Hemispheric geochemical dichotomy of the mantle is a legacy of austral supercontinent assembly and deep subduction of continental crust

Oceanic hotspots with extreme enriched mantle radiogenic isotopic signatures—including high 87Sr/86Sr and low 143Nd/144Nd indicative of ancient subduction of continental crust—are restricted to the southern hemispheric mantle. However, the mechanisms responsible for concentrating subducted continental crust in the austral mantle are unknown. We show subduction of sediments and subduction eroded material, and lower continental crust delamination, cannot generate this spatially coherent austral domain. However, late Neoproterozoic to Paleozoic continental collisions—associated with the assembly of Gondwana and Pangea—were positioned predominantly in the southern hemisphere during the late Neoproterozoic appearance of widespread continental ultra-high-pressure (UHP, >2.7 gigapascals) metamorphic terranes, which marked the onset of deep subduction of upper continental crust. We propose that deep subduction of upper continental crust at ancient rifted-passive margins during austral supercontinent assembly, from 650-300 Ma, resulted in enhanced upper continental crust delivery into the southern hemisphere mantle. In contrast, EM domains are absent in boreal hotspots, for two reasons. First, continental crust subducted after 300—when the continents drifted into the northern hemisphere—has had insufficient time to return to the surface in plumes feeding northern hemisphere hotspots. Second, before the appearance of continental UHP rocks at 650 Ma, upper continental crust was not subducted to great depths, thus precluding its subduction into the northern hemisphere mantle during the Precambrian when continents may have been located in the northern hemisphere. Our model implies a recent formation of the austral EM domain, explains the geochemical dichotomy between austral and boreal hotspots, and may explain why austral hotspots outnumber boreal hotspots.

Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece

Retrograde metamorphic rocks provide key insights into the pressure–temperature (P–T) evolution of exhumed material, and resultant P–T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P–T conditions of retrograde metamorphic rocks has historically been challenging and has resulted in debate about the conditions experienced by these rocks. In this work, we combine elastic thermobarometry with oxygen isotope thermometry to quantify the P–T evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ∼1.4–1.7 GPa between 500–550 ∘C, retrograde epidote growth at ∼1.3–1.5 GPa between 400–500 ∘C, and a second stage of retrograde epidote growth at ∼1.0 GPa and 400 ∘C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist–eclogite facies and subsequent retrogression under blueschist–greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure–low-temperature conditions. These P–T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P–T-deformation conditions of retrograde mineral growth in high-pressure–low-temperature (HP/LT) metamorphic terranes.

A Globally Significant Potential Megascale Geopark: The Eastern Australian Mantle Hotspot Interacting with a North-Migrating Heterogeneous Continental Plate Creating a Variety of Volcano Types, Magmas, Xenoliths, and Xenocrysts

Australia commenced separating from Antarctica some 85 million years ago, finally separating about 33 million years ago, and has been migrating northwards towards the Eurasian plate during that time. In the process, Australia, on its eastern side, progressively passed over a mantle hotspot. A magma plume intersected a variable lithocrust with various lithologic packages such as Phanerozoic sedimentary basins, fold belts and metamorphic terranes, and Precambrian rocks. As such, there was scope for compositional evolution of magmas through melting and assimilation, as well as plucking of host rocks to include xenoliths, and xenocrysts. The volcanic chain, volcanoes, and lava fields that are spread latitudinally along 2000 km of eastern Australia present a globally-significant volcanic system that provides insights into magma and crust interactions, into the variability of xenoliths and xenocrysts, into magma evolution dependent on setting, and into the mantle story of the Earth. The Cosgrove Volcano Chain is an example of this, and stands as a globally-unique potential megascale geopark.

