Geochemistry of diabase dikes from the Lewis Hills Massif, Bay of Islands ophiolite: evidence for partial melting of oceanic crust in transform faults

The Lewis Hills Massif of the Bay of Islands ophiolite is interpreted to have had a transform fault history in an oceanic fracture zone prior to its obduction. Diabase dikes from the Lewis Hills Massif are divisible into two groups on the basis of their trace element compositions. One group is similar to mid-ocean ridge basalts and is termed the "LH normal" suite. The other group, termed the "LH depleted" suite, is characterized by high CaO, Ti/Zr and Y/Zr with relatively low Si02, Zr and P205. The low Si02 abundances are suggestive of silica-undersaturated rocks of the alkali basalt-nephelinite series of magmas, but the very low abundances of incompatible trace elements in these magmas contrast with their high abundances in most silica-undersaturated rocks. We interpret the geochemical characteristics of these diabase dikes to indicate that the magmas from which they solidified were produced by partial melting of adcumulate gabbroic and troctolitic rocks at low pressures (2-3 kbar) within the ophiolitic fracture zone. The effects of partial melting of gabbroic and troctolitic rocks have not been documented in rocks described from present-day oceanic fracture zones, but samples have not been obtained from those structural levels of fracture zones where such evidence might be found.