Cooling history and differentiation of a thick North Mountain Basalt flow (Nova Scotia, Canada).

A thick (<175 m) North Mountain Basalt flow at McKay Head, Nova Scotia (Canada) shows ∼25-cm-thick differentiated layers separated by ∼130 cm of basalt in its upper 34m. Upper layers (∼5 m below the lava top) are highly vesicular whereas lower ones are pegmatitic and contain a thin (∼2 cm) rhyolite band. The layering of the flow closely resemble that of some Hawaiian lava lakes. The eesicular basalts and mafic pegmatites are inferred to be liquid-rich segregations which drained into horizontal cracks that formed within a crystalline mush. The cracks resulted from a thermal contraction associated with cooling and shrinkage of the mush. Rhyolites were formed by in situ differentiation. Gas overpressures fractured the pegmatites and gas effervescence filter pressing forced silicarich residual liquid from pegmatite interstices into the fractures creating bands. Chemical differences between the pegmatitic layers and early formed, highly differentiated upper vesicular layers may reflect a role for volatiles in the differentiation process along with crystal fractionation. [ABSTRACT FROM AUTHOR]