Metamorphic, geochronologic and tectonic evolution of deep crustal rocks within the Caledonian Orogen, western Norway.

Abstract

The temporal and tectonic evolution of the deepest portions of orogens has been the focus of considerable interest and debate. Regional exposures of granulite to eclogite transitional rocks in the Bergen Arcs of western Norway provide a unique opportunity to study the processes affecting lowermost continental crustal rocks in the roots of a continent-continent collisional orogen. Pseudotachylyte veins within the partially eclogitized granulites are interpreted to represent the products of fracturing, faulting and seismicity at depths of $>$60 km in the thickened continental crust. The rapid relaxation of stresses caused by the eclogitization process may have resulted in the formation of the pseudotachylytes and deep crustal earthquakes. Incremental heating and laser ablation $\sp{40}$Ar-$\sp{39}$Ar degassing experiments on muscovite from the eclogites place constraints on the behavior of argon in deep crustal rocks. The combined data yield compelling evidence for local, heterogeneous incorporation of excess argon during high pressure metamorphism ($>$15 kbar), possibly related to the introduction of infiltrating fluids. The U-Pb, Sm-Nd and $\sp{40}$Ar-$\sp{39}$Ar isotopic data from eclogite minerals document a short duration of the eclogite forming event in the Bergen Arcs immediately followed by rapid cooling and exhumation. This suggests that in addition to the limited interaction with fluids, the duration of the eclogite event was important in the preservation of the arrested stages of eclogite development within the granulites. Hornblende and muscovite $\sp{40}$Ar-$\sp{39}$Ar data from regionally distributed eclogite and amphibolite samples document an abrupt displacement of about 40 Myr across the Bergen Arcs shear zone, which separates the overlying Caledonian Nappes (upper plate) from the underlying (para)autochthonous basement gneisses of the Western Gneiss Region (lower plate). Moreover, the argon isotope data indicate that eclogites within upper plate were exhumed and cooled between about 450 and 430 Ma coincident with the main phase of continent-continent collision. In contrast, eclogites within the basement gneisses of the lower plate were exhumed between about 410 and 390 Ma related to late syn- to post-orogenic extension. Thus, temporally and possibly tectonically distinct mechanisms are indicated for cycling of continental crust through the deepest portions of the Caledonian Orogen of western Norway.