Distinct formation history for deep mantle domains reflected in geochemical differences

Doucet, Dr. Luc1, Li, Pr. Zheng-Xian1, El Dien, Hamed Gamal1,2, Pourteau, Dr. Amaury1, Murphy, Pr. Brendan1,3, Collins, Pr. William1, Mattielli,Pr. Nadine4, Olierook,Dr. Hugo5,6, Spencer,Asso. Pr. Christopher 1,7, Mitchell, Asso. Pr. Ross8

1Earth Dynamics Research Group, TIGeR, School of Earth and Planetary Sciences, Curtin University, Perth WA 6845, Australia, Bentley, Australia, 2Geology Department, Faculty of Science, Tanta University, Tanta, Egypt, 3Department of Earth Sciences, St. Francis Xavier University, Antigonish, , Canada, 4Laboratoire G-Time, Université Libre de Bruxelles, Brussels, Belgium, 5Timescales of Mineral Systems, Centre for Exploration Targeting – Curtin Node, Curtin University, Bentley, Australia, 6John de Laeter Centre, Curtin University,, Bentley, Australia, 7Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Canada, 8State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

The Earth’s mantle is currently divided into the African and Pacific domains, separated by the circum-Pacific subduction girdle and each domain features a large low shear-wave velocity provinces (LLSVPs) in the lower mantle. However, it remains controversial as to whether the LLSVPs have been stationary through time or dynamic, changing in response to changes in global subduction geometry. Here we compile radiogenic isotope data on plume-induced basalts from ocean islands and oceanic plateaus above the two LLSVPs which show distinct Pb, Nd and Sr isotopic compositions for the two mantle domains. The African domain shows enrichment by subducted continental material during the assembly and breakup of the supercontinent Pangaea, whereas no such feature is found in the Pacific domain. This deep-mantle geochemical dichotomy reflects the different evolutionary histories of the two domains during the Rodinia and Pangaea supercontinent cycles and thus supports a dynamic relationship between plate tectonics and deep mantle structures.


Biography

After a PhD in St Etienne, France (2012), Luc moved to Brussels to apply the “non-traditional” stable-isotope systematics on Archean igneous. After an academic career break, he joined Professor Li and the Earth Dynamics Research Group (Curtin University) to work on the present-day and past connections between Earth’s mantle, supercycles.

Paleogeography of the western margin of Rodinia: New findings from Madagascar and Seychelles

Zhou, Dr Jiu-long1, Li, Prof. Xian-Hua1,2, Li,Prof. Zheng-Xiang3

1State Key Laboratory Of Lithospheric Evolution, Institute Of Geology And Geophysics, Chinese Academy Of Sciences, Beijing, China, 2College of Earth Sciences, University of Chinese Academy of Sciences, Beijing, China, 3Earth Dynamics Research Group, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, Australia

* Email: zhoujl_geo@126.com

We present new and existing geological and geochemical observations from Madagascar and Seychelles, both are believed to have faced the Mirovoi superocean during Rodinia time, in order to shed new light on the evolution of Rodinia’s western margin. (1) 870-750 Ma plutonic rocks in central Madagascar (i.e., the Imorona-Itsindro Suite) have a general bi-modal nature. The felsic components have a ferroan A-type composition, were likely derived from ancient continental crust, and show a positive zircon εHf(t) excursion during a 790 Ma high-flux magmatic pulse. The gabbroic components host layered Fe-Ti-V oxides and chromitites, and were likely derived from an enriched lithospheric mantle with no clear asthenospheric input. (2) Clastic sedimentary rocks in central Madagascar coeval with the Imorona-Itsindro Suite contain detrital zircons of dominantly Archean to Paleoproterozoic ages. (3) Neoproterozoic granitoids in Seychelles were emplaced during two discrete magmatic pulses: an older and weaker one at 810 Ma and a younger and more intense one at 760–745 Ma. Primary low-δ18O signals are absent from the 810 Ma granitoids but characterize the 750 Ma granitoids. The 750 Ma Mahé-type (Nd-isotopically juvenile) granitoids appear to display a stronger 18O-depletion than the 750 Ma Praslin-type (Nd-isotopically enriched) granitoids. Some of the 750 Ma 18O-depleted rocks even document intra-zircon δ18O decreases from core to rim. Thus, the Seychelles granitoids likely acquired their low-δ18O signatures through the dynamic magma process of crustal cannibalization, instead of through source inheritance from a pre-existing basement. (4) Neoproterozoic magmatism in the Bemarivo Terrane (northern Madagascar) also has a two-episodes age-distribution: an older 760–745 Ma one represented by the Antsirabe Nord Suite, and a younger 740–700 Ma one represented by the Manambato Suite and the Daraina-Milanoa Group. The Antsirabe Nord granitoids are ferroan A-type in chemistry, have negative ɛHf(t) values and low-δ18O signatures, and were likely generated by remelting of an ancient basement source. In contrast, the Manambato granitoids and Daraina-Milanoa rhyolites are chemically calc-alkaline and Hf-isotopically juvenile, with progressively disappearing low-δ18O signatures. Generation of these rocks involves a significant input of asthenospheric materials.