Metamorphic microdiamond formation is controlled by water activity, phase transitions and temperature

Metamorphic diamonds hosted by major and accessory phases in ultrahigh-pressure (UHP) metamorphic terranes represent important indicators of deep subduction and exhumation of continental crust at convergent plate boundaries. However, their nucleation and growth mechanisms are not well understood due to their small size and diversity. The Bohemian microdiamond samples represent a unique occurrence of monocrystalline octahedral and polycrystalline cubo-octahedral microdiamonds in two different metasedimentary rock types. By combining new and published data on microdiamonds (morphology, resorption, associated phases, carbon isotope composition) with P–T constraints from their host rocks, we demonstrate that the peak P–T conditions for the diamond-bearing UHP rocks cluster along water activity-related phase transitions that determine the microdiamond features. With increasing temperature, the diamond-forming medium changes from aqueous fluid to hydrous melt, and diamond morphology evolves from cubo-octahedral to octahedral. The latter is restricted to the UHP-UHT rocks exceeding 1100 °C, which is above the incongruent melting of phengite, where microdiamonds nucleate along a prograde P–T path in silicate-carbonate hydrous melt. The observed effect of temperature on diamond morphology supports experimental data on diamond growth and can be used for examining growth conditions of cratonic diamonds from kimberlites, which are dominated by octahedra and their resorbed forms.

Even the low-T garnet from the iconic Barrow’s zone is tetragonal

<p>Common (anhydrous) Fe-Mg-Ca-Mn garnet, the archetypal cubic mineral, has been recently discovered to be tetragonal in metapelites and metabasites from low-temperature regional metamorphic terranes (Cesare et al., 2018).</p><p>Despite the differences in bulk rock composition and pressure conditions, such low-T tetragonal garnets share common chemical features, namely high grossular (>25 mol%) and low pyrope (<7 mol%) contents. Similar compositions are documented in other contexts worldwide, both in blueschists-eclogites and in phyllites, including the metapelites from the garnet zone of the iconic Barrovian metamorphism of the Scottish highlands (Viete et al., 2011).</p><p>We have analysed a garnet crystal from a chlorite-biotite schist collected at the Barrow’s garnet zone in Glen Esk. The unit cell parameters were refined using diffraction reflections between 1.20 and 0.55 Å providing a tetragonal cell with a = 11.5731(5) Å and c = 11.5887(8) Å and volume V = 1552.15(15) Å3. Systematic absences analysis on complete intensity data collected up to 2theta = 80° indicated I41/acd space group confirming the cell parameters refinement.</p><p>Therefore, the garnet is tetragonal and not cubic, as suggested by its weak birefringence under crossed polarizers.</p><p>These results show that the tetragonal structure of common Fe-Mg-Ca-Mn garnet is verified whenever this mineral displays the Ca-rich, Mg-poor composition often observed in low-T metamorphic rocks. And support the hypothesis that the lowering of symmetry is composition-dependent.</p><p> </p><p>References</p><p>Cesare, B., et al. Garnet, the archetypal cubic mineral, grows tetragonal. Sci Rep <strong>9</strong>, 14672 (2019).</p><p>Viete, D.R., et al. The nature and origin of the Barrovian metamorphism, Scotland: Diffusion length scales in garnet and inferred thermal time scales. J. Geol. Soc. London <strong>168</strong>, 115–132 (2011).</p><p> </p>

Large-scale fluid circulation in deep subduction interfaces: implications on fast and slow earthquake-related processes

