Petrogenesis of late Cenozoic volcanic rocks from the Raton-Clayton volcanic field, northeastern New Mexico and southeastern Colorado
Stormer, John C., Jr.
Doctor of Philosophy
Late Cenozoic volcanism on the Great Plains in northeastern New Mexico was closely related to the tectonic and thermal deformation along the transitional zone between the Colorado Plateau and southern Basin and Range province, which produced a broadly synchronous volcanism along the Jemez lineament. During late Miocene to Holocene volcanism on the Great Plains, a wide variety of magmas were erupted in the Raton-Clayton volcanic field (RCVF) on the eastern flank of the Rio Grande rift, northeastern New Mexico and southeastern Colorado. Late Cenozoic volcanic rocks from the RCVF range in composition from nephelinite through basalt to dacite and rhyolite; many are alkalic. Mafic feldspathoidal magmas were derived from an enriched lithospheric mantle by very small degrees of melting. While some show evidence of apparent olivine fractionation, most were not significantly fractionated and/or contaminated by the crust and provide the best representation of the isotopic composition of the lithospheric mantle. Alkali and subalkali basalts are also quite magnesian (i.e., MgO $>$ 8%), indicating very limited fractionation. Compositional variations of the mafic volcanic suite from the RCVF, therefore, were primarily produced by differential melting of their lithospheric mantle source. Intermediate to silicic volcanic rocks were generated by fractionation from mafic magmas derived from the mantle. Isotopic compositions of earlier volcanic rocks reflect their hypothetical lithospheric mantle source, whereas those of the basaltic trachyandesites indicate a second, more depleted mantle source that was involved in the latest stage of the magmatism. While very small isotopic deviation from the lithospheric mantle of the silicic rocks indicates a relatively closed magmatic system, limited isotopic variation of mafic rocks probably resulted from their short residence in the crust. Regional petrogenetic comparison of early silicic rocks also shows that late Cenozoic silicic magmatism on the Great Plains evolved in a more closed system.
Geochemistry; Geology; Mineralogy