The mid-Proterozoic or "boring billion" exhibited extremely stable environmental conditions, with little change in atmospheric oxygen levels, and mildly oxygenated shallow oceans. A limited number of passive margins with extremely long lifespans are observed from this time, suggesting that subdued tectonic activity-a plate slowdown-was the underlying reason for the environmental stability. However, the Proterozoic also has a unique magmatic and metamorphic record; massif-type anorthosites and anorogenic Rapakivi granites are largely confined to this period and the temperature/pressure (thermobaric ratio) of granulite facies metamorphism peaked at over 1500 °C/GPa during the Mesoproterozoic. Here, we develop a method of calculating plate velocities from the passive margin record, benchmarked against Phanerozoic tectonic velocities. We then extend this approach to geological observations from the Proterozoic, and provide the first quantitative constraints on Proterozoic plate velocities that substantiate the postulated slowdown. Using mantle evolution models, we calculate the consequences of this slowdown for mantle temperatures, magmatic regimes and metamorphic conditions in the crust. We show that higher mantle temperatures in the Proterozoic would have resulted in a larger proportion of intrusive magmatism, with mantle-derived melts emplaced at the Moho or into the lower crust, enabling the production of anorthosites and Rapakivi granites, and giving rise to extreme thermobaric ratios of crustal metamorphism when plate velocities were slowest.
Metamorphism associated with orogenesis provides a mineral record that may be inverted to yield ambient apparent thermal gradients. On modern Earth, tectonic settings with lower thermal gradients are characteristic of subduction zones whereas those with higher thermal gradients are characteristic of backarcs and orogenic hinterlands. The duality of thermal environments reflects the asymmetry of one-sided subduction, which is the hallmark of modern plate tectonics; a duality of metamorphic belts is the characteristic imprint of one-sided subduction in the geological record. Apparent thermal gradients derived from inversion of age-constrained metamorphic P-T data may be used to identify tectonic settings of ancient metamorphism and to evaluate Precambrian geodynamic regimes. The Neoarchean records the first occurrences of granulite facies ultra-high temperature metamorphism (G-UHTM) and medium-temperature eclogite-high-pressure granulite metamorphism (E-HPGM), signifying a change in geodynamics that generated sites of higher and lower heat flow than those implied by apparent thermal gradients recovered from the older geological record. GUHTM is dominantly a Neoarchean-Cambrian phenomenon inferred to have developed in settings analogous to backarcs and orogenic hinterlands. In addition to Proterozoic occurrences, E-HPGM is common in the Paleozoic Caledonides and Variscides, and is inferred to record subduction-to-collision orogenesis. The occurrence of G-UHTM and E-HPGM belts since the Neoarchean signifies a change to one-sided subduction of oceanic lithosphere, possibly beginning as early as the Paleoarchean, and widespread transfer of water into the upper mantle. This change registers the beginning of a 'Proterozoic plate tectonics regime' that evolved during a Neoproterozoic transition to the 'modern plate tectonics regime' characterized by colder apparent thermal gradients and deep subduction of continental crust. The age distribution of metamorphic belts is not uniform, recording amalgamation of continental lithosphere into supercratons or supercontinents.
If we accept that a critical condition for plate tectonics is the creation and maintenance of a global network of narrow boundaries separating multiple plates, then to argue for plate tectonics during the Archean requires more than a local record of subduction. A case is made for plate tectonics back to the early Paleoproterozoic, when a cycle of breakup and collision led to formation of the supercontinent Columbia, and bimodal metamorphism is registered globally. Before this, less preserved crust and survivorship bias become greater concerns, and the geological record may yield only a lower limit on the emergence of plate tectonics. Higher mantle temperature in the Archean precluded or limited stable subduction, requiring a transition to plate tectonics from another tectonic mode. This transition is recorded by changes in geochemical proxies and interpreted based on numerical modeling. Improved understanding of the secular evolution of temperature and water in the mantle is a key target for future research. ▪ Higher mantle temperature in the Archean precluded or limited stable subduction, requiring a transition to plate tectonics from another tectonic mode. ▪ Plate tectonics can be demonstrated on Earth since the early Paleoproterozoic (since c. 2.2 Ga), but before the Proterozoic Earth's tectonic mode remains ambiguous. ▪ The Mesoarchean to early Paleoproterozoic (3.2–2.3 Ga) represents a period of transition from an early tectonic mode (stagnant or sluggish lid) to plate tectonics. ▪ The development of a global network of narrow boundaries separating multiple plates could have been kick-started by plume-induced subduction.
