Lithospheric magmatism in southern Colorado and northern New Mexico.
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Wegert, Daniel James.
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This dissertation looks at the formation of three different volcanic centers throughout southern Colorado and northern New Mexico to evaluate potential sources for their subduction-like trace element signatures. These subduction signatures are found in volcanic rocks formed well away from concurrent subduction and are, in many cases, rift related. The Laramide-age McDermott Formation is a lahar deposit deposited into the San Juan Basin formed through melting of Proterozoic subduction-modified lithospheric mantle. Near zero εNdt values (-1.94 and 0.47) rule out a crustal source for these magmas, while trace element ratios suggest a subduction-modified continental lithosphere source. Geochemical analyses suggest a possible relationship to the La Plata Mountains intrusive complex, though weathering and sericitization have made confirming this hypothesis impossible. The Nathrop domes are crustal melts formed due to partial melting associated with crustal extension along the Arkansas Valley Graben segment of the Rio Grande Rift. εNdt values (-10.1, Bald Mountain and -13.9, Precambrian granite) suggest that the magmas ancestral to those of the Nathrop Domes were largely the result of partial melting of Precambrian crustal rock. These crustal melts then evolved through protracted crystal fractionation of observed phenocrysts, yielding the large enrichment/depletion patterns observed in incompatible element diagrams. The Raton-Clayton volcanic field lies on the Jemez Lineament, on the eastern flank of the Rio Grande Rift. The Jemez Lineament coincides with the ancient Proterozoic boundary between the Matzatzal and Yavapai terranes, which have been interpreted as accreted arc complexes. The subduction signature present in the RCVF lavas is attributed to modification during this event. The RCVF magmas show evidence that both assimilation and fractional crystallization played significant roles in their evolution. εNd values for all samples indicate derivation from lithospheric mantle. Olivine back-calculation results were used to determine that magmas ancestral to Raton-Clayton basalts were formed between 50 and 217 km inside the Earth.