We re-evaluate the Neoproterozoic, Pacific-type accretionary complex on Anglesey and in the Lleyn peninsula (Wales, UK), by reconstructing its ocean plate stratigraphy (OPS). Three types of distinctive OPS were successively emplaced downwards in an accretionary wedge: the oldest at the top formed when an ocean opened and closed from a ridge to a trench, the central OPS was subjected to deep subduction and exhumed as blueschists, and the youngest at the bottom is an olistostrome-type deposit that formed by secondary gravitational collapse of previously accreted material. The three types formed by successive eastward subduction of young oceanic lithosphere at the leading edge of Avalonia. The downward growth of the accretionary complex through time was almost coeval with exhumation of the blueschist unit at 550-560 Ma in the structural centre of the complex on Anglesey. From balanced sections we have reconstructed the ocean plate stratigraphy on Llanddwyn Island from which we calculate that about 8 km of lateral shortening of ocean floor took place during imbrication and accretion; this is comparable with the history of plate subduction around the Pacific Ocean. We also calculate that the age of the subducted lithosphere was very young, probably less than 10 Ma.

The tectonic evolution of the Northern Shimanto belt, central Shikoku, Japan, was examined based on petrological and geochronological studies in the Oboke area, where mafic schists of the Kawaguchi Formation contain sodic amphibole (magnesioriebeckite). The peak P-T conditions of metamorphism are estimated as 4-4.5 kbar (15-17 km depth), and 240-270°C based on available phase equilibria and sodic amphibole compositions. These metamorphic conditions are transitional between blueschist, greenschist and pumpellyite-actinolite facies. Phengite K-Ar ages of 64.8 ± 1.4 and 64.4 ± 1.4 Ma were determined for the mafic schists, and 65.0 ± 1.4, 61.4 ± 1.3 and 63.6 ± 1.4 Ma for the pelitic schists. The metamorphic temperatures in the Oboke area are below the closure temperature of the K-Ar phengite system, so the K-Ar ages date the metamorphic peak in the Northern Shimanto belt. In the broad sense of the definition of blueschist facies, the highest-grade part of the Northern Shimanto belt belongs to the blueschist facies. Our study and those of others identify the following constraints on the possible mechanism that led to the exhumation of the overlying Sanbagawa belt: (i) the Sanbagawa belt is a thin tectonic slice with a structural thickness of 3-4 km; (ii) within the belt, metamorphic conditions varied from 5 to 25 kbar, and 300 to 800°C, with the grade of metamorphism decreasing symmetrically upward and downward from a structurally intermediate position; and (iii) the Sanbagawa metamorphic rocks were exhumed from 60 km depth and emplaced onto the Northern Shimanto metamorphic rocks at 15-17 km depth and 240-270°C. Integration of these results with those of previous geological studies for the Sanbagawa belt suggests that the most probable exhumation mechanism is wedge extrusion.

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The 560-550 Ma blueschists and associated rocks in Anglesey, UK were derived from a subductionaccretion complex. The blueschist unit is divided into three mineral zones by two newly mapped metamorphic isograds; zone I sub-greenschist facies, (crossite isograd), zone II blueschist facies, (barroisite isograd), zone III epidote-amphibolite facies. The zones and isograds dip gently to the east, and decrease in metamorphic grade from the central high-pressure zone III to lower grade zones II and I to the west and east. The P-T conditions estimated from zoned amphibole indicate an anticlockwise P-T path following adjustment to a cold geotherm. This path is well preserved in the compositional zoning of Na-Ca amphibole that have a core of barroisite surrounded by a rim of crossite, although this is only locally developed. The sense of subduction was to the east and exhumation to the west, as indicated by the metamorphic isograds. The symmetrical arrangement of the metamorphic zones with the deepest high-pressure rocks in the middle suggests an isoclinal antiformal structure that formed by wedge extrusion during exhumation in the subduction zone.