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

Abstract

Most of the Phanerozoic orogenic belts exhibit HP to UHP metamorphism. First discovered in the Alps and the Norwegian Caledonides, ultra-high pressure rocks are now described in various Paleozoic belts including the Kazakhstan Caledonides, the Uralides, and the Variscides. In these orogenic belts both oceanic and continental, crustal and mantle rocks underwent HP to UHP metamorphism in a variable range of temperatures. The PT conditions require subduction of these rocks to depths sometimes, in the case of continental crust, exceeding 100 km. In the Uralides as in the Variscides, the HP metamorphism occurred very early, 100 to 110 m.y. before the end of the orogeny. In the Uralides, there is a linear HP belt west of the main ophiolitic suture. The HP metamorphism (15-18 kbar/500-700°C) developed in supracrustal 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 arc. The island arc is well preserved and the volcanites underwent only low-grade metamorphism. The Variscides exhibit two kinds of HP rocks: (1) the lower allochthonous units, beneath ophiolitic nappes, are characterized by HP to UHP/LT metamorphism (12-25 kbar/400-700°C), developed in the supracrustal rocks of a thinned continental margin, (2) the upper unit includes oceanic rocks and arc crust and mantle with a HP/MT granulitic metamorphism (10-15 kbar/700-900°C). For the northern Iberian and Massif Central sections, this duality may be explained by the following model: HP/UHP rocks of the lower allochthonous units may be interpreted as parts of a thinned passive margin of the Gondwana supercontinent, deeply subducted (between 400 and 380 Ma) beneath an oceanic and island-arc lithosphere, part of a Galicia-Massif Central ocean. The plate tectonic setting is similar to that of the Urals and Oman. Granulitic rocks of the upper unit may represent the deep crust and mantle of an island arc, itself subducted at about the same time beneath the conjugate thinned passive margin (Avalon) of the Galicia-Massif Central ocean.