Geochemical and tectonic transitions in the evolution of the Mogollon-Datil Volcanic Field, New Mexico, U.S.A.

Abstract

The mafic to intermediate rocks of the Mogollon-Datil Volcanic Field have been subdivided petrologically and geochemically into three groups: Pre-30 Ma, 30-20 Ma and Post-20 Ma. The Pre-30 and 30-20-Ma rocks are broadly calc-alkaline, with high LILE/HFSE ratios and enriched radiogenic isotope compositions. Their Pb isotope ratios define a linear array on a PbPb diagram with a slope corresponding to an age of ∼ 1.7 Ga and they appear to have been derived largely from within the mantle lithosphere. The Post-20-Ma lavas are small-volume alkali olivine basalts, that typically have OIB-like isotope and trace-element ratios and are therefore inferred to have been derived from the underlying asthenosphere. Most of the igneous rocks of the Mogollon-Datil Volcanic Field were generated in a period of extensional tectonics. The peak of extension occurred at ∼ 28.5 Ma and, although there was a small contribution from asthenosphere-derived magmas in the 29-20-Ma rocks, such magmas only became dominant (≥75%) at ∼ 14 Ma, i.e. 15 Ma after the peak of extension. Published models are used to evaluate the temperature of the upper mantle at the time of magmatism and in particular whether the presence of OIB-like magmas should be regarded as evidence for a mantle plume. Partial melting in the mantle lithosphere is thought to have taken place in the presence of small amounts of H 2O and CO 2, whereas the underlying asthenosphere is assumed to have been anhydrous. For mechanical boundary layer thicknesses in the range 80–100 km and β<2, partial melting in the asthenosphere is initiated at the relatively low potential temperatures of ∼ 1320°C. This is consistent with the low eruption rates in the Mogollon-Datil Volcanic Field and it is concluded that magmatism took place in response to lithospheric extension over mantle that was only slightly hotter than that beneath mid-oceanic ridges.