Uranium-lead zircon geochronology of the Yellowknife volcanic belt and subjacent rocks, N.W.T., Canada: Constraints on the timing, duration, and mechanics of Greenstone belt formation.

摘要

U-Pb zircons dating of supracrustal rocks and subjacent gneisses near Yellowknife indicate a complex history of volcanism, sedimentation, deformation and plutonism that spans more than 340 million years. The supracrustal rocks can be divided into at least four distinct sequences separated by considerable increments of time (40-100 million years). On the basis of U-Pb dating of zircons, the lowermost part of the Yellowknife volcanic belt (Kam Group) can be further divided into two sequences. The youngest is a 2712-2701 Ma sequence of metabasalts, gabbroic intrusions, and interlayered felsic tuffs. The older sequence underlies a distinctive ash layer (Ranney chert). In the southern part of the belt, the chert is dominated by volcanic zircons that are ca. 2712 Ma whereas in the north there is a large detrital component with ages as old as 2.87 Ga.;The Dwyer Group, a sequence of quartzite, metarhyolite, and banded iron formation, occurs structurally beneath the Yellowknife greenstone belt and unconformably upon basement. Detrital zircons from the quartzite yield Pb-Pb ages ranging from 2.95 to ;The youngest volcanic rocks in the Yellowknife belt are the intermediate rocks of the Banting Group. U-Pb ages from volcanic rocks as well as quartz-feldspar porphyry dike swarms yield ages of ca. 2.66 Ga. The Jackson Lake Formation consists of sandstones and conglomerates exposed along the boundary between the Kam and Banting Groups. An age of 2605 Ma determined for a granitic clast provides a maximum age of deposition for this unit.;A model involving deposition of the lower Kam in a basin between a continental fragment (Anton) and an island arc (upper Kam) is proposed. The Banting Group and Jackson Lake Formation must be related to much younger events perhaps along the tectonically active margins of the Slave Craton.