ASSESSING PRECAMBRIAN CRUST THROUGH A GEOCHEMICAL INVESTIGATION OF MESOZOIC PLUTONS IN THE NORTHWESTERN CORDILLERA: IMPLICATIONS FOR CONTINENTAL RECONSTRUCTIONS

The relative contribution of tectonically and magmatically accreted crust in the evolution of continents and continental crust is a fundamental, outstanding problem in geology. For Laurentia, one major significant question is the nature of continental crust that underlies Paleozoic and Mesozoic rocks of West-Central North America and its implications for the identity of the Proterozoic conjugate craton that separated from SW Laurentia in the Neoproterozoic (e.g., Australia, South China, and Siberia). One key, but poorly constrained, crustal boundary is the one dividing Archean and Proterozoic basement rocks west of the Wyoming Craton and east of the rifted Neoproterozoic margin. If the boundary between Archean and Proterozoic lithosphere of distinct ages and/or histories in this region can be determined, then new and valuable constraints can be placed on the numerous paleogeographic/paleotectonic reconstructions proposed for this margin. The nature of this largely buried basement is only accessible, however, via study of the numerous Mesozoic/Tertiary plutons in this region. Granitoids can provide a reliable means of sampling the chemical and isotopic composition of the intermediate and deeper parts of the continental crust. Granitoids with SiO2 between 55 and 73 weight percent contents were collected from plutons west of the exposed Wyoming Craton and east of the Neoproterozoic rifted margin. Trace elements analyzed using ICP-MS indicate REE patterns lack a Eu anomaly, suggesting a lack of plagioclase in the source. HREE contents are <10x chondrites, indicating a lower crustal source containing garnet, amphibole, and/or pyroxene. Depletion in HFSE (e.g., Nb) suggests this source was originally formed in a convergent margin. The time of formation is difficult to specify; however, a secondary Pb isochron yields an age of 2082 ± 160. U-Pb and Lu-Hf values analyzed from zircons will help further constrain the characteristics of the source material.