2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 4
Presentation Time: 2:15 PM


KONTAK, Daniel J., Mineral Resources Branch, Nova Scotia Department of Nat Rscs, P. O. Box 698, Halifax, NS B3J 2T9, kontakdj@gov.ns.ca

The 201 Ma North Mountain Basalt (NMB) of southern Nova Scotia, part of the Central Atlantic Magmatic Province (CAMP), outcrops as a prominent cuesta along the southern margin of the Bay of Fundy and is conformable with a basin fill sequence of underlying (Triassic) and overlying (Jurassic) terrestrial sedimentary rocks. The ca. 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. The LFU (≤180 m) is massive and dominantly holocrystalline with colonnade and complex entablature joint patterns. The LFU contains layers of comb-textured, pyroxene-rich (En10Fs78Wo12 ), mafic pegmatite (≤1-3 m) with concordant or discordant rhyolite or granophyre seams (≤3 cm). The MFU (≤165 m) contains multiple (≤15-20), thin (≤15 m), geometrically-complex flows with abundant, zonally-arranged, zeolite-bearing amygdules. The UFU (≤150 m), not everywhere present, consists of 1 or 2 flows and is massive, has colonnade and entablature jointing, and generally contains ≤30-40% mesostasis in a medium-grained, ophitic-textured host. Within the flow base the UFU locally contains felsic (ca. 70-74 wt. % SiO2) segregation pipes (3-60 cm; 10-15/m2) that are sometimes cored by agate. Whereas field evidence (e.g, stacked lobes, vesicle zonation, flow arhcitecture) suggests MFU consists of inflated flows sheets, such evidence is lacking for the LFU and UFU which, instead, represent ponded flows that filled structural depressions. Mineralogically, the NMB is dominated by augite (Wo40-30En60-20Fe10-50), calcic plagioclase (An70-30), equant- to skeletal Fe-Ti oxides and trace opx (Wo5En75Fe20) in the LFU. Variable amounts (to 30-50%) of mesostasis occurs, with skeletal-textured Fe-rich augite (toWo20En10Fe70), plagioclase (An30-60) and Fe-Ti oxide. Locally, abundant (30%) red-brown felsic (74 wt. % SiO2) glass with skeletal apatite (≤50 µm) and pyroxene (to 43 wt. % FeO) occur. Textural evidence of silicate-liquid immiscibility includes the presence of hedenbergite-like pyroxene and Fe-Ti-P - rich spheres in intermediate- to felsic glass. Mobilization of the intermediate to felsic, Fe-rich interstitial melt probably generated the layered pegmatites and segregation pipes of the LFU and UFU, respectively.