Preface 1. Introduction Michael Brown and Tracy Rushmer 2. Structure of the continental lithosphere Alan Levander, Adrian Lenardic and Karl E. Karlstrom 3. Thermo-mechanical controls on heat production distributions and the long-term evolution of the continents Mike Sandiford and Sandra McLaren 4. Composition, differentiation, and evolution of continental crust: constraints from sedimentary rocks and heat flow Scott M. McLennan, Stuart Ross Taylor and Sidney R. Hemming 5. The significance of Phanerozoic arc magmatism in generating continental crust Jon P. Davidson and Richard J. Arculus 6. Crustal generation in the Archean Hugh Rollinson 7. Structural and metamorphic processes in the lower crust: evidence from a deep-crustal isobarically-cooled terrane, Canada Michael L. Williams and Simon Hanmer 8. Nature and evolution of the middle crust: heterogeneity of structure and process due to pluton-enhanced tectonism Karl E. Karlstrom and Michael L. Williams 9. Melting of the continental crust: fluid regimes, melting reactions and source-rock fertility John D. Clemens 10. Melt extraction from lower continental crust of orogens: the field evidence Michael Brown 11. The extraction of melt from crustal protoliths and the flow behavior of partially molten crustal rocks: an experimental perspective Ernie H. Rutter and J. Mecklenburgh 12. Melt migration in the continental crust and generation of lower crustal permeability: inferences from modeling and experimental studies Tracy Rushmer and Steve Miller 13. Emplacement and growth of plutons: implications for rates of melting and mass transfer in continental crust Alexander R. Cruden 14. Elements of a modeling approach to the physical controls on crustal differentiation George W. Bergantz and Scott A. Barboza.
Barrow (1893) introduced three important ideas that furthered understanding of metamorphic processes: (i) the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism; (ii) the concept of progressive metamorphism; and (iii) the concept of magmatic advection of heat as a possible cause of regional metamorphism. This article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the P–T–t paths of metamorphic rocks, and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts. Understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum T and maximum P. Orogenic belts characterized by clockwise P–T paths achieved maximum P before maximum T, the metamorphic peak normally post-dated early deformation within the belt and additional heating above the ‘normal’ conductive flux has been related to the amount of overthickening. By contrast, orogenic belts characterized by counterclockwise P–T paths achieved maximum T before maximum P, the metamorphic peak normally pre-dated or was synchronous with early deformation within the belt and additional heating above the ‘normal’ conductive flux has been related to the emplacement of plutons. Techniques used to constrain portions of P–T–t paths include: the use of mineral inclusion suites in porphyroblasts and reaction textures; thermobarometry; the use of fluid inclusions; thermodynamic approaches such as the Gibbs method; radiogenic isotope dating; fission track studies; and numerical modelling. We can utilize specific mineral parageneses in suitable rocks to determine individual P–T–t paths, and a set of P–T–t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism. Recent advances are reviewed with reference to collision metamorphism, high-temperature–low-pressure metamorphism, granulite metamorphism, and subduction zone metamorphism, and some important directions for future work are indicated.
Significant volume of wet melting requires an influx of H 2 O-rich volatile phase. In hydrate-breakdown melting, initial melt accumulation is diffusion-controlled and melt accumulates around peritectic phases in low-pressure sites. As the melt-bearing rock weakens, it becomes porous at a few volume per cent melt, initiating an advective flow regime; as melt volume reaches the melt connectivity transition, melt may be lost from the system in the first of several melt-build-up–melt-loss events. Using mineral equilibria modelling, major and accessory phase controls on melt chemistry are evaluated. In residual migmatites and granulites, microstructures indicate the former presence of melt whereas leucosome networks record melt extraction pathways. Sites of initial melting nucleate shear instabilities; strain and anisotropy of permeability control the form of millimetre- to centimetre-scale inferred melt channels and strong anisotropy promotes high fluid focusing. Focused melt flow occurs by dilatant shear failure of low melt volume rocks, which leads to the formation of melt flow networks allowing accumulation and storage of melt, and forming the link for melt flow from grain boundaries to ascent conduits. Melt ascent is via ductile fractures, which may propagate from dilation or shear bands. Fractures are characterized by blunt tips; also they may exhibit zigzag geometry close to the tips and petrographic continuity between leucosome in the host and granite in the dyke. Horizontal tabular and wedge-shaped intrusions commonly are associated with the brittle–ductile transition zone. Vertical lozenge-shaped intrusions represent congelation of magma in ascent conduits and blobby plutons record lateral expansion localized by instability; these intrusion types characterize emplacement at deeper levels in the crust.
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