and dated and analyzed for Hf isotopes by LA-ICPMS to characterize temporal and spatial changes in magma sources and contaminants, and thereby the changes in the lower crustal architecture caused by relamination.