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At least 73 records · Page 4Linked to original sources
USGS1982

Fission-track ages of late Cenozoic distal tephra beds in the Yukon Territory and Alaska

Six distal tephra beds from the Yukon Territory and Alaska have been dated by the fission-track method. Zircon and glass ages were determined for the Fort Selkirk and Lost Chicken tephra beds, but only glass ages for the others.Assuming that no track fading has occurred in the glass, Old Crow and Dawson tephra beds are younger than 120 000 and 52 000 years BP, respectively. Mosquito Gulch tephra is 1.22 Ma old, Fort Selkirk tephra is about 1 Ma old, the Ester Ash Bed is 0.45 Ma old, and the best estimate of the age of Lost Chicken tephra is the range 1.7–2.6 Ma.It is evident from these results and from the known abundance of tephra beds within late Cenozoic deposits of the Yukon Territory and Alaska that application of the fission-track method to distal tephra, in conjunction with detailed characterization studies, offers great potential for elucidation of the late Cenozoic geologic history of Alaska and the Yukon Territory.

Alaska, Yukon Territory↗
USGS1983

Holocene history of the estuarine area surrounding Portage, Alaska, as recorded in a 93 m core

A 93 m core obtained at Portage, Alaska records four prograding cycles of estuarine deposition for the past 8230 ± 100 years. Analyses of texture, mineralogy, paleontology, and sedimentary structures enable definition of eight lithologic units. Mineralogic studies show that past and present sedimentation at Portage has been largely mud and sand from the Susitna River on the northwest side of Cook Inlet. Six radiocarbon dates from concentrated organic debris in the core and on the surface enable determination of sedimentation rates for four intervals and show rates to be higher and to vary more at depth than rates nearer the surface. The glacier-carved Turnagain Arm fiord has been receiving sediment for at least the past 14 000 years, as estimated by extrapolation of calculated sedimentation rates to the thickness of unconsolidated sediment drilled at Portage, which totals approximately 300 m. Presently, deposition is accomplished "instantly" as the result of tectonic subsidence and compaction of sediment caused by periodic earthquakes, in combination with a turbid estuarine system.

Alaska↗
USGS1983

Distribution of mineral deposits in accreted terranes and cratonal rocks of western United States

The western margin of the conterminous United States, covering roughly 300 000 mi 2 (777 000 km 2 ), is an agglomeration of tectonostratigraphic terranes accreted to the North American craton mainly during Mesozoic time. The terranes represent a number of fundamental crustal types: oceanic crust, island-arc crust, mélange, various combinations of the preceding three, batholithic, miogeoclinal, and platform.The distribution patterns of types of mineral deposits show that miogeoclinal terranes of the craton are characterized by replacement and vein-type lead–zinc–silver, skarn tungsten deposits, molybdenum, and tin, whereas accreted terranes contain all the known volcanogenic massive sulfide deposits, all chromite and chert-associated manganese, and all the large gold quartz-vein deposits, except Goldfield, Nevada. Carlin-type disseminated fine-grained gold deposits occur mostly in windows of Paleozoic miogeoclinal rocks in Nevada, but the only known fine-grained gold deposit in California is in very youthful volcanic rocks overlying oceanic-crust terrane. Large bedded-type barite deposits, although in the same general area and showing the same general trend as fine disseminated gold deposits in Nevada, are in allochthonous oceanic terrane. Mercury and antimony are dominantly in accreted terranes but antimony, in particular, also forms important deposits in cratonal rocks. Most of the large iron deposits are in the craton but a few are in accreted island-arc terranes.

California, Idaho, Nevada, Oregon, Utah, Washingto↗
USGS1983

Correlation of metal occurrence and terrane attributes in the northwestern conterminous United States

The regional distribution patterns for 20 critical metals derived by an areal-pattern recognition technique leads me to conclude that metals are regularly distributed preferentially in the accreted and cratonic terranes studied in and surrounding the state of Oregon in the northwestern United States. The distribution patterns along distinctively different zones differ in these contrasting crusts across a concealed boundary that is established by strontium isotope data. Copper and gold appear to be abundant in both terranes; nickel, cobalt, chromium, and the platinum-group metals predominate in accreted terranes; lead, zinc, tin, and tungsten are concentrated locally along the boundary zone along the edge and in the interior of the cratonic terrane. The association of metals with accreted terranes and with restricted areas or lineaments in the craton further supports the metallogenetic hypothesis that metal materials are derived mainly from underlying ensialic and ensimatic crusts, and anticipates the possibility for remobilization of these materials along the same broad linear zones over geologic time.

