Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny

Magmatic arcs are terrestrial environments where lithospheric cycling and recycling of metals and volatiles is enhanced. However, the first-order mechanism permitting the episodic fluxing of these elements from the mantle through to the outer Earth’s spheres has been elusive. To address this knowledge gap, we focus on the textural and minero-chemical characteristics of metal-rich magmatic sulfides hosted in amphibole-olivine-pyroxene cumulates in the lowermost crust. We show that in cumulates that were subject to increasing temperature due to prolonged mafic magmatism, which only occurs episodically during the complex evolution of any magmatic arc, Cu-Au-rich sulfide can exist as liquid while Ni-Fe rich sulfide occurs as a solid phase. This scenario occurs within a ‘Goldilocks’ temperature zone at ~1100–1200 °C, typical of the base of the crust in arcs, which permits episodic fractionation and mobilisation of Cu-Au-rich sulfide liquid into permeable melt networks that may ascend through the lithosphere providing metals for porphyry and epithermal ore deposits

Tectonics and melting in intra-continental settings

Most of the geodynamic theories of deformation aswell asmetamorphismandmelting of continental lithosphere are concentrated on plate boundaries and are dominated by the effects of subduction upon deformation of the margins of continental lithospheric blocks. However, it is becoming increasingly apparent that suture zones, or mobile belts, presumably representing fossil subduction zones, or other types of pre-deformation which occur far from present plate boundaries, play a key role in intra-continental deformation. In such zones, the crust is strongly sheared and the mantle lithosphere is metasomatized. Reworking of such settings reveals a surprisingly large range of instabilities that develop in compressed lithosphere with lateral heterogeneities inherited from previous deformational processes. Structural complexity arises,which is sensitive to lithospheric age and tectonic setting. This complexity influences localization of deformation, topographic evolution, melt generation, and melt generation, ascent and emplacement. In this paper, using fully coupled petrological–thermomechanical modeling, various tectonic responses are correlated with magmatic events in intra-continental settings and are compared to observed intra-cratonic orogenies and magmatic events

Tectonics and melting in intra-continental settings

Most of the geodynamic theories of deformation aswell asmetamorphismandmelting of continental lithosphere are concentrated on plate boundaries and are dominated by the effects of subduction upon deformation of the margins of continental lithospheric blocks. However, it is becoming increasingly apparent that suture zones, or mobile belts, presumably representing fossil subduction zones, or other types of pre-deformation which occur far from present plate boundaries, play a key role in intra-continental deformation. In such zones, the crust is strongly sheared and the mantle lithosphere is metasomatized. Reworking of such settings reveals a surprisingly large range of instabilities that develop in compressed lithosphere with lateral heterogeneities inherited from previous deformational processes. Structural complexity arises,which is sensitive to lithospheric age and tectonic setting. This complexity influences localization of deformation, topographic evolution, melt generation, and melt generation, ascent and emplacement. In this paper, using fully coupled petrological–thermomechanical modeling, various tectonic responses are correlated with magmatic events in intra-continental settings and are compared to observed intra-cratonic orogenies and magmatic events

Ultra-hot Mesoproterozoic evolution of intracontinental central Australia

The Musgrave Province developed at the nexus of the North, West and South Australian cratons and its Mesoproterozoic evolution incorporates a 100 Ma period of ultra-high temperature (UHT) metamorphism from ca. 1220 to ca. 1120 Ma. This was accompanied by high-temperature A-type granitic magmatism over an 80 Ma period, sourced in part from mantle-derived components and emplaced as a series of pulsed events that also coincide with peaks in UHT metamorphism. The tectonic setting for this thermal event (the Musgrave Orogeny) is thought to have been intracontinental and the lithospheric architecture of the region is suggested to have had a major influence on the thermal evolution. We use a series of two dimensional, fully coupled thermo-mechanical-petrological numerical models to investigate the plausibility of initiating and prolonging UHT conditions under model setup conditions appropriate to the inferred tectonic setting and lithospheric architecture of the Musgrave Province. The results support the inferred tectonic framework for the Musgrave Orogeny, predicting periods of UHT metamorphism of up to 70 Ma, accompanied by thin crust and extensive magmatism derived from both crustal and mantle sources. The results also appear to be critically dependent upon the specific location of the Musgrave Province, constrained between thicker cratonic masses

Mineral systems prospectivity modelling for gold and nickel in the Halls Creek Orogen, Western Australia

International audienceGeodynamic models, geological-geophysical interpretations, and isotope analysis illustrate that there are links between the nickel and gold mineral systems in the Halls Creek Orogen, Western Australia. Whole-rock Nd and 40Ar/39Ar analysis of rocks throughout the region, when compared with the geodynamic models suggest that nickel and gold mineralization in the Halls Creek Orogen can be related to basin development and subsequent basin closure during the convergence of North Australian Craton and Kimberley Craton, respectively. Whole-rock Nd analysis confirmed the input of juvenile melts in the centre of the orogen before the 1835–1805 Ma Halls Creek Orogeny, supporting the upwelling of decompression mantle melts during the basin development. These analyses also revealed the spatial links between nickel and gold mineralization and lithological units with positive εNd values. Spatially the link between these mineral systems appears to be related to the presence of deep-seated shear zones that formed early in the history of the orogen and were later reactivated. The results of geodynamic models, geophysical interpretation, and isotopic analysis are used to understand the critical processes in the gold and nickel mineralization, which are presented by predictor maps. The GIS-based knowledge-driven fuzzy logic method used to integrate the predictor maps and create the prospectivity maps. Herein we show that mafic-ultramafic units prospective for nickel mineralization formed by upwelling of decompression mantle melt during crustal thinning and extension during basin development, and typically consist of the most juvenile magmas in the region. Whereas, gold deposits formed during the compressional regime and basin closure, and are located along a major shear zone separating two terranes. This deep crustal-scale shear zone is implied to be the site of multiple stages of deformation that acted as fluid migration pathways during basin closure and subsequent collision. Another critical element that appears to be related to gold prospectivity is the presence of lithologies with a juvenile signature. In contrast to nickel analyses which are closely related to mafic-ultramafic units, the source component seems less influential when attempting to target orogenic gold deposits

Evolution of Earth’s tectonic carbon conveyor belt

Concealed deep beneath the oceans is a carbon conveyor belt, propelled by plate tectonics. Our understanding of its modern functioning is underpinned by direct observations, but its variability through time has been poorly quantified. Here we reconstruct oceanic plate carbon reservoirs and track the fate of subducted carbon using thermodynamic modelling. In the Mesozoic era, 250 to 66 million years ago, plate tectonic processes had a pivotal role in driving climate change. Triassic–Jurassic period cooling correlates with a reduction in solid Earth outgassing, whereas Cretaceous period greenhouse conditions can be linked to a doubling in outgassing, driven by high-speed plate tectonics. The associated ‘carbon subduction superflux’ into the subcontinental mantle may have sparked North American diamond formation. In the Cenozoic era, continental collisions slowed seafloor spreading, reducing tectonically driven outgassing, while deep-sea carbonate sediments emerged as the Earth’s largest carbon sink. Subduction and devolatilization of this reservoir beneath volcanic arcs led to a Cenozoic increase in carbon outgassing, surpassing mid-ocean ridges as the dominant source of carbon emissions 20 million years ago. An increase in solid Earth carbon emissions during Cenozoic cooling requires an increase in continental silicate weathering flux to draw down atmospheric carbon dioxide, challenging previous views and providing boundary conditions for future carbon cycle models