BASAL LHERZOLITES IN THE BAY OF ISLANDS OPHIOLITE - ORIGIN BY DETACHMENT-RELATED TELESCOPING OF A RIDGE-PARALLEL MELTING GRADIENT

Decompression melting under a spreading centre should generate a resid ual upper mantle section with a weak vertical gradient in the degree o f chemical depletion. Chemical data from the 6 km-thick ophiolitic man tle section of the Table Mountain massif (Bay of Islands Ophiolite, Ne wfoundland) reveal, however, a depletion gradient of 1% melting/100 m in the basal mantle section of the massif. Within the framework of a s egmented ridge we suggest that during ophiolite detachment intermitten t accretion of progressively less depleted peridotites occurred to the (former) base of the ophiolite, amounting to a several hundred metre- thick Iherzolitic sole. In this view, the basal mantle section in Tabl e Mountain extends a structural concept previously established for the metamorphic sole: both metamorphic gradient in the sole and melting g radient in the basal mantle rocks are apparent and of tectonic nature. Like the sole, the basal mantle sequence may thus expose mantle mater ial derived from a scale larger than the preserved ophiolite complex.