California, Idaho, Montana, Nevada, Oregon, Utah, ↗
USGS1984

Paleogene geology and chronology of southwestern Umnak Island, Aleutian Islands, Alaska

The rocks of southwestern Umnak Island (Nikolski area) consist mainly of a slightly deformed marine sedimentary sequence intruded by hypabyssal quartz diorite sills and small plutons. Laminated and thin-bedded argillaceous and tuffaceous mudstone and siltstone reflect volcanic arc sedimentation from late Eocene to early Oligocene time. Zeolite facies minerals formed during diagenesis include heulandite, analcime, quartz, chlorite, and smectite. Radiometric dates from sill rock and plutons yield apparent ages of about 30 Ma, i.e, middle Oligocene. Chemical data from igneous rocks exhibit calc-alkaline and tholeiitic volcanic arc differentiation trends. Dikes of pyroxene basalt yield K/Ar apparent ages as old at 17 Ma, and a columnar-jointed plug of pyroxene basalt is as young as 1 Ma. The fossil ages and radiometric dates from southwestern Umnak Island are similar to those reported from other central and eastern Aleutian islands, and indicate uniformity in the chronology and tectonic development of the archipelago during the Paleogene. Paleomagnetic data suggest possible northward movement but remain equivocal and more work is indicated.

Alaska↗
USGS1985

A heat-flow reconnaissance of southeastern Alaska

Heat flow was measured at nine sites in crystalline and sedimentary rocks of southeastern Alaska. Seven of the sites, located between 115 and 155 km landward of the Queen Charlotte – Fairweather transform fault, have an average heat flow of 59 ± 6 mW m −2 . This value is significantly higher than the mean of 42 mW m −2 in the coastal provinces between Cape Mendocino and the Queen Charlotte Islands, to the south, and is lower than the mean of 72 ± 2 mW m −2 for 81 values within 100 km of the San Andreas transform fault, even farther south. This intermediate value suggests the absence of significant heat sinks associated with Cenozoic subduction and of heat sources related to either late Cenozoic tectono-magmatic events or significant shear-strain heating. At Warm Springs Bay, 75 km from the plate boundary, an anomalously high heat flow of 150 mW m −2 can most plausibly be ascribed to the thermal spring activity from which its name is derived. At Quartz Hill, 240 km landward of the plate boundary, a value of 115 mW m −2 might indicate a transition to a province of high heat flow resulting from late Tertiary and Quaternary extension and volcanism.

Alaska↗
USGS1985

Review of radiometric data from the Yukon crystalline terrane, Alaska and Yukon Territory

The results of more than 20 years of geochronological studies in the Yukon Crystalline Terrane in east-central Alaska and the western Yukon Territory suggest at least six igneous and thermal (metamorphic?) events. Plutonism during Mississippian, Early Jurassic, mid-Cretaceous, Late Cretaceous, and early Tertiary times is indicated. Evidence also indicates that Mississippian, Early Jurassic, late Early Cretaceous, and late Cretaceous thermal (metamorphic?) events have affected parts of the terrane. The western part of the terrane was affected by a significant regional metamorphic event in late Early Cretaceous time, followed by a terrane-wide mid-Cretaceous plutonic event. The pattern of K–Ar ages allows division of the terrane into domains, bounded by northeast-trending lineaments.

