Chasing the Late Jurassic APW Monster Shift in Ontario Kimberlites

A 30\ub0 gap was recognized in a composite APW path when global poles from predominantly igneous rocks were assembled in North American coordinates using plate reconstructions (Kent & Irving 2010 JGR). The 'monster shift' occurred between a 160-190 Ma cluster of mean poles at 75-80\ub0N 90-110\ub0E to a 140-145 Ma grouping centered at 60-65\ub0N ~200\ub0E. There are hardly any intermediate igneous poles whereas the rather divergent directions from the Late Jurassic Morrison Formation published by Steiner & Helsley (1975 GSA Bulletin) are subject to adjustments for Colorado Plateau rotation and sedimentary inclination error, neither of which are precisely known for this redbed unit sampled in Colorado. On the other hand, similar large rapid swings have been recognized in the Late Jurassic APW path for Adria (Channell et al. 2010 Paleo3), suggesting a global phenomena. In an effort to fill the data gap between ~145 and 160 Ma, we sampled accessible outcrops/subcrops of kimberlites in the Timiskaming area of Ontario, Canada, that are associated with high precision U-Pb perovskite ages (Heamon & Kjarsgaard 2000 EPSL). We report initial results from two of the intrusions: the 153.6\ub12.4 Ma Peddie kimberlite from outcrop and the Triple B kimberlite that was accessible by trenching and is assumed to be the same age as the nearby 153.7\ub11.8 Ma Seed kimberlite as delineated by aeromagnetic surveys and borings. Systematic progressive thermal demagnetization indicated in each unit a dominant characteristic component with unblocking temperatures to 575\ub0 that presumably reflect a magnetite carrier that will be checked by further rock magnetic experiments. Samples from the Peddie kimberlite had stable downward (normal polarity) magnetizations whose mean direction gives a paleopole at 73\ub0N 184\ub0E. In contrast, samples from the Triple B kimberlite have upward (reverse polarity) magnetizations with a well-grouped direction whose (north) paleopole is 78\ub0N 197\ub0E, proximal to the Peddie pole. The normal and reverse polarities suggest that sufficient time elapsed between emplacement of the Triple B and Peddie to give an opportunity to average secular variation. The combined ~154 Ma Triple B and Peddie pole encouragingly lies about halfway between igneous poles from North America nearest in age: the 169 Ma Moat volcanics and the 142 Ma Ithaca kimberlites. However, preliminary paleomagnetic results from some of the other sampled kimberlite bodies are more problematical and require further paleomagnetic and geochronological work

In situ oxygen-isotope, major-, and trace-element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from Roberts Victor, Kaapvaal Craton

A subducted oceanic crustal origin for most eclogite xenoliths in kimberlites has long been a cornerstone of tectonic models for craton development. However, eclogite xenoliths often have protracted and complex histories involving multiple metasomatic events that could overprint some of the key geochemical indicators typically taken as evidence of a subducted origin (e.g., garnet δ18O-values and mineral 87Sr/86Sr compositions). To assess the potential for disturbance of oxygen isotopic compositions in mantle eclogites via diamond-forming and other possible metasomatic fluids, we have conducted a multi-technique in situ study of a diamondiferous eclogite xenolith from the Roberts Victor kimberlite, S. Africa. Using SIMS we provide the first texturally-controlled in situ measurements of δ18O-values in eclogitic garnet in close proximity to diamond. Garnet and clinopyroxene modal proportions are heterogeneous in the xenolith and garnet compositions vary from Mg# = 75.8–79.2; grossular proportions = 8.05–10.14 mol.%, and omphacitic pyroxene has Jd13–24 and Mg# = 86.6–90.0. Rare earth element patterns of minerals across the xenolith, including grains close to diamond, are typical LREE-depleted garnets and markedly LREE-enriched pyroxenes. These silicate minerals also record detectable intra- and inter-grain LREE abundance variations. Clinopyroxenes of the studied xenoliths show HFSE and Sr abundance variations that are decoupled from LREE contents and major-element variations. Mineralogical constraints and bulk-rock reconstructions indicate that the studied sample likely experienced selective incompatible element enrichment during small-volume (<<0.03 wt.%) infiltration of metasomatic fluid(s) potentially linked to ancient diamond evolution. Intra-grain major-element, LREE and HFSE variations in clinopyroxene resulted from late-stage metasomatism. Oxygen isotope compositions in garnet are decoupled from all major- and trace-element variations, with garnet δ18O-values being uniform across the xenolith in a wide variety of textural settings. Garnet δ18O-values of +6.5 ± 0.2 ‰ are higher than the mean (+5.19 ± 0.26 ‰) of the mantle garnet range (+4.8–5.5 ‰). Modelling of the buffering effect of mantle peridotite on CO2-rich and H2O-rich metasomatic fluids at temperatures within the diamond stability field indicates that the likelihood of a metasomatic fluid with exotic oxygen isotopic composition arriving at a mantle eclogite body with its isotopic composition unmodified, after percolative flow through dominantly peridotitic mantle at great depth, is very low. As we find no evidence of metasomatically induced garnet oxygen isotope variations in the studied diamondiferous eclogite xenolith we conclude that the most likely origin for the elevated garnet δ18O-values is via inheritance from a crustal protolith altered at relatively low temperatures. These results have broader relevance and support the hypothesis of a low-pressure protolith for mantle eclogite xenoliths, demonstrating the robust nature of garnet oxygen isotope compositions – even in diamond-bearing eclogites