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Title: Metamorphic facies map of Alaska

Abstract

A metamorphic-facies of Alaska has been compiled, following the facies-determination scheme of the Working Group for the Cartography of the Metamorphic Belts of the World. Regionally metamorphosed rocks are divided into facies series where P/T gradients are known and into facies groups where only T is known. Metamorphic rock units also are defined by known or bracketed age(s) of metamorphism. Five regional maps have been prepared at a scale of 1:1,000,000; these maps will provide the basis for a final colored version of the map at a scale of 1:2,500,000. The maps are being prepared by the US Geological Survey in cooperation with the Alaska Division of Geological and Geophysical Surveys. Precambrian metamorphism has been documented on the Seward Peninsula, in the Baird Mountains and the northeastern Kuskokwim Mountains, and in southwestern Alaska. Pre-Ordovician metamorphism affected the rocks in central Alaska and on southern Prince of Wales Island. Mid-Paleozoic metamorphism probably affected the rocks in east-central Alaska. Most of the metamorphic belts in Alaska developed during Mesozoic or early Tertiary time in conjuction with accretion of many terranes. Examples are Jurassic metamorphism in east-central Alaska, Early Cretaceous metamorphism in the southern Brooks Range and along the rim of the Yukon-Kovyukukmore » basin, and late Cretaceous to early Tertiary metamorphism in the central Alaska Range. Regional thermal metamorphism was associated with multiple episodes of Cretaceous plutonism in southeastern Alaska and with early Tertiary plutonism in the Chugach Mountains. Where possible, metamorphism is related to tectonism. Meeting participants are encouraged to comment on the present version of the metamorphic facies map.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Geological Survey, Menlo Park, CA
OSTI Identifier:
6164210
Report Number(s):
CONF-8505215-
Journal ID: CODEN: AAPGB
Resource Type:
Conference
Resource Relation:
Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States); Journal Volume: 69:4; Conference: AAPG-SEPM-SEG Pacific section meeting, Anchorage, AK, USA, 22 May 1985
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; ALASKA; METAMORPHIC ROCKS; CRETACEOUS PERIOD; GEOLOGIC HISTORY; JURASSIC PERIOD; MAPS; METAMORPHISM; TECTONICS; TERTIARY PERIOD; CENOZOIC ERA; FEDERAL REGION X; GEOLOGIC AGES; MESOZOIC ERA; NORTH AMERICA; ROCKS; USA 580100* -- Geology & Hydrology-- (-1989)

Citation Formats

Dusel-Bacon, C., O-Rourke, E.F., Reading, K.E., Fitch, M.R., and Klute, M.A.. Metamorphic facies map of Alaska. United States: N. p., 1985. Web.
Dusel-Bacon, C., O-Rourke, E.F., Reading, K.E., Fitch, M.R., & Klute, M.A.. Metamorphic facies map of Alaska. United States.
Dusel-Bacon, C., O-Rourke, E.F., Reading, K.E., Fitch, M.R., and Klute, M.A.. 1985. "Metamorphic facies map of Alaska". United States. doi:.
@article{osti_6164210,
title = {Metamorphic facies map of Alaska},
author = {Dusel-Bacon, C. and O-Rourke, E.F. and Reading, K.E. and Fitch, M.R. and Klute, M.A.},
abstractNote = {A metamorphic-facies of Alaska has been compiled, following the facies-determination scheme of the Working Group for the Cartography of the Metamorphic Belts of the World. Regionally metamorphosed rocks are divided into facies series where P/T gradients are known and into facies groups where only T is known. Metamorphic rock units also are defined by known or bracketed age(s) of metamorphism. Five regional maps have been prepared at a scale of 1:1,000,000; these maps will provide the basis for a final colored version of the map at a scale of 1:2,500,000. The maps are being prepared by the US Geological Survey in cooperation with the Alaska Division of Geological and Geophysical Surveys. Precambrian metamorphism has been documented on the Seward Peninsula, in the Baird Mountains and the northeastern Kuskokwim Mountains, and in southwestern Alaska. Pre-Ordovician metamorphism affected the rocks in central Alaska and on southern Prince of Wales Island. Mid-Paleozoic metamorphism probably affected the rocks in east-central Alaska. Most of the metamorphic belts in Alaska developed during Mesozoic or early Tertiary time in conjuction with accretion of many terranes. Examples are Jurassic metamorphism in east-central Alaska, Early Cretaceous metamorphism in the southern Brooks Range and along the rim of the Yukon-Kovyukuk basin, and late Cretaceous to early Tertiary metamorphism in the central Alaska Range. Regional thermal metamorphism was associated with multiple episodes of Cretaceous plutonism in southeastern Alaska and with early Tertiary plutonism in the Chugach Mountains. Where possible, metamorphism is related to tectonism. Meeting participants are encouraged to comment on the present version of the metamorphic facies map.},
doi = {},
journal = {Am. Assoc. Pet. Geol., Bull.; (United States)},
number = ,
volume = 69:4,
place = {United States},
year = 1985,
month = 4
}