Alaska, Yukon Territory↗
USGS1985

The age and origin of felsic intrusions of the Thetford Mines ophiolite, Quebec

The Thetford Mines ophiolite in southern Quebec was obducted in Early Ordovician time during the closing of the proto-Atlantic. The tectonized peridotite lower unit of the ophiolite is intruded by felsic dikes and pods including isolated lenses of massive rodingite, small bodies of strongly deformed diorite, and younger, less deformed quartz monzonite. These intrusions are found only near the base of the ophiolite, do not intrude the surrounding country rock, and are rootless; for these reasons they are considered to have been emplaced in the ophiolite before it reached its present location.The younger group of intrusions consists of biotite–muscovite quartz monzonite and leuco–quartz monzonite. Analyzed samples have high K 2 O contents, high (K 2 O × 100)/(Na 2 O + K 2 O) ratios, and high initial strontium ratios (0.7171–0.7179), indicating that the magma source region was continental and that these felsic rocks formed by partial melting of continental sediments. Whole-rock and mineral isochron ages suggest that the felsic intrusions are about 456 ± 4 Ma old and that they were metamorphosed about 418 ± 7 Ma ago.The detachment of the ophiolite occurred about 491 ± 3 Ma ago and is recorded by the age of the metamorphic aureole beneath the ophiolite. The felsic dikes were intruded some 35 Ma years later during the Taconic Orogeny. The lengthy time between detachment and final nappe emplacement recorded by the felsic dikes may be a requirement for the formation of abundant asbestiform chrysotile.

Quebec↗
USGS1986

Precambrian basement geology of North and South Dakota

Combined analysis of drill-hole, gravity, and magnetic data indicates that the buried Precambrian basement rocks of the Dakotas can be divided into several lithotectonic terranes. Eastern North Dakota and northeastern South Dakota are underlain by Archean gneiss. Except for the Black Hills region of South Dakota, where Archean rocks are also exposed, the western third of both Dakotas is underlain mainly by Early Proterozoic gneiss and metasedimentary rocks. Part of this region is underlain by Archean crust with an Early Proterozoic tectonic overprint. A broad transition zone of strongly overprinted Archean crust occurs between the Proterozoic rocks to the west and the Archean rocks to the east. South central South Dakota is underlain by an Early Proterozoic batholith. Early Proterozoic felsic volcanic rocks occur in southeast South Dakota. The bootheel portion of South Dakota contains a diverse assemblage of basement rocks that are partly Archean in age.Churchill Province rocks of the Trans-Hudson foldbelt project into the western Dakotas. The Thompson nickel belt and the Pickwitonei gneiss belt correlate with the western and eastern halves, respectively, of the transition between Archean and Proterozoic crust, and the Archean Glennie – Hanson Lake microcontinent of the Churchill Province likely extends into western North Dakota. Archean rocks of Minnesota extend into the eastern Dakotas, and the Wyoming craton extends to the Black Hills region. The Cheyenne foldbelt projects into southwest South Dakota. The Penokean foldbelt of Michigan and Wisconsin does not extend into the Dakotas, but it most likely extends into northwest Iowa.Tectonic evolution of the Early Proterozoic terrane in the Dakotas was most likely similar to plate tectonic models for the evolution of the Trans-Hudson foldbelt in the Churchill Province. As in the Churchill Province, the western Dakotas are underlain by Early Proterozoic rocks, but it is not known whether these rocks formed as a result of rifting and subsequent closure of a once extensive Archean crust or as a result of collision of once widely separated blocks of Archean crust.

North Dakota, South Dakota↗
USGS1986

Fission-track dating of the tectonic development of the San Juan Islands, Washington

The San Juan Islands of Washington State form a geologically complex province located between the north Cascades, Vancouver Island, and the Olympic Peninsula. We have obtained 53 fission-track dates from the San Juan Islands province that help constrain its late Paleozoic to early Cenozoic tectonic and sedimentary history and its relationship to neighboring geologic terranes. The San Juan Islands can be divided into two main blocks separated by the Haro fault. South of the Haro fault, complexly deformed, metamorphosed, and probably exotic early Paleozoic to early Late Cretaceous rocks form four imbricate thrust plates separated by south- and east-dipping late Early to Late Cretaceous thrust faults. Reset zircon fission-track dates indicate that thrusting may have produced an upside-down geothermal gradient in the uppermost plate, the Decatur terrane. If present, this gradient was probably produced by conductive or frictional heating associated with a now-eroded overlying thrust fault and hot thrust plate. Cretaceous thrusting in the southern San Juan Islands was accompanied by uplift and resetting of apatite fission-track dates. In contrast to correlative rocks of the southern San Juan Islands, Upper Triassic to Lower Cretaceous rocks in and north of the Haro fault zone are essentially unmetamorphosed and only broadly folded. Apatite dates from the Upper Triassic Haro Formation and the Upper Jurassic and Lower Cretaceous Spieden Group indicate they did not participate in Late Cretaceous uplift of the southern San Juan Islands. Together with their basement (the Wrangellia terrane?), these rocks probably acted as a backstop to thrusting. The synorogenic Late Cretaceous Nanaimo basin formed north of the Haro fault in front of the advancing San Juan Islands thrust system. The age of Nanaimo deposition matches uplift (apatite) dates in the southern San Juan Islands, and detrital zircons from the Nanaimo Group yield dates consistent with southern San Juan Islands sources. Burial led to resetting of apatite dates in what is probably the deeper part of the Nanaimo basin.

