Ultramafic rocks in the Bay of Islands Ophiolite complex, Newfoundland Appalachians.

Abstract

The Bay of Islands Ophiolite Complex of the western Newfoundland Appalachians is exposed in four massifs, from north to south: Table Mountain, North Arm Mountain, Blow-me-Down Mountain, and the Lewis Hills. Each massif contains a mantle tectonite unit dominated by harzburgite with subordinate lherzolitic rocks, with minor dykes and lenses of dunite and pyroxenite. In the Blow-me-Down Mountain massif, a thick dunite layer (up to 5 km), separates the gabbroic unit from harzburgitic mantle tectonites. This paper focuses on the harzburgitic and dunitic facies of the Blow-me-Down Mountain massif, with additional samples from the North Arm Mountain and Table Mountain massifs. Bulk rock compositions and mineral chemistry of olivine and spinel indicate that the harzburgites are mantle restites, whereas dunites are cumulates formed from mafic melt. Concentrations of Ir-type PGE are consistently high for harzburgite and variably low for dunite, supporting this interpretation. Values of spinel Cr# in harzburgites and subordinate lherzolites show a wide range (0.3 to 0.9). Mantle harzburgites with low spinel Cr# (< 0.6) have mineral-chemical signatures similar to abyssal peridotites formed at a slow spreading ridge. The magmatism during this period produced overlying tholeiitic gabbros and basalts. High spinel Cr# values (> 0.6) in harzburgite require extensive partial melting in a subduction setting. This change in the tectonic setting is supported by clinopyroxene compositions and dunite dykes with high Cr# (< 0.9) cutting earlier formed mantle harzburgite and gabbro units.