PLATINUM-GROUP ELEMENT GEOCHEMISTRY OF CONTINENTAL THOLEIITES - ANALYSIS OF THE LONG-RANGE DYKE SWARM, NEWFOUNDLAND, CANADA

The rift-related, Late Proterozoic Long Range continental dykes of western Newfoundland have platinum-group element (PGE; Pd, Pt, Rh, Ru and Ir determined) concentrations that correlate with indicators of magma evolution [ e.g., Mg' = Mg/(Mg + 0.9Fe(tot))] and magma alkalinity (La/Sm), suggesting that metamorphism had a minimal effect on the PGE. The low-melting-point (LMP) PGE (Pd, Pt and Rh) and Cu behaved incompatibly as magma evolved, indicating that sulphide fractionation did not occur and that the dykes hold little economic promise. Hypotheses that augite (a major fractionating phase) has an affinity for the PGE are not supported. Primitive rocks tend to have lower Ir than evolved rocks and all samples display a negative high-melting-point (HMP) PGE (Ru and Ir) anomaly on mantle-normalized diagrams. Olivine and early chromite fractionation may have decreased the concentrations of these elements. The Long Range dykes confirm that Pd/Ir ratios and absolute LMP PGE concentrations tend to decrease with increasing hot-spot magma alkalinity. The lower PGE concentrations in most MORB compared with hot-spot basalts imply that there are at least two mantle reservoirs for the PGE. Lithosphere overlying mantle sources (continental or oceanic) has little effect on PGE concentrations. The apparently similar source-region PGE concentrations for hot-spot basalts suggest that differences in PGE concentrations between MORB and hot-spot basalts reflect incomplete segregation of the PGE into the core during its formation and rise of the homogeneously dispersed elements to upper-mantle depths in deep mantle plumes.