COMPOSITIONAL AND TEXTURAL EVIDENCE OF AN IGNEOUS ORIGIN FOR OLIVINE CORONAS IN LHERZOLITE FROM MILE 91 CANYON

Orthopyroxene rims are nearly ubiquitous in separating olivine from intercumulate amphibole in the more evolved samples of lherzolite and websterite in peridotite located at mile 91 in the Grand Canyon, USA. These “olivine coronas” have a general outward succession of olivine, orthopyroxene (bronzite), pargasitic amphibole (in some cases symplectically intergrown with orthopyroxene), and albitic plagioclase. Igneous origin for olivine coronas in lherzolite from Mile 91 Canyon is supported by microtextural evidence such as olivine-orthopyroxene contacts that are convex toward olivine and are locally strongly cuspate and columnar orthopyroxene, as well as compositional evidence including extremely sodic plagioclase that is not in equilibrium with reactant olivine (and the complete absence of plagioclase from some corona textures). The progression from cumulus to intercumulus assemblages and then from amphibole to plagioclase closely mimics the expected trend for fractional crystallization for all of the major oxides except for K2O.

The probable sequence for the formation of the olivine coronas in the mile 91 lherzolite first involved the interaction of cumulus olivine with an evolved, silica-rich liquid resulting in dissolution of the olivine to form a rim of columnar bronzite via an inward-moving dissolution front. The similarity of Al2O3 concentration in corona-forming orthopyroxene in the mile 91 lherzolite to olivine rather than to cumulate orthopyroxene crystals suggest that the rims were formed by dissolution of olivine. This liquid then either directly precipitated pargasitic amphibole and albitic plagioclase or precipitated an intermediate symplectite. In most instances, symplectite reacted fully to form amphibole. With the crystallization-dictated shift in melt composition, albitic plagioclase crystallized toward the interior of interstitial pore spaces. The regional metamorphic history of the Proterozoic rocks of southwestern North America would be consistent with the slow-cooling conditions that optimize corona-forming reactions.