Geology topics
Travis Hudson
Publications and source records attributed to Travis Hudson.
Alaska resource data file: Dillingham quadrangle
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Calc-alkaline plutonism along the Pacific rim of southern Alaska
Field, petrology, and age data on southern Alaska plutonic rocks now enable the delineation of eight calc-alkaline plutonic belts. These belts of plutons or batholithic complexes are curvilinear to linear and trend parallel or subparallel to the continental margin. The belts represent the principal loci of emplacement for plutons of specific ages and although there is spatial or temporal overlap in some cases, they are more commonly spatially and temporally distinct. Intermediate lithologies such as quartz diorite, tonalite, and granodiorite dominate in most of the Belts but granodiorite and granite characterize one. The belts are of Mesozoic or Cenozoic age and plutonism began in six of them at about 195, 175, 120, 75, 60, and 40 m.y. ago; age relations in two are poorly known. Recognition of the belts is important for future studies of regional geology, tectonism, and magmatism along the Pacific rim of southern Alaska.
Igneous and metamorphic rocks of the Serpentine Hot Springs area, Seward Peninsula, Alaska
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Map and tables describing areas of mineral resource potential, Seward Peninsula, Alaska
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Preliminary geologic map of Seward Peninsula, Alaska
This preliminary map is the first of several data compilations being prepared as a foundation for evaluating the mineral resource potential of Seward Peninsula. In addition to the sources of geologic data credited in the index map, Sainsbury's recent (1975) summary of his extensive geologic studies on Seward Peninsula has been particularly helpful. The map, unit descriptions, and correlations reflect as closely as possible the geology as published in the principal sources of geological data. Important sources of additional and more detailed descriptive data are referenced in the description of map units.
Genesis of a zoned granite stock, Seward Peninsula, Alaska
A composite epizonal stock of biotite granite has intruded a diverse assemblage of metamorphic rocks in the Serpentine Hot Springs area of north-central Seward Peninsula, Alaska. The metamorphic rocks include amphibolite-facies orthogneiss and paragneiss, greenschist-facies fine-grained siliceous and graphitic metasediments, and a variety of carbonate rocks. Lithologic units within the metamorphic terrane trend generally north-northeast and dip moderately toward the southeast. Thrust faults locally juxtapose lithologic units in the metamorphic assemblage, and normal faults displace both the metamorphic rocks and some parts of the granite stock. The gneisses and graphitic metasediments are believed to be late Precambrian in age, but the carbonate rocks are in part Paleozoic. Dating by the potassium-argon method indicates that the granite stock is Late Cretaceous. The stock has sharp discordant contacts, beyond which is a well-developed thermal aureole with rocks of hornblende hornfels facies. The average mode of the granite is 29 percent plagioclase, 31 percent quartz, 36 percent K-feldspar, and 4 percent biotite. Accessory minerals include apatite, magnetite, sphene, allanite, and zircon. Late-stage or deuteric minerals include muscovite, fluorite, tourmaline, quartz, and albite. The stock is a zoned complex containing rocks with several textural facies that are present in four partly concentric zones. Zone 1 is a discontinuous border unit, containing fine- to coarse-grained biotite granite, that grades inward into zone 2. Zone 2 consists of porphyritic biotite granite with oriented phenocrysts of pinkish-gray microcline in a coarse-grained equigranular groundmass of plagioclase, quartz, and biotite. It is in sharp, concordant to discordant contact with rocks of zone 3. Zone 3 consists of seriate-textured biotite granite that has been intruded by bodies of porphyritic biotite granite containing phenocrysts of plagioclase, K-feldspar, quartz, and biotite in an aplitic groundmass. Flow structures, pegmatite and aplite segregations, and miarolitic cavities are common in the seriate-textured granite. Zone 4, which forms the central part of the complex, consists of fine- to medium-grained biotite granite and locally developed leucogranite. Small miarolitic cavities are common within it. Eight textural facies have been defined within the complex, and mineralogic, petrographic, modal, and chemical variations are broadly systematic within the facies sequence. Study of these variations shows that the gradational facies of zones l and 2 systematically shift toward more mafic compositions inward within the complex. Seriate-textured rocks of zone 3 are similar in composition to those of zone 2, but porphyritic rocks of zone 3 and rocks of zone 4 mark shifts to more felsic compositions. These late-crystallizing felsic rocks are products of an interior residual magma system. This system was enriched in water and certain trace elements including tin, lithium, niobium, lead, and zinc. The complex as a whole has higher concentrations of these elements than many other granites. The nature of this geochemical specialization is particularly well demonstrated by the trace-element composition of biotite. The crystallization history of the pluton was complex. The available data suggest that this history could have included: (1) chilling and metasomatic alteration adjacent to the contact, (2) in-situ crystallization in several marginal facies accompanied by some transfer of residual constituents toward interior parts of the pluton, (3) slight upward displacement of magma that was subjacent to the crystallized walls, accompanied by disequilibrium crystallization and local vapor saturation, (4) upward displacement of part of the residual water-rich interior magma, accompanied by rapid loss of a separated vapor phase, and (5) displacement of the margins of the pluton by normal faults, accompanied by loss of an exsolved vapor phase from th
Intrusive rocks of the Yakutat-St. Elias area, south-central Alaska
Twenty-three plutons, exposed over a total area of nearly 1200 km 2 , have been studied in the Alaska part of the St. Elias Mountains between long 138° and 141°W. Results of potassium-argon age determinations combined with field relations, petrography, and major- and trace-element chemistry suggest six major intrusive events: (1) late Paleozoic gabbro to quartz diorite intruded Paleozoic metamorphic rocks that are probably equivalent to the Kaskawulsh Group in adjacent areas of Canada, (2) Triassic quartz diorite formed one small pluton in undated metamorphic rocks near Mt. St. Elias, (3) Jurassic tonalite and granite intruded upper Paleozoic(?) and lower Mesozoic(?) metamorphic rocks, (4) Late Cretaceous or Tertiary altered tonalite formed three widely separated plutons in metasedimentary rocks of Jurassic(?) and Cretaceous age in the Yakutat Group, (5) Eocene granodiorite and granite, and (6) late Cenozoic tonalite and granodiorite intruded both the Yakutat Group and upper Paleozoic(?) and lower Mesozoic(?) metamorphic rocks. The Paleozoic, Jurassic, and Cretaceous or Tertiary plutonic suites are restricted to particular geologic terranes, and the Jurassic and Eocene suites correlate with regional plutonic belts present elsewhere in southern Alaska. The distribution of the Tertiary plutons does not require large-scale horizontal displacements along the Fairweather and other major high-angle faults. The available data indicate that the mineral resource potential of the Yakutat-St. Elias area is low for those deposits that are generally related to magmatic processes.
Metamorphic rocks of the Yakutat-St. Elias area, south-central Alaska
Metamorphic rocks in the Yakutat-St. Elias area range in grade from zeolite to amphibolite facies. Radiometric age determinations on selected metamorphic rocks have helped to identify two major metamorphic events, one in Late Cretaceous time that was characterized by penetrative deformation, and one in the Miocene Epoch that resulted in widespread recrystallization and igneous intrusions. Mapped facies units show many discontinuities across major faults, and their distribution indicates major differential uplift between several structurally bound crustal blocks within a strongly deformed mobile belt west of the Art Lewis fault. Late Cretaceous metamorphic gradients indicate that the blocks have been tilted to the west or northwest parallel to their general structural grain. As mineral assemblages that were recrystallized during Miocene time occur on both sides of the Fairweather fault, large-scale, post-Miocene strike-slip movement along the fault is not indicated.