<p>Devolatilization and fluid-rock interaction processes along subduction interfaces, in particular at depths where episodic tremor and slip events (ETS) are inferred, are evidenced by the occurrence of metamorphic veins in exhumed metamorphic terranes. We investigate the late Cretaceous lawsonite blueschist-facies Seghin complex, part of the Zagros suture zone (Iran), a well-preserved paleo-subduction mélange composed of an antigorite-rich matrix wrapping foliated metatuffs and minor carbonate-bearing metasediments. We first focus on characterizing the relative chronology, conditions of deformation and potential fluid source(s) of Lws+Cpx+Gln veins and aragonite-filled explosive hydraulic breccias. Petrological, geochemical as well as O-C and Sr-Nd isotopic systematics of silicate-rich veins suggest formation mostly from internal devolatilization. This stage is followed at near peak burial conditions by pervasive, externally-derived fluid influx events, with fluids characterized by REE enrichments, and geochemical signatures indicating mixing between metasedimentary-derived fluids and far-traveled mafic-ultramafic-derived fluids. Our geochemical and petrological observations suggest that a host rock-buffered isotopic homogenization occurred between the infiltrating fluids and the rock matrix.</p><p>The high pore fluid pressures that enabled the formation of these deep veins also enabled the formation of shallower fault-related rocks including breccias, foliated cataclasites and fluidized ultracataclasites, intimately associated with extensional Gln-bearing veins and Lws+Gln+Ph+Ab fluid-filled pockets. Mineral assemblages reveal that this faulting occurred upon exhumation throughout the lawsonite blueschist-facies (i.e. 35 to 20 km depth). Crosscutting relationships among multiple generations of fluidized ultracataclasites and extensional veins show that episodic seismic faulting and hydrofracturing were contemporaneous processes. Mechanical modelling confirms that the studied fault-related features can only form under nearly lithostatic pore fluid pressure conditions, maintaining the system in a critically unstable regime that promotes recurrent seismic faulting. We propose a large-scale tectonic model in which deeply produced H<sub>2</sub>O-rich fluids are transported as highly pressurized “pulses” over tens of km parallel to the subduction interface, triggering episodic hydrofracturing and host rock-buffered isotopic homogenization within the ETS region. The mechanical consequence of these events is the triggering of unstable slip within the seismogenic window, as deduced in this unique record of blueschist-facies crustal paleo-earthquakes. These results shed a new light on the physical nature of the numerous moderate magnitude events (Mw=3-6) that are extensively recorded nowadays in Mariana-type plate boundary systems.</p>

How did the Archean crust evolve? Insights from the structure and petrology of the Lewisian of Scotland

<p>High-grade Archean gneiss terranes expose mid to lower crustal rocks and are generally dominated by tonalite-trondhjemite-granodiorite (TTG) gneisses. Occurrences of mafic-ultramafic bodies and garnet-bearing felsic gneisses within these environments have been interpreted as supracrustal or near-surface rocks requiring a tectonic process involving mass transfer from the near-surface to the mid-crust. However, there is significant uncertainty regarding the nature of this mass transfer, with suggestions including a range of uniformitarian and non-uniformitarian scenarios.  One non-uniformitarian scenario, ‘sagduction’, has been proposed as a possible mechanism (Johnson <em>et al.,</em> 2016, and references therein), although the dynamics of sagduction are still relatively unexplored.</p><p>This study focuses on mafic, ultramafic and garnet-bearing felsic gneiss bodies in the central region in the Lewisian Gneiss Complex of northwest Scotland as test cases to investigate the behaviour of possibly supracrustal rocks in a mid-crustal environment. Existing datasets of TTGs (Johnson <em>et al.,</em> 2016), mafic gneisses (Feisel <em>et al.,</em> 2018) and ultramafic gneisses (Guice <em>et al.,</em> 2018) from across the central region were utilised in addition to felsic and mafic gneiss samples obtained in this study from the ~10 km<sup>2</sup> Cnoc an t-Sidhean (CAS) suite. The CAS suite is the largest reported supracrustal in the Lewisian, and dominantly comprises garnet-biotite felsic gneiss assemblages and an associated two-pyroxene mafic gneiss. Field mapping was undertaken to collect samples representative of the observed heterogeneity of the suite, and to assess field associations between possible supracrustals and surrounding TTGs. Phase equilibria modelling was conducted on all lithologies to ascertain peak pressure-temperature (<em>P-T</em>) conditions, and to calculate the density of the modelled rocks at peak conditions.</p><p>The results obtained in this study indicate peak metamorphic conditions of 950 ± 50 °C and 9 ± 1 kbar for the CAS suite, consistent with the central region of the Lewisian Complex (Feisel <em>et al.,</em> 2018). Density contrasts at mid-crustal conditions of 0.12–0.56 gcm<sup>-3</sup> were calculated between TTGs and the other lithologies and used to estimate the buoyancy force that drives density-driven segregation. This allowed us to investigate the rates of vertical motion that result from density contrasts, as a function of the effective viscosity during metamorphism. Independent viscosity estimates were attained using mineral flow-laws and our estimated <em>P-T</em> conditions, and from examination of modern-day regions of crustal flow. We were therefore able to estimate the conditions under which sagduction could have been a viable mechanism for crustal evolution in the Lewisian and similar high-grade metamorphic terranes. We conclude that sagduction was unlikely to have operated in the Lewisian under the dry conditions implied by preserved mineral assemblages.</p><p> </p><p> </p><p>Feisel, Y., et al. 2018. New constraints on granulite facies metamorphism and melt production in the Lewisian Complex, northwest Scotland. Journal of Metamorphic Geology. <strong>36</strong>, 799-819</p><p>Guice, G.L., et al. 2018. Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland. Geosciences, <strong>8, </strong>338.</p><p>Johnson, T.E., et al. 2016. Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland. Precambrian Research, <strong>283</strong>, 89-105.</p>

