Cretaceous mafic rocks are present in several wells on the Grand Banks of Newfoundland, near the southwest Grand Banks Jurassic transform margin and along the trend of the Collector magnetic anomaly. In the Brant and Mallard wells, felsic volcanic, subvolcanic, and pyroclastic rocks are several hundreds of meters thick. Seismic reflection profiles show that the volcanic rocks are locally bounded by rejuvenated faulted margins of early rift basins beneath the mid-Cretaceous, postrift unconformity. The Early Cretaceous age of the volcanic rocks is confirmed by K/Ar geochronology.

Trace element and isotopic composition of both mafic and felsic rocks are quite different from those of plume-related Cretaceous rocks elsewhere on the eastern North American margin. Mafic rocks have a Pb isotopic composition unusually enriched in 207Pb/204Pb, which is similar to Late Triassic-Early Jurassic mafic rocks along the eastern Appalachians. Nd isotopic compositions are closer to typical ocean island basalt (OIB) sources. Observed isotopic compositions suggest mafic magma was partially melted from continental margin lithospheric mantle enriched with small amounts of hydrous or silica-rich phases of OIB affinity. Whole-rock geochemistry, isotopic composition, and mineral chemistry show that the felsic rocks resemble alkaline A-type granite. Those from Mallard have some crustal contamination; those from Brant, which is located closer to the continental margin edge, have almost pristine mantle isotopic characteristics indicating an OIB source magma.

The spatial distribution of Cretaceous volcanic products suggests that magma was generated along the reactivated transform margin and that major faults in continental crust acted as magma conduits. Partial melting of continental margin lithospheric mantle resulted from extensional decompression, probably aided by a weak plume-related thermal anomaly. As on other transform margins, magmatic products reflect a complex mantle history.

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