Continental subduction and exhumation of HP rocks in Paleozoic orogenic belts: Uralides and Variscides

Most of the Phanerozoic orogenic belts exhibit HP to UHP metamorphism. Firs t discovered in the Alps and the Norwegian Caledonides, ultra-high pressure rocks are now described in various Paleozoic belts including the Kazakhsta n Caledonides, the Uralides, and the Variscides. In these orogenic belts bo th oceanic and continental, crustal and mantle rocks underwent HP to UHP me tamorphism in a variable range of temperatures. The PT conditions require s ubduction of these rocks to depths sometimes, in the case of continental cr ust, exceeding 100 km. In the Uralides as in the Variscides, the HP metamor phism occurred very early, 100 to 110 m.y. before the end of the orogeny. I n the Uralides, there is a linear HP belt west of the main ophiolitic sutur e. The HP metamorphism (15-18 kbar/500-700 degrees C) developed in supracru stal continental rocks outcropping beneath an ophiolitic nappe. This is the result of the eastward subduction of the East European continental margin beneath a Uralian oceanic lithosphere and an island are. The island are is well preserved and the volcanites underwent only low-grade metamorphism. Th e Variscides exhibit two kinds of HP rocks: (1) the lower allochthonous uni ts, beneath ophiolitic nappes, are characterized by HP to UHP/LT metamorphi sm (12-25 kbar/400-700 degrees C), developed in the supracrustal rocks of a thinned continental margin, (2) the upper unit includes oceanic rocks and are crust and mantle with a HP/MT granulitic metamorphism (10-15 kbar/700-9 00 degrees C). For the northern Iberian and Massif Central sections, this d uality may be explained by the following model: HP/UHP rocks of the lower a llochthonous units may be interpreted as parts of a thinned passive margin of the Gondwana supercontinent, deeply subducted (between 400 and 380 Ma) b eneath an oceanic and island-are lithosphere, part of a Galicia-Massif Cent ral ocean. The plate tectonic setting is similar to that of the Urals and O man. Granulitic rocks of the upper unit may represent the deep crust and ma ntle of an island are, itself subducted at about the same time beneath the conjugate thinned passive margin (Avalon) of the Galicia-Massif Central oce an.