New insight into the timing and history of diatreme-dike complexes of the northeastern Navajo volcanic fiedl, southwestern Colorado

Abstract

The northern extent of the Navajo volcanic field (NVF) is defined by several diatreme complexes that evolved from swarms of north-northeast-trending mafic dikes exposed near Mesa Verde National Park (MVNP). Field evidence indicates that the eruptive phases were generated by multi-stage degassing and fluidization of mantle magmas. This produced diatreme buds that developed along dikes, lateral eruptions that scoured and were injected into adjacent sedimentary strata, and the formation of dome-shaped blind diatremes. New 40Ar/39Ar age constraints on mafic dikes exposed near MVNP indicate a time of emplacement of ~24 Ma, within the accepted age range of magmatism for the NVF. Swarms of mafic dikes with similar composition to NVF rocks are exposed as far north as Placerville, approximately 75 kilometers north of MVNP. Mafic dikes exposed near Mt. Wilson yield a 40Ar/39Ar age of ~7 Ma. The younger dikes are either part of NVF magmatism or another distinct period of potassic magmatism in the region. Oligocene to Miocene mafic dikes exposed from MVNP to Placerville are potassic to ultrapotassic alkaline basalts to basaltic andesites with mineral associations that are similar to those found in minette dikes in other parts of the Navajo volcanic field. All of these rocks have chemical affinities that are consistent with partial melting of metasomatized lithospheric mantle with possible minor contributions from other mantle sources. The protracted period of Oligocene to Miocene potassic magmatism in southwestern Colorado makes up part of mantle magmatism that led to the emplacement of mafic-dike swarms across the northern margin of the San Juan basin after 30 Ma. The dominant north-to-northeast trends of these dike swarms lend evidence for incipient west to northwest rifting in southwestern Colorado that is aligned with a regional zone of crustal anisotropy (Colorado Mineral belt), and high heat flow related partial melting in the mantle (the Aspen anomaly). Regional extension and influx of mantle magmas into the crust in southwestern Colorado in the Middle to Late Tertiary were a probable catalyst to crustal melting in the adjacent San Juan volcanic field, and potassic mantle magmas in particular may have made an important contribution to the production of these crustal magmas.