John W.F. Ketchum

Evolution of the southern Abitibi greenstone belt based on U–Pb geochronology: autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation

Abstract New mapping and U–Pb zircon geochronological results in the southern Abitibi greenstone belt (SAGB) support an autochthonous regional stratigraphy comprised of nine supracrustal assemblages, rather than the collage of allochthonous terranes proposed in recent publications. Based on lithological and geochronological criteria these supracrustal assemblages also encompass the Swayze, Shining Tree and Montcalm greenstone belts indicating that, despite separation by large granitic intrusions, they are in fact part of one extensive greenstone belt. In conjunction with the mapping, the geochronological results support coherent, upward-facing stratigraphic sections with about 20% of the samples containing inherited zircons with ages similar to those found in underlying assemblages. The seven oldest assemblages represent semi-continuous volcanism from 2750 to 2697 Ma. The volcanism is compositionally diverse ranging from komatiite and tholeiitic basalt to calc–alkaline mafic to felsic lavas. Intimate intermingling of the different magma clans occurs throughout much of the stratigraphic section. Regional fault control on the distribution of the volcanic assemblages provides evidence for early dip–slip movement associated with volcanic extension dating back to at least 2725 Ma. The two youngest assemblages are dominantly sedimentary and were unconformably deposited on the volcanic assemblages in close proximity to regional faults. The earlier sedimentary assemblage consists of turbidites and minor iron formation deposited from 2696 to 2692 Ma, and the youngest one of subaerial conglomerates, fluvial sandstones and alkalic to shoshonitic volcanic rocks ranging in age from 2687 to 2675 Ma. The sedimentary assemblages are broadly contemporaneous with the emplacement of syntectonic granitic plutons, regional folding and reactivated movement on the regional faults probably related to accretion of the Abitibi to the Superior Province craton. Autochthonous repetition of different geodynamic environments over the 50 million years of volcanic activity with five dominantly tholeiitic ± komatiitic assemblages and two dominantly calc–alkaline assemblages, as well as the extensive intermingling of these different magma clans in a number of the assemblages, appears to be unique to the Archean. It suggests complex, large-scale and long-lived interaction between mantle plumes and subduction zone magmas. The Nd isotope data indicate a derivation of the diverse volcanic magmas from homogeneously depleted sources with eNd=2.5±0.5 demonstrating a lack of contamination by ancient enriched components.

Structural evolution of the Makkovik Province, Labrador, Canada: Tectonic processes during 200 Myr at a Paleoproterozoic active margin

In the Makkovik Province, the structures and plutonic rocks of the reworked southern edge of the Archean Nain craton (parautochthon) and an adjacent juvenile domain form a spatially concentrated record of ∼ 200 Ma at a Paleoproterozoic active margin. D1 thin- and thick-skinned structures, formed during terrane collision before ca. 1896 Ma, were nearly erased by D2 transpressive structures present across the entire parautochthon that had been thermally softened by emplacement of arc plutons at ca. 1895–1870 Ma. D3 transpressive inversion of a back arc rift and D4 strike-slip shearing formed structures at the oceanward margin of the parautochthon and were responses to events in the active arc that was now widely separated from the parautochthon by accreted terranes. The tectonic isolation of the parautochthon ended by the time of D5 (ca. 1740–1710 Ma) and D6 with the reappearance there of granite and low-grade shear zones. Structural and plutonic style varied at each stage, primarily controlled by the crustal level attained.

Field relations, geochemistry, and age of Paleoproterozoic igneous rocks in the northeastern Kaipokok Bay area, Makkovik Province, Labrador

