Nd isotopes and the origin of 1.9-1.7 Ga Penokean continental crust of the Lake Superior region
Nd isotopic data on 26 samples demonstrate the origin of the Early Proterozoic crust of the Penokean orogen. The three major components are (1) the Marquette Range Supergroup, a predominantly miogeoclinal continental-margin sequence deposited before 1.85 Ga on Archean basement; (2) ca. 1.88 Ga felsic metavolcanic rocks of the Wisconsin magmatic terrane to the south, of evolved island-arc affinity; and (3) 1.87-1.76 Ga granitoids that intrude the metavolcanic rocks. Initial ϵ Nd values for the metavolcanic rocks of the magmatic terrane range between 0.0 and +2.4 and T DM ages between 1.9 and 2.2 b.y. The volcanic rocks primarily represent new crustal material that had only a limited Archean input, probably through mixing of subducted sediments into the magma source area. ϵ Nd (T) values for sedimentary rocks in the lower part of the Marquette Range Supergroup indicate an Archean source, most likely the 2.7 Ga Superior province to the north. On the other hand, the sedimentary rocks in the upper part of the Marquette Range Supergroup (graywackes of the upper Michigamme Formation) have initial ϵ Nd values between -0.8 and +1.5, indicative of an Early Proterozoic source. These graywackes probably are foredeep deposits derived from the volcanic rocks of the Wisconsin magmatic terrane to the south. Thus, the time of deposition of the upper Michigamme Formation dates the final convergence of the Wisconsin magmatic terrane with the continental margin. The granitoids of the Wisconsin magmatic terrane have a wide range of ϵ Nd (T) values, from -4.5 to +4.0. They represent mixtures of variable amounts of new crustal material and recycled Archean detritus. The more negative ϵ Nd (T) values occur in samples close to the northern boundary of the Wisconsin magmatic terrane, which is marked by a mylonitic shear zone (the Niagara fault zone). We suggest that the miogeoclinal lower Marquette Range Supergroup rocks having an Archean Nd signature became involved in magma genesis close to the collisional boundary, whereas increasingly lesser amounts of this older material were available for mixing farther away from the suture. The 1.9-1.7 Ga Penokean events involved major growth of new crust from the mantle.