Reconstruction of the geometry of accretion during formation of the Bay of Islands Ophiolite Complex

Abstract

Two types of highly allochthonous ophiolitic terrains crop out adjacent to one another in the Bay of Islands region of southwestern Newfoundland, Canada. The first, called the Coastal Complex (CC), is exposed as a long, linear belt along the west coast. It consists of a highly deformed ophiolitic assemblage interpreted as oceanic crust and upper mantle with a previous transform fault tectonic history. The second, called the Bay of Islands Complex (BOIC), crops out to the east and consists of a series of four relatively undeformed ophiolitic massifs interpreted as oceanic crust and upper mantle with a more normal seafloor spreading tectonic history. An autochthonous relationship can be demonstrated between the two types of ophiolitic terrains in the southernmost of these massifs, indicating that each evolved along a single ridge‐transform system. The internal structure of both the BOIC and the CC is consistent with the above interpretations. The NE‐SW structural trends in the CC transform are oriented approximately parallel to the sea‐floor spreading direction inferred from the average orientation of the sheeted diabase dikes within the BOIC. One‐way chilling statistics from the sheeted diabase dike unit, growth directions of dendritic crystals along inclined comb layers in the plutonic section, and the sense of shear along the CC transform all indicate that the BOIC was accreted on the NE side of a NW‐SE trending spreading center (present geographic reference frame). Juxtaposition of the BOIC with the NE‐SW trending CC occurred at a ridge‐transform intersection. The transform exhibited a left‐lateral offset. Magma chamber profiles have been identified in the layered gabbroic sections of the BOIC by using the patterns outlined by the attitudes of fine‐scale igneous layering. These patterns depict arcuate structures which appear to define roughly a cross‐sectional view of a systematically oriented half‐cylinder; the long axis of which is oriented parallel with the plane of the sheeted dikes and major lithologic unit contacts. Thus we believe that a large cylindrical‐shaped crustal level subaxial magma chamber extended in a direction parallel with the ridge axial‐plane at the time of formation of the BOlC. The CC marks the abrupt truncation of this long, linear magma chamber. Petrologic constraints demonstrate that the ultramafic and transition zone cumulates which form the base of the plutonic section, crystallized at a range of high to intermediate pressures possibly along a long narrow conduit extending to 30‐km depth beneath the crustal level magma chamber. After formation, these cumulates were deformed at high temperatures in the zone of mantle upwelling and divergence, then tectonically transported to crustal or near crustal levels to form the basement upon which the remainder of the ophiolite stratigraphy was accreted. A geometric and tectonic model is presented for the accretion process responsible for formation of the Bay of Islands Ophiolite.