LU-HF ISOTOPIC TRENDS DURING SHALLOW SUBDUCTION AND FOUNDERING OF THE FARALLON SLAB AND THE RESULTANT MAGMATIC SWEEP ACROSS THE SOUTHWESTERN U.S

Previous research on Samarium-Neodymium (Sm-Nd) isotopic ratios of granitic bodies emplaced during the shallow-subduction and then roll-back of the Farallon slab (Late Cretaceous to Middle Miocene) in the Southwestern United States shows similar radiometric signatures to the basement terranes upon which the plutons were intruded regardless of the tectonic configuration at the time. This pattern records a decreasing trend of εNd values from the coast of California to the border of Arizona, and an increasing trend from the border of Arizona to West Texas. This trend, however, is not well-established in the Lutetium-Hafnium (Lu-Hf) isotopic system. The two radiometric systems are similar; however, changes in εHf values have been used in other orogenic systems (e.g. Tibet) to infer changes in tectonic environment. Is it possible that these trends reflect basement terrane composition? We collected and analyzed ~30 samples from granitic rocks along a transect from the Sierra Nevada to West Texas in the Southern U.S. Cordillera. Our initial results show that there is only a limited difference in εHf between periods of under-thrusting (contraction) and roll-back (extension). Instead, the age and isotopic composition of the basement terranes into which the plutons were intruded is an important control on the isotopic composition. These results imply that the spatial pattern of magmatism is important to consider when interpreting temporal isotopic shifts.