Extreme metamorphism and metamorphic facies series at convergent plate boundaries: Implications for supercontinent dynamics

Abstract

Crustal metamorphism under extreme pressure-temperature conditions produces characteristic ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) mineral assemblages at convergent plate boundaries. The formation and evolution of these assemblages have important implications, not only for the generation and differentiation of continental crust through the operation of plate tectonics, but also for mountain building along both converging and converged plate boundaries. In principle, extreme metamorphic products can be linked to their lower-grade counterparts in the same metamorphic facies series. They range from UHP through high-pressure (HP) eclogite facies to blueschist facies at low thermal gradients and from UHT through high-temperature (HT) granulite facies to amphibolite facies at high thermal gradients. The former is produced by low-temperature/pressure (T/P) Alpine-type metamorphism during compressional heating in active subduction zones, whereas the latter is generated by high-T/P Buchan-type metamorphism during extensional heating in rifting zones. The thermal gradient of crustal metamorphism at convergent plate boundaries changes in both time and space, with low-T/P ratios in the compressional regime during subduction but high-T/P ratios in the extensional regime during rifting. In particular, bimodal metamorphism, one colder and the other hotter, would develop one after the other at convergent plate boundaries. The first is caused by lithospheric subduction at lower thermal gradients and thus proceeds in the compressional stage of convergent plate boundaries; the second is caused by lithospheric rifting at higher thermal gradients and thus proceeds in the extensional stage of convergent plate boundaries. In this regard, bimodal metamorphism is primarily dictated by changes in both the thermal state and the dynamic regime along plate boundaries. As a consequence, supercontinent assembly is associated with compressional metamorphism during continental collision, whereas supercontinent breakup is associated with extensional metamorphism during active rifting. Nevertheless, aborted rifts are common at convergent plate boundaries, indicating thinning of the previously thickened lithosphere during the attempted breakup of supercontinents in the history of Earth. Therefore, extreme metamorphism has great bearing not only on reworking of accretionary and collisional orogens for mountain building in continental interiors, but also on supercontinent dynamics in the Wilson cycle.

First finding of continental deep subduction in the Sesia Zone of the Western Alps and implications for subduction dynamics

“…Previous studies found that the EMC reached peak metamorphic P–T conditions of 1.5–2.4 GPa and 500–600°C [ 24 , 25 ], which are converted to thermobaric ratios of 250–333°C/GPa. They correspond to low geothermal gradients of 7.5–10°C/km, which belong to Alpine-type eclogite facies metamorphism [ 26 ]. The eclogite facies HP metamorphism took place at 80–65 Ma according to the studies of different slices [ 25 , 27 , 28 ].…”

“…A prograde HP metamorphic age of 85.8 ± 1.0 Ma was obtained at P–T conditions of 1.9–2.0 GPa and 540–550°C [ 25 ], which are converted to thermobaric ratios of 275–284°C/GPa. They correspond to low geothermal gradients of 8.3–8.5°C/km, which belong to Alpine-type eclogite facies metamorphism [ 26 ]. For a slice of the Sesia Zone exposed at the locality of Fondo, south of Quincinetto, a pressure cycling at eclogite facies was proposed with two episodes of HP metamorphism at 80–75 and 70–65 Ma, respectively, with a time lag of less than ca.…”

“…7A ), corresponding to thermobaric ratios of 158–161°C/GPa. Therefore, the target orthogneiss was subducted to mantle depths of >80 km, experiencing Alpine-type eclogite facies UHP metamorphism at low geothermal gradients of 4.7–4.8°C/km [ 26 ]. Nevertheless, the majority of UHP metamorphic rocks were retrogressed into HP ones.…”

Growth of continental crust in intra-oceanic and continental-margin arc systems: Analogs for Archean systems

“…Yet another setting for post-collisional magmatism is in metamorphic core complexes, which may form on overthickened crust, or over regions such as backarcs experiencing extension from slab rollback (Vanderhaeghe and Teyssier, 1997;Whitney et al, 2004Whitney et al, , 2013Lamont et al, 2020;Soleimani et al, 2021). In most metamorphic core complexes, the magmas form the cores of gneiss domes, typically forming migmatitic to leucogranite sheets derived from remelting of pre-accreted crust with little or no additional contributions from the mantle (Zheng and Chen, 2021;Zheng and Gao, 2021).…”

Significance of selective crystal entrainment and differential crystal‐melt separation in petrogenesis of granites from the Tongbai orogen

“…Hacker et al, 2015; Huang et al, 2013; Rudnick & Gao, 2014). It is generally accepted that partial melting of crustal rocks can effectively differentiate their protolith composition by generation, segregation and migration of felsic magmas from their sources, giving rise to granites in the shallow level and granulites in the deep level (Brown, 2013; Brown & Solari, 1998; Cipar et al, 2020; Clemens, 1990; Sawyer, 1994; Sawyer et al, 2011; Zheng & Chen, 2017, 2021). Although the geodynamic mechanism of intracrustal differentiation can be deciphered by investigating the structure and composition of granites such as autochthonous granites, granitic veins and leucosomes in well‐exposed granulite and migmatite sections (e.g.…”