Mineral chemistry and petrogenesis of cumulate dunites from Blow Me Down Mountain, Newfoundland
Mineral compositions of relic olivine, chrome-spinel, and clinopyroxene have been determined with an electron microprobe for a suite of eighty-five partially serpentinized dunites from Blow Me Down Mountain, Newfoundland. Olivine compositions range from Fo 91.7 to 89.4 with 0.45-0.26 NiO (wt. %). Spinels have Cr-numbers of 20.2 to 61.3 and Mg-numbers of 77.4 to 51.5 with 0.05 to 0.45 wt. per cent TiO2. The origin of the unit from which these samples were collected has been disputed in the literature with both cumulate and metasomatic/residual origins proposed. Variations in mineral compositions in this suite of samples display the kind of systematic changes with stratigraphic height that are characteristic of cumulates. These variations form what is called cryptic layering (Wager and Deer, 1939; Irvine, 1982) and are caused by crystal fractionation of the magmas that precipitated the cumulates. Reversals in cryptic trends indicate mixing between resident magmas and replenishments of more primitive magma. Olivine compositions from this suite suggest that very primitive to primary magmas crystallized to form these lowermost cumulates. Picritic (high MgO) basalts are required to account for the great thickness of ultramafic cumulates found at the Blow Me Down Mountain massif of the Bay of Islands ophiolite.