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Peridotite and Pyroxenite Xenoliths from the Muskox

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Peridotite and Pyroxenite Xenoliths from the Muskox PERIDOTITE AND PYROXENITE XENOLITHS FROM THE MUSKOX KIMBERLITE, NORTHERN SLAVE CRATON, CANADA by David Edward Newton BSc. The University of Massachusetts Amherst, 2007 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in The Faculty of Graduate and Postdoctoral Studies (Geological Sciences) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) August 2015 © David Edward Newton, 2015 Abstract Petrography, mineralogy and thermobarometry are reported for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite includes 15% coarse spinel peridotite, 4% coarse spinel-garnet peridotite, 4% coarse garnet peridotite, 9% porphyroclastic peridotite, 60% websterite and 8% orthopyroxenite. Peridotites are composed primarily of forsteritic olivine (Fo 89-94), enstatite (En 89-94), Cr-diopside, Cr- pyrope garnet and chromite spinel. Pyroxenites are composed primarily of enstatite (En 90-92), Cr-diopside and Cr-pyrope garnet. Thermobarometric estimates were made using two-pyroxene, garnet-clinopyroxene and Ca-in-orthopyroxene thermometers and garnet-orthopyroxene barometer. Results suggest that coarse peridotites equilibrated at 650-1220 °C and 23-63 kbar; porphyroclastic peridotites equilibrated at 1200-1350 °C and 57-70 kbar; pyroxenites equilibrated at 1030-1230 °C and 50-63 kbar. Muskox xenoliths are compared with xenoliths recovered from the neighboring Jericho kimberlite, erupted 15 km away from and at the same time as Muskox. Contrasts in the characteristics of these two suites of mantle samples include: 1) higher levels of depletion throughout the Muskox mantle column based on the contents of MgO in olivine and orthopyroxene and Cr2O3 in garnet; 2) the presence of a shallow zone of metasomatism in the spinel stability field in the Muskox mantle; 3) a higher proportion of pyroxenitic versus peridotitic rock types at the base of the mantle column beneath the Muskox kimberlite and higher Cr2O3 in all minerals in pyroxenites; and 4) lower levels of deformation in the Muskox mantle. We interpret these contrasts as representing small scale heterogeneities in the bulk composition of the mantle, as well as the local effects of kimberlite formation and ascent. If percolation of asthenosphere-derived pre-kimberlitic fluids in the less permeable ii Muskox mantle was impeded, localization of this fluid may have resulted in higher proportions of pyroxenitic rock types here, as well as lower degrees of deformation of the peridotitic mantle. iii Preface This thesis is original, unpublished, independent work by the author, D. Newton. iv Table of Contents Abstract...........................................................................................................................................ii Preface……………………………………………………………………………………………iv Table of Contents...........................................................................................................................v List of Tables.................................................................................................................................vii List of Figures...............................................................................................................................viii Acknowledgments..........................................................................................................................ix Chapter 1: Introduction...................................................................................................................1 1.1 Background and objective..............................................................................................1 1.2 Geological setting..........................................................................................................2 Chapter 2: Petrography...................................................................................................................5 2.1 Coarse peridotite............................................................................................................5 2.1.1 Coarse spinel peridotite..................................................................................5 2.1.2 Coarse spinel-garnet peridotite.......................................................................7 2.1.3 Coarse garnet peridotite..................................................................................8 2.2 Porphyroclastic Peridotite.............................................................................................8 2.3 Pyroxenite....................................................................................................................10 2.3.1 Websterite.....................................................................................................10 2.3.2 Orthopyroxenite............................................................................................11 Chapter 3: Mineral Chemistry......................................................................................................13 3.1 Analytical methods......................................................................................................13 3.2 Results......................................................................................................................... 13 3.2.1 Olivine...........................................................................................................13 v 3.2.2 Orthopyroxene..............................................................................................14 3.2.3 Clinopyroxene...............................................................................................15 3.2.4 Garnet and Spinel..........................................................................................16 3.2.5 Amphibole.....................................................................................................17 Chapter 4: Thermobarometry.........................................................................................................18 4.1 Methodology................................................................................................................18 4.2 Results.........................................................................................................................24 Chapter 5: Discussion....................................................................................................................26 5.1 Depletion of the Muskox peridotitic mantle...............................................................26 5.2 Two depth zones of metasomatism.............................................................................27 5.2.1 Shallow zone of metasomatism....................................................................27 5.2.2 Deep zone of metasomatism.........................................................................31 5.3 Deformation of the Muskox peridotitic mantle...........................................................33 5.4 Characteristics of the Muskox pyroxenitic mantle......................................................36 5.5 Concluding remarks.....................................................................................................39 Figures............................................................................................................................................42 Figure Captions..............................................................................................................................53 References……..............................................................................................................................58 Appendix A: Muskox Xenolith Sample List and Macrospecimen Descriptions...........................67 Appendix B: Petrographic Descriptions........................................................................................72 Appendix C: Major Element Chemistry of Minerals in Muskox Xenoliths................................149 vi List of Tables Table 1 Pressure and temperature estimates for Muskox samples containing two pyroxenes and garnet.................................................................................................20 Table 2 Pressure and temperature estimate for Muskox samples containing two pyroxenes...........................................................................................................................22 Table 3 Pressure and temperature estimates for Muskox samples lacking either A. clinopyroxene or B.orthopyroxene...............................................................................23 Table 4 Evidence for metasomatism in Muskox peridotite xenoliths............................................30 vii List of Figures Figure 1 Photoplate 1.....................................................................................................................51 Figure 2 Photoplate 2.....................................................................................................................52 Figure 3 Olivine and orthopyroxene Mg# histograms for Muskox and Jericho peridotites and pyroxenites.........................................................................................................................53 Figure 4 Mg# vs. Cr2O3 wt.% for minerals in Muskox xenoliths.................................................54 Figure 5 CaO vs. Cr2O3 wt.% for garnets in Muskox xenoliths....................................................55 Figure 6 Thermobarometric comparison plots for Muskox xenoliths...........................................56
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