GEOCHEMICAL AND PETROLOGICAL ANALYSIS OF NORTH MOUNTAIN BASALTS (UPPER FLOW) AT SCOTS BAY, NOVA SCOTIA

Rift basins that formed ~200 Ma during the breakup of Pangea at the Triassic-Jurassic boundary are located along the east coast of North America from Nova Scotia to South Carolina. The basins contain Newark Supergroup sedimentary rocks that are intercalated with tholeiitic basalts associated with the Central Atlantic Magmatic Province (CAMP). In Nova Scotia, CAMP volcanic flows of the North Mountain (NM) basalts comprise three major units. In this preliminary study petrographic, XRF and SEM-EDS data were collected on 10 samples from the uppermost flow at Ross Creek near Scots Bay (Cape Split, Bay of Fundy) to expand the data set on NM basalts and to assess the conclusion of others that assimilation does not vary along strike within the region.

Primary phases are plagioclase and clinopyroxene in subequal amounts, minor opaques and varied amounts of devitrified glass. Secondary zeolitization is common; samples have LOI values < 1.5% and are interpreted to be ~ unaltered. Petrographic and SEM-EDS analysis indicate some plagioclases are zoned concentrically whereas others exhibit normal zoning; rims of zoned pyroxenes are more calcic. Using a Fe3+/Fe2+ CIPW normalization factor of 0.15, eight samples are quartz normative (up to 6 wt.%) with Mg#’s of 54-57 and two samples are olivine normative (2 and 4 wt.%) with Mg#’s of 57. Major and trace element geochemical data (e.g., Ba/La, Th/La and Rb/Sr ratios and concentrations of Rb and Ba) suggest the Scots Bay basalts/basaltic andesites are typical continental tholeiites and agree with previous studies of NM basalts. REE patterns show moderate enrichment in LREE vs. HREE and plots of abundance ratios (element/Yb; chondrite normalized) show negative Nb anomalies. All patterns are similar to those of previous studies; however our ratios/concentrations are elevated. Additionally, Scots Bay samples have slightly higher K₂O and Rb/Sr.

All initial results indicate the Scots Bay basalts likely resulted from partial melting of an upper mantle source similar to that of oceanic tholeiites, with evidence of crustal contamination. However, our samples have ~ low Ce/Pb ratios (3.9-6.2) vs. most continental tholeiites – which may indicate even greater crustal contamination than reported by others for NM sites further south.