A late- to post-Caledonian hydrothermal pebble breccia from the basal gneiss region of Nord-Trøndelag, Central Norway

A composite, near-vertical, hydrothermal breccia body penetrating Precambrian granitic gneiss near
Skrøyvstad, Nord-Trøndelag, consists of 1. 5-2 m wide outer zones of fractured and negligibly disrupted,
stilbite-veined country rock, and a 0.5 m-thick central zone of subrounded to rounded, abraded fragments or 'pebbles' encrusted with radiate stilbite. The two zones are representative of shatter breccia and pebble breccia, respectively. The central pebbly zone is cemented by stilbite and also by later, low-density aphanitic quartz, possibly a type of chalcedony or in part devitrified opal. The in situ shattering of the gneiss is thought to be due to hydraulic fracturing and rupture following the build-up of pressure from ascending, trapped hydrothermal solutions. The resulting failure and accompanying decompression led to propagation of the main fissure and the violent ingress of ascending hydrothermal fluids. In this highly mobile, fluidised conduit system the rock fragments were abraded and milled, and subsequently coated with zeolite nucleating from the cooling solutions. Comparable composite breccia bodies described from many parts of the American cordilleras and from SW England occur above the apical portions of hydrous granitoid plutons and some carry rich mineralisations. Although the age of the Skrøyvstad breccia is unknown, arguments are presented for a Devonian or younger emplacement. There are no upper constraints on the age and it may even be as late as Tertiary if comparisons with breccia dykes occurring in SE Greenland are taken into account. The breccia is believed to overlie a hidden granite body. lndications of the presence of such a subsurface pluton just south of Skrøyvstad are found in gravimetric data.