ND AND SR ISOTOPIC STUDY OF THE BAY OF ISLANDS OPHIOLITE COMPLEX AND THE EVOLUTION OF THE SOURCE OF MIDOCEAN RIDGE BASALTS

The time of crystallization of the complex is 508 + or - 6 m.y. and 501 + or - 13 m.y. using two Sm/Nd mineral isochrons from gabbros of the Blow-me-Down massif. This is the time of formation of this bit of former Iapetus oceanic crust. K/Ar ages show it was obducted 470 m.y. ago. The LIL and isotopic data show the rocks are oceanic. The variable Sr- but constant Nd-isotopic values indicate major interaction with sea-water, and show that early Palaeozoic ocean-floors were important sinks for radiogenic crustal Sr.M.J.Le B.