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

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66 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 is discordant, having PbN ages of 367 and 370 Ma and a 207~b/206~b age of 386 Ma. The concordant monazite date of about 381 Ma is regarded as a minimum age for emplacement of the pluton because this date may represent (1) the time of cooling through the closure temperature of the U-Pb system in monazite (-720°C; Pamsh, 1990) or (2) the time of growth of metamorphic monazite. In either case, the crystallization age of the pluton must be equal to or older than the monazite age. We thus constrain the time of emplacement of the Baby Creek orthogneiss body to be between 381 and 398 Ma. In contrast to the euhedral, light- to medium-brown zircons in other orthogneisses, the zircons in sample 88TM137 (Middle Fork Koyukuk River orthogneiss body) are subhedral, dark brown, and deformed (table 1). Isoto- 0.065 3 w 0.063 - N \ n a D 0.065 A 90TM408 Geroe Creek orthogneiss body weighted average of Pb/238 U ages = 39 1 i 1 Ma l~eference chord (0 and 390 Ma) I I I I I I I I 0.43 0.44 0.45 0.46 0.47 0.48 0.49 0.50 Twin Lakes pluton of Baby Creek orthogneiss body Reference chord .42 0.44 0.46 0.48 pic data from four fractions are scattered, with 207~b/206~b ages between 392 and 612 Ma and PbN ages ranging from 360 to 466 Ma (table 2, fig. 30). They have uranium concentrations (333 to 438 ppm; table 2) intermediate between those of the hornblende-biotite orthogneisses and the Baby Creek (two-mica) orthogneiss body. No sphene or monazite was found in this sample. The poor preservation of the zircon and the scattered isotopic systematics preclude determining a precise date for this rock. However, the fraction with the youngest 207~b/206~b age (about 392 Ma) is only about 2 percent discordant. Thus, we suggest a maximum age of emplacement for the Middle Fork Koyukuk River orthogneiss body of about 392 Ma. The absence of sphene or monazite prevents an independent verification of this assumed age. B 90TM498 0.065 Horace Mountain plutons D 88TM137 Middle Fork Koyukuk 0.074 /// River orthogneiss 440 0.070 420' 400. ' ' " Reference chord 360 : / (0 and 390 Ma) Figure 3. Concordia plots of U-Pb data from orthogneiss of the Coldfoot terrane, eastern Brooks Range. Sample localities shown on figure 2. '9
U-Pb AGES OF ZIRCON, MONAZITE, AND S ;PHENE FROM BROOKS RANGE, ALASKA METAFELSITES OF THE HAMMOND TERRANE Two samples of fine-grained, foliated, porphyritic rocks (identified as felsites in the field) were collected from the Nutirwik Creek unit of the Hammond terrane to test their temporal correlations with granitic orthogneisses in the Coldfoot terrane. One or both of the samples may be metavolcanic in origin and thus may also provide con- straints for the age of the stratigraphic section. Isotopic data from zircons from eight samples of fine-grained felsic rocks in the southern and eastern Brooks Range have yielded a composite discordia with an upper intercept age of 396+20 Ma (Dillon and others, 1987). Two of their 0.065 plagioclase felsite 2 0.063 N \ a 0.061 weighted average of PW % ages = 393 + 2 ~a 0.059 0.43 0.44 0.45 0.46 0.47 0.48 0.49 0.50 207 pb) 235 u INTERCEPTS AT 1015 k 103 and 388.7 k 3.2 Ma (MSWD = 1.4) Figure 4. Concordia plots of U-Pb data from metafelsites of the Hammond terrane, eastern Brooks Range. Sample localities shown on figure 2. MSWD, mean square of weighted deviates. samples, from Whiteface Mountain and Wiehl Mountain, are about 25 km west and 30 km south, respectively, of our sample 90TM409. Sample 90TM409 is a buff-colored, strongly foliated felsic rock. It is about 25-40 cm thick and is interlayered with gray, green, and purple shale, and interfolded (or faulted) with limestone and marble. It is compositionally homogeneous and contains augen of partly polygonized quartz phenocrysts, albitized feldspar phenocrysts, and volcanic rock fragments in a fine-grained, foliated matrix of metamorphic quartz, feldspar, calcite, and white mica. The lithologic characteristics and field occurrence have led to the interpretation that this rock is volcanic (extrusive) in origin, although it is possible, given the superimposed deformation and very limited outcrop, that it is a sill. Zircons from our sample are euhedral, light pink, adamantine, and have Yw of 2-4 (table 1). We analyzed three fractions, all of which are concordant (fig. 4A), despite having fairly high uranium concentrations of 431-987 ppm. The weighted average of the 207~b/206~b ages is 39322 Ma, interpreted as the time of crystallization of the rock. Sample 90TM497 is a plagioclase porphyry (table I) interpreted in the field as a hypabyssal felsite plug, flow, or sill. It contains feldspar and amphibole phenocrysts in a fine-grained groundmass and has been metamorphosed under apparent static conditions to albite, epidote, quartz, white mica, and pale amphibole. Zircons from this sample are similar in morphology to zircons in metafelsite sample 90TM409 (euhedral, light pink, adamantine) except that they are more prismatic with llw of 4-8 (table 1). One fraction is concordant at about 385 Ma; two other fractions have 207~b/206~b ages of 400 and 469 Ma (table 2). The isotopic data define a chord with a lower-intercept age of 389+3 Ma (fig. 4B); the upper-intercept age of 1,015+103 Ma may be geologically meaningless because the oldest known rocks in the central and eastern Brooks Range are Late Proterozoic (Dillon and others, 1987; Karl and others, 1989; Karl and Aleinikoff, 1990). Thus, the age of the sample may be best estimated by the 206~b/238~ age (385+2 Ma) of the concordant fraction. In summary, a11 six samples dated in this study are Early to Middle Devonian in age. The ages of the hornblende-biotite orthogneisses are identical within uncertainty (391+_1 and 393+_5 Ma) and the rocks may be related. The two-mica orthogneisses were more difficult to date, because of inheritance of radiogenic Pb, high uranium contents, and poorly preserved zircon grains. However, concordant monazite and two discordant zircon fractions in the Baby Creek orthogneiss body constrain the emplacement age to be between 381 and 398 Ma. A slightly discordant zircon fraction in the Middle Fork Koyukuk River orthogneiss body suggests a maximum crystallization age of about 390 Ma. The ages of the two metafelsites are 393+2 Ma and probably about 385-390 Ma, respectively. 67
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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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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
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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 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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