The northeastern part of Kaipokok Bay is located in the Makkovik Province of Labrador, in the boundary zone between the Kaipokok and Aillik domains. The Kaipokok domain is characterized by reworked Archean gneiss, ca. 2100 Ma mafic metavolcanic and supracrustal metasedimentary rocks, and ca. 1895–1870 Ma plutonic rocks. The Aillik domain to the southeast is characterized by metavolcanic and metasedimentary rocks of the ca. 1860 Ma Aillik Group and younger 1800 Ma (syn-Makkovikian), 1720 Ma (post-Makkovikian), and 1670–1640 Ma (Labradorian) plutonic rocks. In the study area, the Aillik Group is dominated by metasandstone with less abundant metaconglomerate and meta-rhyolite, and rare calc-silicate and mafic volcanic rocks. The largest plutons in the area are the ca. 1802 Ma Long Island Quartz Monzonite and Kennedy Mountain granite, part of the Kennedy Mountain Intrusive Suite of the Aillik domain. A U-Pb (zircon) age of 1800.6 ± 2.3 Ma for the Kennedy Mountain granite confirms that it is the same age as the Long Island Quartz Monzonite, as inferred by earlier workers in the area. Plutons of similar age (Drunken Harbour and Hares Islands granites) have been documented farther northwest in the Kaipokok domain. The presence of compositionally similar 1800 Ma plutons in both the Aillik and Kaipokok domains is consistent with tectonic models which show that the domains were juxtaposed by that time. These plutons have within-plate, A-type characteristics, and were likely emplaced during regional transpression that was focussed on the Kaipokok Bay shear zone between the Kaipokok and Aillik domains. Labradorian-age (1670–1640 Ma) plutonic rocks occur in both the Kaipokok and Aillik domains and range in composition from ultramafic to gabbro and granite. These plutons have diverse petrological characteristics and their ages are not well constrained. Their tectonic setting during emplacement is uncertain, as they are located far from the focus of Labradorian orogenic activity in the Grenville Province. Resume La partie nord-ouest de la baie de Kaipokok est situee dans la province de Makkovik au Labrador, a l’interieur de la zone bordiere separant les domaines de Kaipokok et d’Aillik. Le domaine de Kaipokok se caracterise par la presence de gneiss archeen, de roches metasedimentaires supracrustales et metavolcaniques mafiques d’environ 2100 Ma, ainsi que de roches plutoniques ayant environ 1895 a 1870 Ma. Le domaine d’Aillik au sud-est est caracterise par des roches metavolcaniques et metasedimentaires du groupe d’environ 1860 Ma d’Aillik et des roches plutoniques plus recentes de 1800 Ma (synmakkovikiennes), de 1720 Ma (post-makkovikiennes) et de 1670 a 1640 Ma (labradoriennes) plutoniques. Dans le secteur d’etude, le groupe d’Aillik est principalement compose de metagres et d’une quantite moins abondante de metaconglomerat et de metarhyolite, ainsi que de rares roches volcanomafiques et silicates calciques. Les plutons les plus volumineux dans le secteur sont l’adamellite de l’ile Long d’environ 1802 Ma et le granite du mont Kennedy, qui fait partie du cortege intrusif du mont Kennedy du domaine d’Aillik. Une datation U-Pb (zircon) de 1800,6 ± 2,3 Ma du granite du mont Kennedy confirme qu’il a le meme âge que l’adamellite de l’ile Long, comme l’avaient suppose des chercheurs anterieurs dans le secteur. Des plutons d’un âge similaire (granite de Drunken Harbour et des iles Hares) ont ete documentes plus au nord-ouest dans le domaine de Kaipokok. La presence de plutons de 1800 Ma de composition analogue dans les domaines d’Aillik et de Kaipokok est conforme aux modeles tectoniques montrant que les domaines etaient juxtaposes a cette epoque. Ces plutons possedent les caracteristiques du type A intra-plaque; ils se sont probablement mis en place au cours de la transpression regionale qui etait vraisemblablement concentree sur la zone de cisaillement de la baie de Kaipokok, entre les domaines de Kaipokok et d’Aillik. Des roches plutoniques remontant au Labradorien (1670 – 1640 Ma) sont presentes dans les domaines de Kaipokok et d’Aillik; leur composition varie des roches ultramafiques au gabbro et au granite. Ces plutons presentent des caracteristiques petrologiques diversifiees et leurs âges ne sont pas bien definis. On ignore quel etait exactement leur milieu tectonique pendant leur mise en place, car elles se trouvent loin du foyer de l’activite orogenique labradorienne a l’interieur de la province de Grenville.