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Radiogenic isotopic systematics of layered intrusions : application to the Mesoproterozoic Kiglapait intrusion of coastal Labrador, Canada, and to mafic-ultramafic rock reference materials Fourny, Anaïs


Mafic layered intrusions in the Earth’s crust are natural laboratories for evaluating differentiation processes of mantle-derived magma. The Mesoproterozoic Kiglapait intrusion (Labrador, Canada) represents a remarkable case study of how these intrusions form under closed-system crystallization of basaltic magma. The Kiglapait intrusion is the largest and youngest troctolitic intrusion contained within the vast Nain Plutonic Suite, one of the best-preserved examples of Proterozoic anorthosite massifs that distinguish magmatic activity in the Earth’s crust from ~1.8 to 0.9 billion years ago. The radiogenic isotope ratios of Pb-Sr-Nd-Hf are powerful geochemical tools for identifying magma sources, detecting contamination, and tracing mixing processes in igneous rocks. To measure accurate and precise isotopic ratios by MC-ICP-MS, the analysis of sample–matrix-matched reference materials is required. A new comprehensive isotope database of mafic to ultramafic reference materials is provided to assess the accuracy of Pb-Sr-Nd-Hf isotopes in Kiglapait samples and to be used as a reference dataset for the isotopic study of other terrestrial, and extraterrestrial, mafic-ultramafic rocks. Integration of Pb-Sr-Nd-Hf isotope and trace element geochemistry of whole rocks and mineral separates allows for definition of the Kiglapait source and parent magma composition. An event of post-crystallization addition of radiogenic Pb is distinguished, the effects of which are effectively leached from plagioclase during sample pre-treatment. An in situ LA-ICP-MS technique is also developed for measuring Pb isotope ratios at high spatial resolution in minerals with very low Pb concentrations, such as plagioclase and clinopyroxene. Combined, the solution-based Pb-Sr-Nd-Hf and in situ Pb isotopic results demonstrate that the primary Kiglapait magma was mantle-derived, with minor assimilation of lower crust during ponding and ascent, and that assimilation of local country rocks, as recorded primarily in Sr isotopic variations, was limited to the uppermost gabbros and ferrosyenites during the final stages of crystallization. This multi-isotopic and trace element geochemical framework developed for the Kiglapait intrusion, and at a larger scale for the entire Nain Plutonic Suite, can be adapted to layered intrusions and Proterozoic anorthosite plutonic suites worldwide to better constrain a wide range of geological issues from mantle heterogeneity to crustal differentiation to Proterozoic geodynamics.

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