Evolution of an Archean basement complex and its autochthonous cover, southern Slave Province, Canada

The Sleepy Dragon Complex (SDC) is one of several antiformal exposures of a 4.0–2.85 Ga basement block underlying the west-central region of the Archean Slave Province, northwestern Canada. This basement block, the Central Slave Basement Complex (CSBC), is overlain by an autochthonous to locally parautochthonous, dominantly quartz-rich clastic sequence, the 2.85–2.80 Ga Central Slave Cover Group. Together these units comprised a fundamental building block during Neoarchean growth and assembly of the Slave craton. ll-exposed Patterson Lake–Morose Lake area represents a type locality for study of the CSBC, Central Slave Cover Group, and overlying rocks of the Yellowknife Supergroup. Field mapping and U–Pb geochronology, employing both thermal ionisation mass spectrometry (TIMS) and sensitive high resolution ion microprobe (SHRIMP), document a prolonged crustal history. Oldest intact basement units consist of foliated to gneissic tonalite dated in two places at 2955 ± 12 Ma and 2944 ± 9 Ma. Indirect evidence for a ca. 3150 Ma basement component is obtained both from xenocrystic zircon in tonalite and detrital zircons from the overlying Central Slave Cover Group, which here consists of the Patterson Lake Formation. A granite boulder from this formation is dated at 2934 ± 3 Ma and may be locally derived. A sheared mafic volcanic unit along the basement-cover contact contains 2942 ± 3 Ma metamorphic titanite which provides a minimum deposition age. This volcanic unit is therefore also a part of the basement complex. r events in the SDC mainly reflect tonalite-trondhjemite-granodiorite plutonism during construction of the overlying Cameron River and Beaulieu River greenstone belts of the Yellowknife Supergroup. A unit of K-feldspar megacrystic granodiorite is dated at 2726 ± 3 Ma, and tonalite and granodiorite bodies located toward the centre of the SDC have primary crystallization ages of 2683 + 3/−2 Ma, 2677 ± 2 Ma, and 2672 + 7/−6 Ma. The core of the southern SDC is occupied by a late-tectonic to post-tectonic granite pluton previously dated at 2586 ± 2 Ma. An intravolcanic unconformity along the southern margin of the SDC (developed in part on 2.7 Ga granodiorite) is constrained by existing U–Pb data to have formed sometime between ca. 2685–2660 Ma. An abundance of synvolcanic plutons of similar age and the presence of a >3-km thick mafic volcanic sequence overlying a less dense, magmatically active felsic crust suggest that gravitational instability and magmatic diapirism may have played important roles in doming and uplift of the SDC prior to regional D1 deformation. The younger-toward-centre zonation of plutonic rocks in the SDC is documented in other domal granite–gneiss complexes of Archean age (e.g., Pilbara craton, western Australia) where it is attributed to upward and outward displacement of older crust during repeated magma channeling into a growing mid-crustal dome. In the west-central Slave craton, such a process might account for the regional presence of local unconformities of apparently similar age. The domal basement–plutonic complexes would have been further amplified after 2660 Ma during the development of regional F1- and F2-fold belts.