Amphibole and plagioclase chemistry in two Proterozoic dyke swarms and its relation to the uplift history of the Kapuskasing Structural Zone

Abstract

The gneisses of the Chapleau portion of the Kapuskasing Structural Zone (KSZ) and of the Wawa Gneiss Terrane (WGT) can be partitioned into subregions on the basis of magnetic polarity of 2.45 Ga Matachewan diabase dykes emplaced within them. West of the Ivanhoe Lake fault zone (ILFZ) the polarity sequence across the trend of the dyke swarm is normalreversenormal (N -R -N) over a traverse distance > 100 km. Highly correlated with dyke N polarity is the presence of tea-coloured groundmass plagioclase and hornblende with high Al, Na, and Ti content. Reversely magnetized dykes within the WGT and in dykes of either polarity in regions far removed from the KSZ have groundmass plagioclase with hydrous alteration and relatively Al-poor amphiboles. Although N and R magnetized dykes have groundmass plagioclase with comparable Fe contents, the plagioclase of N dykes and Kapuskasing dykes in the highest grade country rock contain discrete, micrometre-sized, Ti-poor magnetite particles. The mineralogical variations are independent of whole-rock bulk chemistry and are ascribed to greater crystallization depth for N magnetized dykes. The magnitude of the amphibole compositional change in dykes at the western N-R boundary within the WGT is comparable to that across the ILFZ. The western N -R boundary is wholly within the WGT, whereas the ILFZ juxtaposes upper and lower crustal levels. As a consequence, only some of the differential uplift between the KSZ and the Abitibi belt can be accounted for in post-Matachewan dyke time. Limited data from the amphiboles in the 2.04 Ga east-northeast-trending Kapuskasing dykes suggest that this phase of faulting along these block boundaries postdates Kapuskasing dyke emplacement.Une Ctude de la polarit6 magnttique des dykes de la diabase de Matachewan, PgCs de -2,45 Ga, qui recoupent les gneiss de la rCgion de Chapleau, dans la Zone scructurale de Kapuskasing (ZSK) et du Terrane gneissique de Wawa (TGW), a permis de rCpartir ces gneiss en sous-rtgions. A l'ouest de la zone de failles du lac Ivanhoe (ZFLI), la sCquence des polaritCs qui traverse la direction de l'essaim de dykes est normaleinversenormale (N -I -N) sur une distance transversale de > 100 km. I1 existe une excellente correlation entre les dykes de polarit6 N et la presence d'une matrice, de couleur du thC vert, formCe de plagioclase et de hornblende ayant une teneur ClevCe d'Al, Na et Ti. Les dykes d'aimantation inverse dans le TGW, et les dykes des polaritCs N et I dans les rCgions Cloignees de la ZSK, poss&lent une matrice formCe de plagioclase alttrCe en argile et des amphiboles contenant relativement peu d'Al. Quoique les teneurs en Fe dans la matrice de plagioclase soient comparables dans les dykes de polarit6 normale et inverse, le plagioclase des dykes de polarit6 normale et des dykes de Kapuskasing dans les roches encaissantes les plus intenskment mCtamorphisCes renferme des particules discrktes, de la taille du micrombtre, de magnetite pauvre en Ti. Les variations minCralogiques sont indkpendantes de la composition chimi...