The pre-745 Ma Neoproterozoic igneous rocks in central Madagascar, the Bemarivo Terrane, and Seychelles bear strong resemblance to each other in geochemistry and petrogenesis, and likely have been produced within continental rift(s). However, the post-745 Ma igneous rocks, only found in the Bemarivo Terrane, likely signified the onset of arc magmatism. We thus conclude that Rodinia’s western margin experienced a transition from a protracted passive margin setting at 870–745 Ma to an arc setting at 740–700 Ma at this segment of the eastern edge of the Mirovoi superocean. The 740 Ma  subduction initiation age here is significantly younger than previously proposed.


Biography

Jiu-Long Zhou obtained his Ph.D. degree from the China University of Geosciences (Beijing) in 2016, and then received postdoctoral training at the Institute of Geology and Geophysics, Chinese Academy of Sciences. His research interests focus on utilizing geochemical methods to understand supercontinent cycles and associated igneous processes.

Linking supercontinents to a convective mantle framework

Martin, Erin L.1,3, Cawood, Peter A.1, Murphy, J. Brendan2,3

1School of Earth, Atmosphere and Environment Science, Monash University, Clayton, Australia, 2Department of Earth Sciences, St Francis Xavier University, Antigonish, Canada, 3Earth Dynamics Research Group, The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, Australia

The amalgamation of continental fragments into supercontinents can occur by processes of introversion, involving the closure of interior oceans, extroversion, in which the exterior ocean closes, or orthoversion, entailing formation 90° from the centroid of the previous supercontinent. However, individual supercontinents are often defined as forming by contradictory mechanisms; for example, Pangea has been argued as forming by introversion and by extroversion. Conflicting interpretations arise, in part, from attempting to define an ocean as interior or exterior based on paleogeography, or the age of oceanic crust relative to the time of supercontinent breakup. We argue that interior and exterior oceans should be defined relative to the peripheral subduction ring and its associated accretionary orogens that surround the amalgamated supercontinent. The subduction ring broadly divides the Earth into two cells, which conform to the spherical harmonic degree-2 structure of the mantle: one associated with supercontinent assembly, and therefore dominated by continental crust with only minor oceanic crust, and the other containing almost exclusively oceanic crust, which is subducted to create peripheral accretionary orogens at the margin of the supercontinent. All oceans within the cell that contains continental blocks are interior oceans, as they are interior to the continental cell of the degree-2 planform. By contrast, the exterior ocean is the oceanic cell antipodal to the continental cell, separated by the subduction ring. Interior oceans close following asymmetrical subduction and collisional orogenesis. However, for the exterior ocean to close, the subduction ring must collapse upon itself, leading to the juxtaposition of long-lived accretionary orogens within the core of the supercontinent. Employing this geodynamic definition for interior and exterior oceans, Rodinia formed by extroversion, but all other supercontinents formed by introversion which cannot occur without orthoversion.


Biography

Erin Martin is a research associate working with as part of the Pulse of the Earth ARC Laureate Fellowship team at Monash University. Erin completed her PhD at Curtin University with the Earth Dynamics Research Group.

