Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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214 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 Wrangellia environments, as encompassed in the age range of the Gastineau Volcanics. Moreover, their inclusion of samples with a 420-90 Ma age range in their "Wrangellia," and with lithologies of mudstone to grano- diorite (Samson and others, 1990, p. 75 1, 755), makes in- terpretation of their data in terms of tectonic setting for the Juneau-area rocks uncertain. The magmatic environment that they inferred, however, is one that seems to be sup- ported by much of our data. In particular, the lack of a subduction-zone component for all units of this study is indicated by Ta abundance in the Hf-Th-Ta diagram of fig- ure 22. The typical depletion of the high-field-strength elements Ta and Nb relative to Th of subduction-related volcanic rocks (Pearce, 1983) is similarly not evident in MORB-normalized element variations (fig. 21). Even tholeiites of arcs of a primitive stage of development show strongly negative Nb anomalies (Arculus, 1987). The tectonic settings suggested by different chemical signatures of the different metabasalt units of this part of Wrangellia are conflicting, as seen by comparing many of the diagrams of this report. However, we follow Wood (1980), Pearce (1983), Arculus (1987), and others in con- sidering that the relatively immobile HFSE such as Th, Hf, Nb, and Ta afford the best available indicators of tectonic setting. Thus, an arc-derived origin as reported for upper Paleozoic volcanic rocks of Wrangellia elsewhere, such as by Jones and others (1986, 1987) and Monger and Berg (19871, is not indicated for the older part of the Gastineau Volcanics of the terrane near Juneau, nor for the Triassic basalts of the terrane as implied by interpretations of a back- or fore-arc setting of that volcanism in other areas (Davis and Plafker, 1985; Vallier, 1986; Barker and others, 1989). The basaltic volcanism of Wrangellia near Juneau, lacking evidence of a subduction component (fig. 22), is inferred to be instead largely of oceanic within-plate (OIB) type and possibly related to oceanic mantle-plume processes. Such activity might account for the immense scale of Wrangellian volcanism (Richards and other, 1991). Wrangellia was amalgamated with the Alexander terrane by Carboniferous time (Gardner and others, 1988), if not having originated within that terrane and always a part of it. However combined, the composite Alexander plus Wrangellia terrane (Brew and Ford, 1993) subsequently accreted to the western edge of North America by the time of the Cretaceous to early Tertiary plutonism in the Coast Mountains (Debiche and others, 1987), in an event that was possibly associated with the regional Barrovian metamorphism described by Himmelberg and others (1991) or with volcanism of the Douglas Island Volcanics. In summary, geochemical data of our study show that basaltic rocks of the Wrangellia terrane are much more widespread in northern southeastern Alaska than previ- ously recognized, and are represented in the Juneau area by the Gastineau Volcanics and metabasalts of Lions Head Mountain, rather than the Taku terrane as interpreted by others. The rocks show little geochemical influence of subduction processes. Distinct chemical differences between the Permian(?) and Triassic(?) lower and the Upper Trias- sic upper parts of the Gastineau Volcanics reflect differences in tectonic settings of the Juneau-area Wrangellia volcanism. Rocks of the lower part of the formation show LILE-enriched abundances and other trace-element charac- teristics of ocean-island basalt, whereas Upper Triassic rocks of the unit and correlative metabasalts show enriched MORB and other features that may be related to an exten- sional-rift setting, as suggested for correlative Triassic units of other Wrangellia areas, or to oceanic mantle- plume activity. The metabasalts of the Juneau area record volcanism of diverse tectonic settings and stages of development within a long-lived ocean-island volcanic environment, with earlier activity related to ocean-island volcanism and later events to rifting activity possibly associated with ancestral events preceding amalgamation with the Alexander terrane. Accretion of the combined Alexander plus Wrangellia terrane to North America may have been associated with major Barrovian regional metamorphism near the end of Cretaceous time or with Cretaceous subduction-related volcanism of the Douglas Island Volcanics. REFERENCES CITED Arculus, R.J., 1987, The significance of source versus process in the tectonic controls of magma genesis: Journal of Volca- nology and Geothermal Research, v. 32, p 1-12. Badger, R.L., 1993, Fluid interaction and geochemical mobility in metabasalts: An example from the central Appalachians: Journal of Geology, v. 101, p. 85-95. Baedecker, P.A., ed., 1987, Methods for geochemical analysis: U.S. Geological Survey Bulletin 1770, p. A1-K5. Barker, Fred, 1957, Geology of the Juneau (B-3) quadrangle, Alaska: U.S. Geological Survey Geologic Quadrangle Map GQ-100, scale 1:63,360. Barker, Fred, Sutherland Brown, A., Budahn, J.R., and Plafker, George, 1989, Back-arc with frontal-arc component origin of Triassic Karmutsen basalt, British Columbia, Canada: Chemical Geology, v. 75, p. 81-102. Berg, H.C., 1981, Upper Triassic volcanogenic massive-sulfide metallogenic province identified in southeastern Alaska, in Albert, N.R.D., and Hudson, Travis, eds., The United States Geological Survey in Alaska. Accomplishments during 1979: U.S. Geological Survey Circular 823-B, p. B104-B108. Brew, D.A., Drew, L.J., Schmidt, J.M., Root, D.H., and Huber, D.F., 1991, Undiscovered locatable mineral resources of the Tongass National Forest and adjacent lands, southeastern Alaska: U.S. Geological Survey Open-File Report 91-10.370 p. Brew, D.A., and Ford, A.B., 1977, Preliminary geologic and metamorphic isograd map, Juneau B-1 quadrangle, Alaska: Geological Survey Miscellaneous Field Studies Map MF- 846, scale 1:31,680. 1984, Tectonostratigraphic terranes in the Coast plutonic- metamorphic complex, southeastern Alaska, in Bartsch- Winkler, Susan, and Reed, K.M., eds., The U.S. Geological
GEOCHEMICAL CHARACTER OF UPPER PALEOZOIC AND TRIASSIC ROCKS, WRANGELLIA TERRANE Survey in Alaska: Accomplishments during 1982: U.S. Geological Survey Circular 939, p. 90-93. -1985, Preliminary reconnaissance geologic map of the Juneau, Taku River, Atlin, and part of the Skagway 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 85-395.23 p. 1993, Regional distribution and geologic context of volcanic-rock-hosted massive-sulfide deposits in southeastern Alaska and adjacent parts of British Columbia [abs.]: Abstracts of Professional Papers, Alaska Miner's Association Conference Juneau 1993, Juneau, Alaska, April 21-23, 1993, p. 33-34. Brew, D.A., Ford, A.B., and Himmelberg, G.R., 1989, Evolution of the western part of the Coast plutonic-metamorphic complex, south-eastern Alaska, USA, in Daly, J.S., Cliff, R.A., and Yardley, B.W.D., eds., Evolution of metamorphic belts: Geological Society Special Publication 43, p. 447-452. Brew, D.A., and Grybeck, Donald, 1984, Geology of the Tracy Arm-Fords Terror Wilderness study area and vicinity, Alaska: U.S. Geological Survey Bulletin 1525, p. 23-52. Brew, D.A., Karl, S.M., Barnes, D.F., Jachens, R.C., Ford, A.B., and Homer, Robert, 1991, A northern Cordilleran ocean-continent transect: Sitka Sound, Alaska, to Atlin Lake, British Columbia: Canadian Journal of Earth Sciences, v. 28, p. 840-853. Brew, D.A., Ovenshine, A.T., Karl, S.M., and Hunt, S.J., 1984, Preliminary reconnaissance geologic map of the Petersburg and parts of the Port Alexander and Sumdum 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 84-405, 2 sheets, with 43 p. pamphlet. Brouxel, Marc, and Lapierre, He~ette, 1988, Geochemical study of an early Paleozoic island-arc-back-arc basin system. Part 1: The Trinity Ophiolite (northern California): Geological Society of America Bulletin, v. 100, p. 11 11-1 119. Buddington, A.F., and Chapin, Theodore, 1929, Geology and mineral deposits of southeastern Alaska: U.S. Geological Survey Bulletin 800, 398 p. Condie, K.C., 1989, Plate tectonics and crustal evolution: New York, Pergamon, 476 p. Davis, Alick, and Plafker, George, 1985, Comparative geochemistry and petrology of Triassic basaltic rocks from the Taku terrane on the Chilkat Peninsula and Wrangellia: Canadian Journal of Earth Sciences, v. 22, p. 183-194. Debiche, M.G., Cox, Allan, and Engebretson, D.C., 1987, The motion of allochthonous terranes across the north Pacific basin: Geological Society of America, Special Paper 207.49 p. Desmons, J., Delaloye, M., Desmet, A., Gagny, C.I., Rocci, G., and VoIdet, P., 1980, Trace and rare earth element abundances in Troodos lavas and sheeted dikes, Cyprus: Ofioliti, v. 5, p. 35-56. Douglass, S.L., Webster, J.H., Burrell, P.D., Lanphere, M.L., and Brew, D.A., 1989, Major element chemistry, radiometric values, and locations of samples from the Petersburg and parts of the Port Alexander and Sumdum quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 89-527, 66 p., map scale 1:250,000. Ewart, A., 1982, The mineralogy and petrology of Tertiary-Recent orogenic volcanic rocks: with special reference to the andesite-basaltic compositional range, in Thorpe, R.S., ed., Andesites: New York, John Wiley & Sons. p. 25-95. Flower, Martin, 1991, Magmatic processes in oceanic ridge and intraplate settings, in Floyd, P.A., ed., Oceanic basalts: New York, Van Nostrand Reinhold, p. 116-147. 