Gavin S. Sinclair

Fieldwork was carried out in the Makkovik Bay area of the Paleoproterozoic Aillik domain of the Makkovik Province, Labrador, to determine the field relationships of units in the study area, which aided collection of samples for geochemical analysis and argon thermochronology. The Aillik domain is the middle and least understood of the three domains of the Makkovik Province. The Kaipokok domain to the north of the Aillik domain is composed of mainly Archean crust and the Paleoproterozoic Cape Harrison domain to the southeast is composed mainly of juvenile crust. The Aillik domain in the Makkovik Bay area consists mainly of the ca. 1860 Ma Upper Aillik Group, Measles Point Granite, Kennedy Mountain Granite and a mafic dyke swarm.

The Upper Aillik Group consists of a felsic-dominated bimodal volcanic suite with minor mafic and sedimentary rocks. Sedimentary rocks are associated mainly with the mafic volcanic rocks and although their stratigraphy is not fully understood, the limited stratigraphic information indicates that the sediments were deposited in a rift basin during extrusion of mafic lavas and minor felsic lavas. The felsic volcanic rocks which occur above the rift sediments represent the largest volume of volcanic rocks. Closely associated with the felsic volcanic pile is the Measles Point Granite, a hypabyssal intrusion recognised during the present study. Intruding only the Upper Aillik Group and Measles Point Granite is a mafic dyke swarm. Following intrusion of the dykes the Upper Aillik group was thrust westwards onto the Kaipokok domain. The Kennedy Mountain Granite is not intruded by the mafic dykes and field relationships indicate it was emplaced after thrusting. A significant feature of all these rocks is that they were metamorphosed regionally to amphibolite facies, and have locally retrogressed to greenschist facies within and near the straightened zone, a zone of shearing which cuts through the study area and was formed syn- thrusting and was reactivated following the intrusion of the Kennedy Mountain Granite.

Major, trace and rare element geochemical analysis of samples of felsic and mafic volcanic rocks of the Upper Aillik Group, amphibolite dykes, Measles Point Granite and Kennedy Mountain Granite has yielded insight into the development of the Makkovik Province. The A-type geochemical characteristics of the felsic volcanic rocks and Measles Point Granite indicate that they formed in a within-plate tectonic setting. The chemistry of the mafic dykes supports this interpretation, but the mafic samples from the Upper Aillik Group display a mixed chemical signature ranging from MORB to volcanic arc. The geochemical signature of the Kennedy Mountain Granite is similar to that of the felsic and Measles Point Granite samples, although it is interpreted to be younger.

⁴⁰Ar/³⁹Ar thermochronology of amphibole and muscovite separates from samples collected across the study area has revealed two domain-wide thermal events following the intrusion of the Kennedy Mountain Granite. The amphibole separates dated by incremental heating record an age range of 1740-1720 Ma, correlative with province-wide ca. 1720 Ma granitic plutonism. The muscovite separates were dated by incremental heating and with a laserprobe, producing an age range of 1640-1660Ma which correlates with ca. 1650 Ma and ca. 1640 Ma intrusions in the Aillik and Cape Harrison domains. This age range lies within the error ranges of a ca. 1635 Ma amphibole separate from the straightened zone.

The combined field study and geochemical data are interpreted to indicate that the Aillik domain was formed by rifting of a continental margin. The mechanism for rifting is postulated to be slab-rollback of oceanic crust, which could explain the mixed chemical signature in the mafic volcanic samples. Following rifting, the Upper Aillik Group was thrust onto the Kaipokok domain (postulated to have occurred around 1820 Ma) and argon thermochronology indicates the Aillik domain was tectonically active for at least 150 Ma following thrusting. The nature of these later tectonic events is unknown.

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Supervisor: Nicholas Culshaw