The Evje-Iveland pegmatite-hosted but meteoric fluid-generated megabreccias: Markers of Permian-Triassic rifting in South Norway

In the Proterozoic Evje-Iveland pegmatite field of southern Norway, five pegmatites have been found to exhibit features of large-scale hydraulic brecciation that have been previously overlooked or misinterpreted. We provide the first detailed description and characterization of these breccias from a study of the Landsverk 1 pegmatite, with additional insights from Gudmundsgruve, Haugen 1 and Haugen 2 and Storsynken. This is based on studies of the breccia cement, specifically oxygen isotope, trace element and cathodoluminescence data for quartz, 40 Ar/ 39 Ar dating of K-feldspar and trace elements in fluorite. The clast-supported breccias in the Landsverk 1 pegmatite consist of millimetre- to meter-sized fragments of pegmatite (~95 vol%) and wall rocks (~5 vol%) incorporated due to sidewall collapse. Fluid flow through the breccias led to infilling and replacement mostly by milky white quartz and epidote, but also left a high percentage of open space, including cavities up to 1.5 m in size. The process responsible for breccia formation was fluid-assisted hydraulic fracturing caused by a single pulse of stress at pressures between 50 and 250 MPa. 40 Ar/ 39 Ar dating of K-feldspar crystals from cavities in the cement yielded an early Triassic age of 246.1 ± 0.9 Ma, which is approximately 660 Ma younger than the emplacement of the pegmatite-forming melts. This indicates that, contrary to previous interpretations, brecciation was not part of the magmatic evolution of the pegmatite. From the δ 18 O values of breccia cement quartz (−8.38 ± 0.28‰), fluids involved in the initial stages of cementation had a temperature of 549 ± 23 °C and were meteoric in origin. So far, five megabreccias have been identified within a 12 km long, NNW-SSE-striking corridor in the Evje-Iveland area, indicating that the brecciation event is a regional phenomenon. The orientation and timing indicate formation under E–W to ENE–WSW extensional stress, with dolerite dykes emplaced along associated faults during the South-Scandinavian Permian–Triassic rift (phase 1). • Pegmatite-hosted but meteoric-fluid-generated mega-breccias occur in southern Norway. • Breccia-forming fluids had a temperature of 549 ± 23 °C and were meteoric in origin. • Age dating of breccia cement yields ~246 Ma, ~660 Myr younger than the host pegmatite. • These breccias record Permian–Triassic rifting related to Atlantic opening.