ISOTOPIC AND TRACE-ELEMENT CONSTRAINTS ON THE GENESIS OF A BONINITIC SEQUENCE IN THE THETFORD MINES OPHIOLITIC COMPLEX, QUEBEC, CANADA

The Mont Ham Massif (part of the Thetford Mines ophiolite, south Quebe c) represents a magmatic sequence made up of tholeiitic and boninitic derived products. A geochemical study confirms the multicomponent mixi ng models that have been classically advanced for the source of bonini tes, with slab-derived components added to the main refractory harzbur gitic peridotite. An isochron diagram of the boninitic rocks is interp reted as a mixing trend between two components: (i) a light rare earth element (LREE) enriched component (A), interpreted as slab-derived fl uid-melts equilibrated with sedimentary materials (epsilon(Nd) = -3, S m-147/Nd-144 = 0.140), and (ii) a LREE-depleted component (B) (0.21 < Sm-147/Nd-144 < 0.23), interpreted as slab-derived fluid-melt equilibr ated with recycled Iapetus oceanic crust and equated to the Nd-isotope characteristics of the Iapetus mantle (epsilon(Nd) = 9) A multicompon ent source is also necessary to explain the Nd-isotope and trace eleme nt composition of the tholeiites, which are explained by the melting o f a more fertile, lherzolitic mantle and (or) mid-ocean ridge basalt s ource (component C), characterized by a large-ion lithophile element d epleted pattern and an Iapetus mantle Nd isotopic composition (epsilon (Nd) = 9), mixed in adequate proportions with the two previously infer ed slab-derived components (A and B). The genesis of the boninites of Mont Ham is not significantly different from those of boninites locate d in the Pacific. An intraoceanic subduction zone appears to be an app ropriate geodynamic environment for the Mont Ham ophiolitic sequence.