Andrew J. D. Kendall

B. Sc. Honours Thesis

The White Rock Formation Metavolcanics at Cape St. Mary, Nova Scotia: Petrography, Geochemistry, and Geological Affinities.

(PDF - 7.1 Mb)

A 156 meter thick section of the Silurian White Rock Formation occurs at Cape St. Mary, southern Nova Scotia in the core of a southwest plunging syncline. It includes four metabasite units identified as basaltic flows, and a basal metafelsite unit, identified as an ash flow. The nearby St. Alphonse gabbroic plug has tectonized margins, indicating a pre-deformational (Acadian) age, and seems to be related to pre-Acadian White Rock metavolcanics.

Petrographic examination of the metavolcanics reveals them to be pervasively altered, sheared, and metamorphosed to the lower greenschist facies. Primary titanaugites survive in the uppermost metabasite unit and are petrographically and chemically similar to those of the gabbroic plug.

Examination of the major, minor, and trace element geochemistry of the metavolcanics and the gabbro was undertaken in order to determine the original nature of the metavolcanics, and their paleotectonic setting. Major element mobility (particularly the alkalis) was found to be significant, and therefore their use as paleotectonic discriminator elements is limited. However, some of the trace elements (REE, Hf, Ta, Th, Ti) are found to be relatively immobile.

The rare earth elements (REE's) show the metabasites to be the metamorphosed equivalents of alkali basalts, and the basal metafelsite to have rhyolitic affinities. Other trace element and minor element discriminators (Hf-Ta-Th, TiO₂-P₂O₅, K₂O-P₂O₅-TiO₂) reveal that the metavolcanics have been erupted in a within-plate continental environment. Trace element geochemistry of the St. Alphonse gabbro and pyroxene chemistry indicates it to be geochemically related to the metabasites, but slightly more evolved.

Trace element similarities between the Cape St. Mary metavolcanics and those in the Yarmouth syncline suggest the possibility of correlation on a geochemical basis. Correlations of the more evolved metabasites of the upper parts of the White Rock at Yarmouth to the Cape St. Mary exposures suggest that the gabbro is most likely a feeder for equivalent metavolcanics which once occurred at Cape St. Mary.

Keywords:
Pages: 141
Supervisor: Gunter Muecke