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

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GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 Figure 22. Th-Hf-Ta diagram showing fields of major volcanic settings of Wood (1980). SRV, subduction-related volcanics; WPV, within-plate volcanics; NMORB and EMORB, normal and enriched ocean-ridge basalt. Letter symbols, table 1. the Gastineau at Bishop Point and other metabasalt units of the area (LILE-group 1). Table 3 summarizes the chemical comparisons of the metabasalt units. Signatures particu- larly apparent for identifying the older parts of the Gastineau Volcanics are (1) higher K20 content (fig. 3); (2) higher P205 content (fig. 4); (3) generally higher TiO, content (figs. 5, 20); (4) generally higher Sr and Rb (figs. 7, 9); (5) generally lower RbISr (fig. 8A, B); (6) higher Zr (figs. 9, 18, 20); (7) more enriched chondrite-normalized LREE patterns (fig. 10); (8) distinctly higher La (figs. 14, 15); (9) higher Th (figs. 15, 16); (10) generally higher Hf (fig. 16); (1 1) generally higher Th/Yb and TaIYb (fig. 17); (12) distinctly enriched MORB-normalized abundances of elements between Th and P (fig. 21); and (13) distinctly more abundant Nb and Ta (figs. 21, 22). Major- and trace- element geochemical signatures such as these and as shown in table 3 can aid geologic mapping of the abundant greenstone and greenschist units of this region that are generally difficult to distinguish by field and petrographic characteristics alone. INTERPRETATIONS AND CONCLUSIONS Metabasalts of this study are closely comparable to those of Wrangellia in other southeastern Alaskan and nearby areas. The Wrangellia identification of the metabasalt of the Chilkat Peninsula (Plafker and others, 1989) and chemical comparisons of this study clearly indicate that at least the Triassic upper part of Wrangellia extends much farther to the south (fig. 1) than previously docu- mented. The discontinuity of the Wrangellia greenstone belt between Lions Head Mountain and Blackerby Ridge near Juneau (fig. 1) is due to plutonic intrusions in inter- vening areas (Knopf, 191 1; Brew and Ford, 1985) and as indicated in this study also to the involvement of Late Cretaceous to early Tertiary amphibolite-facies Barrovian regional metamorphism (Brew and others, 1989; Himmel- berg and others, 1991). The amphibolites of western Heintzleman Ridge show many geochemical similarites with greenschist-facies metabasalts of the Chilkat Penin- sula and Lions Head Mountain, and the metabasalt of the Bishop Point area of the Gastineau Volcanics (tables 1-3; figs. 2, 5, 9-13, 19-22). The amphibolites have a tholeiitic composition similar to that of Triassic greenschist-facies metavolcanic rocks of the area (figs. 2-6, 9-10, 17, 19- 22). Like the greenschist-facies rocks, they show back-arc- basin, within-plate, or ocean-floor basalt composition (figs. 5, 20, 22). Their unusually high Ba content suggestive of calc-alkaline nature (figs. 14, 18) may be an effect of alter- ation. The amphibolites of the Juneau area lack the island- arc character reported (Stowell and Hooper, 1991 ; Stowell and others, 1992) for other amphibolites of the west mar- gin of the metamorphic and plutonic belt of the Coast Mountains (JA, figs. 15, 17, 20-22). MORB-normalized geochemical patterns (fig. 21) are those of oceanic within- plate basalt (OIB) that lack the Ta and Nb depletion relative to Th and Hf typical of island-arc basalt (Pearce, 1983; Condie, 1989). An absence of metamorphic equivalents of the voluminous andesites or other more differentiated volcanic rocks typically associated with island-arc suites (Gill, 1981)-and lacking from MORB and intraplate oceanic-basalt associations (Perfit and others, 1980bis a strong argument against an evolved island-arc origin not only for the arnphibo- lite but for all metabasalts of this study. The two groups of REE patterns for the metabasalts of this study (fig. 10) lie entirely within the spread of REE patterns reported by Davis and Plafker (1985) for Triassic- age Wrangellia samples from the Chilkat Peninsula and the Nikoiai Greenstone. HREE patterns of all units of this study and LREE patterns of the Upper Triassic part of the Gastineau Volcanics approximate those of basalt of the Karmutsen Formation given in Barker and others (1989). The Permian(?) and Triassic(?) lower part of the Gastineau Volcanics, however (fig. 10, GVn, GVc, GVs), differs lack evidence of subduction origin (fig. 22). The many cherni- cal differences between the Permian(?) and Triassic(?) western, older part of the Gastineau Volcanics and the Up- per Triassic Bishop Point area of the Gastineau (figs. 3-5, 7-10, 15-17, 20-22) suggest variations in tectonic setting during the formation of this volcanic sequence. The Wrangellia terrane of mainland southeastern Alaska is found in our study to extend from the Chilkat Peninsula at least as far south as the vicinity of Taku Inlet (fig. I), and constitutes much of the Taku terrane shown by others in studies described earlier. We believe that the Wrangellia terrane extends even farther south, at least to the vicinity of Tracy Arm, where little-studied Permian
