Evolution of Navajo eclogites and hydration of the mantle wedge below the Colorado Plateau, southwestern United States

Abstract

Eclogite and pyroxenite xenoliths from ultramafic diatremes of the Navajo province on the Colorado Plateau have been analyzed to investigate hydration of continental mantle and effects of low‐angle subduction on the mantle wedge. Xenoliths have been characterized by petrographic and electron probe analysis and by Sm‐Nd, Rb‐Sr, K‐Ar, and O isotopic analysis of mineral separates from one eclogite and by U‐Pb isotopic analysis of zircons from three samples. K‐Ar analysis of phengite establishes eruption of a Garnet Ridge, Arizona, diatreme at 30 Ma. Sm‐Nd and Rb‐Sr analyses of clinopyroxene and garnet from that eclogite document recrystallization shortly preceding eruption. Three zircon fractions have been analyzed from that eclogite and from two others representing the nearby Moses Rock and Mule Ear diatremes. Seven of nine small multigrain fractions scatter about a poorly fit discordia between ca. 35 Ma and 1515 Ma (fractions range from overlapping concordia at the lower intercept to a 207Pb/206Pb age of ca. 1220 Ma). The discordant fractions establish a mid‐Proterozoic zircon component in each eclogite, inconsistent with an origin from basalt of the Farallon plate. The pressure recorded by one of these eclogites (3.3 GPa) exceeds that of an eclogite previously attributed to the Farallon plate. Nonetheless, each of the eclogites contains a fraction of nearly concordant zircons with ages in the range 35 to 41 Ma, and one rock also contains a fraction that is nearly concordant at 70 Ma. These concordant ages are interpreted to record episodic zircon growth during recrystallization of Proterozoic mantle. The concordant zircon ages are consistent with published data that establish recrystallization of Navajo eclogites from 81 to 33 Ma, a time interval similar to that of the Laramide orogeny. The eclogite‐facies recrystallization and growth of new zircon are attributed to the catalytic effects of water introduced into the mantle from the Farallon slab. Water penetrated fracture zones extending for at least tens of kilometers into the mantle wedge above the Farallon slab during low‐angle subduction. Magmatism in the San Juan volcanic field to the northeast of the diatremes may be related to similar hydration.