Ravindra P. Sinha

The Triassic volcanic rocks of North Mountain, Nova Scotia, consist of four basalt flows which extend from Cape Split to Brier Island. .Additional outcrops of the basalts are located in Colchester and Cumberland counties. Between the third flow and the youngest flow, interflow sediments occur which are continuous throughout North Mountain. This indicates that the eruptions of the basalts have been in two distinct phases. The thickness of the flows varies considerably depending upon the pre-eruption local topography.

The basalts erupted along fissures; no volcanic necks which might have formed central vents were found. Where the flows have extruded subaqueously, pillow lavas have formed. These occur at Scots Bay, Horse Shoe Cove and several other localities. Rheomorphic dykes and rootless dykes can be observed in a number of outcrops. The rocks near McKay Head are volcanogenic they consist of peperites and volcanic breccia; earlier these rocks were considered to be sedimentary conglomerates.

The basalts are tholeiitic. The chief mineral constituents of the basalts are plagioclase, monoclinic pyroxene and magnetite. The composition of the plagioclase varies from Ar33 (microlites) to An73 (microphenocrysts). A trend in the anorthite content of the plagioclase microphenocrysts is observed in the flows. The clinopyroxenes consist of Ca-rich pigeonite, subcalcic augite, and augite. The three Pyroxenes are found to co-exist together in the North Mountain basalts.

Zoning is common among the clinopyroxenes; the core is either pigeonite or subcalcic augite. No conspicuous modal or textural differences between the flows have been observed. A change in the slope of the crystallization curves of plagioclase and pyroxene is attributed to the interplay of nucleation and crystal growth. Variation diagrams plotted from fifty-five chemical analyses appear to show that the two phases differ in calcium content. The flows also differ in their ferrous and ferric oxide content; this is considered to be due to differential oxidation of FeO in different flows.

Evidence of assimilation is found in the samples from Cape d'Or region. The assimilated material is conceivably sandstone or marly sandstone.

The abundance of trace elements in the North Mountain basalts indicate a similarity between the continental tholeiitic basalts and oceanic tholeiitic basalts.

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Supervisor: C. Friedlaender