Washington↗
USGS1986

U-Pb geochronology of two augen gneiss terranes, Idaho: New data and tectonic implications

U–Pb isotopic analyses of cogenetic zircon suites revise the ages of two augen gneiss terranes in central and northern Idaho. The augen gneiss of Priest River yields an upper-intercept concordia age for only moderately discordant zircons of 1576 ± 13 Ma; there is no evidence of an inherited xenocrystic lead component. Because this unit probably is in basement beneath Belt–Purcell strata, it provides a maximum age for Belt and Purcell sedimentation. However, high-grade metamorphism and structural complexity permit several tectonic interpretations, all of which should be treated with caution pending further detailed studies.Zircons from the augen gneiss of Red River contain inherited radiogenic lead, but by analogy with lithologically and isotopically similar augen gneiss and porphyritic granite near Salmon and Shoup, Idaho, an age of about 1370 Ma is suggested. However, the uncertainty associated with this value is unknown and could be large. Intrusion preceded or was synkinematic with at least part of the deformation and metamorphism that affected the metasedimentary host rocks, thought by some to be Belt–Purcell strata.

Idaho↗
USGS1987

Holocene and latest Pleistocene glacial chronology, Glacier National Park, Montana.

Moraines of two different age groups have been identified fronting the present-day glaciers and snowfields in Glacier National Park, Montana. The subdued, vegetated moraines of the older group have been found at 25 sites, mainly in the central part of the Lewis Range. These older moraines are in places overlain by the Mazama ash. Although the exact age of the moraines has not been determined by radiocarbon dating, vegetative evidence and correlation with other pre-altithermal age moraines in the Rocky Mountains suggest that these older moraines date from 10 000 BP or earlier. Whether these moraines are the product of a separate advance after the end of the Wisconsin glaciation or are simply the product of the last advance or stillstand of Wisconsin glaciers before final deglaciation is not known.Moraines of the younger group, consisting of fresh bouldery rubble, are common throughout Glacier Park. Tree-ring analyses indicate that some of these younger moraines were deposited by advances that culminated during the mid-19th century. At that time there were more than 150 glaciers in Glacier Park. This episode of mid-19th century climatic cooling resulted in the most extensive glacial advance in this region since the end of the Wisconsin glaciation.Present-day glaciers have shrunk drastically from their mid-19th century positions; more than half the glaciers present during that time no longer exist. Much of this retreat occurred between 1920 and the mid-1940's, corresponding to a period of above-average summer temperatures and below-average annual precipitation in this region. Between 1966 and 1979, several of the larger glaciers in the Mount Jackson area of Glacier Park advanced as much as 100 m.

Montana↗
USGS1987

Lead isotopic fingerprinting of tectono-stratigraphic terranes, east-central Alaska

Common lead isotopic compositions have been determined on feldspars from meta-igneous rocks from nine tectono-stratigraphic terranes or subterranes in east-central Alaska. Most of the terranes have distinct and well-defined signatures in terms of isotopic composition; thus, most can be distinguished on conventional lead isotopic diagrams. Lead isotopic ratios provide evidence for (1) possible sources for the igneous rocks, (2) time of metamorphism, (3) correlation of terrane fragments, and (4) delineation of juxtaposed terranes. Determination of lead isotopic ratios from igneous rocks can be useful in characterizing tectono-stratigraphic terranes (as to mantle or crustal origin) and in correlation, particularly where terrane relationships are enigmatic.

