The problem of mantled gneiss domes

Abstract

In many orogenic zones there occur domes having a superincumbent mantle of sedimentary strata layered parallel to the dome contacts and the foliation of the gneiss. In the Karelidic zone of East Finland are numerous domes showing somewhat different development. In some, the lowest horizon of the mantle consists of basal conglomerate with boulders of the same gneiss that forms the dome; in others, the basement stratum is a layer of quartzite, above which follow dolomite and micaschist; and in still others, dolomite forms the basement. Ivariably the domes are more or less inclined according to the general tilt of the orogenic zone. In some domes the gneiss, or rather granite, has apparently been preserved as it was when the sediments were deposited upon the croded surface of the plutonic mass. In most cases, however, it has become migmatized and granitized during the doming, and now shows a veined structure and has a potash-rich ideal-granitic composition, although its original composition may have been granodioritic or quartz-dioritic. In some cases massive granites break through the domes, and at the contacts the palingenic gneissose granite may display an intrusive relation to the mantle rocks. On this account certain domes were formerly thought to be younger intrusions.