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  • The Cordilleran orogen in southeastern Alaska includes 24 distinct magmatic belts, ranging in age from Cambrian to Holocene, that are defined by map relations, lithology, age, and chemical composition. The youngest magmatic features are Quaternary-age pre- and post-glacial volcanic rocks that occur in three major fields in the region, as well as in isolated locations. Cenozoic magmatic features consist of four major and three minor belts. The major Tkope-Portland Peninsula belt of Oligocene age includes both volcanic and plutonic rocks. The major calcalkalic Coast Mountains belt of early and middle Eocene age is the single largest magmatic feature of themore » region. Early Tertiary and latest Cretaceous magmatism is represented by the major calcalkalic great tonalite sill belt, a remarkable long and narrow feature along the west side of the Coast Mountains. Cretaceous and Jurassic intrusive rocks occur in five major belts and two minor belts in the region and Paleozoic intrusive rocks occur in four major and two minor belts. The three major plutonic-metamorphic complexes (PMC), from east to west, are: the Coast PMC in the Coast Mountains; the Glacier Bay-Chichag of plutonic complex (Chugach MC) in the northern outer islands. The Coast PMC records dynamothermal and regional contact metamorphic events related to regional plutonism within several juxtaposed terranes; its lengthy and complicated history is related to the Late Cretaceous collision of the Alexander and Wrangellia terranes and the Gravina overlap assemblage to the west against the Yukon prong and Stikine terrane to the east. The relatively simple Glacier Bay PC history is recorded as the roots of a Late Jurassic through late Early Cretaceous island arc that probably developed during the early stages of the above tectonic event. The complicated Chugach MC history developed during and after the Late Cretaceous collision of the Chugach terrane with the Wrangellia and Alexander terranes.« less
  • Upholding the Central Range of Taiwan is the Cenozoic Slate Belt which originated during the orogenic collision of the Taiwan Tertiary argillaceous geosyncline by the Luzon Arc on the east. Previous K/Ar studies of slate and phyllite suggest their metamorphism peaked 6-3 m.y. ago from concordant size fraction measurements. The far eastern foothills contain large bodies of higher rank gneisses, schists, and marbles of definite pre-Cenozoic metamorphic origin. In between these and the Slate Belt is the Schist Belt. Structural evidence for multiple metamorphism, and the occurrence of a discontinuous conglomerate near the schist/slate contact, has suggested the Schist Beltmore » is part of the pre-Cenozoic basement further east. To the contrary K/Ar studies indicate the Schist Belt paralleling the Slate Belt is a higher facies equivalent of the Slate Belt because of an identical pattern of concordant 6-3 m.y. ages. In contrast are discordant older apparent ages from the higher rank schist and gneiss bodies further east closer to the actual collision. Biotite typically gives half the measured age of muscovite. The contrasting pattern is due to variable partial resetting of the older basement metamorphic rocks during the Luzon collision which created the Slate and Schist Belts.« less
  • The western metamorphic belt is part of the Coast plutonic-metamorphic complex of western Canada and southeastern Alaska that developed as a result of tectonic overlap and/or compressional thickening of crustal rocks during collision of the Alexander terrane and Gravina assemblage on the west against the Yukon Prong and Stikine terranes to the east. Sub-greenschist to lower greenschist facies metabasalts exposed along the west end of the western metamorphic belt near Juneau, Alaska record the earliest metamorphic event (M1). These low-grade rocks are gradational with younger, higher-grade assemblages that define an inverted metamorphic gradient (metamorphic event M5). The most common metamorphicmore » mineral assemblages are chlorite-epidote-actinolite with or without pumpellyite and stilpnomelane. There is no systematic distribution of metamorphic mineral assemblages in the study area, and all assemblages are in the pumpellyite-actinolite facies near the transition to the lower greenschist facies. Different low-variance assemblages can be attributed to minor differences in pressure, temperature, or X[sub CO[sub 2]]. Mineral chemistry and phase equilibria suggest that thermal peak metamorphism of pumpellyite-bearing assemblages occurred at about 325 C and 2 to 4.5 kbar. The geologic setting, the pumpellyite-actinolite to lower greenschist facies mineral assemblages, and the deduced P and T of peak metamorphism are all compatible with burial metamorphism of the Douglas Island Volcanics at a depth of 15 to 20 km. Preservation of the low-grade metamorphic mineral assemblages during collisional crustal thickening and during establishment of the inverted metamorphic gradient is attributed to instantaneous thickening displacing the pre-thickening isotherms to substantially greater depths and rapid uplift after establishment of the inverted gradient.