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The Late Cretaceous Middle Fork Caldera, Its Resurgent Intrusion, and Enduring Landscape Stability in East-Central Alaska

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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 Charles R. Bacon1, Cynthia Dusel-Bacon2, John N. Aleinikoff3, and John F. Slack4 1U.S. Geological Survey, Volcano Science Center, 345 Middlefi eld Road, MS 910, Menlo Park, California 94025-3561, USA 2U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, 345 Middlefi eld Road, MS 901, Menlo Park, California 94025-3561, USA 3U.S. Geological Survey, Central Mineral and Environmental Resources Science Center, MS 963, P.O. Box 25585, Denver Federal Center, Denver, Colorado 80225-0585, USA 4U.S. Geological Survey, Eastern Mineral and Environmental Resources Science Center, 12201 Sunrise Valley Drive, MS 954, Reston, Virginia 20192-0002, USA ABSTRACT and provides evidence of mafi c magma and of the Middle Fork block, the Mount Veta thermal input. block has been uplifted suffi ciently to expose Dissected caldera structures expose thick The Middle Fork is a relatively well pre- a ca. 68–66 Ma equigranular granitic pluton. intracaldera tuff and, uncommonly, co genetic served caldera within a broad region of Farther to the southeast, in the Kechumstuk shallow plutons, while remnants of cor- Paleozoic metamorphic rocks and Meso- block, the fl at-lying outfl ow tuff remnant in relative outfl ow tuffs deposited on the pre- zoic plutons bounded by northeast-trending Gold Creek and a regionally extensive high eruption ground surface record elements of faults. In the relatively downdropped and terrace indicate that the landscape there has ancient landscapes. The Middle Fork caldera less deeply exhumed crustal blocks, Creta- been little modifi ed since 70 Ma other than encompasses a 10 km × 20 km area of rhyo- ceous–Early Tertiary silicic volcanic rocks entrenchment of tributaries in response to lite welded tuff and granite porphyry in east- attest to long-term stability of the landscape. post–2.7 Ma lowering of base level of the central Alaska, ~100 km west of the Yukon Within the Middle Fork caldera, the granite Yukon River associated with advance of border. Intracaldera tuff is at least 850 m porphyry is interpreted to have been exposed the Cordilleran ice sheet. thick. The K-feldspar megacrystic granite by erosion of thick intracaldera tuff from an porphyry is exposed over much of a 7 km × asymmetric resurgent dome. The Middle INTRODUCTION 12 km area having 650 m of relief within the Fork of the North Fork of the Fortymile western part of the caldera fi ll. Sensitive River incised an arcuate valley into and Large calderas are the sources of voluminous high-resolution ion microprobe with reverse around the caldera fi ll on the west and north ignimbrites and contain vast thicknesses of geometry (SHRIMP-RG) analyses of zircon and may have cut down from within an orig- intracaldera tuff. Silicic magma of such ignim- from intracaldera tuff, granite porphyry, inal caldera moat. The 70 Ma land surface brites is widely considered to be related to more and outfl ow tuff yield U-Pb ages of 70.0 ± is preserved beneath proximal outfl ow tuff voluminous crystal-rich magma or mush that 1.2, 69.7 ± 1.2, and 71.1 ± 0.5 Ma (95% confi - at the west margin of the caldera structure eventually solidifi es as a pluton (Smith, 1979; dence), respectively. An aeromagnetic survey and beneath welded outfl ow tuff 16–23 km Bachmann et al., 2007; de Silva and Gregg, indicates that the tuff is reversely magnetized, east-southeast of the caldera in a paleovalley. 2014). Yet directly relating an ignimbrite to a and, therefore, that the caldera-forming erup- Within ~50 km of the Middle Fork caldera specifi c pluton is complicated by failure of ero- tion occurred in the C31r geomagnetic polar- are 14 examples of Late Cretaceous (?)–Ter- sion to both preserve volcanic rocks and expose ity chron. The tuff and porphyry have arc tiary felsic volcanic and hypabyssal intru- subjacent plutons and by processes that affect geochemical signatures and a limited range in sive rocks that range in area from <1 km2 plutons between the time of an eruption and 2 SiO2 of 69 to 72 wt%. Although their pheno- to ~100 km . Rhyolite dome clusters north when they eventually solidify. Deep erosion of crysts differ in size and abundance, similar and northwest of the caldera occupy tectonic caldera structures may expose shallow plutons, quartz + K-feldspar + plagioclase + biotite basins associated with northeast-trending but commonly those plutons are signifi cantly mineralogy, whole-rock geochemistry, and faults and are relatively little eroded. Lava of