Periodic Reporting for period 2 - CHRONOTEC (Conditions and duration of High tempeRature metamOrphism in large hot orogeNs assessed through petrochronolOgy : insights from Grenville TECtonics)
Okres sprawozdawczy: 2022-09-01 do 2023-08-31
Orogenic belts are among the most striking evidence of Earth Tectonics. During convergence processes, the crust can be buried, thickened and heated up, leading to creation and recycling of continental material. This is particularly well exemplified in Large Hot Orogens (LHOs), the roots of which can attain and maintain temperatures higher than 800°C over several tens of million years (m.y.) causing partial melting, crustal flow and potential development of orogenic plateaux. Exhumed core zones of ancient orogens give direct access to formerly deep and hot levels of crust that are still buried in modern analogues, offering unparalleled opportunities to both assess the condition and duration of high-T processes and investigate the tectonic evolution of orogenic roots. The Mesoproterozoic Grenville Province is a type example of an ancient LHO, active for over 100 m.y (~1090 – 980 Ma). Geodynamic modelling has addressed key tectonic features of this orogen, but metamorphic data, crucial for tectonic interpretations, are patchy and incomplete.
In the last decades, the development of thermodynamic modeling and instrumental advances in elemental and isotopic analysis have open new avenues to unravel the Pressure-Temperature-time (P-T-t) paths of rocks, to assess the rates and duration of large-scale orogenic processes. In high-grade metamorphic rocks, the time dimension of P-T-t paths is mostly documented by zircon U-Pb dating coupled to its trace element record, that can be linked to the garnet growth, based on partitioning of trace elements. However, decoupling between trace element content and ages in zircon are documented, especially during high-grade metamorphism, resulting in misinterpretations and hampering P-T correlation and large-scale tectonic interpretation. This can be resolved by documenting the chronological record of U–Pb in zircon and Lu–Hf, Sm–Nd in garnet from the same sample, but such multifaceted studies are rare, calling to investigate both garnet and zircon chronometers in a variety of tectonic settings to bring better constraints from natural samples.
The objectives of CHRONOTEC are dual: (i) to bring new insights on the fate of the continental crust in Large Hot Orogens, by assessing the timing, duration, and conditions of High-Temperature (high-T) metamorphism during continental collision, and (ii) to assess the age, chemical and isotopic record and links between chronometers zircon and garnet in high-T mafic rocks across a Pressure (P) gradient.
In the last decades, the development of thermodynamic modeling and instrumental advances in elemental and isotopic analysis have open new avenues to unravel the Pressure-Temperature-time (P-T-t) paths of rocks, to assess the rates and duration of large-scale orogenic processes. In high-grade metamorphic rocks, the time dimension of P-T-t paths is mostly documented by zircon U-Pb dating coupled to its trace element record, that can be linked to the garnet growth, based on partitioning of trace elements. However, decoupling between trace element content and ages in zircon are documented, especially during high-grade metamorphism, resulting in misinterpretations and hampering P-T correlation and large-scale tectonic interpretation. This can be resolved by documenting the chronological record of U–Pb in zircon and Lu–Hf, Sm–Nd in garnet from the same sample, but such multifaceted studies are rare, calling to investigate both garnet and zircon chronometers in a variety of tectonic settings to bring better constraints from natural samples.
The objectives of CHRONOTEC are dual: (i) to bring new insights on the fate of the continental crust in Large Hot Orogens, by assessing the timing, duration, and conditions of High-Temperature (high-T) metamorphism during continental collision, and (ii) to assess the age, chemical and isotopic record and links between chronometers zircon and garnet in high-T mafic rocks across a Pressure (P) gradient.
Type samples from the Central Grenville were selected from distinct levels of the High-P segment and from distinct levels of the Mid-P segment, corresponding to the lower and middle orogenic crust respectively. For each sample, the researcher, Caroline Lotout, performed textural observation chemical analyses of major and trace elements in major mineral phases, acquired SEM-MLA mineral maps of the thin sections and X-ray composition maps of garnet and matrix, carried out garnet Lu-Hf and Sm-Nd dating, Zircon CL imaging, U-Pb dating and trace element analysis, and O stable isotope analyses in zircon and garnet. P-T estimations using pseudosection modelling have been completed on the high-Pressure samples. As a result, a first article on the evolution of the high-Pressure segment during the Grenville orogeny has been published in Precambrian research (Lotout et al. 2023). The results obtained during CHRONOTEC have been promoted by five presentations in 4 conferences.
CHRONOTEC documents the petrogenetic link between garnet and zircon using numerous samples across a Pressure gradient in a Large Hot Orogen setting, using a large range of independent tools (trace elements and O isotopes in zircon and garnet, garnet Lu-Hf and Sm-Nd dating, zircon U-Pb dating, P-T estimates, SEM-MLA mineral maps) to meticulously detail their metamorphic history. Moreover, the P-T-t paths obtained using cutting edge techniques are among the first in the Grenville Province of Canada and constitute therefore an important contribution to our understanding of the geodynamic evolution of the orogen.