| Summary: | The layered series of the Muskox Intrusion can be divided into four megacyclic units on the basis of field mapping, each beginning with a thick olivine cumulate and succeeded up section by a variety of pyroxenitic and gabbroic cumulates. A systematic change in the order of appearance of orthopyroxene in the pyroxenite units of the layered series is correlated with a decrease in Mg no. (Mg/Mg + $ Sigma$Fe in cation units) at which clinopyroxene joins olivine on the liquidus, and spike-like increases in LIL/HFS ratios, which require that the composition of the magma of the Muskox Intrusion changed through time across the layered series. Mixing of residual liquid with primary liquids replenishing the chamber can explain the upsection decrease in the Mg no. of clinopyroxene at the base of successive pyroxenite units, but this mechanism cannot account for increased ratios of LIL to HFS incompatible elements observed in the olwebsterite of Megacycle #3 and the first websterite of Megacycle #4, because these ratios are not significantly fractionated by closed-system crystal fractionation. Contamination of the parental magma with melts from the paragneiss country rocks can reproduce the observed increase in the ratios of K/Ti and K/Zr. The gabbroic layers of Megacycle #1, #2, and #3 are especially enriched in LIL/HFS ratios, indicating the presence of a highly contaminated magma layer above the uncontaminated magma which crystallized much of the ultramafic series. The spike-like enrichment in LIL/HFS ratios in the pyroxenites suggests that episodic events of mixing occurred between an evolving uncontaminated magma and a highly contaminated magma, accumulated at the roof of the magma chamber.
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