Calc-alkaline plutonism along the Pacific rim of southern Alaska

Abstract

Field, petrology, and age data on southern Alaska plutonic rocks now enable the delineation of eight calc-alkaline.ptutonioobelts. .-.These belts of plutons or batholithic complexes are curvilinear to linear and trend parallel or subparallel to the continental margin. The belts represent the principal loci of emplacement for plutons of specific ages and although there is spatial or temporal overlap in some cases, they are more commonly spatially and temporally distinct. Intermediate lithologies such as quartz diorite, tonalite, and granodiorite dominate in most of the belts but granodiorite and granite characterize one. The belts are of Mesozoic or Cenozoic age and plutonism began in six of them at about 195, 175, 120, 75, 60, and 40 m.y. ago; age relations in two are poorly known. Recognition of the belts is important for future studies of regional geology, tectonism, and magmatism along the Pacific rim of southern Alaska.

Geochronology of plutonic rocks and their tectonic terranes in Glacier Bay National Park and Preserve, southeast Alaska

“…As part of how the plutonic belts in Glacier Bay National Park and Preserve fit into previous, but now-discarded, regional-belt nomenclature, we note that Hudson (1979Hudson ( , 1983 and Brew and Morrell (1983) suggested that a distinct belt of Middle Jurassic plutons extended into the Glacier Bay region from the north and continued southward into the northeastern part of Chichagof Island. Hudson assigned the Jurassic rocks to his Tonsina-Chichagof belt and Brew and Morrell (1983) termed it the Muir-Chichagof Belt II.…”

“…This belt includes some plutons east of Glacier Bay that were assigned to the Chilkat-Prince of Wales province by Sonnevil (1981). All of these late Early Cretaceous rocks were assigned to the Nutzotin-Chichagof belt by Hudson (1979Hudson ( , 1983. We have assigned or reassigned all of the plutons described in this report to the belts that are described here.…”

“…G. Plafker (unpublished data, 2002) established that K-Ar ages on this family of plutons farther north are closely concordant and range from 46.8 to 51.1 Ma. This pluton is considered part of the SanakBaranof plutonic belt (Hudson, 1983), which has no other representatives in Glacier Bay National Park and Preserve.…”

“…The felsic-intermediate belt has analogs to the north in the Yakutat and Skagway quadrangles (Wilson and others, 1994). Brew and Morrell (1983) considered the rocks of the felsicintermediate belt to be part of the Sanak-Baranof belt of Hudson (1983); however, newer (and better) age controls and other data suggest that the bodies in that belt may be slightly older and are all oceanward of those of the Muir-Fairweather felsic-intermediate belt (Bradley and others, 1993).…”

Geologic studies in Alaska by the U.S. Geological Survey, 1990

“…To refine the timing of this deformation event, we have undertaken an 40 ArP 9 Ar study of rocks in southeast Alaska, based on similar ages and compositions for the whole group (Loney and others, 1967;Dodds and Campbell, 1988;Karl and others, 1988). Hudson (1979) named the belt the Tonsina-Chichagof belt; the segment of the belt that intrudes the combined Wrangellia-Alexander terrane in Canada is referred to as the Saint Elias plutonic group (Dodds and Campbell, 1988). Concordant biotite and hornblende K-Ar dates from single samples range from approximately 160 to 130 Ma, with most of the hornblende K-Ar dates falling between 140 and 160 Ma.…”