GEOCHEMISTRY AND REGIONAL SIGNIFICANCE OF THE EARLY CRETACEOUS BIMODAL BASALT-FELSIC ASSOCIATIONS ON GRAND-BANKS, EASTERN CANADA

Cretaceous mafic rocks are present in several wells on the Grand Banks of Newfoundland, near the southwest Grand Banks Jurassic transform ma rgin and along the trend of the Collector magnetic anomaly. In the Bra nt and Mallard wells, felsic volcanic, subvolcanic, and pyroclastic ro cks are several hundreds of meters thick. Seismic reflection profiles show that the volcanic rocks are locally bounded by rejuvenated faulte d margins of early rift basins beneath the mid-Cretaceous, postrift un conformity. The Early Cretaceous age of the volcanic rocks is confirme d by K/Ar geochronology. Trace element and isotopic composition of bot h mafic and felsic rocks are quite different from those of plume-relat ed Cretaceous rocks elsewhere on the eastern North American margin. Ma fic rocks have a Pb isotopic composition unusually enriched in Pb-207/ Pb-204, which is similar to Late Triassic-Early Jurassic mafic rocks a long the eastern Appalachians. Nd isotopic compositions are closer to typical ocean island basalt (OIB) sources. Observed isotopic compositi ons suggest mafic magma was partially melted from continental margin l ithospheric mantle enriched with small amounts of hydrous or silica-ri ch phases of OIB affinity. Whole-rock geochemistry, isotopic compositi on, and mineral chemistry show that the felsic rocks resemble alkaline A-type granite. Those from Mallard have some crustal contamination; t hose from Brant, which is located closer to the continental margin edg e, have almost pristine mantle isotopic characteristics indicating an OIB source magma. The spatial distribution of Cretaceous volcanic prod ucts suggests that magma was generated along the reactivated transform margin and that major faults in continental crust acted as magma cond uits. Partial melting of continental margin lithospheric mantle result ed from extensional decompression, probably aided by a weak plume-rela ted thermal anomaly. As on other transform margins, magmatic products reflect a complex mantle history.