Alaska↗
USGS1988

Ferromanganese deposits from the Gulf of Alaska Seamount Province: Mineralogy, chemistry, and origin

Ferromanganese-oxide deposits dredged from four seamounts (Welker, Miller, Murray, and Patton) in the Gulf of Alaska Seamount Province include poorly crystallized microlaminated crusts on basalt substrate, well-crystallized Mn-oxide veins in epiclastic sedimentary rocks, and crystalline Mn-oxide layers and micronodules in phosphorite. The principal rock types dredged are alkali-basalt pillow fragments and tuffaceous conglomerate and sandstone. The glassy rims of pillow fragments, the glassy goundmass of large volcanic clasts, and the tuffaceous component of the sediment are altered to palagonite. Other low-temperature alteration products include phillipsite, smectite, and carbonate-apatite.Thick (10–50 mm) Fe–Mn crusts consist mainly of δ-MnO 2 ; straight and cuspate growth laminae indicate variable growth rates and periods of nondeposition. A larger number of detrital particles toward the top of thick crusts record the increasing influence of active volcanoes of the Aleutian arc during northwestward movement of the Pacific plate. Thick crusts on basalt substrate have higher Mn/Fe ratios and lower Co content than Fe–Mn crusts from low-latitude seamounts of the central Pacific region. Thin (< 10 mm) crusts on volcaniclastic substrate contain todorokite and birnessite and have higher Mn/Fe ratios, Ni, and Cu and lower Fe and Co than thick Gulf of Alaska crusts.Veins of todorokite and cryptomelane with complex internal structure occur in altered tuffaceous sandstone and conglomerate from Miller Seamount. Fibrous todorokite has a composition similar to those of other marine examples but may contain up to 7% Mn 2+ in M2 sites. Microprobe analysis of the marine cryptomelane indicates a composition that is approximately (K,Ba) 1–2 (Mn 4+ ,Co) 7–8 O 16 ∙ x (H 2 O).A third type of Fe–Mn deposit in phosphorite is an intimate mixture of todorokite (and minor δ-MnO 2 )-bearing layers and micronodules, carbonate-apatite, and phillipsite that encloses grains of altered volcanic glass and lithic fragments.The microlaminated structure, mineralogy (predominantly δ-MnO 2 ), and composition (Mn/Fe ratio and transition metal, rare earth element, U, and Th contents) of the thick crusts are characteristic of hydrogenetic Fe–Mn crusts elsewhere in the Pacific. Conversely, the crystallinity and chemical composition of the Mn-oxide veins and thin crusts indicate formation during diagenesis of the volcanogenic sediment substrate. Mn and other transition metals are mobilized during low-temperature oxidative alteration (palagonitization) of basaltic volcanic glass; the oxidation of Fe 2+ to Fe 3+ during palagonitization and the dissolution of the dilute biogenic fraction of the sediment combine to lower the Eh of ambient pore fluid and enhance the mobility of Mn 2+ . Diagenesis in the phosphatic sandstone from Patton Seamount involves organic-rich sediment and pore waters elevated in phosphorus owing to upwelling above a large volcanic edifice.

Alaska↗
USGS1989

Multiple hydrothermal and metamorphic events in the Kidd Creek volcanogenic massive sulphide deposit, Timmins, Ontario: evidence from tourmalines and chlorites

