North Mountain basalt from Digby, Nova Scotia: models for a fissure eruption from stratigraphy and petrochemistry

Abstract

The early Mesozoic, quartz normative, North Mountain basalts in southwestern Nova Scotia (Digby area) form three units: a coarse massive lower flow (~190 m) bearing minor lenses of mafic pegmatite, a middle unit of thin amygdaloidal basaltic flows (~50 m), and an upper flow unit of massive phenocryst-rich basalt (~160 m). The two thick units show phenocrysts of orthopyroxene (bronzite) and (or) pigeonite, augite, and zoned plagioclase in a granular matrix of augite, pigeonite, and plagioclase. Variation diagrams and chondrite-normalized rare-earth-element patterns relate all chemical diversity between and within flows to removal and (or) accumulation of plagioclase and pyroxene phenocrysts (~1:1). High K, Rb, and Ba, appear related to assimilation of continental crust. Constancy of fractionation-independent element ratios and variations in phenocryst content vertically and along the 200 km strike of the basalts suggest (1) crystal settling and accumulation together with assimilation and mixing in a lower crustal magma chamber, (2) rise to upper crustal levels in a central conduit followed by northeast-ward emplacement along a tension-induced dyke system, and (3) extrusion along a fissure in two major and numerous minor pulses that formed the lower, upper, and middle units. Assimilation did not occur as magma moved through the dyke system, for assimilation-related variations in composition do not occur along strike.

40Ar/39Ar age of the Jurassic North Mountain Basalt, southwestern Nova Scotia

“…The coincidence of these chronometers and implication of rapid post-eruption cooling of the basalt sequence based on estimated cooling times (Papezik et al 1988) are consistent with the inferred duration of 580 ka for the Mesozoic basaltic magmatism in the Newark Supergroup (Olsen and Fedosh 1988;Olsen 1997). We have no reason to doubt the younger ages reported earlier for the NMB (Carmichael and Palmer 1968;Hayatsu 1979); however, given the presence of alteration within these rocks it may be that this phenomenon is more signifi cant than realized.…”

“…Somewhat similar geological relationships are observed on the north side of the Bay of Fundy, but in places the NMB rests on basement rocks of Carboniferous age and is overlain by sandstone and siltstone of the Jurassic McCoy Brook Formation (Greenough et al 1989;Olsen et al 1987). The stratigraphy of the NMB has recently been reviewed by Kontak (2002) based on recent fi eld studies which follow on the earlier work of Lollis (1959), Papezik et al (1988), and Greenough et al (1989), and a summary of this stratigraphy follows.…”

Layered rhyolite bands in a thick North Mountain Basalt flow: the products of silicate liquid immiscibility?

“…comm.). The basalts are chemically typical of high-Ti quartz-normative tholeiites from the Early Jurassic, Eastern North American dolerite province (Dostal and Dupuy, 1984;Papezik et al, 1988).…”

Geochemical Effects of Alteration on Mafic Rocks from Indian Ocean Site 706

“…Rare earth element mobility in basaltic rocks is well documented for a number of geologic environments (Floyd, 1977;Hellman et al, 1979;Nystrom, 1984), including the ocean floor. Glass tends to be more significantly affected than crystalline basalt (Frey et al, 1974 It is well known that Cu is highly mobile during zeolite facies metamorphism of basalts (e.g., Papezik et al, 1988). During low-grade metasomatism of ocean-floor basalts, Cr and Ni are either immobile or slightly removed whereas Cu and Pb tend to be added and Zn removed (Frey et al, 1974;Mottl, 1983).…”