Abstracts
Abstract
The ca. 370 Ma Eastern Shore dykes include six single or multiple (two or three) parallel spessartite dykes that trend NNW in the Sheet Harbour area of eastern mainland Nova Scotia. Spessartite is a variety of calc-alkaline lamprophyre that characteristically occurs in dykes associated with granitoid rocks and gold mineralization in orogenic belts. Spessartite dykes typically contain crustal xenoliths and xenocrysts, a feature shown by three of the Eastern Shore dykes (Popes Harbour, Tuff Island, and Borgles Island). The Eastern Shore dykes are mineralogically and chemically typical of spessartite but show wide chemical variation and higher MgO, lower TiO2 and P2O5, and flatter chondrite-normalized rare-earth-element patterns than are typical of spessartite. The pair of dykes at Sober Island show internal variations that are evidence of magma evolution resulting from plagioclase and amphibole fractionation; the East Jeddore and Little Harbour Road and Coast dykes are similar to the least evolved Sober Island spessartite. The three xenolith/xenocryst-bearing dykes show anomalous chemical features such as more enrichment in light rare-earth elements (REE), higher Sr, Y, Zr, and Hf, and more varied compositions compared to the other dykes. The Popes Harbour dyke is characterized by low Ni. Despite these differences, mainly parallel chondrite-normalized REE patterns suggest that the Eastern Shore dykes are related and derived from hydrous large-ion-lithophile- and high-field strength-element-enriched garnet-bearing mantle-derived magma that experienced variable crustal contamination.
Keywords:
- Meguma terrane spessartite,
- calc-alkalic lamprophyre,
- whole-rock geochemical data,
- subcontinental li
Résumé
Les dykes de la côte est, qui remontent à environ 370 Ma, comprennent six dykes parallèles de spessartite, simples ou multiples (deux ou trois), orientés vers le nord-nord-ouest dans le secteur de Sheet Harbour, dans l'est de la partie continentale de la Nouvelle-Écosse. La spessartite est une variété de lamprophyre calcoalcalin habituellement présent dans des dykes associés à des roches granitoïdes et à une minéralisation aurifère dans les ceintures orogéniques. Les dykes de spessartite renferment généralement des xénolites et des xénocristaux crustaux, une caractéristique relevée dans trois des dykes de la côte est (Popes Harbour, île Tuff et île Borgles). Les dykes de la côte est sont similaires à la spessartite sur les plans minéralogique et chimique, mais ils présentent une diversité marquée des compositions chimiques, des teneurs supérieures en MgO, des teneurs inférieures en TiO2 et P2O5, et des spectres des terres rares normalisés aux valeurs chondritiques plus plats que ceux habituellement observés dans la spessartite. La paire de dykes de l’île Sober affiche une diversité interne qui témoigne d’une évolution du magma résultant du fractionnement du plagioclase et de l'amphibole; les dykes d’East Jeddore et de la côte et du chemin Little Harbour sont analogues à la spessartite de l’île Sober la moins évoluée. Les trois dykes abritant des xénolites/xénocristaux présentent des caractéristiques chimiques anomales telles qu'un enrichissement plus important en éléments des terres rares légers (ÉTR), des concentrations plus élevées en Sr, Y, Zr et Hf, ainsi que des compositions plus variées que les autres dykes. Le dyke de Popes Harbour se distingue par une faible teneur en Ni. Malgré ces différences, les spectres principalement parallèles des terres rares normalisés aux valeurs chondritques laissent supposer que les dykes de la côte est sont apparentés et dérivés d'un magma mantellique grenatifère hydraté, enrichi en éléments lithophiles à grand rayon ionique et à haute intensité, ayant subi une contamination crustale variable.
Appendices
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