Development of William’s Ridge, Kerguelen Plateau and Broken Ridge: tectonics, hotspot magmatism, microcontinents, and Australia’s Extended Continental Shelf

Coffin, Millard F1.; Whittaker, Joanne1; Daczko, Nathan2; Halpin, Jacqueline1; Bernardel, George3; Picard, Kim3; Gardner, Robyn2; Gürer, Derya4; Brune, Sascha5; Gibson, Sally6; Hoernle, Kaj7; Koppers, Antonius8; Storey, Michael9; Uenzelmann-Neben, Gabriele10; Magri, Luca1; Neuharth, Derek5; Christiansen, Sascha Høegh9; and Easton, Laura3

1Institute For Marine & Antarctic Studies, University Of Tasmania, Hobart, Australia, 2Macquarie University, Sydney, Australia, 3Geoscience Australia, Canberra, Australia, 4University of Queensland, Brisbane, Australia, 5GFZ German Research Centre for Geosciences, Potsdam, Germany, 6University of Cambridge, Cambridge, United Kingdom, 7GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, 8Oregon State University, Corvallis, United States of America, 9Natural History Museum of Denmark, Copenhagen, Denmark, 10Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

William’s Ridge, a ~300-km-long salient extending southeast from the Central Kerguelen Plateau, and Broken Ridge are conjugate divergent margins in the southern Indian Ocean that separated at ~43 Ma. In early 2020, scientists aboard Australia’s Marine National Facility, RV Investigator, acquired multichannel seismic reflection (MCS), sub-bottom profiling, multibeam bathymetry, and gravity data on these margins, as well as dredged rock samples, on a 57-day voyage. The research project constitutes the first-ever case study of conjugate oceanic plateau end-member tectonic plates, with the goal of advancing knowledge of lithospheric rifting, breakup, and initial plate separation processes. The first-ever dedicated multibeam mapping of William’s and Broken ridges encompassed ~52,000 km2 and ~43,000 km2, respectively. Four new RV Investigator MCS profiles (500 line-km) across William’s Ridge complement one legacy RV Rig Seismic and three new RV Sonne MCS profiles; five new RV Investigator MCS profiles (603 line-km) across the conjugate portion of Broken Ridge are the first to be acquired on that feature. Multibeam bathymetry and MCS transects of William’s Ridge show multiple linear ridges and troughs interpreted as horst and graben. In contrast, multibeam bathymetry and MCS transects of Broken Ridge show a prominent E-W scarp (Diamantina Escarpment) with a complex morphology of emanating en echelon crustal blocks and depressions at the base of the scarp. Prominent angular unconformities (middle Eocene hiatus?) characterize the sedimentary section on some ridges, and dipping reflection sequences within interpreted igneous basement suggest subaerial basalt flows. Rock dredges on the facing conjugate margin fault scarps targeted all stratigraphic levels exposing basement rocks. Nine on William’s Ridge yielded both oceanic and (in situ?) continental rocks; eight on Broken Ridge yielded solely oceanic rocks. The new geophysical data and geological samples may justify a new or revised submission to the United Nations Commission on the Limits of the Continental Shelf to extend Australia’s marine jurisdiction on and around William’s Ridge under the United Nations Convention on the Law of the Sea.


Biography

Marine geophysicist Mike Coffin investigates interactions between the solid Earth and the oceanic environment. Educated at Dartmouth College (A.B.) and Columbia University (M.A., M.Phil., Ph.D.), he has pursued an international career in Australia, France, Japan, Norway, the UK, and the USA. His 35 research voyages span the global ocean.