Tourmaline and chlorite are the principal ferromagnesian silicate minerals in the Kidd Creek massive sulphide deposit. Tourmaline is most common in sphalerite-rich peripheral margins of the chalcopyrite stringer zone. Within the north orebody, samples typically contain <1% tourmaline, but small areas (hand-specimen scale) may have 10–20%. Chlorite is more widely distributed and in places constitutes 30–50% of rock volumes. Associated assemblages may include quartz, sulphides (principally chalcopyrite, sphalerite, and (or) pyrite), carbonate, albite, sericite, and rare fluorite, allanite, or zoisite(?).The tourmalines and chlorites record a series of multiple hydrothermal and metamorphic events. Paragenetic studies suggest that tourmaline was deposited during several discrete stages of mineralization, as evidenced by brecciation and cross-cutting relationships. Most of the tourmalines have two concentric growth zones defined by different colours (green, brown, blue, yellow). Some tourmalines also display pale discordant rims that cross-cut and embay the inner growth zones and polycrystalline, multiple-extinction domains. Late sulphide veinlets (chalcopyrite, pyrrhotite) transect the inner growth zones and pale discordant rims of many crystals. The concentric growth zones are interpreted as primary features developed by the main ore-forming hydrothermal system, whereas the discordant rims, polycrystalline domains, and cross-cutting sulphide veinlets reflect post-ore metamorphic processes.Detailed electron microprobe analyses of tourmalines show a wide compositional range, from Fe-rich dravite nearly to end-member schorl, with Fe/(Fe + Mg) ratios varying from 0.33 to 0.92; only minor amounts of Ca are present, yielding uniformly high Na/(Na + Ca) ratios of 0.84–0.99. Two sets of chemical zoning trends are identified in the tourmalines, involving systematic changes in Fe/(Fe + Mg), Na/(Na + Ca), Al, and Ti that are believed to reflect internal coupled substitutions (e.g.,  + Ti = Na + Al) and local mineral equilibria (e.g., tourmaline–chlorite). Analyses of the pale discordant reaction rims show consistent depletion of Fe, Ca, and Ti, presumably by fluid–solid reactions during post-ore metamorphism.Chlorites also show an extensive range in composition, from ripidolite nearly to end-member daphnite, with Fe/(Fe + Mg) ratios of 0.43–0.98 and Si cation values of 5.00–5.39. Chlorites from the fringes of the footwall stringer zone have narrow compositional ranges, whereas chlorites near footwall rhyolite sills in the core of the stringer zone display major variations in Fe/(Fe + Mg) ratios, including one sample with a range of 0.68–0.95. The former group of chlorites has Fe/(Fe + Mg) ratios that correlate well with those of coexisting tourmalines (exclusive of late reaction rims). Data for the latter group, in contrast, fall off equilibrium K D curves, indicating that the tourmalines and chlorites within these samples are not in chemical equilibrium. The chlorites are believed to have been altered (overprinted) by Fe-rich hydrothermal fluids apparently generated during intrusion of the rhyolite sills. The tourmalines, however, are unaffected and retain primary chemical signatures.Variations in mineral proportions and mineral chemistry within the deposit mainly depend on fluctuations in temperature, pH, water/rock ratios, and amounts of entrained seawater. The major proposed control is mixing between high-temperature, Fe-rich end-member hydrothermal fluids and cold, Mg-rich entrained seawater. Fe/(Fe + Mg) variations in footwall tourmalines (and equilibrium chlorites) are believed to largely reflect the progressive infiltration of Mg-rich seawater into the margins and top of the hydrothermal system. The more Fe-rich compositions of Kidd Creek tourmalines relative to those from sediment-hosted massive sulphide deposits (e.g., Sullivan, British Columbia) may be related to the preferential generation of end-member hydrothermal fluids in proximal volcanic environments like that at Kidd Creek.

Ontario↗
USGS1989

Turbidity-current channels in Queen Inlet, Glacier Bay, Alaska

Queen Inlet is unique among Glacier Bay fjords because it alone has a branching channel system incised in the Holocene sediment fill of the fjord floor. Bathymetry and seismic-reflection profiles show that four channels begin on, or at the base of, the delta front of this marine-outwash fjord. By midpoint of the fjord, these channels have merged into one main channel that extends the length of the fjord. The main channel cuts deeply into the scarp that separates this hanging tributary from the West Arm of Glacier Bay. The Queen Inlet channel ends on the main fjord floor as several small distributaries that form part of a lobate-fan deposit. Sand from the channel and lobate fan, in contrast to fjord-floor mud, plus steep truncated channel walls indicate that turbidity currents created this apparently active channel system. Queen Inlet and other known channel-containing fjords are marine-outwash fjords; the tidewater glacial fjords do not have steep delta fronts on which slides are generated and may not have a sufficient reservoir of potentially unstable coarse sediment to generate channel-cutting turbidity currents. Presence or absence of channels, as revealed in the ancient rock record, may be one criterion for interpreting types of fjords.

