Major oxide, minor oxide, trace element, rare-earth element, and geochemical data from rocks collected in Eagle and Tanacross quadrangles, Alaska in 2000

Werdon, M. B.; Szumigala, D. J.; Newberry, R. J.; Grady, J. C.; Munly, W. C.

doi:10.14509/2729

Cited by 2 publications

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“…6), but no samples or fi eld notes are available. Close association of rhyolite and alkali basalt (Werdon et al, 2000) in the four ~3-10 km 2 areas of unit Tf near Bullion Creek 26-33 km east-northeast of the caldera (Fig. 15, loc.…”

Section: Other Cretaceous-early Tertiary Felsic Volcanicsmentioning

confidence: 99%

“…8). Werdon et al (2000) described the tiny hilltop Tf patch south of the head of Montana Creek as biotite-bearing, crystal-lithic tuff and reported compositions that broadly resemble those of welded tuffs of the Middle Fork caldera; thus, it is conceivable that this locality preserves outfl ow tuff from that source. Six small areas of Tf are mapped (Foster, 1976) south of the Middle Fork caldera (Fig.…”

Section: Other Cretaceous-early Tertiary Felsic Volcanicsmentioning

confidence: 99%

“…Alkali rhyolite (72.5) n.a. Lava remnants Northeastern exposures devitrifi ed porphyritic rhyolite with collapsed pumice clasts and relict perlitic texture (72.5 wt% SiO 2 ) associated with basalt (52.3 wt% SiO 2 ); southwestern beneath basalt (50.5 wt% SiO 2 ); northwestern possibly intrusive; chemical analyses from Werdon et al (2000). 7…”

Section: Landscape Evolutionmentioning

confidence: 99%

“…High-silica rhyolite (78) Qz Tuff Southeastern exposures associated with alkali basalt (50.0 wt% SiO 2 ); chemical analyses from Werdon et al (2000). 8…”

Section: Landscape Evolutionmentioning

confidence: 99%

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The Late Cretaceous Middle Fork caldera, its resurgent intrusion, and enduring landscape stability in east-central Alaska

et al. 2014

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Dissected caldera structures expose thick intracaldera tuff and, uncommonly, co genetic shallow plutons, while remnants of correlative outfl ow tuffs deposited on the preeruption ground surface record elements of ancient landscapes. The Middle Fork caldera encompasses a 10 km × 20 km area of rhyolite welded tuff and granite porphyry in eastcentral Alaska, ~100 km west of the Yukon border. Intracaldera tuff is at least 850 m thick. The K-feldspar megacrystic granite porphyry is exposed over much of a 7 km × 12 km area having 650 m of relief within the western part of the caldera fi ll. Sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) analyses of zircon from intracaldera tuff, granite porphyry, and outfl ow tuff yield U-Pb ages of 70.0 ± 1.2, 69.7 ± 1.2, and 71.1 ± 0.5 Ma (95% confidence), respectively. An aeromagnetic survey indicates that the tuff is reversely magnetized, and, therefore, that the caldera-forming eruption occurred in the C31r geomagnetic polarity chron. The tuff and porphyry have arc geochemical signatures and a limited range in SiO 2 of 69 to 72 wt%. Although their phenocrysts differ in size and abundance, similar quartz + K-feldspar + plagioclase + biotite mineralogy, whole-rock geochemistry, and analytically indistinguishable ages indicate that the tuff and porphyry were comagmatic. Resorption of phenocrysts in tuff and porphyry suggests that these magmas formed by thermal rejuvenation of near-solidus or solidifi ed crystal mush. A rare magmatic enclave (54% SiO 2 , arc geochemical signature) in the porphyry may be similar to parental magma and provides evidence of mafi c magma and thermal input.The Middle Fork is a relatively well preserved caldera within a broad region of Paleozoic metamorphic rocks and Mesozoic plutons bounded by northeast-trending faults. In the relatively downdropped and less deeply exhumed crustal blocks, Cretaceous-Early Tertiary silicic volcanic rocks attest to long-term stability of the landscape. Within the Middle Fork caldera, the granite porphyry is interpreted to have been exposed by erosion of thick intracaldera tuff from an asymmetric resurgent dome. The Middle Fork of the North Fork of the Fortymile River incised an arcuate valley into and around the caldera fi ll on the west and north and may have cut down from within an original caldera moat. The 70 Ma land surface is preserved beneath proximal outfl ow tuff at the west margin of the caldera structure and beneath welded outfl ow tuff 16-23 km east-southeast of the caldera in a paleovalley. Within ~50 km of the Middle Fork caldera are 14 examples of Late Cretaceous (?)-Tertiary felsic volcanic and hypabyssal intrusive rocks that range in area from <1 km 2 to ~100 km 2 . Rhyolite dome clusters north and northwest of the caldera occupy tectonic basins associated with northeast-trending faults and are relatively little eroded. Lava of a latite complex, 12-19 km northeast of the caldera, apparently fl owed into the paleovalley of the Middle Fork of the North Fork of the Fortymile River. To...

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Section: Other Cretaceous-early Tertiary Felsic Volcanicsmentioning

confidence: 99%

Section: Other Cretaceous-early Tertiary Felsic Volcanicsmentioning

confidence: 99%

Section: Landscape Evolutionmentioning

confidence: 99%

“…High-silica rhyolite (78) Qz Tuff Southeastern exposures associated with alkali basalt (50.0 wt% SiO 2 ); chemical analyses from Werdon et al (2000). 8…”

Section: Landscape Evolutionmentioning

confidence: 99%

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The Late Cretaceous Middle Fork caldera, its resurgent intrusion, and enduring landscape stability in east-central Alaska

et al. 2014

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Geochemistry, petrography, and zircon U–Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon–Tanana terrane

2013

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We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

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Major oxide, minor oxide, trace element, rare-earth element, and geochemical data from rocks collected in Eagle and Tanacross quadrangles, Alaska in 2000

Abstract: Note: This report (including all analytical data, tables, and map sheets) is available in digital format from the DGGS web site (http://wwwdggs.dnr.state.ak.us) at no charge. The digital data are available as PDF files and Excel spreadsheets.

Cited by 2 publications

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The Late Cretaceous Middle Fork caldera, its resurgent intrusion, and enduring landscape stability in east-central Alaska

The Late Cretaceous Middle Fork caldera, its resurgent intrusion, and enduring landscape stability in east-central Alaska

Geochemistry, petrography, and zircon U–Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon–Tanana terrane

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