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

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226 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 Brew's (1988) suggestion that these basalts trace south- eastward across Lynn Canal and into Berners Bay (fig. 1). Davis and Plafker (1985) present major- and minor- element abundances of seven samples of Upper Triassic basalt from Chilkat Peninsula. Their average major elements and ranges of minor elements are shown beside our corresponding values for Upper Triassic basalts in table 1. Comparison of the data for basalts of the Taku terrane and the two sets of basalts of the Wrangellia terrane shows all three units to be very similar in both major- and minor- element abundances. The Chilkat Peninsula rocks do con- tain higher Ti02 and Sc, and lower AI2O3 and (mobile) Na20, but the remaining 25 elements show similar values or ranges. Thus, as discussed below, the geochemical data are consistent with lithic correlation of all three sequences. Major element contents of two analyses of basalt northeast of Bemers Bay given by Ford and Brew (1988) are like those of Davis and Plaiker (1985) and our values shown in table 1. In regard to the correlation of Triassic strata of the Taku and Wrangellia terranes in southeastern Alaska with strata of the Wrangellia terrane elsewhere, it should be noted that Barker and others (1989; in press) have divided basalts of the Wrangellia terrane into island-arc type tholeiite and back-arc basin basalts. The Triassic sequences in southeastern Alaska are chemically similar to the back-arc basin components of the Karmutsen Forma- tion and the Nikolai Greenstone. CONCLUSIONS The geochemical signatures of Permian and Triassic basalts along the west flank of the Coast Mountains offer important insights into the tectonic setting in which they formed and their regional - correlation. Several first-order conclusions can be reached from this study: First, the Permian basalts of the Taku terrane have a different chemical signature from Triassic basalts of the Taku and Wrangellia terranes: the Permian rocks show distinctly higher abundances of Ti, P, Zr, Th, Ta, and light rare-earth elements. If confirmed through more detailed studies, this may provide a powerful means of discriminating Permian from Triassic components of Taku rocks in regions where age control is lacking. In terns of their chemical affinities, the Permian volcanic rocks are highmagnesian alkaline basalts, whereas the Triassic basalts of both the Taku and Wrangellia terranes are similar to midocean-ridge or within-plate tholeiite. Second, the lithologic correlation of basalts along the east shore of Lynn Canal and in Berners Bay with the Chilkat Peninsula section (Plafker and Hudson, 1980), and therefore with the Wrangellia terrane, is supported by the similar geochemical signatures reported by Ford and Brew (1988) and herein for these two sequences. Third, Triassic basalts of the Taku terrane in central southeastern Alaska are geochemically indistinguishable from those of the Wrangellia terrane that lie along strike to the northwest. In addition, as Gehrels and others (1992) pointed out, the pre-Permian(?), Permian, and Triassic strata of the Taku terrane show general lithologic similari- ties to rocks of the Wrangellian terrane--even though indi- vidual sequences cannot be correlated. Hence, it appears likely that rocks of the Taku terrane in the Juneau-Port Houghton area may be a part of the Wrangellia terrane. Such a correlation necessitates a facies change in the region southeast of Berners Bay, presumably where the Fanshaw fault was obliterated by intrusive rocks of the Coast batholith (see figure 1). The facies change would involve gradation of thick, subaerial to shallow-marine Up- per Triassic basalt flows of the Chilkat Peninsula into a basinal sequence of basaltic pillow flows and breccia, black carbonaceous shale and limestone, and gray chert- as now exposed near Juneau and to the south. Documenta- tion of such a correlation requires additional studies, especially of the pre-Triassic rocks of the Taku terrane. If future work does not support this correlation, and the Wrangellia and Taku terranes remain distinct entities, their boundary is at least locally the Fanshaw fault. REFERENCES CITED Barker, Fred, Arth, J.G., and Stem, T.W., 1986, Evolution of the Coast Batholith along the Skagway traverse, Alaska and British Columbia: American Mineralogist, v. 71, p. 632-643. Barker, Fred, Nokleberg, W.J., Plafker, George, and Leeman, W.P., in press, Wrangellia terrane, in Barker, Fred, ed., Some accreted volcanic rocks of Alaska and their elemental abundances, in Plafker, George, and Berg, H.C., eds., Geology of Alaska: Geological Society of America, The Geology of North America Barker, Fred, Sutherland Brown, A., Budahn, J.R., and Plaflcer, George, 1989, Back-arc with frontal-arc component origin of Triassic Karmutsen basalt, British Columbia, Canada: Chemical Geology, v. 75, p. 81-102. Brew, D.A., and Ford, A.B., 1984, Tectonostratigraphic terranes in the Coast plutonic-metamorphic complex, southeastern Alaska, in Bartsch-Winkler, S., and Reed, K.M., The U.S. Geological Survey in Alaska: Accomplishments during 1982: U.S. Geological Survey Circular 939, p. 90-93. Brew, D.A., and Ford, A.B., 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. 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-A, p. 21-52. Davis, Alict, and Plafker, George, 1985, Comparative geochemistry of Triassic basaltic rocks from the Taku terrane on the Chilkat Peninsula and Wrangellia: Canadian Journal of Earth Sciences, v. 22, p. 183-194.