Alaska↗
USGS1989

Petrology and age of volcanic-arc rocks from the continental margin of the Bering Sea: Implications for Early Eocene relocation of plate boundaries

Eocene volcanic flow and dike rocks from the Beringian margin have arc characteristics, implying a convergent history for this region during the early Tertiary. The extrusive rocks are basalt, basaltic andesite, andesite, and minor dacite and rhyolite. The intrusive sample is from a quartz diorite dike intruding serpentinized peridotite. Major-element oxide contents, particularly FeO*/MgO versus SiO 2 , identify both tholeiitic and calc-alkalic basalt; more silicic lavas have calc-alkalic affinities. Consistent with volcanic-arc compositions, spidergrams show pronounced Nb–Ta depletion and alkali enrichment relative to light-rare-earth-element ( LREE ) abundance. Chondrite-normalized REE plots show relatively flat patterns, with only slight LREE enrichment for tholeiitic compositions and greater LREE enrichment and lower heavy-rare-earth-element ( HREE ) abundance for calc-alkalic compositions. The samples, particularly those with calc-alkalic compositions, are rich in plagioclase that is strongly zoned; the more silicic samples contain orthopyroxene, clinopyroxene, and primary amphibole. The quartz diorite dike contains iron-rich almandine phenocrysts that appear to be magmatic, suggesting emplacement at great depth near the base of the crust or upper mantle.Chemical and mineralogical compositions are similar to those of modern Aleutian-arc lavas. They also resemble volcanic-arc compositions from western mainland Alaska, although greater chemical diversity and a stronger continental influence are observed in the Alaskan mainland rocks.Early Eocene ages of 54.4–50.2 Ma for the Beringian samples are well constrained by conventional K–Ar ages of nine plagioclase separates and by concordant 40 Ar/ 39 Ar incremental heating and total-fusion experiments. A concordant U–Pb zircon age of 53 Ma for the quartz-diorite dike is in good agreement with the K–Ar data.Plate motion studies of the North Pacific Ocean indicate more northerly directed subduction prior to the Tertiary and a continuous belt of arc-type volcanism extending from Siberia, along the Beringian margin, into mainland Alaska. Around 56 Ma (chron 25–24), subduction changed to a more westerly direction and subduction-related volcanism ceased for most of mainland Alaska. The increasingly oblique angle of convergence should have ended subduction along the Beringian margin as well. However, consistent ages of 54–50 Ma indicate a final pulse in arc-type magmatism during this period of plate adjustment, which may be explained by three different models: (1) The northern and central part of the Beringian margin maintained a higher angle of convergence, allowing a final pulse of arc-type magmatism. (2) The rocks erupted in an early, or proto, Aleutian arc and were rafted against the continental margin along transform faults. (3) The rocks erupted along a leaky transform fault, analogous to calc-alkalic volcanism in the southern California borderland.

Alaska↗
USGS1989

Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen

The Early Proterozoic Penokean Orogen developed along the southern margin of the Archean Superior craton. The orogen consists of a northern deformed continental margin prism overlying an Archean basement and a southern assemblage of oceanic arcs, the Wisconsin magmatic terranes. The south-dipping Niagara fault (suture) zone separates the south-facing continental margin from the accreted arc terranes. The suture zone contains a dismembered ophiolite.The Wisconsin magmatic terranes consist of two terranes that are distinguished on the basis of lithology and structure. The northern Pembine–Wausau terrane contains a major succession of tholeiitic and calc-alkaline volcanic rocks deposited in the interval 1860–1889 Ma and a more restricted succession of calc-alkaline volcanic rocks deposited about 1835 – 1845 Ma. Granitoid rocks ranging in age from about 1870 to 1760 Ma intrude the volcanic rocks. The older succession was generated as island arcs and (or) closed back-arc basins above the south-dipping subduction zone (Niagara fault zone), whereas the younger one developed as island arcs above a north-dipping subduction zone, the Eau Pleine shear zone. The northward subduction followed deformation related to arc–continent collision at the Niagara suture at about 1860 Ma. The southern Marshfield terrane contains remnants of mafic to felsic volcanic rocks about 1860 Ma that were deposited on Archean gneiss basement, foliated tonalite to granite bodies ranging in age from about 1890 to 1870 Ma, and younger undated granite plutons. Following amalgamation of the two arc terranes along the Eau Pleine suture at about 1840 Ma, intraplate magmatism (1835 Ma) produced rhyolite and anorogenic alkali-feldspar granite that straddled the internal suture.

Wisconsin↗