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Eclogite xenoliths from the Jericho and Muskox kimberlites, Nunavut, Canada Beausoleil, Yvette Léa


A total of 109 eclogite xenoliths from the Jericho and Muskox kimberlites (Nunavut, Canada) were studied petrographically and mineralogically to constrain their depth distribution within the Northern Slave mantle. The eclogites are dominated by pyrope-almandine and omphacite with accessory rutile, apatite and olivine. Garnet-clinopyroxene thermobaromtry suggests that Northern Slave eclogites formed at 670 -1300 °C and 25 – 70 kbar. Eclogites were classified into Group A, B, or C based on mineral composition and into massive and foliated textural types. Group A Northern Slave eclogites may have formed as cumulates from mantle mafic melts, whereas Group B and C eclogites are interpreted as modified subducted oceanic crust. All Northern Slave eclogites were subjected to partial melting and recrystallization, which produced secondary high-MgO garnet and clinopyroxene, phlogopite, amphibole carbonates and spinel group minerals. The recrystallization was caused by an influx of carbonatitic and hydrous hot fluid. The most recent heating event immediately predating kimberlite eruption resulted in garnet and clinopyroxene zoning. Diamondiferous eclogites from the Northern Slave are always massive and belong mostly to Group A. The majority of diamondiferous eclogites from the Northern Slave occur at shallower depths than those from the Central Slave craton. The criteria for distinguishing diamondiferous eclogites based on high Na₂O content in garnet and high K₂O content in clinopyroxenes can be applied only to Muskox eclogites. The high Mg content in both garnet and clinopyroxene best distinguishes the diamondiferous eclogites from Jericho. A model with multiple subducted slabs of oceanic crust below the Slave craton is proposed. The deepest subducted slab (190 – 210 km) dated at 1.88 – 1.84 Ga below the Central Slave extends to shallower depths of 170 – 185 km below the Northern Slave. Another slab (1.95 – 1.91 Ga) that occurs at 140 – 160 km below the Central Slave may extend to the north where it becomes progressively thicker from imbrication. The shallowest (120 – 130 km) and oldest (2.67 – 2.6 Ga) slab occurs only below the Northern Slave. Eclogites of mantle origin formed in mafic magma chambers, which existed only below the Northern Slave at 135 – 150 km depths.

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