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

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68 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY. 1992 DISCUSSION AND TECTONIC IMPLICATIONS Despite the unwise practice of using data from several peraluminous and metaluminous magmatic suites that are distinct and unrelated to each other. However, the similar U-Pb ages derived from samples from these suites, the present spatial proximity of the metagranitic bodies, and plutons to calculate a composite regression and concordia their similar porphyritic textures suggests to us that the intercept age, the Devonian age (391+20 Ma) of Dillon Horace Mountain plutons, Geroe Creek orthogneiss body, and others (1987) for orthogneisses in the eastern Brooks and Baby Creek orthogneiss body once formed parts of a Range is accurate. Our new data for each pluton are more large Early Devonian batholith that was dismembered durprecise; much of our data are concordant, and other frac- ing the Brookian orogeny in the Jurassic or Early Cretations are mostly less than 10 percent discordant. However, ceous. This interpretation is supported by the tabular despite very careful hand-picking of very small samples, shapes of the metagranitic bodies and the presence of genwe were unable to avoid some grains that contain inherit- tly dipping faults both above and below several of the ance of older radiogenic Pb, in even the most pristine zir- bodies. The compositional differences between the con grains. The age of inheritance is unknown, although orthogneisses may be due to assimilation of different the only known older rocks are Late Proterozoic (about amounts of pelitic wall rocks. The assimilation of pelitic 700-750 Ma). Zircons in the hornblende-biotite ortho- material is supported by the broad range of the 207~b/206~b gneisses (fig. 3A, B) contain only small amounts of inher- ages for the Baby Creek orthogneiss body discussed above. ited material, as evidenced by 207~b/206~b ages 393 and Thus, we suggest that the various orthogneisses represent dif- 412 Ma. Zircon fractions in the two-mica orthogneisses ferent parts of a variably contaminated batholith. (fig. 3C, D) have a much broader range of 207~b/206~b Although the Coldfoot and Hammond terranes have ages (398 to 612 Ma), and we were unable to obtain con- different stratigraphies and different metamorphic and cordant data from any fraction. Although these plutons are deformational histories, the almost exact agreement in ages not S-type granites in the strict sense because their of the orthogneisses and felsic metavolcanic rocks begs the mineralogic and chemical characteristics were later found question of the relationship between the two terranes. to be nondiagnostic (Dillon, 1989, p. 183), they are Dillon and others (1987) and Dillon (1989) suggested that peraluminous and probably were derived from pelitic sedi- the volcanogenic rocks that comprise the Nutirwik Creek ments. Detrital zircons in sedimentary rocks may be de- unit represent the extrusive volcanic centers of the rived from many provenances (for example, quartzite in metagranitic rocks in the Coldfoot terrane to the south. the Yukon-Tanana terrane contains zircons that range in Balanced cross sections through the Brooks Range, which age from 1.2 to 3.4 Ga; Mortensen, 1990), and thus, inherited regard the Coldfoot and Hammond terranes as basement material in the zircons may be a mixture of many ages. for the imbricated passive continental deposits of the The orthogneisses are strongly foliated, a deformation Endicott Mountains terrane to the north, support this view that apparently occurred in the middle Mesozoic on the baby placing the Coldfoot terrane beneath the Hammond tersis of K-Ar ages (Armstrong and others, 1986). Our U-Pb rane in their reconstruction (Oldow and others, 1987). The data on sphene indicate that this metamorphism did not ex- Early Devonian Nutirwik Creek unit, however, is inticeed about 550°C, because the sphenes have not been reset mately interleaved with, and possibly may have been deto a Mesozoic age. The sphene and monazite may be ig- posited on, Ordovician to Devonian carbonate platform neous minerals that cooled through their closure tempera- deposits. The Coldfoot terrane into which the metagranitic tures shortly after emplacement of the magmas, or they rocks were emplaced, in contrast, consists partly of calcarmay have formed during a penetrative deformation in the eous quartz schist of Silurian (?) and Early Devonian age. Devonian. The coarse-grained, euhedral morphology of Taken at face value, these relations suggest that the volcathese minerals suggests that they are igneous in origin. nic rocks were deposited in a different early to middle Pa- The -390-Ma ages of the felsites may