UBC Theses and Dissertations
Eclogite xenoliths from the Chidliak kimberlite, Baffin Island, Nunavut, Canada Pobric, Vedran
I studied the petrography, major element chemistry, geothermobarometry, and trace element chemistry of 19 eclogite xenoliths from Chidliak kimberlites (Baffin Island, Nunavut, Canada). These granoblastic, partially melted, mostly bi-mineralic eclogites are composed of pyrope and omphacite, with accessory orthopyroxene, kyanite, and rutile, as well as secondary chlorite, serpentine, phlogopite, amphibole, and spinel. The Chidliak eclogites are classified in the Coleman’s groups A, B, and C based on the garnet composition, and into groups A and B based on the clinopyroxene composition. Parameters of the eclogite formation were calculated by projecting clinopyroxene-garnet temperatures onto a geotherm constrained using Chidliak peridotite xenoliths. Thermobarometry of Chidliak eclogites yields temperatures of 855-1390°C and pressures of 45 70 kbar, i.e. in the diamond stability field. The temperatures may have been overestimated by 50-120°C due to the inability to account for Fe³⁺ in clinopyroxene and garnet. Chidliak eclogites come from the depths similar to those for eclogites of the Slave craton. Whole rock major and trace element composition was reconstructed based on the compositions of clinopyroxene, garnet, and accessory minerals analyzed on the electron microprobe and laser ablation inductively coupled plasma mass spectrometer. Whole rock major element composition of the eclogites suggests that their protoliths could be Archean basalts or oceanic gabbros, modified by partial melting and carbonatitic mantle metasomatism. Metasomatism was responsible for the introduction of Mg into Chidliak eclogites with MgO>16.5 wt. % and the high Zr/Hf values that correlate with the high MgO bulk content. Based on REE and HFSE concentrations, Chidliak eclogites were divided into several groups that show evidence for: 1) the origin in a shallow, low-P, garnet-free source protolith, 2) partial melting and extraction of arc melts, 3) cumulation of plagioclase, and 4) ancient mantle metasomatism by alkali and carbonatitic fluids. I propose two possible tectonic settings for the formation of the studied eclogites: 1) metamorphosed mid-ocean ridge gabbros subducted under the Hall Peninsula Block during the 1.8-1.9 Ga Trans-Hudson orogeny, or alternatively 2) oceanic plateau basalts metamorphosed to Archean greenstone belts of the North Atlantic Craton that sank to the mantle via delamination of the lower crust.
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