Geochemistry of late Middle Jurassic–Early Cretaceous igneous rocks on the eastern North American margin

Abstract

Geochemical analyses have been made of late Middle Jurassic to Early Cretaceous volcanic rocks on the eastern North American margin from Baltimore Canyon northeastward to the Newfoundland Seamounts. Samples from the Newfoundland Seamounts were obtained from dredge hauls, from the New England Seamounts from Deep Sea Drilling Project holes, and from the Grand Banks, Scotian Shelf, Georges Bank, and Baltimore Canyon area from exploratory oil wells. All of the rocks analyzed are alkali basalts and related mafic rocks. The analytical data allow the rocks to be divided into two distinct igneous suites. The New England suite includes the Baltimore Canyon, Georges Bank, and New England Seamounts. Rocks from these areas show marked enrichment in light REE and many incompatible trace elements. The Eastern Canadian suite, consisting of rocks from the Scotian Shelf and Grand Banks, show slightly convex REE spectra and less enrichment in incompatible elements. The rocks from the New England suite contain high-pressure clinopyroxene phenocrysts, some of which show complex zoning; these are absent in rocks from the Eastern Canadian suite. The data available from these offshore areas is limited and does not permit a unique petrogenetic explanation for the geochemical differences between the two suites. The Eastern Canadian suite resembles early-formed alkali rocks from Loihi Seamount, interpreted as the products of a low degree of partial melting of a MORB-type mantle source. This source was slightly depleted in LREE in order to produce the observed convex LREE spectra. The New England suite closely resembles many ocean island basalt suites. Our data do not allow discrimination between various current hypotheses for the origin of relative enrichment in incompatible elements of alkali olivine basalts. Variations in element ratios for geochemically very similar element pairs (such as La/Ce and Zr/Hf) suggest that the mantle beneath the two suites may have different compositions. The mantle composition inferred for the New England suite is similar to that postulated by many authors as resulting from mantle plumes. The localization of the volcanism in the Early Cretaceous, however, is related to the development of fractures, some of which resulted from reactivation of old fracture zones.