Geochemical-Seismological Imaging of Volatile-Assisted Melting at the Southern Margin of the Colorado Plateau, USA

Abstract:

Pliocene-Quaternary basalt-dominated volcanic rocks of the San Francisco-Morman Mountain fields (SFMM) erupted at the southern margin of the Colorado Plateau (CP) of the USA, a region of high continental elevation. The volcanic field is aerially extensive (>8,500 km²), and the youngest portion is located above the depth where the lithosphere-asthenosphere boundary (LAB) shoals to ~70 km. The volcanic fields also lie above and inboard of a band of low V_sthat encircles more than half of CP at <80 to 150 km depth. We obtained comprehensive new geochemical data for magnesian basalts (9-13 wt.% MgO) in order to better understand mantle melting associated with foundering of continental mantle lithosphere.

Melting of peridotite-dominated domains rather than lithologies that might have been introduced into the mantle by Laramide-aged shallow subduction are indicated by major element and transition metal signatures. The influence of subduction may nevertheless be recorded by moderately enriched isotopic compositions (ε_Nd = -1.5 to +4.2; ε_Hf= +5.5 to +11.8) that diverge from the mantle array. Trace element characteristics indicate variable mixtures of small degree melts (≤1%) generated in the garnet-stability field and melts generated at spinel stability conditions ± melt-rock reaction at the latter conditions.

Thermobarometry results for clinopyroxene-bearing SFMM basalts indicate magmatic temperatures of 1250-1330°C at or just below the Moho. Melting conditions delimited from olivine-melt equilibra mostly range from ~90 km to just above the seismic proxy for the LAB. More deeply equilibrated melts appear to have greater contributions from asthenosphere-derived melts, and melt contributions may by hybridized by localized convective upwelling in response to LAB migration. Melt equilibration temperatures are 80-130°C higher than for clinopyroxene crystallization, assuming that melts have water contents (~0.5 wt.%) inferred from mantle xenoliths. This could be reconciled by melting/melt reaction involving a more hydrous CP mantle, capable of producing melts with ~3 wt.% water, potentially the remnant of Laramide-aged flat-slab subduction.

This project is supported by NSF EarthScope Grant EAR-1109826.