Abstract
Studies of mineral equilibria in metamorphic rocks have given valuable insights into the tectonic processes operating at convergent plate margins during an orogeny. Geodynamic models simulating orogenesis and crustal thickening have been constrained by temperature and pressure estimates inferred from the mineral assemblages of the various lithologies involved along with age constrains from increasingly precise geochronological techniques. During such studies it is assumed that the pressure experienced by a given rock is uniquely related to its depth of burial. This assumption has been challenged by recent studies of high pressure (HP) and ultrahigh pressure (UHP) rocks. Here, we describe an example of Caledonian HP metamorphism from the Bergen Arcs in western Norway, and show that the associated formation of Caledonian eclogites at the expense of Proterozoic granulites was related to local pressure perturbations rather than burial, and that the HP metamorphism resulted from fluid-induced weakening of an initially dry and highly stressed lower crust when thrust upon the hyperextended margin of the Baltic shield.
Highlights
The last two decades of research have shown that prior to orogeny the lower continental crust is dry and mechanically strong[1,2,3]
We show that rheology-induced pressure increases to eclogite facies conditions above 2 GPa are consistent with the pressure-temperature-time history experienced by this crustal volume without invoking tectonic burial beyond ca 55 km depth
Amphibolite facies metamorphism associated with late pegmatite intrusions occurred near 600 °C at 423.6 ± 1 Ma, some 5 million years after eclogite formation
Summary
The last two decades of research have shown that prior to orogeny the lower continental crust is dry and mechanically strong[1,2,3]. We present observations from the Bergen Arcs in western Norway where fluid induced retrogression of lower crustal granulites followed initial seismic faulting. Retrogression produced both eclogite facies and amphibolite facies lithologies, which became loci of shear zone development. At this initial stage of the Scandian collision, ophiolites and Laurentian island-arc complexes were emplaced onto the Jotun-Lindås microcontinent and its fossiliferous Middle Silurian cover[13] These events caused fluid-induced metamorphism transforming the 930 Ma old anhydrous granulite facies mineralogy[14] to eclogites and amphibolites in shear zones, breccias and along fractures located near the leading edge of this microcontinent. The regional (U)HP metamorphism and metamorphic zonation in the Western Gneiss Complex at 410 ± 10 Ma (see inset map in Fig. 1) formed during the terminal stages of the collision[15,16]
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