GEOLOGICAL SETTING AND PETROLOGY OF THE 201 MA NORTH MOUNTAIN BASALT, SOUTHERN NOVA SCOTIA

The 201 Ma North Mountain Basalt (NMB) of southern Nova Scotia, part of the Central Atlantic Magmatic Province, forms a prominent cuesta along parts of the Bay of Fundy. The NMB is generally conformable with a basin fill sequence of underlying (Triassic) and overlying (Jurassic) red-brown, terrestrial sedimentary rocks. Deposition of the 400 m thick, subaerial sequence of continental tholeiites was rapid (<0.5 Ma), probably fissure fed and is subdivided into lower- (LFU), middle- (MFU) and upper-flow (UFU) units based on field relationships. The LFU (<180 m) is a single, massive, medium-grained, dominantly holocrystalline ponded flow. Based on limited field and drill hole data, it is suggested that LFU deposition was partly into a faulted-bounded trough. In its upper part the LFU locally contains layers of mafic pegmatite (<1 m) with or without thin (cm) rhyolite bands. The MFU (<165 m) contains multiple (<15), thin (1-15 m), geometrically-complex flows with abundant (<30-50%), zeolite-bearing amygdules distributed in a consistent and repetitive manner. Bottom pipe vesicles indicate a consistent NE-movement direction and the MFU has features typical of pahoehoe flows. The UFU (0-150 m) is massive, represents 1 or 2 flows, and generally contains <30-40% mesostasis in a medium-grained, ophitic-textured host. Both the LFU and UFU have well-developed columnar jointing, whereas the UFU may also contain segregation pipes (3-60 cm wide; felsic composition) towards the base. The NMB is dominated by augite (Wo_40-30En_60-20Fe_10-50), calcic plagioclase (An_30-70) and equant- to skeletal Fe-Ti oxides with trace pigeonite (Wo₅En₇₅Fe₂₀). Although uncommon in the LFU, some MFU and most UFU contain a mesostasis with skeletal Fe-rich augite (to Wo₂₀En₁₀Fe₇₀), plagioclase (An_30-60) and skeletal Fe-Ti oxide with additional components related to silicate-liquid immiscibility: (1) an interstitial, variably altered and devitrified glass of intermediate to felsic composition, and (2) spheres of Fe-Ti-P - rich silicate melt may have unmixed to a Fe-Ti-P phase and hedenbergite-like pyroxene (35 wt. % FeO). In several samples of MFU and UFU is abundant (30%) red-brown felsic (i.e., 74 wt. % SiO₂) glass with microlites (<50 µm) of skeletal apatite. Mobilization of this felsic melt probably led to formation of segregation pipes.