A structural reinterpretation of the Aillik Series, Labrador

Clark, A. M. S. (Anthony Miles Stapelton) (1970) A structural reinterpretation of the Aillik Series, Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

The main body of the Makkovik Peninsula, between Makkovik and Kaipokok Bays, Labrador (latitude 55°N, longitude 59°W), was mapped on a scale of 1 in. to 2000 ft., and microscopic investigation carried out on samples collected in the field, in order to determine the geological history of the Aillik Series. -- The structural complexity of the Aillik Series has been shown to be considerably greater than envisaged by previous workers. It has definitely undergone three phases of deformation, and probably more. The occurrence of the first of these three deformations in some of the lithologic units, but not in others, is considered to indicate a post-first deformation age of deposition for the unaffected units. The Aillik Series is thus divided into an older and a younger sequence, both of which have undergone the second and third deformations. -- The older sequence consists of very feldspathic, compositionally banded psammites of unknown origin. The younger sequence consists of feldspathic cross-bedded psammite, conglomerate, feldspathic psammite with quartz phenocrysts, and amphibolitised basic pillow lavas. These quartz phenocrysts (which may be of secondary origin) in one psammite unit indicate an association with high-level acid igneous activity. The very feldspathic nature of all of the psammites of both the older and younger sequences also suggests such an association. -- The first recognisable deformation, and probably earlier deformations, produced complex compositional banding in the older sequence. The second deformation caused the transposition of this banding in the older sequence, the production of a penetrative mineral orientation throughout the area in both sequences, and the development of mylonitic banding in some members of both sequences. A large recumbent anticline, numerous small folds, boudins, small slides, and possibly a large slide, were also developed during the second deformation. A large antiform and several minor structures were produced during the third deformation, though only occasionally occurring, imperfectly formed fabrics were developed. -- The metamorphic grade during the second deformation was variable. Low amphibolite facies metamorphism affected the whole of the Aillik Series, with higher grade metamorphism occurring in parts of the older sequence. -- Early dioritic orthogneiss has been intruded by a pre- or syn-second deformation granitic gneiss. Feldspar porphyritic diorite intrusions and granitic pegmatite veins show second deformation structures. Late-stage basalt, diorite and net-vein diorite intrusions show no deformational structures or fabrics. -- The quartzo-feldspathic nature of the rocks, and the lack of acicular or platey minerals, makes evaluation of the structural history extremely difficult. This difficulty is compounded by the late-stage annealing recrystallisation and development of polygonal fabric, which has almost completely destroyed earlier fabrics.

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