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

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204 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 CHEMICAL CHARACTERISTICS OF THE METABASALTS MAJOR ELEMENTS Average major-element compositions of the units of this study are given in table 1. Additional analyses for rocks of the Chilkat Peninsula are given in Davis and Plafker (1985). The metabasalt units have only a small range in average Si02 content (46-48 percent), except for the metabasalt of the Chilkat Peninsula that has an unusu- ally high Si02 content of 49.9 percent, which is about the same as reported by Davis and Plafker (1985). The metabasalt of the Chilkat Peninsula is quartz-normative in average composition and all other units contain normative hypersthene and olivine (data not shown). The range in FeO*/MgO values is small (1.2-2.0) and the rocks are clearly tholeiitic in terms of SiOz and FeO*/MgO variation (fig. 2). The three western subareas of the Gastineau Volcanics (GVc, GVn, GVs; tables 1, 2, fig. 1) form a group with higher K20, on average, compared with other metabasalts of this study (fig. 3). They also form a group having higher average P205, but with average MgO range over- lapping that of other units (fig. 4). The range of average Mg numbers of the Gastineau Volcanics (Mg# 57-64, table 1) overlaps that of most other metabasalts (CP, 59; LH, 61; JA, 64). Mg numbers of the Gastineau Volcanics and the metabasalts of the Chilkat Peninsula and Lions Head Mountain are somewhat lower than those of ocean- island basalt (OIB) reported by Flower (1991, Mg# 64- 80). Except for the greenschist of the Barlow Cove Forma- tion (Mg# 72), average Mg numbers are generally well below those of probable primary mantle melts (> 73, Gill, 1981) suggesting either melt derivation from other than a primitive source or fractionation prior to eruption. Major-element compositions (table 1) suggest mag- matic settings of units somewhat different from those shown by trace elements that are described later. In terms of TiOz and FeO*/MgO variation, all units show MORB and back-arc-basin basalt compositions, and the three western subareas of the Gastineau Volcanics and the I ' I V I ' I ' I ' 1 ' - CP - - - - LH BC - - [3TA GVn 1 . 1 . 1 . 1 . 1 . 1 . K20, IN WEIGHT PERCENT Figure 3. Variation between average Si02 and K20 content of metabasalt units. Letter symbols, table 1. I - - ' ' - \ \ \ - / Western subareas, \ / Gastineau Volcanics \ - GVS O/ \OGV~ R / \ /- - ---- - - 00 LH - Gvb JA BC 0- - I . . . . I . . . . Figure 2. Variation between average Si02 content and MgO, IN WEIGHT PERCENT FeO*/MgO of metabasalt units, showing dividing line of calc-alkalic and tholeiitic compositions of Miyashiro Figure 4. Variation between average P205 and MgO content (1974). Letter symbols, table 1. of metabasalt units. Letter symbols, table 1. - -
GEOCHEMlCAL CHARACTER OF UPPER PALEOZOIC AND TRIASSIC ROCKS, WRANGELLIA TERRANE 205 metabasalt of Lions Head Mountain have higher Ti02 contents than other units (fig. 5). An abyssal tholeiite composition (=MORB?) of all units is suggested by average FeO* versus FeO*/MgO, according to Miyashiro and Shido's (1975) criteria (fig. 6). The MORB characteristics suggested by such major-element variations are not shown by many trace-element variations discussed below. TRACE ELEMENTS Average trace-element compositions of the units of this study are shown in table 2. Additional trace-element data for the metabasalt of the Chilkat Peninsula and for its correlative NikoIai Greenstone are given in Davis and Plafker (1985). Geochemically, the metabasalts of this study can be divided into two groups on the basis of large- ion-lithophile element (LILE) content. These elements include K, Rb, Sr, Ba, Zr, Th, Ta, and light rare-earth elements (LREE). LZLE-group 1, of generally lower LILE content, includes the metabasalts of the Chilkat Peninsula, Lions Head Mountain, and Barlow Cove, the Gastineau Volcanics of the Bishop Point area, and amphibolites of the Juneau area. LILE-group 2, with generally higher LILE content, comprises the three western subareas of the Gastineau Volcanics (table 3). LILE groups 1 and 2 are somewhat analogous, respectively, to E-type (enriched) MORB and alkaline OIB of Walker (1991) and Floyd (1991). The metabasalts of the three western subareas of the Gastineau Volcanics (GVc, GVn, GVs; tables 1-3, fig. I) differ from other metabasalt units in having generally higher average Sr and Rb contents and lower RbISr (table 2, figs. 7, 8). The metabasalts of the Bishop Point area of the Gastineau Volcanics (GVb), Chilkat Peninsula (CP), Lions Head Mountain (LH), and Barlow Cove (BC) generally group distinctly apart from other units in average K20 and Si02 variation with RbISr (fig. 8). Variations shown in figure 8A clearly discriminate the Bishop Point area Upper Triassic part of the Gastineau Volcanics from the Permian(?) and Triassic(?) lower part of the Gastineau. The Bishop Point area of the Gastineau Volcanics and the metabasalts of Lions Head Mountain and the Chilkat Pen- insula (and other units) have much lower average K20 than the three western subareas of the Gastineau Volcanics (fig. 8A) and higher average Si02 content (table 1, fig. 8B). Average Rb/Sr of the metabasalts of the Chilkat Pen- insula (0.12, Davis and Plafker, 1985; 0.09, table 2) and Lions Head Mountain (0.08, table 2) are higher than those of metabasalts of the Gastineau Volcanics (0.03-0.06, table 2). Zr-Rb variations show especially well marked dis- crimination of the three western subareas of the Gastineau Volcanics, with higher Zr content, from other units (fig. 9). Figure 5. Variation between average TiO, content and FeO*IMgO of rnetabasalt units, show& fields of Figure 6. Variation between average FeO* content and FeO*l midocean ridge and back-arc basin basalts (MORB, MgO of metabasalt units. Tholeiite/calc-alkalic divider from BABB) and island-arc basalt (IAB) of Hawkins and Miyashiro (1974). AB-TH, field of abyssal tholeiite (Miyashiro others (1985). Letter symbols, table 1. and Shido, 1975). Letter symbols, table 1.
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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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Page 180 and 181: Table 1. Isotopic ages of intrusive
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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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