Trace element abundances in mantle-derived minerals which bear on compositional complexities in the lithosphere of the Colorado Plateau

Abstract

Proterozoic crust of the Colorado Plateau, southwestern USA, was intruded in the mid-Tertiary by hypabyssal minettes and ultramafic breccias which contain mantle-derived xenoliths and xenocrysts. Pyrope-rich garnets and Cr-rich diopsides from these fragments collected at Red Mesa, Garnet Ridge and The Thumb diatremes were analyzed by secondary ion mass spectrometry for the rare earth elements (REE), Sr, Ti, Zr, Y, V, and Cr. Typical garnet xenocrysts derived from shallow, low temperature peridotites are light REE (LREE) depleted with convex-upward REE patterns, and have relatively high Y abundances and low Zr and Ti abundances. These xenocrysts are characterized by relatively low Zr/Y and high Al/Cr ratios consistent with a lherzolitic source. Uncommon garnet xenocrysts have “sinusoidal” REE patterns and low Y abundances and are Cr-rich but not subcalcic. Sinusoidal REE patterns also characterize some garnets from deeper, high-temperature garnet peridotites; a zoned garnet from one of these xenoliths preserves a core with a sinusoidal pattern overgrown by a rim with a convex-upward REE pattern. The rim of this garnet is also relatively rich in Ti, Zr and Y. Garnet xenocrysts with sinusoidal REE patterns have relatively low Al/Cr, Ti and Y abundances and some contain chlorite inclusions and probably have relatively refractory, and in some instances, hydrous source rocks. Our data further document the occurrence of these garnets outside Archean cratons and although their petrogenesis remains controversial, the garnets examined here could plausibly have formed by subsolidus processes in refractory peridotite. Garnets from more fertile, high temperature lherzolites have convex upward REE patterns and relatively high abundances of Ti, Y and Zr. Compared to the garnet xenocrysts, garnets from the higher temperature garnet peridotites tend to have lower Al/Cr and higher Zr/Y ratios indicating that the xenocrysts are derived from relatively fertile source rocks. Hence the mantle in this region may be stratified with more refractory rocks at greater depths. The mantle beneath the Colorado Plateau lacks pronounced, widespread silica enrichment and subcalcic garnets and is only rarely enriched in incompatible elements at shallow levels. At the time of minette magmatism, the Colorado Plateau was characterized by a cool root analogous to those beneath Archean cratons but compositionally more akin to modern abyssal peridotites.