« less
  • Magnetic susceptibility (MS) measurements on 194 granitic rock samples from a transect across the Coast plutonic-metamorphic complex near Juneau help define and characterize individual plutons and plutonic units that form 3 major plutonic belts and 6 major subbelts of the region. Plutons along the western margin (Admiralty-Revillagigedo belt) form two subbelts with distinctive MS. A group of small 95-Ma tonalitic plutons have very low MS (< 300 [times] 10[sup [minus]5] Slu) and contain virtually no magnetite, but typically contain sulfides, ilmenite and primary epidote. A second group consists of 2 quartz monzodioritic plutons of moderate MS (1,000--3,000). Adjacent to themore » northeast are large segments of the 55--65 Ma great tonalite sill belt with 2 subbelts; a western subbelt of foliated tonalitic bodies with high MS (> 3,000) and magnetite content, and an eastern subbelt of foliated granodioritic plutons of variable but generally high MS. Three small deformed tonalitic sills located east of Juneau are spatially part of the great tonalite sill belt but exhibit very low MS similar to those tonalites of the Admiralty-Revillagigedo belt. On a regional scale the MS shows no correlation with total iron (FeO) except within suites or subbelts, but it does show a positive correlation with the oxidation state (molecular ratio Fe[sup 3])/(Fe[sup 2]+Fe[sup 3]). The differences in MS between the major plutonic belts appear to be related to differences in the ages of the belts and the host terranes in to which they were emplaced. The differences between subbelts and within subbelts is less clear, but is probably due to variable oxidation states and FeO content caused by differentiation trends and contamination, and possibly related to size of plutons, and the amount of tectonic stress and deformation, and grade of metamorphism during synkinematic emplacement.« less
  • The mid to late mesozoic Brookian orogeny involved southward subduction underneath an island arc and subsequent burial of a passive continental margin. Field mapping, petrography, and geochronology of blueschist-bearing metasedimentary rocks in the Walker Lake region of the southern Brooks Range enables correlation of fabrics with crustal movements that are responsible for their burial and exhumation. Observed early ductile fabrics in the schist belt are consistent with north-vergent transport. These fabrics include (1) S[sub D] (dominant foliation), a south-dipping transposed crenulation cleavage which formed under blueschist facies conditions; (2) outcrop-scale north-vergent folds and an associated mesoscopic south-dipping crenulation cleavage, S[submore » D+1]; (3) kilometer-scale gentle warps and north-vergent, open monoclinal folds. Small north-vergent folds (fabric 2) fold an earlier mineral stretching lineation associated with S[sub D]. The similarity of orientation of the three fabrics and their evolution in time from small and isoclinal to large and open suggests that they are the product of progressive deformation. S[sub D] is likely a burial fabric; later fabrics may be related to uplift along a regionally penetrative system of ductile, north-vergent south-dipping thrusts. A metamorphic overprint, dated at 105--110 Ma, increases to amphibolite facies toward the north and outlasts north-vergent fabrics. Retrograde helicitic albite is commonly rotated by later minor extensional crenulation cleavages. Discrete, narrow zones of ductile deformation in the southernmost schist belt, parallel to S[sub D] but postdating blueschist facies conditions, locally exhibit top-to-the-south sense of shear. This evidence suggest that north-vergent contraction continued during exhumation of the schist belt from deep to mid-crustal levels and that extension may have been responsible for exhumation from middle to shallow crustal levels.« less