High-pressure metamorphism and deep-crustal seismicity: evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas

Abstract

The metamorphic complex of the Western Gneiss Region (WGR), Norway, constitutes the root of the Caledonian mountain belt and experienced temperatures of 700–800 °C and pressures in excess of 20 kbar during peak metamorphism. Mafic bodies surrounded by strongly banded felsic gneisses commonly exhibit variable reequilibration to granulite and eclogite facies conditions and locally preserve igneous minerals and textures. The Kråkeneset gabbro, located on the island of Vågsøy in the mixed HP/UHP zone of the western WGR, display evidence for extensive metastability through the entire prograde and retrograde P, T histories. Eclogite constitutes less than a few percent of the total volume of the body and high-pressure assemblages typically form thin coronas around magmatic phases or occur along localized zones of brittle deformation and fluid infiltration. The gabbro displays pseudotachylyte vein networks that define subparallel brittle fault zones, <50 cm wide, transecting the gabbro body. The pseudotachylytes contain μm- to mm-scale amoeboid and dendrite-like textures of garnet and plagioclase with inclusions of the eclogite facies minerals orthopyroxene, omphacite, amphibole, and dolomite, suggesting rapid disequilibrium growth of minerals during high-pressure conditions. Textural and petrological evidence from pseudotachylytes and corona structures show that the growth of these unusual textures occurred shortly after pseudotachylyte crystallization by a process of rapid solid-state alteration of a microcrystalline pseudotachylyte matrix. The pseudotachylyte-lined fault zones are in close spatial association with numerous amphibole±carbonate-filled hydrofractures with conspicuous fracture-parallel alteration zones defined by hydrous eclogite facies assemblages. These eclogite facies hydrofractures testify to the existence of high fluid pressures and to fluid infiltration following brittle failure during high-grade metamorphic conditions. Geothermobarometric estimates (ca. T=650–700 °C, P=∼20 kbar) and petrological data imply that hydrofracturing, pseudotachylyte crystallization, and the subsequent pseudotachylyte alteration process must have occurred during high-pressure metamorphism. Our observations are suggestive of a deep-crustal earthquake scenario where a high-pressurized fluid phase plays a double role by causing both seismic failure through the embrittlement effect and facilitating eclogitization of the metastable anhydrous gabbro. Metamorphic reaction along hydrofractures and fault planes led to the development of eclogite facies foliation fabrics and illustrate the rheological change from brittle to plastic behavior associated with the gabbro to eclogite transition. The formation of weak deep-crustal shear zones following brittle failure represents an arrested initiation of the physical breakup and metamorphic reequilibration of the Kråkeneset gabbro during its residence deep in the former Caledonian collision zone.