Petrogenesis of the Boundary Intrusions Flin Flon Area, Saskatchewan-Manitoba

Abstract

The Precambrian (Aphebian) Boundary intrusions are a set of dike-like or lensoid, ultramafic to felsic intrusions that occur in a linear zone centred on the town of Flin Flon, Manitoba. The intrusions are emplaced in Amisk metavolcanic rocks and Missi metasedimentary rocks. Cross-cutting relationships, petrography, and chemical features of Boundary rocks indicate the intrusions are comprised of three compositionally distinct, sequentially emplaced rock groups. The mafic (mela-dioritic) group is the oldest; early, melanocratic, augite- and biotite-bearing members of this group were intruded and brecciated by successive differentiates containing augite and, eventually, hornblende. Crystallization at relatively high fH20 and f02 produced an unusual differentiation trend in which successive differentiates became slightly enriched in MgO relative to FeO. Crystal fractionation of augite, possibly subordinate olivine, and minor magnetite from a tholeiitic olivine basalt magma accounts for the observed chemical variation in the mafic eroup. Felsic group rocks (lellco-diorite to granodiorite) cut and brecciate all mafic group rock types. Members of the felsic group are chemically similar to post-Missi granodiorite plutons, and do not represent Si02-rich residual liquids produced by fractionation of augite and olivine from a mafic Boundary magma. The youngest Boundary rocks are members of the olivine- bearing group (wehrlite to olivine gabbro). The major occurrence of wehrlite is in the core of a large mafic Boundary intrusion, with which wehrlite is in intrusive contact. Crystal fractionation of subequal amounts of olivine and clinopyroxene, and minor magnetite, from a tholeiitic olivine basalt magma accounts for the observed chemical variation in the olivine-bearing group. A regular increase in the amount of intercumulus hornblende in wehrlite, from the centre to the margins of the body, indicates that there was a greater water content in the marginal zones of the intrusion. Folding and faulting of the Amisk and Missi Groups occurred during the Hudsonlan Orogeny, and involved three separate phases of deformation. S2 axial-plane foliations (produced during the second phase of deformation) are developed only in the older members of the Boundary mafic group. All Boundary rock types contain structures produced during the third phase of deformation. Emplacement of the intrusions occurred during the interval late-P2 to late-P3.