Rapid exhumation of lower crust during continent-continent collision and late extension: Evidence from ⁴⁰Ar/³⁹Ar incremental heating of hornblendes and muscovites, Caledonian orogen, western Norway

Resolving enigmatic aspects of the age relations of deep crustal rocks in the Bergen Arcs area provides key insights into the controversial tectonic evolution of the Caledonian orogen in western Norway. In the Bergen Arcs area, eclogites of the lower crustal Lindås nappe (upper plate) structurally overlie eclogite-bearing basement gneiss of the Western Gneiss region (lower plate) across the Bergen Arcs shear zone. Laser ⁴⁰Ar/³⁹Ar incremental heating data for hornblendes and muscovites on mineral separates, in tandem with detailed thermobarometry and structural observations, allow evaluation of the tectonic cooling and exhumation histories of the deep crustal rocks as well as offset and movement history across tectonic boundaries. In the Lindås nappe the eclogites formed at ≈700 °C and pressures >17 kbar. Hornblende and three separate muscovite grains from a single eclogite sample reveal well-defined ⁴⁰Ar/³⁹Ar isotope plateaus at 447.7 ± 4.2, 432.6 ± 0.5, 429.2 ± 0.9, and 429.5 ± 0.6 Ma (2σ errors given here and below), respectively, indicating relatively rapid cooling of the high-pressure rocks (10–15 °C/m.y.). Apparently older plateau ages from muscovites from other eclogites at 451.3 ± 1.1 and 462.6 ± 0.8 Ma may be related to indiscernible excess argon. High-grade amphibolites (≈690 °C, 8–12 kbar) of the Lindås nappe yield ages of 439.3 ± 3.5 and 455.4 ± 1.5 Ma. In contrast, hornblende from gneiss in the adjacent Western Gneiss region (≈670 °C, 8 kbar) to the east across the Bergen Arcs shear zone reveal plateau ages of 409.3 ± 2.7 and 394.8 ± 2.2 Ma, similar to other hornblende and muscovite ⁴⁰Ar/³⁹Ar dates in the Western Gneiss region immediately to the north. To the west of the Lindås nappe allochthonous basement gneiss of the Øygarden Gneiss complex (≈670 °C, 8 kbar) yields a plateau age of 408.1 ± 3.4 Ma.

The temperature estimates from all the samples analyzed are above the closure temperatures for hornblende and muscovite and thus indicate that the argon ages reflect cooling after peak metamorphic conditions. The argon ages for the high-pressure eclogites of the Lindås nappe imply extreme crustal thickening (>50 km) prior to ca. 450 Ma. Together the data indicate that subduction of lowermost continental crust, possibly related to continent-continent collision, occurred early (ca. 450 Ma) in the tectonic evolution of the orogen, leading to the development and exhumation of the lower crustal eclogites and high-grade amphibolites within the Lindås nappe (between 450 and 430 Ma). In contrast, the high-pressure eclogites of the Western Gneiss region, which may have formed significantly later (ca. 425 Ma), were exhumed and cooled along with the basement gneiss late in the history of the orogen (ca. 400 Ma). The 40 m.y. offset across the Bergen Arcs shear zone, which separates the Lindås nappe (upper plate) and the Western Gneiss region (lower plate), is consistent with extensional movement along the fault resulting in the rapid exhumation and cooling of the region late in the tectonic evolution of the orogen (ca. 400 Ma). Thus, the deep crustal rocks in the overlying Caledonian nappes in western Norway appear to have been cooled and exhumed rapidly related to continent-continent collision during a predominantly contractional phase early in the tectonic evolution of the orogen, whereas lower crustal rocks of the underlying basement appear to have been exhumed extremely rapidly, coincident with late orogenic to postorogenic extension. Temporally and possibly tectonically distinct mechanisms are illustrated for the exhumation of deep crust within the tectonic evolution of a continent-continent collisional orogen.