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Precambrian Research

Volume 350, November 2020, 105862
Precambrian Research

Metamorphism of the Mougooderra Formation: Implications for Neoarchean tectonics in the western Youanmi Terrane, Yilgarn Craton

https://doi.org/10.1016/j.precamres.2020.105862Get rights and content

Highlights

Abstract

In the Yilgarn Craton, the Mougooderra Formation is an extensive sedimentary sequence that preserves low-variance metamorphic mineral assemblages and unconformably overlies older greenstone sequences within the Youanmi Terrane. Here we present garnet Lu–Hf and Sm–Nd geochronology in combination with pressure–temperature (P–T) estimates of the Mougooderra Formation and the underlying Polelle Group to give insights into the geodynamics of this region at the time of metamorphism. Metamorphic rocks from the Mougooderra Formation have variable metamorphic mineral assemblages that reflect bulk rock concentrations of Fe, Mg, Ti, Al, and K at constant P–T. Based on these variations, the metamorphic rocks of this formation can be separated into a high Fe–Mg group, a high Al group and an average pelitic group that each preserve low-pressure mineral assemblages. Thermobarometry and phase equilibrium modelling indicate that the Mougooderra Formation experienced P–T conditions of 545–580 °C and ~2–3.5 kbar, while the Polelle Group reached 610–650 °C at ~4–7.2 kbar. Garnet Lu–Hf and Sm–Nd geochronology returned respective ages of 2686 ± 18 Ma and 2611 ± 35 Ma for the Mougooderra Formation and ages of 2685 ± 15 Ma and 2590 ± 21 Ma for the Polelle Group. Lu–Hf garnet ages are interpreted to record the timing of late prograde metamorphism and indicate simultaneous metamorphism, whereas the geological significance of the Sm–Nd ages is unclear. Garnet Lu–Hf ages obtained in this study are similar to emplacement ages of syntectonic granites in the region. Although these distal contemporaneous granites may have contributed to elevated heat flow, the high apparent thermal gradients calculated here, as well as synchronous craton-wide magmatism, reflect a regional thermal event at this time. The ages, P–T conditions, and high apparent thermal gradients from the study area suggest metamorphism occurred as the result of high heat flow into thinned crust of the Youanmi Terrane and indicates a complex thermal history within this region of the Yilgarn Craton. These results are consistent with the hypothesis that metamorphism during the Archean was more closely related to heat from mantle-derived magmatism.

Introduction

Archean terranes generally consist of several dominant rock types including alternating felsic and mafic greenstone sequences, granitic intrusions, and sedimentary sequences that unconformably overlie greenstone belts (Anhaeusser, 2014). These greenstone belts are often heavily mineralized areas that host world-class mineral deposits. However, despite the attention that these regions receive, a consensus on the mechanisms behind the formation and tectonic evolution of many of these terranes has not yet been reached (e.g. Bédard, 2018, Wyman, 2018). In addition to the typically well-studied granitic plutons and greenstone sequences, metasedimentary sequences made up dominantly of metasandstone and metaconglomerate units can be found. These metasedimentary sequences may contain aluminous protoliths with pressure (P)–temperature (T) sensitive metamorphic mineral assemblages that can provide information on the thermal and burial history of the sedimentary packages and allows for an opportunity to better understand the metamorphic history of a specific region. Garnet-bearing rocks within these sequences allow constraints to be placed on the absolute timing of metamorphism through the use of Lu–Hf and Sm–Nd geochronology. Garnet geochronology is a powerful tool due to the presence of garnet within a wide variety of rock types and because the thermodynamic processes controlling the formation of garnet grains are well-understood, allowing petrographic observations to be directly linked to the ages and metamorphic conditions of the rock (Baxter et al., 2017, Baxter and Scherer, 2013). In addition, the apparent thermal gradients derived from these metamorphic rocks can also provide important constraints on the tectonic processes on early Earth and can offer insights into the geodynamic processes that have operated throughout Earth’s history (e.g. Brown and Johnson (2018)).

Our understanding of the geodynamic processes that contributed to the formation of the Yilgarn Craton has been largely based on studies within the more mineral-rich terranes of the eastern Yilgarn Craton. In recent years, research has expanded into the more western Youanmi Terrane (Fig. 1; Van Kranendonk and Ivanic, 2009, Ivanic et al., 2010, Ivanic et al., 2012, Van Kranendonk et al., 2013, Zibra et al., 2017, Zibra et al., 2018, Clos et al., 2018, Clos et al., 2019); however, the metamorphic history of the sedimentary sequences within this terrane still remains unclear. The Mougooderra Formation is one of the largest epiclastic metasedimentary sequences located within the Youanmi Terrane of the Yilgarn Craton. This formation is located within the Yalgoo region (Fig. 1), overlies greenstone sequences of the 2820–2800 Ma Norie and the 2790–2740 Ma Polelle groups, and contains a range of metamorphic mineral assemblages, making it an ideal unit for better understanding the metamorphic history of the region. In addition, garnet-bearing rocks from the Polelle Group offer the opportunity to further our understanding of metamorphism within the area by comparing metamorphic conditions and ages between the greenstone sequences and overlying sedimentary sequences.

