The contribution of metamorphic petrology to understanding lithosphere evolution and geodynamics

Abstract

a b s t r a c tIn the early 1980s, evidence that crustal rocks had reached temperatures >1000 C at normal lower crustal pressures while others had followed low thermal gradients to record pressures characteristic of mantle conditions began to appear in the literature, and the importance of melting in the tectonic evolution of orogens and metamorphicemetasomatic reworking of the lithospheric mantle was realized. In parallel, new developments in instrumentation, the expansion of in situ analysis of geological materials and increases in computing power opened up new fields of investigation. The robust quantification of pressure (P), temperature (T) and time (t) that followed these advances has provided reliable data to benchmark geodynamic models and to investigate secular change in the thermal state of the lithosphere as registered by metamorphism through time. As a result, the last 30 years have seen significant progress in our understanding of lithospheric evolution, particularly as it relates to Precambrian geodynamics.EoarcheaneMesoarchean crust registers uniformly high T/P metamorphism that may reflect a stagnant lid regime. In contrast, two contrasting types of metamorphism, eclogiteehigh-pressure granulite metamorphism, with apparent thermal gradients of 350e750 C/GPa, and granulite eultrahigh temperature metamorphism, with apparent thermal gradients of 750e1500 C/GPa, appeared in the Neoarchean rock record. The emergence of paired metamorphism is interpreted to register the onset of one-sided subduction, which introduced an asymmetric thermal structure at these developing convergent plate margins characterized by lower T/P in the subduction channel and higher T/P in the overriding plate. During the Paleoarchean to Paleoproterozoic the ambient mantle temperature was warmer than at present by w300e150 C. Although the thermal history of Earth is only poorly constrained, it is likely that prior to c. 3.0 Ga heating from radioactive decay would have exceeded surface heat loss, whereas since c. 2.5 Ga secular cooling has dominated the thermal history of the Earth. The advent of paired metamorphism is consistent with other changes in the geological record during the Neoarchean that are best explained as the result of a transition from a stagnant lid to subduction and a global plate tectonics regime by c. 2.5 Ga. This interpretation is supported by results from 2-D numerical experiments of oceanic subduction that demonstrate a change to one-sided subduction is plausible as upper mantle temperature declined to <200 C warmer than at present during the late NeoarcheanePaleoproterozoic. This is the beginning of the Proterozoic plate tectonics regime.At 1.0 Ga the ambient mantle temperature was still w150e100 C warmer than at present. Continued secular cooling caused a transition to cold subduction registered in the crustal record of metamorphism by the first appearance of blueschist and high to ultrahigh pressure metamorphism during the Neoproterozoic. Results of 2-d numerical experiments of continental collision demon...

The zircon archive of continent formation through time

“…O'Neill et al 2007;Sizova et al 2010). A gradual change from shallow subduction in the Archaean, to steep subduction after 1 Ga, is compatible with the record of metamorphic gradients (Brown 2014;Sizova et al 2014). As discussed by Korenaga (2013), however, the role of water in the mantle is not always accounted for in numerical models, and if dry enough, a hot mantle may also allow for slab subduction.…”

“…In this instance, the significant record of crustal growth in the Neoarchaean at 2.9-2.7 Ga (see Figs 1 & 10a) may represent the onset of stabilization of Archaean crust , and/or may also represent a true episode of increased crustal growth (Condie 1998;Arndt & Davaille 2013). Concurrent with the preservation of ultra high-pressure metamorphism, the onset of cold, deep and steep subduction like the present-day probably occurred in the Neoproterozoic (Stern 2005(Stern , 2008Brown 2006Brown , 2007Brown , 2014Sizova et al 2010Sizova et al , 2014. So where now?…”

“…Modelling of mantle temperatures suggests that steeper subduction may have begun to occur around 3.2-2.5 Ga (Sizova et al 2010). The metamorphic record shows that eclogite-high pressure and ultrahigh temperature metamorphism started around c. 2.7 Ga (Brown 2006(Brown , 2007(Brown , 2014. Diamond inclusion compositions show a change around 3 Ga that is interpreted as the onset of formation of eclogite through subduction and its capture via continental collision (Shirey & Richardson 2011).…”

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

“…Metamorphic rocks record pressure-temperature (P-T ) conditions that reflect Earth's internal thermal structure during ancient orogenic events (Brown, 2014). Steadystate geotherms for continental crust are curved but have average gradients of~15°C km −1 (Fig.…”

Continent formation through time

“…Indeed, a number of Archaean granitegreenstone terrains are interpreted as volcanic plateaux, either truly oceanic or built on a substrate of older, at least in part sialic, crust ( Fig. 6b; Polat et al 1998;Van Kranendonk et al 2007a, 2014Tessalina et al 2010;Reimink et al 2014). The timing of the first true continents is difficult to ascertain, whether continental crust was derived in arc environments or in volcanic plateaux, it would require significant global plate motions to amalgamate these into large continental blocks ( Fig.…”