Thermal modelling in shallow subduction: an application to low P/T metamorphism of the Cretaceous Shimanto Accretionary Complex, Japan

Abstract

This paper presents the results of numerical modelling to investigate the regional occurrence of prehnite‐bearing metamorphic rocks at shallow levels in subduction zones. The modelling assumes a simple geometrical configuration in which the thermal structure in a prism is controlled by boundary conditions at the top and base of the prism. It is expected that the predominant metamorphic facies in a prism will change with decreasing age of the descending slab. The results of thermal modelling show that the facies boundary between pumpellyite–actinolite and prehnite–actinolite facies (including prehnite–pumpellyite facies) overlaps with an array of P–T conditions in the prism when the age of a descending slab is younger than 10 Myr. This implies that the change of the predominant metamorphic facies from pumpellyite–actinolite to prehnite–actinolite facies will switch drastically. The critical age of the switch depends on subduction parameters. In particular, the critical age of the descending slab is <5 Myr in the case of no shear heating, with a subduction rate of v=75–200 mm y−1 and subduction angle of θ=5–15°. For shear heating (constant shear stress=30 MPa) with a subduction rate of v=75 mm y−1 and subduction angle of θ=10° the critical age is 7 Myr. To test this switching behaviour in the development of prehnite–actinolite facies in the prism, petrologic data from the Cretaceous Shimanto Accretionary Complex (CSAC) in Kyushu, Japan were compiled. The regional occurrence and mineral assemblages of prehnite‐bearing metamorphic rocks suggest that the most of CSAC was metamorphosed under prehnite–actinolite facies. This conclusion is consistent with subduction of a young, hot slab, as has been proposed based on other geological observations. This suggests that the regional extent of the prehnite–actinolite facies metamorphic rocks may be a unique evidence for the subduction of a young, hot slab.

Finding of prehnite-pumpellyite facies metabasites from the Kurosegawa belt in Yatsushiro area, Kyushu, Japan

“…Recent progress of thermal modeling has helped to interpret the thermal structures of the descending plate in various types of subduction zone (e.g., Peacock and Wang, 1999;Gerya et al, 2002;Miyazaki and Okumura, 2002). Linking these results with the metamorphic phase petrology can make it possible to predict what type of metamorphism is going on in the present day subduction system, e.g., the lawsonite -blueschist (LBS) can be formed from mid -ocean ridge basalt (MORB) at around 30 km depths in the cold subduction environment and pumpellyite -actinolite (PA), prehnite -pumpellyite (PrP) or prehnite -actinolite (PrA) facies metabasites can be formed in shallower parts than that of the LBS formation (e.g., Peacock and Wang, 1999).…”

Very low- and low-grade metamorphism of mafic volcanic rocks of the Mira terrane (Avalonia), southeastern Cape Breton Island, Nova Scotia

“…Note that the invariant CHEPPDQ assemblages also have one equilibrium in common with CHEPPAQ (equilibrium 2). Therefore, we calculated CHEPPAQ temperatures Notes: 1 RA(PP) = Prehnite-actinolite facies (Prehnite-pumpellyite subfacies), RA = Prehnite-actinolite facies, GS = Greenschist facies (from Miyazaki and Okumura (2002) for those samples above. The P-T conditions determined using CHEPPDQ should not be significantly different.…”

“…2), whereas low-grade metamorphism is broadly equivalent to the greenschist facies (e.g., . The specific position of the P-T "triple-point" between the prehnite-actinolite, pumpellyite-actinolite, and greenschist facies varies from study to study but is generally between 3 and 4 kb (300-400 MPa) and between 300 °C and 350 °C (e.g., Liou et al 1985Liou et al , 1987Frey et al 1991;Powell et al 1993;Banno 1998;Miyazaki and Okumura 2002;Day and Springer 2005).…”

“…However, high-Al mafic rocks (upper edge of grey area) at temperatures lower than reaction 2 will have the assemblage Prh-Pmp-Chl (chemographic plot b). We follow the example of Miyazaki and Okumura (2002) in denoting this relatively rare assemblage as the prehnitepumpellyite subfacies of the prehnite-actinolite facies. At higher T and P (chemographic plots d and e on Figure 3) most mafic rocks consist of the assemblage Ep-Act-Chl, the standard greenschist-facies assemblage.…”

“…(1996). 4 RA(PP) = Prehnite-Actinolite (Prehnite-Pumpellyite) subfacies; RA = Prehnite-Actinolite facies; GS = Greenschist facies; AMPH = Amphibolite facies (abbreviations from Miyazaki and Okumura (2002)). Table A1.…”