Petrogenesis of Archean granitoids: Insights from inverse and forward modelling
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Date
2022-06-24
Authors
Kendrick, Jillian
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Publisher
University of Waterloo
Abstract
Archean granitoids are the earliest representation of continental crust on Earth. The Archean geological record is dominated by rocks of the tonalite–trondhjemite–granodiorite (TTG) suite, which are thought to have formed by partial melting of hydrated mafic crust. Two end-member settings of TTG formation have been proposed: partial melting of subducting oceanic slabs at mantle depth and partial melting of metabasite at a deep crustal level. Each scenario has different implications for Archean tectonic regimes. Although these scenarios are commonly distinguished based on the trace element compositions of TTGs (e.g., Sr/Y, La/Yb), factors that are unrelated to the setting of partial melting can also influence TTG compositions; further research is required to determine the effect of these additional factors. Furthermore, most exposed Archean crust is limited to formerly upper- to middle-crustal levels. Consequently, connections between TTGs at different crustal levels and the deep crustal processes that may affect or generate TTGs generally cannot be studied directly. In addition to TTGs, granitoids of the Archean sanukitoid suite have been considered as important indicators of ancient tectonic processes. The major and trace element characteristics of sanukitoids indicate a metasomatized mantle source and subduction is suggested as the driving force behind the change in mantle composition. However, the nature of the metasomatic agent introduced into the mantle and the processes that trigger mantle melting and sanukitoid generation remain unclear. In this thesis, the formation of Archean granitoids is explored both generally and in a crustal cross section of the Neoarchean Wawa-Abitibi terrane in the southern Superior Province known as the Kapuskasing Uplift.
Investigation of Archean granitoid generation in general is done in this thesis by combining phase equilibrium modelling with trace element partitioning between minerals and melt to calculate theoretical melt compositions under different conditions. Three potential influences on TTG compositions are explored using this approach: variations in source bulk composition, progressive melt loss at the source, and sequestration of garnet cores from the system. The modelling results indicate that although the trace element compositions of TTGs are sensitive to the pressure (i.e., depth) of melting—as is generally expected—other factors complicate this relationship. Compositions associated with high-pressure melting (elevated Sr/Y and La/Yb and low Nb and Ta) can be achieved at a range of pressure–temperature (P–T) conditions, particularly considering fractionation of trace elements into garnet and a range of possible source compositions. These results indicate that the trace element composition of TTGs may not be diagnostic of a particular geodynamic setting, as the end-member scenarios are generally distinguished by depth of melting.
Suites of TTGs and high-grade metabasites from the Kapuskasing Uplift are investigated using both modelling and geochemical data to better understand the evolution of these rocks and the potential connection between them. The TTGs exposed in a large grey gneiss domain at the mid-crustal level in this region exhibit a range of trace element compositions consistent with variable depths of source melting. However, these samples also show evidence of plagioclase accumulation and fractionation. The results of phase equilibrium and melt trace element modelling demonstrate that variable degrees of separation of early crystallizing plagioclase from coexisting melt can account for the range of TTG compositions. This may indicate that the grey gneiss domain was a long-lived crystal mush complex in which localized mush compactions resulted in the different compositional types of TTGs currently observed. Therefore, linking the trace element characteristics of TTGs to their source may be complicated by the behaviour of plagioclase after emplacement of the magma in the crust.
The Kapuskasing Uplift provides an opportunity to evaluate the isotopic record of TTGs from different crustal levels within a craton. In other cratons, isotopic data from Archean granitoids—typically from upper- to middle-crustal exposures—indicate that repeated reworking of the same crustal material was common. However, in the Kapuskasing Uplift, U–Pb and Hf isotopic data from zircon in TTGs indicates that three generations with different ages (one Neoarchean and two Mesoarchean) are present and each is derived from an isotopically distinct source. The oldest Mesoarchean TTG may represent a rifted fragment from other Mesoarchean subprovinces north of the Wawa subprovince as these rocks are isotopically compatible. The isotopic record is also heterogeneous across the crustal cross section. The youngest and most juvenile TTGs are volumetrically dominant at the upper- to middle-crustal level, and the least radiogenic TTGs (potentially derived from an ancient mafic source) are only found in the lower crust. This result highlights a potential bias in the Archean geological record; the typically inaccessible lower crust may contain volumetrically minor but isotopically distinct TTGs.
Deep-crustal metabasites exposed in the Kapuskasing Uplift show evidence of former partial melting and may represent potential TTG source rocks. This connection is tested by comparing the modelled compositions of anatectic melt, Lu–Hf isotopic data, and geochronological data of the metabasites to data from the natural TTGs at shallower crustal levels in the Kapuskasing Uplift. Based on modelling results, the composition of anatectic melt produced by the metabasites may have been comparable to TTGs. However, considering the different factors that may influence TTG compositions, trace elements alone are insufficient for linking these rocks to their source. The Lu–Hf isotopic data indicate that two isotopically distinct metabasites are present, which together are compatible with the range of Hf isotopic compositions of zircon in TTGs at shallower crustal levels. The timing of metabasite partial melting, however, is coeval with only the youngest TTG magmatism; this metamorphic event likely resulted from late-stage processes during the stabilization of the southern Superior Province. Therefore, the lower crustal metabasites represent compatible source rocks for TTGs at shallower crustal levels in the Kapuskasing Uplift. Greater volumes of these metabasites were likely present previously and juxtaposition and partial melting of the two metabasic sources may have been a two-stage process.
Sanukitoid genesis is investigated using phase equilibrium modelling coupled with trace element partitioning modelling. Metasomatized mantle compositions are estimated assuming two possible metasomatic agents—TTG magma and partial melt from subducted sediments. Model melt reproduces some geochemical features of sanukitoids, but the large variations in natural sanukitoid compositions suggest that the process of sanukitoid generation was different for each craton. Model results indicate that a high degree of mantle melting with no (or minimal) garnet in the residue produces melt that is most similar to primitive sanukitoid compositions. These conditions require relatively low P and high T mantle melting that is consistent with asthenospheric upwelling, but not indicative of a specific tectonic setting.
The results of this thesis have two main implications for understanding Archean granitoids. First, coupled phase equilibrium and melt trace element modelling is a useful tool for exploring the effect of different processes on granitoid trace elements. However, the trace element characteristics of granitoids are the result of a confluence of many factors and do not indicate an unambiguous connection between granitoids and a particular source. Second, Lu–Hf isotopic data (coupled with geochronological data) from TTGs can provide more valuable insights into their sources than trace element data, particularly if Lu–Hf data from potential source rocks are also available.
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Keywords
Archean, TTG, trace elements, zircon, phase equilibrium modelling, Kapuskasing Uplift, garnet