provide con- leozoic stratigraphic setting than the metagranitic rocks straints for the age of the stratigraphic section. Sample and thus may not be consanguineous. Alternatively, if 90TM409 (precisely dated at 393+2 Ma) particularly looks they are assumed to be consanguineous, then large-dislike an interbedded metavolcanic rock and thus would date placement faults must divide the igneous rocks from the the time of deposition of the purple and green phyllites of dated metasedimentary rocks in either or both terranes. the Nutirwik Creek unit. However, geologic mapping The Devonian metagranitic rocks of the central shows that the purple and green phyllite is allochthonous Brooks Range are part of an extensive, discontinuous, linrelative to other rocks of the Hammond terrane and hence ear belt of middle Paleozoic plutonic and volcanic rocks its stratigraphic significance is not understood. All con- that occur in various accreted terranes in the North Ameritacts may be a complex interleaving of fault slivers. can Cordillera (Rubin and others, 1990). In the Brooks On the basis of their contrasting composition and min- Range, Devonian granitic rocks in the Coldfoot, eralogy, Newbeny and others (1986) concluded that the Hammond, and North Slope terranes crop out in a belt of metagranitic rocks of the Chandalar quadrangle comprise over 900 km. Elsewhere in Alaska and western Canada,
U-Pb AGES OF ZIRCON, MONAZITE, AND SPHENE FROM BROOKS RANGE, ALASKA 69 similar middle Paleozoic granitic rocks occur in the Cassiar, Kootenay, Seward, and Yukon-Tanana terranes. These terranes consist mainly of continental-margin, mostly miogeoclinal, sedimentary rocks of Late Protero- zoic and Paleozoic age that are locally intruded by middle Paleozoic granitic rocks and that locally contain middle Paleozoic intermediate and felsic volcanic rocks. Various isotopic and geochemical data suggest that the middle Pa- leozoic igneous rocks formed above continentward-dipping subduction zone(s) (Rubin and others, 1990). It is unknown whether these terranes all formed in the same, extensive linear subduction zone along the former North American continental margin (Rubin and others, 1990) or if some of these terranes formed elsewhere and subse- quently accreted to, and still later were dispersed by strike- slip faulting along, the North American continental margin. The Devonian U-Pb ages of metagranitic rocks in the central Brooks Range provide important age constraints on the origin of a major metallogenic belt of associated granitic magmatism-related mineral deposits, mainly por- phyry, polymetallic vein, and skam deposits. Most of the Devonian orthogneisses that are associated with these lode deposits are highly evolved, peraluminous and metaluminous biotite and two-mica granites. Most interpreta- tions favor formation of the granitic rocks above a continental-margin subduction zone (Newbeny and others, 1986). Possibly associated with the belt of granitic magmatism-related deposits are Devonian(?) Kuroko-type massive sulfide deposits that occur mainly in the Ambler sequence of interbedded Devonian(?) bimodal volcanic rocks and marine sedimentary rocks in the Coldfoot terrane in the western and central Brooks Range (Nokleberg and others, 1987, 1988). Acknowledgments.-We thank Nina Bashir for petro- graphic analysis and Alison Till for help with sample collection. REFERENCES CITED Aleinikoff, J.N., Moore, T.E., Karl, S.M., and Dillon, J.T., 1991, Pb isotopic ratios in Late Proterozoic and Devonian granitic rocks of the Brooks Range, Alaska: Eos, Transactions American Geophysical Union, 1991 Fall Meeting, Program and Abstracts, v. 72, no. 44, p. 299-300. Aleinikoff, J.N., Winegarden, D.L., and Walter, M., 1990, U-Pb ages of zircon rims: A new analytical method using the airabrasion technique: Chemical Geology (Isotope Geoscience Section), v. 80, p. 351-363. Armstrong, R.L., Harakal, J.E., Forbes, R.B., Evans, B.W., and Thurston, S.P., 1986, Rb-Sr and K-Ar study of metamorphic rocks of the Seward Peninsula and Southern Brooks Range, Alaska: Geological Society of America Memoir 164, p. 185-203. Blythe, A.E., Wirth, K.R., and Bird, J.M., 1990, Fission track and 40~r/39~r ages of metamorphism and uplift, Brooks Range, northern Alaska: Geological Association of Canada, Program and Abstracts, v. 15, p. A-12. Box, S.E., 1985, Early Cretaceous orogenic belt in northeastern Alaska: Internal organization, lateral extent, and tectonic interpretation, in Howell, D.G., ed., Tectonostratigraphic terranes of the circum-Pacific region: Circum-Pacific Council for Energy and Mineral Resources (American As- sociation of Petroleum Geologists), Earth Science series, no. 1, p. 137-145. BrosgC, W.P., and Reiser, H.N., 1964, Geologic map and section of the Chandalar quadrangle, Alaska: