Petrology of the Indian Lake and Leadbetter Road plutons, Antigonish Highlands, Nova Scotia
LE3 .A278 2012
Barr, Sandra M.
Bachelor of Science
Earth & Environmental Sciences
The Indian Lake and Leadbetter Road plutons are located in the Antigonish Highlands of northern mainland Nova Scotia. They are separated by the West Barneys River plutonic suite, a large ca. 470 Ma composite pluton. This study describes the petrography and geochemistry of the Indian Lake and Leadbetter Road plutons, and investigates whether or not they are likely to have been comagmatic with each other or with other dated suites in the Antigonish Highlands. The Indian Lake pluton is not reliably dated but is interpreted to have formed at about 615–605 Ma like some other plutons in the Antigonish Highlands. It consists of a calc-alkaline suite of medium-grained quartz diorite and medium-grained monzogranite gradational to granodiorite and tonalite. Both the monzogranite/granodorite/tonalite and quartz diorite contain abundant dioritic inclusions. Chemical data from 15 samples from the Indian Lake pluton show that SiO2 content ranges from about 49% to 74% and reveal trends consistent with crystal fractionation of plagioclase and mafic minerals. The petrographic and chemical characteristics are similar to those of I-type granitoid suites formed in subduction zone settings at active continental margins. In contrast, the Leadbetter Road pluton consists mainly of coarse-grained alkali-feldspar granite with quartz phenocrysts. It is petrographically similar to some components of the West Barneys River plutonic suite. Chemical data from 6 samples shows compositions characteristic of within-plate A-type granite. SiO2 ranges from 75% to 78% and high Zr, Y, Nb, and Ga/Al ratios are distinctive. Also distinctive is the elevated rare-earth element content, with light REE up to 900 times chondritic values and heavy REE up to 60 times chondritic values. These petrological characteristics suggest that the Leadbetter Road pluton is more likely related to West Barneys River suite than to the Indian Lake pluton.CHAPTER 1 Purpose, Methods, and Previous Work 1.1 Introduction The aim of this project is to describe and interpret the petrographic and chemical characteristics of the Indian Lake and Leadbetter Road plutons in the central Antigonish Highlands of northern mainland Nova Scotia (Figs. 1.1, 1.2). The characteristics of the plutons are compared to determine if they are likely to have been co-magmatic. Archibald (2011) suggested that the Leadbetter Road pluton could be part of the Indian Lake pluton that had been separated from the main pluton by intrusion of younger gabbro and granite of the West Barneys River plutonic suite (Fig. 1.2). However, other workers have suggested instead that the Leadbetter Road pluton is part of the West Barneys River plutonic suite (Escarraga 2010; Escarraga et al. 2012; White et al. 2011). The petrographic and chemical characteristics of the plutons are also compared to other dated plutons in the Antigonish Highlands (e.g., Sandy Gunns Lake, Ohio, James River; Fig. 2.2). The Indian Lake pluton has not been reliably dated and its petrological characteristics have not been previously studied in detail, but it has been assumed to be similar to dated ca. 615-605 Ma plutons elsewhere in the Antigonish Highlands (Murphy et al. 1991; Escarraga 2010; White et al. 2011). This study attempts to determine if this inferred correlation is reasonable or not.
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