PETROGENESIS OF THE NAVAJO VOLCANIC FIELD: LESSONS FROM SOUTH AFRICAN KIMBERLITES

Volcanic rocks of the NVF vary widely in composition and style of emplacement. Isotopic and trace element data suggest that they were formed by the partial melting of pyroxene- and mica-rich veins or layers in the continental lithosphere beneath the Colorado Plateau. However, megacrysts in some of the NVF diatremes display isotopic and trace element characteristics of asthenospheric magmas. Similar megacrysts occur abundantly in kimberlites, where they are demonstrably cognate with their host magmas, recording details of the early deep magmatic evolution, including temperature, depth, and wall-rock composition of the crystallization environment. In southern Africa these megacryst-derived parameters vary systematically with age and tectonic history of the lithosphere.

Megacrysts from The Thumb, a mafic minette diatreme in the NVF, display compositional variations analogous to those in southern African kimberlites, and are most similar to those from Proterozoic crustal provinces. Crystallization depths are in the range 90 – 140 km. However, whereas kimberlites and their included megacrysts are isotopically similar in Sr, Nd, Pb and Hf, those of The Thumb are not. This suggests that NVF magmas originated in the asthenosphere and acquired their chemical and isotopic characteristics by reaction with the lithosphere. The unusual reactivity of the lithosphere beneath the NVF is attributable to its straddling a Proterozoic continental suture zone containing abundant hydrous mafic and peridotitic enclaves, as evidenced by xenolith populations. The preferential rifting and magma emplacement along such sutures in continental Large Igneous Provinces implies that melting and assimilation of the lithospheric mélange in such zones could be a major source of trace element and isotopic enrichment seen in early stages of some flood basalt provinces.