Geochemical and Isotopic Heterogeneities along an Island Arc-Spreading Ridge Intersection: Evidence from the Lewis Hills, Bay of Islands Ophiolite, Newfoundland

Abstract

This study focuses on the origin of magma heterogeneity and the genesis of refractory, boninite-type magmas along an arc---ridge intersection, exposed in the Lewis Hills (Bay of Islands Ophiolite). The Lewis Hills contain the fossil fracture zone contact between a split island arc and its related marginal oceanic basin. Three types of intrusions, which are closely related to this narrow tectonic boundary, have been investigated. Parental melts in equilibrium with the ultramafic cumulates of the Pyroxenite Suite are inferred to have high MgO contents and low Al2O3, Na2O and TiO2 contents. The trace element signatures of these Pyroxenite Suite parental melts indicate a re-enriched, highly depleted source with 0 1 mid-ocean ridge basalt (MORB) abundances of the heavy rare earth elements (HREE). Initial eNd values of the Pyroxenite Suite range from y1 5 to ‡0 6, which overlap those observed for the island arc. Furthermore, the Pyroxenite Suite parental melts bear strong similarities to boninite-type equilibrium melts from island arc-related pyroxenitic dykes and harzburgites. Basaltic dykes split into two groups. Group I dykes have 0 6 MORB abundances of the HREE, and initial eNd values ranging from ‡5 4 to ‡7 5. Thus, they have a strong geochemical affinity with basalts derived from the marginal basin spreading ridge. Group II dykes have comparatively lower trace element abundances ( 0 3 MORB abundances of HREE), and slightly lower initial eNd values (‡5 4 to ‡5 9). The geochemical characteristics of the Group II dykes are transitional between those of Group I dykes and the Pyroxenite Suite parental melts. Cumulates from the Late Intrusion Suite are similarly transitional, with eNd values ranging from ‡2 9 to ‡4 6. We suggest that the magma heterogeneity observed in the Lewis Hills is due to the involvement of two compositionally distinct mantle sources, which are the sub-island lithospheric mantle and the asthenospheric marginal basin mantle. It is likely that the refractory, boninite-type parental melts of the Pyroxenite Suite result from remelting of the sub-arc lithospheric mantle at an arc---ridge intersection. Furthermore, it is suggested that the thermal-dynamic conditions of the transtensional transform fault have provided the prerequisite for generating magma heterogeneity, as a result of mixing relationships between arc-related and marginal basin-related magmas.