Trace elements in Cr-pyrope from the Navajo volcanic field of the Colorado Plateau, SW USA, and implications for the mantle wedge during low-angle subduction

Abstract

The zonation of trace elements in Cr-pyrope has been analyzed to investigate histories recorded by garnet in cool mantle and implications for processes in the mantle wedge during low-angle subduction. The garnets are from the Navajo Volcanic Field (NVF) of the Colorado Plateau in the southwestern United States. Their host rocks were emplaced between 30 and 24 Ma, not long after low-angle subduction of the Farallon plate and the Laramide orogeny. Twenty-seven of the 31 garnets are discrete fragments from diatremes of serpentinized ultramafic microbreccia (SUM). Many Ni and Mn values in those grains yield temperatures in and below the range 800 °C to 600 °C, consistent with inclusions of chlorite and other hydrous minerals. Gradients of Na, Ti, Mn, Ni, Y, REE, and other elements are common in the discrete grains. Diffusion calculations constrain possible histories of a garnet with growth zonation of Y and REE and retrograde zonation of Mn and Ni. The garnet must have formed and resided at temperatures below about 900 °C, and that zonation is unlikely to have been preserved from the Proterozoic. The Mn gradient is consistent with cooling during the time period of low-angle Farallon subduction. Many of the discrete garnets grew from sources metasomatically enriched in Li, Na, Ti, and Zr, and they have distinctive high Li/Na. The high Li/Na is a likely consequence of Li metasomatism in the mantle wedge. Nb values in the Cr-pyrope fragments from northern SUM diatremes are lower than those of most garnets from kimberlites, but the low values are similar to and lower than those of pyrope in peridotite emplaced in orogenic belts; those northern diatremes also host lawsonite eclogite and garnetite. Most but not all relatively Cr-rich discrete garnets (4 to 8 wt% Cr2O3) record enrichment in LREE but little enrichment in Ti and Zr; LREE enrichment by an aqueous fluid is consistent with chlorite inclusions in two of these garnets. Oriented lamellae of rutile and other phases are common, and one texture may record fluid interactions associated with interface-coupled dissolution-reprecipitation. Garnets from the four minette-hosted peridotites record markedly hotter temperatures than do the SUM-hosted grains, and they lack the high Li/Na and low Nb. Gradients of trace elements in minette-hosted garnets record metasomatism shortly before crustal emplacement, but that metasomatism resulted in a decrease of Nb and had little effect on Li/Na. The minette-hosted garnet peridotites may represent mantle residual from Proterozoic accretion of the Colorado Plateau. In contrast, at least some of the SUM-hosted garnets probably grew in rock eroded from the forearc wedge by low-angle subduction and emplaced below the NVF.