Paper No. 85-6
Presentation Time: 9:00 AM-5:30 PM
TRACE ELEMENTS IN CR-PYROPE FROM THE NAVAJO FIELD OF THE COLORADO PLATEAU: EVIDENCE FOR MIXING IN THE MANTLE WEDGE DURING LOW-ANGLE SUBDUCTION
Trace elements in pyrope from the Navajo Volcanic Field of the Colorado Plateau in the southwestern US have been analyzed by LA-ICP-MS to investigate the evolution of the mantle wedge during low-angle Farallon subduction. Most analyses are of discrete grains from diatremes of serpentinized ultramafic microbreccia (SUM). Others are of pyrope in minette-hosted peridotite xenoliths. The twenty discrete grains from northern SUM diatremes are unusually low in Nb compared to 6 of 7 garnets from the Green Knobs SUM diatreme in the south and compared to minette-hosted and kimberlite-hosted pyrope; the low Nb values are similar to those of garnets in some orogenic peridotites. SUM-hosted pyrope populations from northern and southern diatremes overlap on a CaO-Cr2O3 plot, but the northern population extends to less-calcic values. That difference and the low Nb values correlate with the presence of lawsonite eclogites only in the northern SUM diatremes. Li, Na, and Ti have strong positive correlations, but the correlations in SUM- and minette-hosted garnets are different, and Li is higher in SUM-hosted samples. In SUM-hosted grains, Ni, HREE, and many other elements are commonly zoned to lower values at margins, and Mn is the only element with well-defined interior-to-edge increases. Trace elements and their zonation patterns indicate growth of SUM-hosted pyrope at temperatures below 900°C followed by slow cooling. Various thermobarometers using compositions of inclusions and their host garnets record pre-eruption temperature in the range from 500°C to 680°C for the SUM-hosted pyrope. Inclusions such as chlorite and chlorapatite in pyrope grains from SUM diatremes are consistent with garnet growth from cool mantle hydrated by subduction sources. Despite much higher pre-eruption temperatures recorded by minette-hosted samples, Ni contents of the interior of garnet in one minette-hosted peridotite are smaller than the high values in five SUM-hosted grains, evidence for a similar low-T heritage. The garnets appear to be from diverse peridotite sources mixed in the wedge as a consequence of low-angle Farallon subduction. Nonetheless, none of the garnets may be from sources within the Farallon plate.