GEOCHEMICAL EVOLUTION OF DEVONIAN-CARBONIFEROUS IGNEOUS ROCKS OF THE MAGDALEN BASIN, EASTERN CANADA - PB-ISOTOPE AND ND-ISOTOPE EVIDENCE FOR MANTLE AND LOWER CRUSTAL SOURCES

Magmatism associated with the extensional Magdalen basin includes volu minous tholeiitic gabbro and basalt and local granite and rhyolite. Pb - and (or) Nd-isotope determinations have been made on 70 igneous rock s from throughout the basin, and a further 15 samples of Avalonian bas ement from the southern margin of the basin, to characterize the contr ibution of lower crustal blocks and mantle sources to the magmatism an d to constrain tectonic models for the basin. Five phases of magmatic evolution are distinguished in the Magdalen basin. (1) Middle to Late Devonian partial melting of lithospheric mantle, producing principally tholeiites and minor alkalic basalt. Tholeiites have Pb isotopic comp ositions similar to that of younger Triassic tholeiites generated from the same mantle, but experienced less crustal contamination. Regional variations in trace element composition of the mantle can be recogniz ed. (2) The mafic magma triggered anhydrous base-of-crust melting, pri ncipally along the transpressive Cobequid and Rockland Brook faults, p roducing A-type granites in which radiogenic Pb increases northeastwar d. (3) In the latest Devonian, a large base-of-crust fractionating mag ma chamber evolved. It contained immiscible mafic and minor felsic mag ma, with uniform Nd isotopes, and high Ti in the mafic magma. (4) Alth ough late Tournaisian dykes are not strongly fractionated, their evolu tion involved more crustal assimilation than earlier mafic rocks. (5) Local Visean-Westphalian alkalic magmas, which ascended along crustal- scale faults, have Pb and Nd isotopic compositions resembling mantle p lumes ol their mixtures with lithospheric mantle sources. Only these y oungest rocks show any isotopic evidence for input from an asthenosphe ric plume source, suggesting that regional extension was responsible f or most of the magmatism.