Magmatic recycling of lithospheric mantle and production of Oligocene-Pliocene alkaline mantle magmas at the end of Laramide subduction, Four Corners region, USA

Abstract:

Middle Cenozoic magmatism in the Four Corners region of the United States produced alkalic mantle magmas that were emplaced in diatreme-dike complexes of the 25-24 Ma Navajo volcanic field (NVF) and small-volume 20-5 Ma dike swarms on the northeastern San Juan Basin and western flanks of the San Juan Mountains. Magmatism involved partial melting of Proterozoic lithospheric mantle that underwent regional metasomatism during Laramide subduction. Emplacement of magmas was focused on zones that were broadly aligned with lithospheric-scale anisotropies (e.g., Colorado lineament). The majority of intrusive rocks produced are minette, but included small amounts of katungite formed by melting of carbonate rich mantle.

A northward decrease in intrusion age is mimicked by subtle variations in major and trace element geochemistry. Compared to the 20-5 Ma dike rocks, NVF intrusions have higher overall Mg# (> 55), K₂O (5-6 wt. %), La/Lu (300–1000), and elevated concentrations of incompatible trace elements (e.g. Rb, Ba, Zr). The katungite samples are marked by extreme silica undersaturation (SiO₂ ~34 wt. %), and higher MgO (~15 wt. %) and FeO (~13 wt. %) than the minettes. Isotopic signatures of the minettes are relatively homogenous, falling within a lithospheric mantle field spanning ⁸⁷Sr/⁸⁶Sr 0.705-0.707 and ε_Nd +2 to -2; except the Dulce dikes which have anomalously lower ε_Nd. Isotopic signatures of the katungites plot between the minettes and average OIB.

The spatial variations in geochemical signatures over time are attributed to either compositional variation in mantle sources ± different degrees of melting of lithospheric mantle; crustal contamination was insignificant. The lower concentrations of incompatible elements in the 20-5 Ma minettes might be explained by depletion of the lithospheric mantle caused by earlier voluminous volcanism in the San Juan Mountains. Alternatively, greater lithospheric fertility in Colorado may have promoted higher percentage partial melts in the mantle diluting the trace element concentrations of the 20-5 Ma minettes.