The Youanmi Terrane, like many Archean terranes, has two endmember tectonic models explaining the formation and evolution of the terrane. The first model involves uniformitarian tectonic processes resulting from subduction and the accretion of terranes onto an older portion of continental crust (Wyman, 2019, Goscombe et al., 2019), while the second model involves thinning of the continental lithosphere, heating of the deep to middle crust and crustal instabilities that resulted in large-scale overturn events (Van Kranendonk et al., 2013, Mole et al., 2019). Recent studies of the granitic rocks within the Yalgoo area have indicated the presence of structures related to doming of granitic material, as well as pervasive north-trending foliations, suggesting that both models have validity and possibly indicating a switch from vertical to horizontal tectonics around ~2730 Ma (Zibra et al., 2017, Clos et al., 2019). There is also a long history of granitic pluton emplacement within the area, and it remains unclear if metamorphism of the Mougooderra Formation occurred due to contact metamorphism resulting from the magmatic history of the area, or if it occurred due to regional tectonic processes.

Here, we constrain the metamorphic history of the Mougooderra Formation through petrography, mineral chemical analysis, phase equilibrium modelling, and garnet geochronology. We also constrain the P–T conditions and garnet ages from the Polelle Group and compare them to those of the overlying Mougooderra Formation to evaluate the heat source for metamorphism in order to test current geodynamic models of the Youanmi Terrane. We provide a revised thermotectonic history of the Youanmi Terrane and discuss the broader implications for the evolution of the Yilgarn Craton.

Section snippets

Geological setting

The Yilgarn Craton (Fig. 1) of Western Australia is one of the largest Archean cratons in the world and consists of seven terranes that preserve greenstone sequences, granitic gneisses, ultramafic to mafic intrusions, and sedimentary sequences between the ages of 3.05 and 2.60 Ga (Pidgeon and Hallberg, 2000, Cassidy et al., 2006). The Narryer and Southwest terranes consist mostly of granitic gneiss, while the domains of the Youanmi Terrane and the four terranes making up the Eastern Goldfields

Whole-rock geochemistry

In total, a suite of sixty-nine samples from the overlying sedimentary sequence and surrounding greenstone sequences were collected during two field seasons. From this sample suite, thirty samples were analysed for whole-rock major and trace elements. Sample locations are listed in Tbale S1 of the Supplementary material. Major element analyses were completed by Bureau Veritas Minerals (Perth, Australia) through the use of X-ray fluorescence (XRF) on a fused glass disc for each sample using

Whole-rock geochemistry

Whole-rock compositions of the Mougooderra Formation are plotted in Fig. 2. Measured whole-rock major element compositions define three general groups; a high Fe + Mg group, a high Al group, and an average pelitic group. Rocks from the high Fe–Mg group have higher concentrations of FeO, MgO, and MnO in comparison to the other samples. Concentrations of FeO in samples of the high Fe–Mg group are generally > 11 wt%, whereas those of the high Al group are <0.35 wt%. Similarly, concentrations of

Significance of garnet ages

Interpreting the geological significance of the ages obtained through garnet geochronology from the Mougooderra Formation and Polelle Group is critical for inferring the geodynamic setting of metamorphism in the Youanmi Terrane. Ages obtained through garnet Sm–Nd and Lu–Hf geochronology can be biased towards the timing of prograde metamorphism (Baxter et al., 2017, Baxter and Scherer, 2013, Smit et al., 2013) or peak metamorphism (Yakymchuk et al., 2015) based on zoning of the parent isotopes

Conclusions

The Mougooderra Formation is one of several large sedimentary sequences within the Youanmi Terrane of the Yilgarn Craton and has a range of metamorphic mineral assemblages controlled by variable whole-rock compositions. Samples from different compositional groups yield similar pressure and temperature conditions for peak metamorphism of 545–580 °C at 2.0–3.5 kbar. P–T estimates from samples of the Polelle Group were 610 °C to 650 °C and 4–7.2 kbar, respectively. Both estimates are consistent

CRediT authorship contribution statement

Stacey Parmenter: Formal analysis, Investigation, Writing - original draft. Tim J. Ivanic: Conceptualization, Resources, Writing - review & editing, Project administration. Fawna J. Korhonen: Conceptualization, Resources, Writing - review & editing. Audrey Bouvier: Investigation, Resources, Writing - review & editing. Jillian L. Kendrick: Investigation, Writing - review & editing. Chris Yakymchuk: Supervision, Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was partially funded by a Natural Sciences and Engineering Research Council of Canada Discovery Grant to CY. We thank M. Smit, Hao Cheng, and two anonymous reviewers for constructive comments on an earlier version of this manuscript. This work is published with the permission of the executive director of the Geological Survey of Western Australia. AB thanks Mouhcine Gannoun for his assistance with the MC-ICPMS in Clermont-Ferrand and Jon Patchett for providing the enriched spikes,

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