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. Migmitized domes of exactly the same character as the Finnish examples, and likewise mantled with sediments, occur in Maryland and elsewhere within the Appalachian range of North America. Very similar also are the Variscan gneiss masses in the Alps, such as the St. Gotthard, Aar, and Mont Blanc massifs. Other Alpine mantled domes have been deformed and drawn out into extensive nappes, as is illustrated by the Simplon Tunnel profile. The manteld domes apparently represent earlier granite intrusions related to a orogenic period. The plutonic mass was later eroded and levelled, and thereafter followed a period of sedimentation. During a subsequent orogenic cycle the pluton was mobilized anew and new granite magma was injected into the plutonic rock at the same time as it was deformed into gneiss, causing its migmatization and granitization, or palingenesis. It seems that a neccessary condition foth the formation of mantled domes is that the area in which they occur should have been subjected to two orogenic revolutions: in Finland the Svecofennidie and Karelidie, in eastern North America the late Pre-Cambrian (or ? Taconic)and the Appalachian, in the Alps the Variscan and the Alpidie. The repeated upheaval of such rather small plutons, a few tens of kilometres in maximum diameter, is a remarkable fact that throws light upon the nature of orogeny, as well as on the genesis of granites, and invites interesting speculations upon the original source of the granitic material.