An investigation into the timing of the Renard kimberlite cluster, Otish field, Canada

Abstract

The timing of a diamondiferous kimberlite cluster within the Archean Superior Province, the Renard cluster (Otish field, Québec), was investigated by ID-TIMS U-Pb dating of groundmass perovskite. Few Otish field kimberlites were previously dated by U-Pb perovskite (Renard 1-3 and Lac Beaver) but the Neoproterozoic dates indicated a much longer kimberlite intrusion history (~100 Myr) than is typically known for kimberlite clusters/fields. Here, a single pipe (Renard 2) is investigated in detail from ten new samples and found to record a punctuated history of intrusions from early-stage, main pipe-infilling stage, and late-stage hypabyssal kimberlites spanning over at least 20 Myr (~652-632 Ma) and possibly longer. In contrast, a new weighted average 206Pb/238U perovskite composite date of 643.8±1.0 Ma is estimated for the main Renard 2 pipe emplacement. Applying this new information to other kimberlites in the cluster (Renard 1-4, Renard 7, Renard 9, G04-296 Anomaly dyke), it was found that the U-Pb perovskite dates can record at least ~124 Myr (~663-539 Ma) of repeated hypabyssal kimberlite intrusions. In particular, the younger Otish field hypabyssal kimberlites (~539 Ma peripheral Renard 2, ~614 Ma Renard 7 and ~551 Ma Lac Beaver) overlap with the timing of the ~620-550 Ma Central Iapetus Magmatic Province LIP and alkaline-carbonatite intrusions along the margin of eastern Canada and western Greenland. The broad scale of kimberlite intrusion at ~632-629 Ma (Renard, Wemindji) across the eastern Superior Province and similar temporal and spatial connection between intrusions in Laurentia and Baltica suggest a large-scale mechanism triggered kimberlite magmas to form (e.g. combined continental extension/rifting and mantle plume(s)). In addition, a hypabyssal kimberlite from the Renard 9 pipe was found to contain grains of kassite, a rare hydrous Ca-Ti oxide (CaTi2O4(OH)2). The first ID-TIMS U-Pb results for kassite are reported with an evaluation of several possible interpretations, including lead loss, initial common lead isotopic composition or gain/loss of intermediate daughter products. The best estimate for the timing of kassite crystallization is taken from two multi-grain kassite fractions recording a weighted average 207Pb/235U date of 456.1±2.1 Ma (2σ).