The first continuous global full-plate animation back to 2 Ga

Li, Zheng-Xiang1, Wu, Lei1,2, Liu, Yebo1, Pisarevsky, Sergei1,

1Earth Dynamics Research Group, TIGeR, School of Earth and Planetary Sciences, Curtin University, Perth WA 6845, Australia, 2Department of Earth & Planetary Sciences, McGill University, 3450 Rue University, Montréal, Québec, H3A 0E8, Canada

Creditable global plate reconstructions back to early Earth are crucial for understanding the evolution and driving mechanisms of the Earth system. Here we present the first continuous global full-plate reconstruction model back to 2 Ga, featuring the assembly and break-up of three successive supercontinents: Nuna (tenure ca. 1600–1300 Ga), Rodinia (900–700 Ma) and Pangaea (320–170 Ma). We revised the configuration of both Nuna and Rodinia based on the updated global palaeomagnetic database with correction applied to inclination shallowing, and calibrated our reconstruction using geological and tectonic databases and prior knowledge. We generally adopt an orthoversion longitudinal rule for supercontinent assembly (i.e., each supercontinent is longitudinally 90° away from the previous one), but provide an alternative for longitudinal evolution between Nuna and Rodinia. We also adopt an alternating introversion and extroversion mechanism for supercontinent assembly, with Rodinia being assembled through introversion, and Pangaea extroversion. For Rodinia reconstruction, we recognise the possible repeated true polar wander events during the tenure of the supercontinent and interpret the phenomenon as inertia interchange true polar wander (IITPW) due to an inherited degree-2 mantle structure from the Nuna cycle. We used the palaeomagnetic reference frame (i.e., a reference frame fixed to the spin axis) for our reconstruction instead of plume or LLSVP reference frames as increasing evidences suggest a dynamic nature of deep mantle structures that are linked to the supercontinent and superocean cycles and plate tectonics.


Biography

Zheng-Xiang Li is an ARC Laureate Fellow and a Co-Leader of IGCP 648. After PhD at Macquarie, he worked for UWA for >17 years before joining Curtin in 2007. The Earth Dynamics Research Group that he leads aims to explore Earth’s evolution since the Proterozoic and the dynamic driving mechanisms.

Structural evolution of a 1.6 Ga orogeny related to the final assembly of the supercontinent Nuna: coupling of episodic and progressive deformation

Volante, Dr Silvia1,2, Collins, Prof William J.1, Pourteau, Dr Amaury1, Li,Prof Zheng-Xiang1, Li, Jiangyu1, Nordsvan, Dr Adam1,3

1Earth Dynamics Research Group, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS) and The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, Australia, 2Institute of Geology, Mineralogy and Geophysics, Ruhr-Universität Bochum, Bochum, Germany 3Department of Earth Sciences, University of Hong Kong, Pokfulam, Hong Kong

The poly-deformed Georgetown Inlier (GTI) in NE Australia has recently been suggested to record a 1.60 Ga orogenic event related to final Nuna assembly. However, the structural evolution of the inlier has remained poorly constrained at the regional-scale, and major tectono-thermal events occurred at c. 1.55 Ga. The GTI is the type-region for conceptualisation of crenulation cleavage development and where the foliation intersection axes (FIAs) approach has been applied. We re-evaluated both concepts by combining a multiscale petrostructural analysis with recent petrological and geochronological data. Three main deformation events (D1, D2, D3) and associated composite fabrics (S1, S2, S3) are identified in the GTI. The original NE-orientation of 1.60 Ga D1 compressional structures is preserved in the low-grade western domain, and the associated composite S1 fabric is retained as microstructural relicts within c. 1.55 Ga D2 low-strain domains to the east. Extensional D2 structures, characterised by a pervasive, high-grade, composite S2 foliation throughout the central and eastern domains, are interpreted as the footwall of a regional N-S-trending, W-dipping crustal-scale detachment zone. Syn-D2 S-type granites formed at 1.55 Ga as the detachment evolved. D1 stage was associated with Nuna assembly, whereas D2 represents post-collisional extension. Progressive foliation development occurred twice in the GTI, at 1.60 Ga (D1) and 1.55 Ga (D2), but the previous FIA analysis only records the 1.60 Ga event and cannot be easily reconciled with the regional structural analysis. This study highlights that a multiscale and multi-disciplinary approach is required to unravel the structural history of orogenic belts.


Biography

Our interest lies in reconstructing the structural, magmatic and metamorphic history of Proterozoic inliers to unravel the evolution of NE Australia during the Mesoproterozoic final assembly of the supercontinent Nuna by applying a multi-disciplinary and multi-scale approach which combines structural analysis with geochronology, metamorphic and igneous petrology and geochemistry.

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