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, cogenetic 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

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 Alask a n 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

i t

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.

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Mount Veta

N Mount Veta Intracaldera block tuff U D Middle Fork caldera M id Granite dl e porphyry F ork of the rth Fork of t N o h e Fortymile R iver Intracaldera tuff

Middle Fork D Mount block Harper U block

Outflow tuff

Figure 2. Perspective view of Middle Fork caldera and vicinity (Google earthTM image).

age of 69.10 ± 0.19 (1σ) Ma for basal outfl ow preted to be early Pleistocene in age and termed zircon geochronology. We also summarize what tuff. In 2010, we revisited the Middle Fork cal- the Charley River advance. According to Weber is known about mapped Cretaceous (?)–Early dera to check key relationships and sample for and Wilson (2012), the peaks within the intra- Tertiary felsic volcanic rocks within 50 km of geochemistry and additional geochronology. caldera tuff near its southern limit, as well as the the caldera. We refi ne initial interpretations of Although exposures are more extensive than in Mount Veta area to the southeast, were sources the caldera geology, igneous geochemistry, and lower-elevation portions of the Fortymile dis- of small alpine glaciers of the early (?) Wiscon- geomorphic signifi cance in the context of the trict, solifl uction debris, colluvium, and vegeta- sin Eagle advance, and all of the major stream geologic history of the Fortymile region and tion obscure much of the fi ll and the wall rocks valleys contain glacial deposits of the Charley the long-lived, post–Late Cretaceous land sur- of the caldera. Traverses of ridge tops reveal River advance. However, glacial erosion was face in this part of east-central Alaska. mainly areas of rubble interspersed between insuffi cient to produce cirques or extensive tundra and low vegetation. True outcrops are cliff exposures in the vicinity of the Middle GEOLOGY common locally at higher elevations and, rarely, Fork caldera. along margins of the valley of the Middle Fork This paper presents new data that we use to Regional Context of the North Fork of the Fortymile River. reconstruct the Middle Fork caldera, includ- Weber (1986) recognized deposits of six gla- ing the extent of granite porphyry within the The Middle Fork caldera lies within a region cial advances in the Yukon–Tanana Upland, the caldera structure, major- and trace-element of Paleozoic metamorphic rocks and Mesozoic oldest and most extensive of which she inter- geochemistry of the igneous rocks, and U-Pb plutons cut by northeast-trending, high-angle

1434 Geosphere, December 2014

faults (Dusel-Bacon et al., 2006, 2009) (Fig. 3). 2013). To the northwest of the caldera, across by northeast-trending faults. This crustal block These regional faults are considered to have left- the Mount Harper fault zone in the Mount contains three mid-Cretaceous calderas identi- lateral strike-slip displacement, antithetic to the Harper block, mid-Cretaceous granitic rocks are ﬁ ed by Bacon et al. (1990), the ages of which throughgoing Denali and Tintina right-lateral , widely exposed, consistent with greater uplift were suggested by Mortensen (2008) to be ca. strike-slip faults (Fig. 1; Page et al., 1995; and exhumation. Southeast of the Middle Fork 108 Ma, similar in age to plutonic rocks to the O’Neill et al., 2010). Many of the northeast- block, the upthrown Mount Veta block exposes southeast in Alaska and Yukon. trending faults also have a component of vertical the same Paleozoic metamorphic units cut by offset (Dusel-Bacon and Murphy, 2001; Siron Early Jurassic and Late Cretaceous granitoids. Geologic Units of the Middle Fork Caldera et al., 2010). The Middle Fork caldera (Figs. 3 Southeast of the Kechumstuk fault zone, the and 4) occurs in the Eagle 1° × 3° quadrangle in geology of the Kechumstuk block is dominated Intracaldera Tuff a downdropped block, here termed the Middle by Early Jurassic and Late Triassic plutons and The dimensions of the caldera structure are Fork block, between northeast-trending faults in Paleozoic amphibolite-facies metamorphic deﬁ ned by the presence of intracaldera welded which the exposed crustal level is dominated by rocks. In the Tanacross 1° × 3° quadrangle, tuff (unit Kr, Fig. 4) because erosion has pro- Paleozoic greenschist- and amphibolite-facies ~60–100 km south of the area of Figure 3, a ceeded to a depth below the original topographic metamorphic rocks intruded by Early Jurassic larger area of felsic volcanic and Paleozoic caldera basin. The densely welded intracaldera and Cretaceous granitoids (Dusel-Bacon et al., metamorphic rocks (Foster, 1970) is bounded tuff has ≤4 mm quartz and feldspar phenocrysts,

64°25′N

EisenmengerEise Fork nme nger For k U D Middle Fork Mount caldera Mount Harper Veta block block U D 64°15′N D MountMount Middle HHarperarper U e n rk zo o Fork GoldG lt ForkF o au ld r f block pe MountMount ar H VetaVeta Tuff in valley t e CreekC n n of Gold Creek re ou o ek zone MountM Harper fault zone z ltlt u Kechumstuk fa