GEOCHEMICAL CHARACTER OF UPPER PALEOZOIC AND TRIASSIC ROCKS, WRANGELLIA TERRANE and Triassic metabasalts are reported (Brew and Grybeck, 1984; Gehrels and others, 1992). Wrangellia is also represented in insular areas of southeastern Alaska. Major- and trace-element composition of the greenschist of the Barlow Cove Formation indicates that rocks of this terrane occur on northern Admiralty Is- land, an area inferred to be the Alexander terrane by some workers (Silberling and others, 1992; Rubin and Saleeby, 1992; Monger and Berg, 1987) but considered to be a composite of the Alexander and Wrangellia terranes by Brew and Ford (1993). The probable correlation of the BarIow Cove rocks with the Upper Triassic Hyd Group of Loney (1964) suggests that this part of Wrangellia extends southward most of the length of Admiralty Island (Lathram and others, 1960; 1965) and probably farther south to other islands of southeastern Alaska, where cor- relative rocks occur (Douglass and others, 1989; Muffler, 1967; Muffler and others, 1969). The Triassic volcanism of Wrangellia has generally been ascribed to varied rift-related tectonic settings. Ac- cording to Davis and Plafker (1985), volcanism of the metabasalt of the Chilkat Peninsula and the Nikolai Green- stone of the Wrangell Mountains occurred in an extensional intraplate or back- or fore-arc setting. Barker and others (1989) proposed an arc-related rift-basin origin also for the Triassic tholeiite of the Karmutsen Formation of the Queen Charlotte and Vancouver Islands. A back-arc origin close behind an arc axis such as in the Wallowa terrane of Oregon and Idaho is suggested by Vallier (1986, in press) for the setting of Karmutsen volcanism. On the other hand, trace-element and Nd isotopic data of the Tri- assic flood basalts of Wrangellia show affinity with OIB, and are more likely related to mantle-plume activity than arc-related processes of a back-arc or rifted-arc environment (Lassiter and DePaolo, 1992). We interpret the geochemical characteristics of rocks of our study to be supportive of very large, oceanic mantle plume (super- plume) activity as hypothesized by Richards and others (1991), even though basalts of such origin are not specifi- cally identified based on chemistry in the lack of available discriminant diagrams for that tectonic setting. The rnetabasalts of the lower-LEE group of this study show relatively flat REE patterns 10-20 times chondrite (LILE-group 1, fig. 10) that lack La and Ce depletion but otherwise approximate those of basalts of midocean ridge and back-arc (marginal) basin settings (Wood and others, 1980; Brouxel and Lapierre, 1988) and are also similar to basalts of rift-related type (Fodor and Vetter, 1984). Metabasalts of the higher-LREE group, of the western (older) subareas of the Gastineau Volcanics, on the other hand, show similarity with island-arc suites (Peccerillo and Taylor, 1976). Both groups of REE patterns for the metavolcanic rocks of this study (fig. 10) show tholeiitic rather than calc-alkaline signatures (Wilson, 1989). How- ever, Wood and others (1980) reported that both LREE- 213 enriched and LREE-depleted patterns can occur in either subduction-zone or mid-ocean ridge environments and thus those elements may not uniquely discriminate a tectonic setting. Many of the major- and trace-element data reported here for the Triassic and Triassic(?) metabasalts of the Chilkat Peninsula, Lions Head Mountain, Barlow Cove, and Bishop Point area of the Gastineau Volcanics, as well as for amphibolite of the Juneau area, are similar to data of Davis and Plafker (1985) used by them to suggest an intraplate or back- or fore-arc extensional setting of the volcanism. The many chemical differences between the Permian(?) and Triassic(?) lower part of the Gastineau Volcanics and its Triassic upper part (tables 1-3 and most figures) are believed to reflect differences in origin. All rock units of this study have ratios of high-field-strength elements versus LILE (represented by NbLa, 1.3-2.1) that are typical of basalts of Wrangellia (Lassiter and DePaolo, 1992). Many back-arc-basin basalts have compositions transitional between MORB and island-arc tholeiite, showing enrichment in Ba, Rb, K, and LREE relative to MORB (Saunders and Tarney, 1991; Ikeda and Yuasa, 1989), as is shown by the lower part of the Gastineau Volcanics (LEE-group 2). Rb/Sr values do not distinguish back-arc from island-arc settings for the metabasalts of this study but do exclude MORB, according to data of Hawkesworth and others (1977). Th/U ratios (1.6-3.0) of the metabasalt samples are about those of the typically low range found in island-arc volcanic rocks and lower than occur in MORB (Taylor and McLennan, 1985). The Permian(?) and Triassic(?) lower part of the Gastineau Volcanics does not show the E-type MORB characteristics (Floyd, 1991) of the Triassic Bishop Point area of the Gastineau and the Triassic(?) Lions Head Mountain, Barlow Cove, and amphibolite units. All units of this study lack the Nb and Ta depletions (figs. 21, 22) characteristic of island-arc tholeiite and related volcanic rocks formed by subduction processes (Pearce, 1983). On the other hand, the Nb and Th contents of all the metabasalts of this study (table 2) show variations reported by Hofmann (1986) for OIB, with Nb, Ta, and Th contents higher by a factor of two in the metabasalts of the western subareas of the Gastineau Volcanics compared with other units. The chemical variations in the northern southeastern Alaskan metabasalts of this study reflect long-lived late Paleozoic and Triassic volcanism of diverse tectonic setting that differs in some respects from that reported for other Wrangellia areas of southern Alaska, British Colum- bia, and Idaho (Jones and others, 1986; Barker and others, 1989; and Vallier, in press). An intra-oceanic, juvenile magmatic environment, with little crustal involvment, is indicated by Nd and Sr isotopic data for the formation of Wrangellia before its accretion to North America (Samson and others, 1990). Samson and others (1990), however, did not distinguish earlier (Permian?) and later (Late Triassic)
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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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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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