GEOCHRONOLOGICAL CONSTRAINTS ON THE DEVELOPMENT OF THE CROSS LAKE GREENSTONE-BELT, NORTHWEST SUPERIOR PROVINCE, MANITOBA

Recent 1:20 000 mapping combined with 11 age determinations from metavolcanic, metaplutonic, and metasedimentary rocks in the Cross Lake greenstone belt provide a framework for interpreting the stratigraphic, structural, intrusive, and metamorphic evolution of the belt. These data, in combination with U-Pb ages from adjacent granitoid domains, are used to model a cratonizing event in the northwest Superior Province. The Cross Lake greenstone belt is made up of three distinct unconformable groups of supracrustal rocks: (i) the Pipestone Lake Group (2760 Ma), a thick sequence of pillowed and massive tholeiitic basalt flows and related high level gabbros, interpreted as a back-arc spreading sequence; (ii) the Gunpoint Group (2730 Ma), a fining-upward sequence of clastic sediments interbedded with rhyodacitic fragmental volcanic rocks; and (iii) the Cross Lake Group (<2710 Ma), a fining-upward, fluvial-marine clastic sedimentary sequence, with shoshonitic volcanic rocks near the top, deposited in a restricted basin. Angular unconformities occur between (i) the Pipestone Lake Group and the overlying Gunpoint and Cross Lake groups, and (ii) the Gunpoint Group and the younger Cross Lake Group. The greenstone belt is flanked by the plutonic Molson Lake Domain on the south and the largely metaplutonic Pikwitonei and granite -greenstone Gods Lake domains on the northwest. The northern contact is a major northeast-trending fault system with a dextral strike-slip component and vertical component indicative of relative uplift of the northwest side. The southern contact is a much broader zone of deformation in which a number of east-southeast-trending dextral fault zones imbricate Pipestone Lake and Gunpoint Group supracrustal rocks and, to a lesser extent, Molson Lake plutonic rocks.