215 Floyd, P.A., 1991, Oceanic islands and seamounts, in Floyd, P.A., ed., Oceanic basalts: New York, Van Nostrand Reinhold, p. 174-2 18. Floyd, P.A., and Winchester, J.A., 1975, Magma type and tectonic setting discrimination using immobile elements: Earth and Planetary Science Letters, v. 27, p. 21 1-218. Fodor, R.V., and Vetter, S.K., 1984, Rift-zone magmatism: petrology of basaltic rocks transitional from CFB to MORB, southeastern Brazil margin: Contributions to Mineralogy and Petrology, v. 88, p. 307-321. Forbes, R.B., Gilbert, Wyatt, and Redman, Earl, 1989, Geologic setting and petrology of the metavolcanic rocks in the northwestern part of the Skagway B-4 quadrangle, southeastern Alaska: Alaska Division of Geological and Geo- physical Surveys, Public-data File 89-14, 43 p. Ford, A.B., and Brew, D.A., 1973, Preliminary geologic and metamorphic-isograd map of the Juneau B-2 quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Stud- ies Map MF-527, scale 1 :3 1,680. 1977, Preliminary geologic and metamorphic-isograd map of parts of the Juneau A-l and A-2 quadrangles, Alaska: U.S. Geological Survey Miscellaneous Field Stud- ies Map MF-847, scale 1:3 1,680. 1988, Major-element geochemistry of metabasalts of the Juneau-Haines region, southeastern Alaska, in Galloway, J.P., and Hamilton, T.D., eds., Geologic studies in Alaska by the U.S. Geological Survey during 1987: U.S. Geologi- cal Survey Circular 1016, p. 150-155. Gardner, M.C., Bergman, S.C., Cushing, G.W., MacKevett, E.M., Jr., Plafker, George, Campbell, R.B., Dodds, C.J., McClelland, W.C., and Mueller, P.A., 1988, Pennsylvanian pluton stitching of Wrangellia and the Alexander terrane, Wrangell Mountains, Alaska: Geology, v. 16, p. 967-97 1. Gehrels, G.E., and Berg, H.C., 1992, Geologic map of southeast- em Alaska: U.S. Geological Survey Miscellaneous Investi- gations Series, Map 1-1867, scale: 1:600,000. Gehrels, G.E., McClelland, W.C., Samson, S.D., Patchett, P.J., and Orchard, M.J., 1992, Geology of the western flank of the Coast Mountains between Cape Fanshaw and Taku In- let, southeastern Alaska: Tectonics, v. 11, p. 567-585. Gill, J.B., 1981, Orogenic andesites and plate tectonics: New York, Springer-Verlag, 390 p. Hawkesworth, C.J., O'Nions, R.K., Pankhurst, R.J., Hamilton, P.J., and Evensen, N.M., 1977, A geochemical study of island-arc and back-arc tholeiites from the Scotia Sea: Earth and Planetary Science Letters, v. 36, p. 253-262. Hawkins, J.W., Moore, G.F., Villamor, R., Evans, C., Wright, E., 1985, Geology of the composite terranes of East and Central Mindanao, in Howell, D.G., ed., Tectono- stratigraphic terranes of the circum-Pacific region: Circum- Pacific Council for Energy and Mineral Resources, Earth Science Series, number 1, p. 437-462. Haydoutov, Ivan, 1989, Precambrian ophiolites, Cambrian island arc, and Variscan suture in the South Carpathian-Balkan region: Geology, v. 17, p. 905-908. Henderson, J.R., Kirkham, R.V., Henderson, M.N., Payne, J.G., Wright, T.O., and Wright, R.L., 1992, Stratigraphy and structure of the Sulphurets area, British Columbia: Geologi- cal Survey of Canada Paper 92-lA, p. 323-332. Himmelberg, G.R., Brew, D.A., and Ford, A.B., 1991, Develop-
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Geologic Studies in Alaska by the U
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CONTENTS Introduction Cynthia Dusel
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CONTENTS CONTRIBUTORS TO THIS BULLE
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GEOLOGIC STUDIES IN ALASKA BY THE U
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LATE HOLOCENE LONGITUDINAL AND PARA
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DEEP-WATER LITHOFACIES AND CONODONT
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Table 1. Locality register for key
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Table 1. Locality register for key
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LITHOFACIES AND CONODONTS OF CARBON
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LITHOFACES AND CONODONTS OF CARBONI
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Table 1. Conodont faunules and lith
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DEPOSITIONAL SEQUENCES IN ATIGUN SY
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U-Pb AGES OF ZIRCON, MONAZITE, AND
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APPEAL FOR NONPROLIFERATION OF ESCA
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FAVORABLE AREAS FOR METALLIC MINERA
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GOLD AND CINNABAR IN HEAVY-MINERAL
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EARLY CENOZOIC DEPOSITIONAL SYSTEMS
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EARLY CENOZOIC DEPOSITIONAL S YSTEM
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RESERVOIR FRAMEWORK ARCHITECTURE, C
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GEOCHEMISTRY OF OPHIOLITIC ROCKS FR
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134 GEOLOGIC STUDIES IN ALASKA BY '
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Page 180 and 181: Table 1. Isotopic ages of intrusive
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Page 219: GEOCHEMICAL CHARACTER OF UPPER PALE
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Page 225 and 226: RECONNAISSANCE GEOCHEMISTRY OF BASA
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Page 243 and 244: RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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