Some chemical and mineralogical aspects of plutonic rocks from the North Arm Mountain Massif, Bay Of Islands Ophiolite, Newfoundland

Whole-rock major and trace element compositions of one basalt, one diabase, and 21 rocks from the magmatic-plutonic units of the North Arm Mountain massif, Bay of Islands ophiolite, Newfoundland, were determined. Mineral compositions of a subset of the plutonic rocks were also determined. The major and trace element compositions of the basalt and diabase are similar to abyssal tholeiites, and this is consistent with the REE data of Malpas (1978) and Suen, et. al. (1979) that suggest the lavas and dikes formed from a depleted or slightly enriched abyssal tholeiitic magma. The alkaline nature of the magma proposed by several previous investigators based on major element chemistry is attributed to alteration. The major primary minerals of the plutonic rocks are approximately in chemical equilibrium with each other, and mineral zoning, where present, is normal. This indicates that the plutonic rocks formed mainly by in situ nucleation and crystallization on or near the margins of the magma chamber rather than by homogeneous nucleation and gravitational sorting. Methods of estimating the amount of trapped liquid in plutonic rocks from incompatible trace element concentrations are discussed. Textures and estimates of trapped liquid indicate many of the plutonic rocks from the area of North Arm Mountain from where the rocks in this study were collected are mesocumulates. This and the thinning of the layered units and thickening of the isotropic gabbros suggest the plutonic rocks in this area formed under conditions of faster cooling than did those to the northeast and southwest. Whole-rock and cryptic mineral variations with pseudo-stratigraphic height suggest the magma chamber was vertically zoned, with. the degree of differentiation increasing upward, but that the extent of fractionation was rather limited. Olivine Fo, plagioclase Ca#, and clinopyroxene Mg# varied by 9, 17, and 13 units, respectively, through a vertical distance of 6 km. through the transition zone and gabbroic units. Mineral compositions determined in this study and others and the gross lithologic layering suggest the general crystallization order of the North Arm parent magma was olivine (± chromite) - clinopyroxene - plagioclase - orthopyroxene. This is inconsistent with phase relations of abyssal tholeiites in which clinopyroxene crystallizes after olivine and plagioclase. No explanation is suggested. Most chemical and mineralogic features of North Arm Mountain plutonic rocks can be attributed to a combination of crystal fractionation and repeated mixing of newly injected parent magma in a large, steady-state chamber at an oceanic spreading center. This is consistent with the geologic evidence (Casey, 1980). However, other processes must be responsible for some of the minor variant mineral assemblages in the plutonic rocks