Metasomatic iron and alkali calcic (MIAC) system frameworks: a TGI-6 task force to help de-risk exploration for IOCG, IOA and affiliated primary critical metal deposits

Australia's and China's resources (e.g. Olympic Dam Cu-U-Au-Ag and Bayan Obo REE deposits) highlight how discovery and mining of iron oxide copper-gold (IOCG), iron oxide±apatite (IOA) and affiliated primary critical metal deposits in metasomatic iron and alkali-calcic (MIAC) mineral systems can secure a long-term supply of critical metals for Canada and its partners. In Canada, MIAC systems comprise a wide range of undeveloped primary critical metal deposits (e.g. NWT NICO Au-Co-Bi-Cu and Québec HREE-rich Josette deposits). Underexplored settings are parts of metallogenic belts that extend into Australia and the USA. Some settings, such as the Camsell River district explored by the Dene First Nations in the NWT, have infrastructures and 100s of km of historic drill cores. Yet vocabularies for mapping MIAC systems are scanty. Ability to identify metasomatic vectors to ore is fledging. Deposit models based on host rock types, structural controls or metal associations underpin the identification of MIAC-affinities, assessment of systems' full mineral potential and development of robust mineral exploration strategies. This workshop presentation reviews public geoscience research and tools developed by the Targeted Geoscience Initiative to establish the MIAC frameworks of prospective Canadian settings and global mining districts and help de-risk exploration for IOCG, IOA and affiliated primary critical metal deposits. The knowledge also supports fundamental research, environmental baseline assessment and societal decisions. It fulfills objectives of the Canadian Mineral and Metal Plan and the Critical Mineral Mapping Initiative among others. The GSC-led MIAC research team comprises members of the academic, private and public sectors from Canada, Australia, Europe, USA, China and Dene First Nations. The team's novel alteration mapping protocols, geological, mineralogical, geochemical and geophysical framework tools, and holistic mineral systems and petrophysics models mitigate and solve some of the exploration and geosciences challenges posed by the intricacies of MIAC systems. The group pioneers the use of discriminant alteration diagrams and barcodes, the assembly of a vocab for mapping and core logging, and the provision of field short courses, atlas, photo collections and system-scale field, geochemical, rock physical properties and geophysical datasets are in progress to synthesize shared signatures of Canadian settings and global MIAC mining districts. Research on a metamorphosed MIAC system and metamorphic phase equilibria modelling of alteration facies will provide a foundation for framework mapping and exploration of high-grade metamorphic terranes where surface and near surface resources are still to be discovered and mined as are those of non-metamorphosed MIAC systems.