TRANSFORMATION OF FELSIC CRUST DURING ULTRAHIGH-PRESSURE METAMORPHISM

It is universally accepted that continental crust can be subducted to the depths of ultrahigh-pressure (UHP) metamorphism. A robust challenge, however, exists in how felsic rocktypes from these terranes apparently lack evidence of metamorphic transformation at peak pressure, even after being subducted to ambient conditions reaching a gigapascal beyond the stability range of their mineral assemblages.

We evaluated the degree of transformation in four felsic rocktypes common to many (U)HP orogenic terranes: paragneiss, pelitic schist, granodioritic and granitic compositions. Thermodynamic modeling (Perpl_X) and LASS–ICPMS zircon geochronology of multiple samples from the Nordfjord HP-UHP province in western Norway reveal that pelitic and paragneissic rocks are the most likely to develop a phase assemblage at least approaching that implied by ambient conditions (known from nearby mafic eclogite), while granitic rocks are less so, and granodioritic compositions almost not at all. Zircon growth is limited to the prograde path, and there is little chronological sign of widespread late retrogression to yield the extant amphibolite-facies parageneses.

Transformation should be advantaged then in hydrated, sediment-dominated cover nappes that are typically subducted and exhumed early in collisional orogens. Dry igneous protoliths, such as deeper crustal levels that enter the subduction zone later, are unlikely to transform significantly, except perhaps in the case of microcontinents that are sandwiched between sedimentary nappes.