U-PB ZIRCON AGES OF ECLOGITES, GARNETITES, AND CENOZOIC ROCK-WATER REACTIONS IN PROTEROZOIC MANTLE BELOW THE COLORADO PLATEAU

Eclogite and garnetite xenoliths from the 30 Ma ultramafic diatremes of the Navajo province provide insights into mantle processes below the Colorado Plateau. Three zircon fractions have been analyzed from each of three eclogites representing the Garnet Ridge, Moses Rock, and Mule Ear diatremes in northeastern Arizona and adjacent Utah. Seven of nine small multigrain fractions from the three samples plot on a poorly-correlated line 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 remaining two fractions plot on a line between ca. 35 and 357 Ma, substantiating a component of Phanerozoic zircon in the rock. The data establish a mid-Proterozoic zircon component in each eclogite that is inconsistent with an origin from basalt of the subducted Farallon plate. Nonetheless, the nearly concordant zircon fractions are interpreted as young crystallization products, consistent with Sm-Nd results that established garnet and omphacite crystallization in one eclogite at about 30 Ma.

Garnetite xenoliths also occur in the diatremes, and most are more than 95% garnet with subequal proportions of Grs, Alm, and Prp endmembers. Garnets have minor compositional zonation with sharp contacts: slightly more magnesian garnet occurs in irregular volumes within grain rims and in intricate networks that appear to have filled fractures. Accessory chlorite contains about 0.30 wt% NiO, like that in chlorite peridotite, and garnet-chlorite Fe-Mg partitioning indicates temperatures from about 400 to 500°C. Mineral compositions and textures are consistent with garnetite formation in the mantle as products of hydrous metasomatism that accompanied recrystallization and zircon growth in the eclogites. The 35-30 Ma timing of eclogite-facies recrystallization coincides with the onset of andesitic volcanism in the nearby southern Rocky Mountains, and both may record upward movement of water introduced during Farallon subduction.