U.S. Geological Sur- vey Miscellaneous Geologic Investigations Map 1-375, scale 1 :250,000. BrosgC, W.P., Reiser, H.N., Dutro, J.T., Jr., and Detterman, R.L., 1979, Bedrock geologic map of the Philip Smith Mountains quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-897B, scale 1 :250,000. Dillon, J.T., 1985, Structure and stratigraphy of the southern Brooks Range and northern Koyukuk basin near the Dalton Highway, in Mull, C.G., and Adams, K.E., eds., Dalton Highway, Yukon River to Prudhoe Bay, Alaska: Alaska Division of Geological and Geophysical Surveys Guide- book 7, V. 2, p. 157-187. Dillon, J.T., BrosgC, W.P., and Dutro, J.T., Jr., 1986, General- ized geologic map of the Wiseman quadrangle, Alaska: U.S. Geological Survey Open-File Report OF 86-219, scale 1:250,000. Dillon, J.T., Harris, A.G., and Dutro, J.T., Jr., Solie, D.N., Blum, J.D., Jones, D.L., and Howell, D.G., 1988, Preliminary geologic map and section of the Chandalar D-6 and parts of the Chandalar C-6 and Wiseman C-1 and D-l quadrangles, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 88-5, scale 1:63,360. Dillon, J.T., Pessel, G.H., Chen, J.H., and Veach, N.C., 1980, Middle Paleozoic magmatism and orogenesis in the Brooks Range, Alaska: Geology, v. 8, p. 338-343. Dillon, J.T., and Reifenstuhl, R.R., 1990, Geologic map of the Wiseman B-l quadrangle, south central Brooks Range, Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 101, scale 1:63,360. Dillon, J.T., Tilton, G.R., Decker, J., and Kelly, M.J., 1987, Re- source implications of magmatic and metamorphic ages for Devonian igneous rocks in the Brooks Range, in Tailleur, I.L., and Weimer, Paul, eds., Alaskan North Slope geology: Bakersfield, California, Pacific Section Society of Eco- nomic Paleontologists and Mineralogists and Alaska Geo- logical Society, Book 50, p. 713-723. Dutro, J.T., Jr., BrosgC, W.P., Reiser, H.N., and Detterman, R.L., 1979, Beaucoup Formation, a new Upper Devonian stratigraphic unit in the central Brooks Range, northern Alaska, in Sohl, N.F., and Wright, W.B., eds., Changes in stratigraphic nomenclature by the U.S. Geological Survey: U.S. Geological Survey Bulletin 1482-A, p. A63-A69. Gottschalk, R.R., 1990, Structural evolution of the schist belt, south-central Brooks Range fold and thrust belt: Journal of Structural Geology, v. 12, p. 453-469. Grantz, Arthur, Moore, T.E., and Roeske, S.M., 1991, Conti- nent-ocean transect A-3: Gulf of Alaska to Arctic Ocean: Boulder, Colorado, Geological Society of America Centen- nial Continent-Ocean Transect 15, 72 p., scale 1:500,000, 3 sheets.
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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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DEEP-WATER LITHOFACIES AND CONODONT
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Page 25 and 26: Table 1. Locality register for key
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Page 37 and 38: LITHOFACIES AND CONODONTS OF CARBON
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Page 51 and 52: Table 1. Conodont faunules and lith
Page 55 and 56: DEPOSITIONAL SEQUENCES IN ATIGUN SY
Page 65 and 66: U-Pb AGES OF ZIRCON, MONAZITE, AND
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Page 77 and 78: APPEAL FOR NONPROLIFERATION OF ESCA
Page 85 and 86: FAVORABLE AREAS FOR METALLIC MINERA
Page 97 and 98: GOLD AND CINNABAR IN HEAVY-MINERAL
Page 107 and 108: EARLY CENOZOIC DEPOSITIONAL SYSTEMS
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RESERVOIR FRAMEWORK ARCHITECTURE, C
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GEOCHEMISTRY OF OPHIOLITIC ROCKS FR
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Table 1. Isotopic ages of intrusive
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Table 1. Isotopic ages of intrusive
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GEOCHEMICAL EVALUATION OF STREAM-SE
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Table 3. Summary of geochemical sig
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GEOCHEMICAL CHARACTER OF UPPER PALE
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RECONNAISSANCE GEOCHEMISTRY OF BASA
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OSTRACODE ASSEMBLAGES FROM MODERN B
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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US. GEOLOGICAL SURVEY REPORTS ON AL
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U.S. GEOLOGICAL SURVEY REPORTS ON A
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U.S. GEOLOGICAL SURVEY REPOR TS ON
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REPORTS ABOUT ALASKA IN NON-USGS PU
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REPORTS ABOUT ALASKA IN NON-USGS PU
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