A detailed petrographic, geochemical and geochronological study of the Hare Bay Gneiss, northeastern Newfoundland

Langille, Amanda E. (2012) A detailed petrographic, geochemical and geochronological study of the Hare Bay Gneiss, northeastern Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Hare Bay Gneiss occurs as three discontinuous northeast-trending belts along the northeastern margin of the Gander Zone and has been described briefly in previous literature as a highly deformed assemblage of paragneiss, orthogneiss and migmatite. Past interpretations of the Hare Bay Gneiss include that it is a highly deformed equivalent of the adjacent Square Pond Gneiss (Blackwood, 1978) and some studies (Blackwood and Kennedy, 1975) proposed it as potential basement to the Gander Zone. Lack of modern documentation and analyses of the rock types that constitute the Hare Bay Gneiss impede definitive interpretations from being made. In an effort to better understand the Hare Bay Gneiss and its importance to the Gander Zone as a whole; this study combines field observations with petrological, geochemical and geochronological data from well exposed sections from Wind Mill Bight Provincial Park Reserve in the north and the town of Hare Bay, in the south. -- Through detailed field mapping and petrographic analysis it is evident that a diverse assemblage of rocks constitutes the studied exposures of the Hare Bay Gneiss. Based on the seven selected study localities the northern belts of the Hare Bay Gneiss predominantly consist of greenschist-facies orthogneiss ranging from granitic to tonalitic in composition, and also includes locally abundant, variably deformed granitic intrusions. The mapped area of the Hare Bay Gneiss contains several minor rock types including tonalite, paragneiss, pegmatite, mafic intrusions, and quartz veins. Nearly all of the orthogneissic and granitic rocks have peraluminous geochemical signatures and REE patterns diagnostic of crustal melts. -- Nine main lithologic units yield concordant ages using the CA-TIMS U-Pb zircon technique. In the Windmill Bight map area four dated units constrain magmatism, ranging from 428 ± 2 Ma for foliated leucogranite to 387 ± 2 Ma for late pegmatite. Leucogranite and At-silicate-bearing orthogneiss in the Greenspond Road section yielded ages of 460 ± 2 Ma and 510 ± 4 Ma, respectively, making the orthogneiss the oldest dated unit so far in the Hare Bay Gneiss. Although lithologically similar, the granitic Valleyfield orthogneiss and the Hare Bay orthogneiss have contrasting concordant U-Pb zircon ages of 465 ± 2 Ma and 491 ± 4 Ma, respectively. Finally a tonalite intrusion cutting an orthogneiss in the "I Love You" section yielded a zircon age of 415 ± 1 Ma. -- The new data demonstrates the complexity of the Hare Bay Gneiss, which contains intrusive rocks ranging in age from Series 2 of the Cambrian to mid- Devonian (Givetian). Based on these data, it is unlikely that these rocks are metamorphic equivalents of adjacent gneisses, or basement rocks to the Gander Zone. Rather they may be partial melts of Gander Group sedimentary rocks and mainly represent pulses of magmatism into crust of the Gander Zone. Many magmatic events recorded in this study are observed elsewhere in the Gander Zone in Newfoundland; specifically Ordovician, Silurian and Devonian ages can be correlated in time with Penobscot, Salinic and Acadian orogenic events recognized elsewhere in the Gander Zone in Newfoundland.

Item Type: Thesis (Masters)
URI: http://research.library.mun.ca/id/eprint/10849
Item ID: 10849
Additional Information: Includes bibliographical references (leaves 151-157).
Department(s): Science, Faculty of > Earth Sciences
Date: 2012
Date Type: Submission
Library of Congress Subject Heading: Gneiss--Newfoundland and Labrador--Analysis; Petrofabric analysis--Newfoundland and Labrador; Analytical geochemistry--Newfoundland and Labrador; Geological time.

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