Petrology of selected Ultramafic Layers in the Reed Lake Pluton, Flin Flon-Snow Lake Greenstone Belt, Manitoba

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Young, Jeffrey
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The early Proterozoic, Reed Lake Pluton is a steeply dipping, mafic-ultramafic, subvolcanic pluton that is 4 km thick and at least 10 km long; the pluton is west facing. In cross-section, the pluton has a rectangular appearance. The pluton comprises a 100 to 300 m thick Lower Mafic Group, 335 to 700 m thick Mafic-Ultramafic Group and 3200 m thick Upper Mafic Group. Original minerals are largely replaced, but in the Mafic-Ultramafic Group, clinopyroxene is commonly relict, as are some oxides and rare olivine. In the Mafic-Ultramafic Group 15 cycles were identified, each of which consists of a lower ultramafic zone and an upper mafic zone; two country rock septa occur within the group. The 15 ultramafic zones comprise abou 20% of the group and range in thickness from 1.3 to 16.3 m. Ultramafic zones consist largely of olivine clinopyroxenite, olivine websterite, olivine-bearing clinopyroxenite, olivine-bearing websterite and clinopyroxenite, with lesser abundances of lherzolite, wehrlite, websterite and several gabbroic lithologies. In each zone there are 1 to 6 layers that range in thickness from 30 cm to 10.2 m. Crystallization in layers followed the following orders: (1) olivine, clinopyroxene, orthopyroxene, plagioclase; or (2) orthopyroxene, clinopyroxene, plagioclase. Clinopyroxene probably crystallized at about the same time as olivine. The crystallization sequence was controlled by phase equilibria and crystallization kinetics. Magmas probably crystallized near or at cotectic boundaries, saturated in either olivine or clinopyroxene. Variations in mineral abundances, chemistry and texture occur at both layer and zone scale. Most layers are ungraded or normally graded with respect to olivine, but some layers have complex olivine/clinopyroxene variations; most layer contacts are sharp. In most graded layers, maximum olivine abundance occurs slightly above to well above the base. Olivine/clinopyroxene trends in most zones varies from ungraded to normally graded to symmetrically graded; in some zones olivine is absent. Two zones, which are reversely graded, have no olivine in the lower part. Clinopyroxene ranges in composition from Wo43.8En48.2Fs8.0 to Wo42.8En43.8Fs13.9. Correlation between mineral variations and clinopyroxene is poor and En/Fs in many layers is similar across layer boundaries. Layers crystallized in situ in a boundary layer at the magma-crystal interface. Small currents probably swept across the floor locally and slower, large scale convection occurred during development of zones. Some crystal settling may have occurred locally. Ungraded layers formed at a constant rate of accumulation during in situ crystallization with efficient removal of elements across the boundary layer. Some ungraded layers represent magma that was more evolved than other layers. Normally graded layers and complexly graded layers probably formed by similar processes. The height above the base of graded layers at which oiivine abundance is highest, may be a function of compositional variations and gradients, fluctuations in temperature and crystallization kinetics. Above the magma-crystal interface a new pulse of nucleation occurred forming the next layer. Crystallization in some layers was probably halted by the onset of nucleation in overlying layers. Migration of varying abundances of intercumulus magma caused re-equilibration of mineral compositions and local deformation at a few zone contacts. Cycles formed by repeated influx of fractionated tholeiitic magma that mixed, to varying degrees, with residual magma or incorporated crystals or other material that had solidified from underlying zones; three zones formed from at least 2 batches of magma. Fractionation within zones is dependent on the size of each batch of magma, time between replenishment of magma batches and the degree of mixing and convection.