Age of the Mulcahy Lake intrusion, northwest Ontario, and implications for the evolution of greenstone-granite terrains

Abstract

Zircon data from a 63 km 2 layered mafic intrusion in the Wabigoon subprovince, the Mulcahy Lake gabbro, show that the gabbro crystallized at 2733.2 −0.9 +1.0 Ma. The largely unaltered and unmetamorphosed gabbro intrudes tholeiites of the Crow Lake-Savant Lake greenstone belt which were generated over at least a 10 Ma time span. Rb Sr and Sm Nd data were obtained for pristine whole rock and mineral separates from the intrusion. The Rb Sr data show disturbances in the system, particularly in the case of pyroxenes. Samples with low Rb/Sr ratios indicate an initial Sr ratio of 0.7007 for an age of 2733 Ma. The Sm Nd age is 2744 ± 55Ma, and an epsilon Nd value of +2.6 ± 1.2 indicates a light rare earth element depleted source. The Sm Nd data show less disturbance than the Rb Sr data. Granitic and mafic intrusives with similar initial ratios occur 80 km to the south in the Quetico subprovince. Primary hornblende was analyzed for 40Ar/ 39Ar. The age obtained is 2703 ± 20Ma, clearly younger than the zircon age indicating some argon loss. The Mulcahy Lake gabbro is one of four layered mafic plutons of similar petrographic character and stratigraphic age which are arrayed in an arc in the Crow Lake-Savant Lake greenstone belt. The plutons appear to represent subvolcanic chambers which developed simultaneously and which crystallized at the end of the volcanic cycle. All intrude basalts and are intruded in turn by the Atikwa batholith, a massive granitic intrusion of over 1000 km 2 in area. Previously published zircon ages for several sample sites within the batholith indicate crystallization at 2731.8 −1.3 +1.6 Ma to 2732.2 ± 2.9Ma. These data show that the batholith overlaps the age of the gabbros or is, at most, 5 Ma younger. This virtual contemporaneity is hidden in the Rb Sr and Sm Nd data. The cessation of mafic magma production and the immediate, if not parallel, voluminous production of calc-alkaline liquid may represent a process important in the development of greenstone-granite belts, a process not apparent when ages are not so precisely determined.