Mineralogy and petrology of kimberlite dyke and sheet intrusions and included peridotite xenoliths from South-West Greenland

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Detailed mineralogical and petrological investigations have been carried out on a suite of micaceous kimberlite dykes and sheets of early Mesozoic age intruded into Precambrian gneisses, supracrustal rocks and a granite complex. Forty-two electron microprobe analyses of kimberlite olivines demonstrate a compositional range from Mg 92Mg 74, with frequent examples of both normal and reverse zoning. Calcite, iron-rich serpentine and phlogopite are important rock-forming phases with accessory apatite and perovskite. Diopside occurs in certain intrusions (up to 25% vol.%). Opaques are commonly composite grains of chromite and magnetite with limited ulvöspinel solid-solution. Eight microscopic diamonds have been recovered from four samples. Xenolith suites are dominated by lherzolitic peridotites, but include rare crustal rocks. Analyses from six garnet peridotites, three spinel peridotites and five phlogipite peridotites demonstrate significant compositional differences between their respective pyroxenes: the Al in diopside of spinel peridotites is 10 times that of diopside in phlogopite peridotites while Al in diopside of garnet peridotites is intermediate in amount. Diopside from phlogopite peridotites is depleted in Ti. These variations are interpreted as evidence of equilibrium between diopside and coexisting garnet, spinel, or phlogopite. With one exception the diopsides are calcic with limited solid solution towards enstatite; kimberlite diopsides are generally significantly more calcic than varieties in peridotite. Pyroxenes from individual peridotite nodules have similar Fe Mg ratios although this ratio varies within the xenolith population. Olivine has a restricted compositional range (about Mg 91). Large crystals in dunite mylonites are significantly more iron-rich. Garnet and spinel peridotites are interpreted as unmodified representatives of the Upper Mantle. Some phlogopite peridotites probably recrystallized during and after incorporation into the kimberlite magma.