RECONNAISSANCE GEOCHEMISTRY OF BASALTS OF THE TAKU AND WRANGELLIA TERRANES 227 Ford, A.B., and Brew, D.A., 1988, Major-element geochemistry of metabasalts of the Juneau-Haines region, southeastern Alaska, in Galloway, J.P., and Hamilton, T.D., eds., Geo- logic studies in Alaska by the U.S. Geological Survey during 1987: U.S. Geological Survey Circular 1016, p. 150-155. Gehrels, G.E., in review, Geology and U-Pb geochronology of the western flank of the Coast Mountains between. Juneau and Skagway, in McClelland, W.C., ed., Tectonics of the Coast Mountains in southeast Alaska and coastal British Columbia: Geological Society of America Special Paper. Gehrels, G.E., McClelland, W.C., Samson, S.D., and Patchett, P.J., and Orchard, M.J., 1992, Geology of the western flank of the Coast Mountains between Cape Fanshaw and Taku Inlet, southeastern Alaska: Tectonics, v. 11, p. 567-585. Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, p. 523-548. McClelland, W.C., Gehrels, G.E., Samson, S.D., and Patchett, P.J., 1992, Protolith relations of the Gravina belt and Yukon-Tanana terrane in central southeastern Alaska: Jour- nal of Geology, v. 100, p. 107-123. Miyashiro, A., 1974, Volcanic rock series in volcanic arcs and active continental margins: American Journal of Science, v. 274, p. 321-355. Monger, J.W.H., and Berg, H.C., 1987, Lithotectonic terrane map of western Canada and southeastern Alaska: U.S. Geo- logical Survey Miscellaneous Field Studies Map MF-1874-B. Mullen, E.D., 1983, Mn0-Ti02-P205: a minor element discrimi- nant for basaltic rocks of oceanic environments and its im- plications for petrogenesis: Earth and Planetary Science Letters, v. 62, p. 53-62. Plafker, George, Blome, C.D., and Silberling, N.J., 1989, Rein- terpretation of lower Mesozoic rocks on the Chilkat Penin- sula, Alaska, as a displaced fragment of Wrangellia: Geology, v. 17, p. 3-6. Plafker, George, and Hudson, Travis, 1980, Regional implica- tions of Upper Triassic metavolcanic and metasedimentary rocks on the Chilkat Peninsula, southeastern Alaska: Cana- dian Journal of Earth Sciences, v. 17, p. 681-689. Redman, Earl, 1984, An unconformity associated with conglom- eratic sediments in the Berners Bay area of southeastern Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 86, p. 1-4. Redman, Earl, Retherford, R.M., and Hickock, B.D., 1984, Ge- ology and Geochemistry of the Skagway B-2 quadrangle, southeastern Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 84-31, 34 p. Rubin, C.M., and Saleeby, J.B., 1988, A new perspective on what is the Taku terrane in southern SE Alaska: Geological Society of America Abstracts with Programs, v. 20, p. 226. 1991, Tectonic framework of the upper Paleozoic and lower Mesozoic Alava sequence: a revised view of the polygenetic Taku terrane in southern southeastern Alaska: Canadian Journal of Earth Sciences: v. 28, p. 881-893. Thompson, R.N., 1985, Asthenospheric source of Ugandan ultrapotassic magma?: Journal of Geology, v. 93, p. 603-608. Wood, D.A., 1980, The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to estab- lishing the nature of crystal contamination of basaltic lavas of the British Tertiary volcanic province: Earth and Plan- etary Science Letters, v. 50, p. 11-30. Reviewers: Richard J. Goldfarb and George Plafker
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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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1 64 GEOLOGIC STUDlES IN ALASKA BY
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Table 1. Isotopic ages of intrusive
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Page 219 and 220: GEOCHEMICAL CHARACTER OF UPPER PALE
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Page 243 and 244: RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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