Suture Zones through Earth History

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Suture zones are first-order tectonic lineaments separating continental fragments that were amalgamated over geological time to form the present-day continents. They typically contain exotic oceanic rocks, indicating that sutures form during the transition from oceanic subduction to continental collision. Despite their fundamental importance as the trace of ancient plate boundaries and as exceptional exposures of oceanic lithosphere, there are many unanswered questions on the significance of suture zones. In particular, the origin of trapped oceanic lithosphere fragments (ophiolites) is still debated, as are the processes leading to their accretion in suture zones. It is also unclear why many sutures, especially deeply eroded ones, do not contain any exotic oceanic rocks. Other controversies include if and how suturing operated in the Early Earth, and if Archean greenstone belts represent such sutures. Additionally, even if the spatial association between suture zones and economically important mineral deposits is easily observed, the role of suturing in the concentration of metals at economic grades is debated. The long term goal of the proposed research is to address such problems by focusing on oceanic and metamorphic rocks.*The research will be based on geological mapping conducted in collaboration, when possible, with geological surveys (QC, NU and Canada). The genesis-alteration of suture rocks will be investigated using major and trace element geochemistry. Emphasis will be put on metamorphic rocks that recorded pre-, syn- and/or post-suturing tectonic processes. Depth of burial (P) and temperature (T) of equilibration will be quantified using state-of-the-art phase equilibria modeling. Burial-exhumation and heating-cooling rates and timing will be derived using a suite of geochronometers (t) including U-Pb in zircon and monazite for protolith and peak metamorphic ages, Ar in hornblende and biotite for cooling ages, and most interestingly the recent Lu-Hf and Sm-Nd in garnet for burial metamorphism ages. Once integrated with the regional deformation patterns (D), the resulting P-T-t-D paths can be compared to predictions from thermal-mechanical numerical models, a recent and very powerful method for the understanding of tectonic processes.*In the next 5 years, three specific themes will be addressed: Theme 1 - Metamorphic soles are slabs found at the base of ophiolites that, based on Ar dating, are thought to form during the inception of a subduction contemporaneously with ophiolite genesis. However, we recently applied, for the first time, Lu-Hf garnet dating to a sole from the India-Asia suture and obtained an age significantly older than expected, shedding doubt on the inferred synchronicity between ophiolite genesis, sole formation and inception of subduction. Research will continue on the India-Asia sole, while archetypal soles in NL and QC will be revisited and numerical models will be designed to test new hypotheses. Theme 2 - Deeply eroded sutures. Projects will focus on the poorly documented Torngat orogen in QC, NL and NU. The research will help localize the suture, identify potential exotic terranes and constrain the different metamorphic episodes and overprinting tectonic processes. Theme 3 - Archean sutures and greenstone belts. One project focuses on the Pontiac sedimentary subprovince, QC, where there is a potential link between gold mineralisations and garnet-grade metamorphism, which is also puzzling in the context of Archean continental suturing. A second project focuses on the La Grande greenstone belt, a poorly studied new mining district in the James Bay area, QC. The research will first focus on the strong unexplained metamorphic gradient that is observed from the core of the belt to its margins.