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The age and origin of megacrysts in the Jericho kimberlite (Nunavut, Canada) Marković, Goran

Abstract

Fourteen samples of megacrysts from Jericho kimberlite have been studied. The study includes petrography, geochemistry of major and minor elements, thermobarometry and Sr-Nd-Hf isotopic analyses. The purpose of the study is to determine the relationship between megacrysts and kimberlites (xenocrystal vs cognate) and shed light on the nature of melts parental to kimberlite megacrysts. The Jericho megacrysts include garnet, clinopyroxene, olivine, ilmenite and orthopyroxene. A unique feature of Jericho megacrysts is its gradual transition from discrete megacrysts to megacrystalline pyroxenites. Equilibrium temperatures and pressures were calculated for eight megacryst samples. All calculated P-T place megacrysts into deep garnet-bearing mantle, with T=1200-1280° C and P=60-71 kbar. The P-T estimates for orthopyroxene-bearing samples are identical to P-T estimates for orthopyroxene-free samples, with 195-230 km depth range. Thermobarometric data on Jericho megacrysts cannot give a definitive answer about their origin. The ratios of Rb and Sr isotopes define a slope that corresponds to the age of 179 ± 21 Ma, Sm-Nd system gives an age of 177 ± 7.3 Ma and Lu-Hf ratios define a line with a slope that corresponds to the age of 169 ± 63 Ma. The Sm-Nd apparent isochron age of megacrysts (177 ± 7.3 Ma) falls within the brackets of the Jericho kimberlite age, as determined from the Rb-Sr isotopic systematics of phlogopite (171.9 ± 2.6 Ma). Isotopic ratios of megacrysts and kimberlite are different, supporting a view that megacrysts could not crystallize from kimberlite magma. On the Sr-Nd-Hf isotopic diagrams, the majority of megacrysts plot within the mixing array of HIMU mantle and EM I and thus can be produced by melting of the metasomatically altered CLM that experienced preferential extraction of Rb and Pb by CO₂-rich fluids (HIMU reservoir) and addition of lower continental crust (EMI reservoir). On the Sr-Nd-Hf isotopic diagrams kimberlites plot within mixing array of HIMU mantle and EM II. A protolith for the kimberlites can be the metasomatically altered CLM (HIMU) that incorporated some subducted terrigenous sediments of the upper crust (EMII reservoir). The difference in Sr-Nd systematics of Jericho megacrysts and kimberlites can be explained by varied contribution of EMI or EMII to prevalent HIMU-type mantle. Results obtained in this study suggest that Jericho megacrysts did not crystallize from host kimberlite. Even though megacrysts are not phenocrysts, they should be considered cognate to kimberlites having crystallized from associated quasi- contemporaneous melts rather than being xenocrysts totally unrelated by the age.

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