Time's arrow, time's cycle: Granulite metamorphism and geodynamics

Abstract

Although the thermal evolution of the mantle before c. 3.0 Ga remains unclear, since c. 3.0 Ga secular cooling has dominated over heat production—this is time's arrow. By contrast, the thermal history of the crust, which is preserved in the record of metamorphism, is more complex. Heat to drive metamorphism is generated by radioactive decay and viscous dissipation, and is augmented by the influx of heat from the mantle. Notwithstanding that reliable data are sparse before the Neoarchean, we use a dataset of temperature (T), pressure (P) and thermobaric ratio (T/P at the metamorphic ‘peak’), and age of metamorphism (t, the timing of the metamorphic ‘peak’) for rocks from 564 localities ranging in age from the Cenozoic to the Eoarchean eras to interrogate the crustal record of metamorphism as a proxy for the heat budget of the crust through time. On the basis of T/P, metamorphic rocks are classified into three natural groups: high T/P type (T/P >775°C/GPa, mean T/P ~1105°C/GPa), including common and ultrahigh-temperature granulites, intermediate T/P type (T/P between 775 and 375°C/GPa, mean T/P ~575°C/GPa), including high-pressure granulites and medium- and high-temperature eclogites, and low T/P type (T/P <375°C/GPa, mean T/P ~255°C/GPa), including blueschists, low-temperature eclogites and ultrahigh-pressure metamorphic rocks. A monotonic increase in the P of intermediate T/P metamorphism from the Neoarchean to the Neoproterozoic reflects strengthening of the lithosphere during secular cooling of the mantle—this is also time's arrow. However, temporal variation in the P of intermediate T/P metamorphism and in the moving means of T and T/P of high T/P metamorphism, combined with the clustered age distribution, demonstrate the cyclicity of collisional orogenesis and cyclic variations in the heat budget of the crust superimposed on secular cooling since c. 3.0 Ga—this is time's cycle. A first cycle began with the widespread appearance/survival of intermediate T/P and high T/P metamorphism in the Neoarchean rock record coeval with amalgamation of dispersed blocks of lithosphere to form protocontinents. This cycle was terminated by the fragmentation of the protocontinents into cratons in the early Paleoproterozoic, which signalled the start of a new cycle. The second cycle continued with the progressive amalgamation of the cratons into the supercontinent Columbia and extended until the breakup of the supercontinent Rodinia in the Neoproterozoic. This cycle represented a period of relative tectonic and environmental stability, and perhaps reduced subduction during at least part of the cycle. During most of the Proterozoic the moving means for both T and T/P of high T/P metamorphism exceeded the arithmetic means, reflecting insulation of the mantle beneath the quasi-integrated lithosphere of Columbia and, after a limited reorganisation, Rodinia. The third cycle began with the steep decline in thermobaric ratios of high T/P metamorphism to their lowest value, synchronous with the breakup of Rodinia and the formation of Pannotia, and the widespread appearance/preservation of low T/P metamorphism in the rock record. The thermobaric ratios for high T/P metamorphism rise to another peak associated with the Pan-African event, again reflecting insulation of the mantle. The subsequent steep decline in thermobaric ratios of high T/P metamorphism associated with the breakup of Pangea at c. 0.175 Ga may indicate the start of a fourth cycle. The limited occurrence of high and intermediate T/P metamorphism before the Neoarchean suggests either that suitable tectonic environments to generate these types of metamorphism were not widely available before then or that the rate of survival was low. We interpret the first cycle to record stabilisation of subduction and the emergence of a network of plate boundaries in a plate tectonics regime once the balance between heat production and heat loss changed in favour of secular cooling, possibly as early as c. 3.0 Ga in some areas. This is inferred to have been a globally linked system by the early Paleoproterozoic, but whether it remained continuous to the present is unclear. The second cycle was characterised by stability from the formation of Columbia to the breakup of Rodinia, generating higher than average T and T/P of high T/P metamorphism. The third cycle reflects colder collisional orogenesis and deep subduction of the continental crust, features that are characteristic of modern plate tectonics, which became possible once the average temperature of the asthenospheric mantle had declined to <100°C warmer than the present day after c. 1.0 Ga.

The Evolution of the Continental Crust and the Onset of Plate Tectonics

“…The bimodality in the Proterozoic and late Archean is taken to reflect destructive plate margins but displaced to higher T/P, perhaps because the Earth was hotter (Holder et al, 2019). However, a number of examples of high T/P conditions in the Phanerozoic are associated with crustal domes and metamorphism in response to thickening, uplift and erosion, rather than to plate margins FIGURE 3 | Variations in the temperatures and pressures from the peak of metamorphism estimates from Brown (2006Brown ( , 2007, as in Brown and Johnson (2019), The facies series are from Zheng and Zhao (2020).…”

“…(see data compilation in Brown and Johnson, 2019), and paired high and intermediate T/P metamorphism can be developed in models with lithosphere peel-back associated with lithospheric convergence (Chowdhury et al, 2020). Holder et al (2019) argued that bimodal metamorphism was well developed by 2 Ga, and that plate tectonics may have started by the Late Archean.…”

“…Some approaches rely on the recognition of boninitic magmas (Cameron et al, 1979;Polat and Kerrich, 2001;Smithies et al, 2004), and others on stratigraphic associations (Turner et al, 2014) that are found in recent subduction-related settings. Yet many rely on distinctive relatively immobile trace elements and ratios such as Th/Nb, which are elevated in modern subduction-related magmas reflecting their distinctive negative anomalies in Nb and Ta, due to fluid-fluxed melting in the mantle wedge between Brown and Johnson (2019). The crustal reworking curve is based on the Hf isotope ratios of zircons in different time slices (Dhuime et al, 2012), and the gray areas represent the ages of supercontinents plotted in Figure 1B after Campbell and Allen (2008).…”

Enigmatic Mid‐Proterozoic Orogens: Hot, Thin, and Low

“… Duration of deformation compared with metamorphic thermobaric ratio for Proterozoic and Phanerozoic orogens. T/P values of individual metamorphic rocks (Brown & Johnson, 2019) are assigned to specific Proterozoic orogens (from Condie, 2021) and compared to the range of T/P values for Phanerozoic orogens (Brown & Johnson, 2019). Final collision age is from Condie (2021).…”

“…During the Phanerozoic, the metamorphic imprint left in the continental crust by orogenesis is accepted as a clear manifestation of plate tectonic convergence. Thus, secular change in metamorphism may offer clues about the style of plate tectonics operating in the Proterozoic (Brown & Johnson, 2019; Holder et al., 2019). A contrast in the style of plate tectonics between the Proterozoic and the Phanerozoic was identified in the metamorphic record (Brown, 2006).…”