younger, and thus they are diffi cult to relate analytically indistinguishable ages indicate a latite complex, 12–19 km northeast of the directly to ignimbrites (e.g., Questa, Zimmerer that the tuff and porphyry were comagmatic. caldera, apparently fl owed into the paleo- and McIntosh, 2012). However, resurgence of Resorption of phenocrysts in tuff and por- valley of the Middle Fork of the North Fork unerupted magma commonly produces struc- phyry suggests that these magmas formed by of the Fortymile River. To the northwest tural doming of caldera fl oors soon after volu- thermal rejuvenation of near-solidus or solidi- of the Middle Fork caldera, in the Mount minous eruption and caldera collapse (Smith fi ed crystal mush. A rare magmatic enclave Harper crustal block, mid-Cretaceous plu- and Bailey, 1968). A few caldera structures are (54% SiO2, arc geochemical signature) in the tonic rocks are widely exposed, indicating conveniently eroded to expose resurgent intru- porphyry may be similar to parental magma greater total exhumation. To the southeast sions but not to have removed intracaldera tuff Geosphere; December 2014; v. 10; no. 6; p. 1432–1455; doi:10.1130/GES01037.1; 16 fi gures; 3 tables; 2 supplemental fi les. Received 5 February 2014 ♦ Revision received 6 August 2014 ♦ Accepted 15 October 2014 ♦ Published online 12 November 2014 1432 For permission to copy, contact [email protected] © 2014 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/6/1432/3339775/1432.pdf by guest on 29 September 2021 The Late Cretaceous Middle Fork caldera, its resurgent intrusion, and landscape stability (e.g., Grizzly Peak, Fridrich et al., 1991). The being mid-Cretaceous (Bacon et al., 1990; Bacon et al., 2009). Many of the identifi ed Middle Fork caldera (Bacon and Lanphere, Mortensen, 2008). Preservation of thick caldera occurrences of sulfi de mineralization are associ- 1996), named for the Middle Fork of the North fi ll and, locally, of outfl ow tuff indicates that the ated with igneous rocks similar in age to those Fork of the Fortymile River, is such a resurgent Middle Fork caldera has not been subjected to of the Middle Fork caldera (Day et al., 2014). caldera structure that encompasses a 10 km × major uplift and deep erosion since the caldera- The intracaldera tuff and the granite porphyry 20 km area of rhyolite welded tuff and granite forming eruption ca. 70 Ma. illustrate what may have existed, prior to exhu- porphyry ~100 km west of the Yukon border The Fortymile mining district, which includes mation, above granitic plutons exposed nearby (Figs. 1 and 2). The Middle Fork caldera is situ- the Middle Fork caldera and much of the Eagle and throughout the district. ated within a broad region of Alaska and adja- and Tanacross 1° × 3° quadrangles, has long Volcanic rocks of the Middle Fork caldera cent Yukon within the Yukon–Tanana Upland been known for occurrences of placer gold and and vicinity were mapped in reconnaissance that contains Late Triassic, Early Jurassic, base- and precious-metal sulfi des (Yeend, 1996; by Foster (1976). On the basis of this geologic and Cretaceous plutons and, in the less deeply Werdon et al., 2004, and references therein). map, and on thin sections and fi eld notes of Fos- exhumed blocks, silicic volcanic rocks. Among The region south of the caldera to the Mosquito ter and coworkers, Bacon et al. (1990) identi- the identifi ed caldera structures, the Middle Fork of the Fortymile River is one of active fi ed the caldera structure. Bacon and Lanphere Fork is the only one known to be of latest Cre- exploration for Zn-Pb-Ag-Cu-Au skarn and (1996) presented an overview of the geology taceous age, the others that have been dated carbonate-replacement sulfi de deposits (Dusel- of the caldera and reported a 40Ar/39Ar biotite Arctic Bering Ocean Strait B ro ok s R ange r ve YukonY RiverRi uk on TintinaTi YukonYu nt kon i RiverR na i DenaliDe YukonY FaultF v n e ali u a FaultF r a u ul FairbanksFairbanks TananaT k t l a o t n la a n A sk a n R a –Tanana– Upland a T n a g n e a AnchorageAnchorage RiverR i n v a e DawsonDawson r W U r a p n l M a g ac M n k e e Gulf d o l n l z of u – i n S e t a Alaska a i WhitehorseWhitehorse M i n n t o s u E n l i t a a s i Pacific n s Ocean Figure 1. Relief map of continental Alaska and adjacent Canada showing Denali and Tintina dextral faults, Yukon and Tanana Rivers, and location of Middle Fork caldera (yellow star). Adapted from Duk-Rodkin et al. (2010, Fig. 1) © Canadian Science Publishing or its licensors. Geosphere, December 2014 1433 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/6/1432/3339775/1432.pdf by guest on 29 September 2021 Bacon et al.
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