Interactions between melt and upper-mantle peridotites in the North Arm Mountain massif, Bay of Islands ophiolite, Newfoundland, Canada: Implications for the genesis of boninitic and related magmas

Abstract

Upper-mantle tectonites forming the basal part of the North Arm Mountain (NAM) massif ophiolitic sequence are dominantly refractory harzburgites, grading upward to Iherzolites near the contact with the overlying crust, and downward to coarse-grained lherzolite close to the metamorphic sole. The tectonites are cut by numerous pyroxenitic dykes ranging from orthopyroxenite, to websterite, to clinopyroxenite, and containing variable amounts of olivine, spinel and disseminated FeNi-sulphides. Microprobe traverses were executed from the core to the rim of several pyroxenite dykes, through the wallrock peridotites and into distal host peridotites. Wallrock peridotites recorded complex mineral modal and chemical variations suggesting that major dissolution-precipitation reactions occurred between minerals of the wallrock peridotites and intrusive melts, involving CrAl and MgFe solid solution exchanges between olivines, pyroxenes, spinels and migrating melts, and selective dissolution of clinopyroxene from wallrock peridotites into migrating melts. There is a general trend of modal clinopyroxene increase and FeAl enrichment among minerals of the pyroxenites along the mantle section, from deeper orthopyroxenites toward websterites and clinopyroxenites located near the crust-mantle interface. This suggests that clinopyroxene-bearing intrusions crystallized from the most evolved members of magmas related to the pyroxenites. Fractionation and interaction processes could both have led NAM migrating melts to Al 2O 3 and FeO enrichment, and CaO enrichment leading towards saturation in clinopyroxene. Magma compositions that would be in equilibrium with intro-mantle intrusive pyroxenites show similarities in composition and evolutionary trends with western Pacific boninites and high-Mg# andesites. If this interpretation is correct then the NAM ophiolitic massif would bear an intraoceanic fore-arc component. We suggest that modern boninites may partly have inherited their unusual magma chemistry by similar interactions and continuous re-equilibration with upper-mantle rocks during their ascent through the mantle wedge above a subduction zone.