The relationship between the Aillik Group and the Hopedale Complex, Kaipokok Bay, Labrador

Marten, B. (Brian Ernest) (1977) The relationship between the Aillik Group and the Hopedale Complex, Kaipokok Bay, Labrador. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Hopedale Complex, of Archean age, is a heterogeneous assemblage of banded biotite gneiss, migmatite, amphibolite and granite. A polyphase structural history that pre-dates deposition of the Aillik Group is recognised in the Complex. -- The structurally overlying Aillik Group is an Aphebian volcano sedimentary sequence comprising a lower division of metabasaltic lavas and metasediments, overlain disconformably by an upper division of conglomerate, rhyolite and acid volcanogenic sediments. The Aillik Group is in the order of 3,600 m. thick in the Kitts-Post Hill belt. -- Deformation during the Hudsonian orogeny was characterised by the formation of major tectonic slides and by remobilisation of the Hopedale Complex beneath the Aillik Group, with syntectonic intrusion of a major suite of acid plutons into the Aillik Group. The first two events, D₁ and D₂, were essentially restricted to the basement-cover contact zone, and to certain discrete stratigraphic horizons in the Aillik Group that developed as tectonic slides. Intense flattening and transposition of Archean structures in the Hopedale Complex occurred in a zone up to 700 m. wide at the basement-cover boundary. The Hopedale Complex-Aillik Group unconformity was obliterated and the contact is now marked by a concordant gradational zone of tectonically interleaved refoliated gneiss and amphibolite (Aillik Group metavolcanics). The D₁-D₂ structural zones appear to have been originally subhorizontal, and they are thought to signify essentially translative movement between basement and cover. Major D₁-D₂ thrust slices in the Aillik Group are inferred; some involve wedges of basement gneiss. There is also tenuous evidence for major nappe structures. -- The culminative event was D₃, a regional east-northeast-trending shear-belt style deformation dominated by subvertical tectonic slides that replace the limbs of major folds. Metamorphic conditions were in the middle amphibolite fades, Migmatisation and anatexis of the Hopedale Complex was synchronous with and controlled by the third deformation in a zone subconcordant with the basement-cover contact. Sequential emplacement of three major acid intrusive bodies in the Aillik Group was also synchronous with D₃. -- Uranium mineralisation occurs locally in D₁-D₂ tectonic slides that were major dilational zones at an early stage in their formation. It is thought that uranium was hydrothermally mobilised from the acid volcanic rocks into the D₁-D₂ dilational zones, with reduction and deposition of uranium in amphibolitic, graphite- and sulphide-bearing lithologies. -- Correlation of the lower Aillik Group with the Moran and Mugford Groups in Labrador, and with the Vallen and Sortis Groups in Greenland is suggested. The upper Aillik Group resembles Aphebian acid volcanics in the south end of the Labrador Trough, and Ketilidian supracrustal rocks in southwest Greenland.

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