Earth's anomalous middle-age magmatism driven by plate slowdown

Abstract

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.

Some thoughts about eclogites and related rocks

“…The paucity of eclogites from the mid-Paleoproterozoic essentially until the late Neoproterozoic coincides with the period of low cooling and exhumation rates discussed earlier and is also related to the warmer style of orogenesis associated with the tenure of the supercontinents Columbia and Rodinia (Spencer et al, 2021) and the resulting warmer subcontinental mantle due to continental insulation (Tamblyn et al, 2022a;Zou et al, 2023) and an associated plate slowdown (Sobolev and Brown, 2019;O'Neill et al, 2022). The absence of blueschists prior to the late Neoproterozoic is enigmatic.…”

“…The secular trend that emerges from these studies reveals faster cooling/exhumation rates in the Phanerozoic than in the mid-Paleoproterozoic, with a low from the late Paleoproterozoic to the early Neoproterozoic (Zou et al, 2023) that coincides with a dearth of low-T /P metamorphic localities in Mesoproterozoic orogens (Brown et al, 2022). The mid-Proterozoic low is associated with the Columbia and Rodinia supercontinents, where continental insulation (Tamblyn et al, 2022a;Zou et al, 2023) and a plate slowdown (Sobolev and Brown, 2019;O'Neill et al, 2022) reduce the rate of secular mantle cooling leading to a warmer style of orogenesis (Spencer et al, 2021).…”

“…These changes may relate to lubrication of subduction by an increased sediment supply at continental margins and in trenches after the major surface erosion events in the Paleoproterozoic, the Huronian glaciations from 2.45 to 2.2 Ga, leading to the formation of the Columbia supercontinent and, in the Neoproterozoic, the "snowball" Earth glaciations from 0.75 to 0.63 Ga, leading to the formation of the Pangaea supercontinent (Sobolev and Brown, 2019). The mid-Proterozoic from about 1.75 to 0.75 Ga was likely a period of delayed mantle cooling due to continental insulation and reduced plate tectonic activity, which may explain the scarcity of eclogites during this interval (Tamblyn et al, 2022a;O'Neill et al, 2022;Zou et al, 2023).…”

Hot, Wide, Continental Back-arcs Explain Earth’s Enigmatic mid-Proterozoic Magmatic and Metamorphic Record

“…These include an apparent high in thermobaric ratios recorded in metamorphic rocks Johnson, 2018, 2019, Figure 1b), and intrusion of voluminous massif anorthosite suites (Ashwal and Bybee, 2017, Figure 1c). It has been proposed that many of these features can be explained by the combined influence of mantle heating due to supercontinental insulation (Brown and Johnson, 2018), a slowdown in plate motions (O'Neill et al, 2022), and a temporary return to a single-lid tectonic regime following an attempted start of plate tectonics in the Paleoproterozoic (Stern, 2020). However, Roberts et al (2022) questioned the necessity for supercontinental insulation and demonstrated that plate tectonics, albeit dominated by accretionary orogenesis, was active throughout the mid-Proterozoic.…”

Coupled fates of Earth’s mantle and core: Early sluggish-lid tectonics and a long-lived geodynamo

“…Compilation of geologic Earth evolution observations. (A) Reconstructed plate velocities, U p [pink lines,(11); blue lines,(12,13); green lines,(62)]. (B) Mantle potential temperature, T p , inferred from non-arc basalts and komatiites(63,64).…”