MollyM

r o u DiamondDiamond t K block e l e l stuks fault zone ch v y u MountainMountain i m m R Middle u s ly h tu a Creek c u He k C echumstuke fa KechuK r e e k

Creek

0 5 10 MILES KKechumstukechumstuk 0 5 10 15 KILOMETERS MMountainountain 64°00′N 144°00′W 143°30′W 143°00′W 142°30′W EXPLANATION Quaternary Mid-Cretaceous Paleozoic Alluvium and colluvium Granite and granodiorite Qtz-mica schist, quartzite, marble, greenschist

Tertiary Jurassic Bt schist, quartzite, marble, amphibolite Intermediate volcanic rocks Granodiorite, monzonite, Augen gneiss and orthogneiss Late Cretaceous Triassic and granite Felsic gneiss and granitoids, undivided Felsic volcanic rocks Granodiorite, tonalite, Fault–high angle, dashed where inferred, dotted Granite and quartz monzodiorite where concealed Fault–thrust, dashed where inferred, dotted where concealed

Figure 3. Generalized geologic map of the southwestern part of the Eagle 1° × 3° quadrangle. Geology modiﬁ ed after Foster (1976), Dusel- Bacon et al. (2013), and Day et al. (2014).

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143° 143° 15′W Pzm 00′W Pzm Mount ortymile Harper f the F Rive Kgd rk o r Qu block Fo Pzm h rt o Pzm N 69.9 e Jg th ±0.5 Ma f o rk 64° Kro o 64° 20′N F 20′N

e l Pzm d 69.7 Middle

d U i ±1.2 Ma D M Kgp Fork Kgp Kr caldera Kro 70.0 Kb? Mount ±1.2 Ma Veta Kb Kr block Kgd Qu Pzm Kb? Jg

64° 64° 15′N 15′N Pzm Pzm Middle Fork 65.8 Kg ±1.5 Ma block D U 67.9 65.8 ±1.4 Ma Pzm ±1.1 Ma 0 5 10 km Pzm Jg Mt. Jg Pzm Veta 143° 143° 15′W 00′W

EXPLANATION Late Cretaceous Middle Fork caldera Mid-Cretaceous Quaternary Kgp Resurgent granite porphyry Kgd Granite and granodiorite Qu Unconsolidated deposits Jurassic Kr Intracaldera rhyolite tuff Late Cretaceous Jg Granodiorite, monzonite, and granite Kb Intracaldera megabreccia Kg Granite Paleozoic Kro Outflow rhyolite tuff Pzm Metamorphic rocks Biotite 40Ar/39Ar age High-angle fault, dashed where inferred, dotted where concealed Zircon SHRIMP U–Pb age Contact, dashed where approximate

Figure 4. Geologic map of the Middle Fork caldera and vicinity. Geology modiﬁ ed after Foster (1976) and Day et al. (2014). Radiometric age uncertainties are given at the 95% conﬁ dence level. SHRIMP—sensitive high-resolution ion microprobe.

cm-sized fíamme visible in thin section and on of the structurally subsided caldera block. The The contact location is particularly uncertain in some weathered joint surfaces, and common original topographic caldera would have been the low-elevation vegetated country on the north lithic fragments that typically are ≤1–2 cm. larger, to the extent that it may not have been and east. There, and west of the high peaks on Maximum exposed thickness of intracaldera ﬁ lled by ponded intracaldera tuff, owing to the south, it is unclear whether mapped meta- tuff, measured from the deepest valley to the inward landsliding of the unstable caldera walls morphic rocks are in fact outside of the caldera highest peak, is 850 m (Fig. 5A). At a minimum, (Lipman, 1997). Contacts of intracaldera tuff or are instead wall-rock blocks within mega- the extent of intracaldera tuff indicates the area with wall rocks are well deﬁ ned only locally. breccia in which the tuff matrix between blocks

1436 Geosphere, December 2014

Intracaldera A welded tuff, D south-center of N caldera U D Resurgent granite porphyry Base of outflow tuff

M idd le Fo rk of th e North Fork of the Fortymile River

Intracaldera welded tuff at western margin of caldera E B

Marble megabreccia block in welded tuff near eastern margin of caldera

F Resurgent granite Intracaldera Jurassic C porphyry welded tuff monzonite

tflow tuff Base of ou Resurgent granite porphyry

Fortymile River k of the Middle Fork of the North For

Figure 5. Field photographs. (A) View east from near western margin of intracaldera tuff, across Fortymile River to resurgent granite porphyry and high country of densely welded intracaldera tuff at south margin of Middle Fork caldera. (B) Part of a >100 m block of Paleozoic marble surrounded by welded tuff (not visible) near eastern margin of Middle Fork caldera (small area of unit Kb in Fig. 4). (C) Aerial view east showing north end of southwestern area of welded outfl ow tuff (Kro) resting on mid-Cretaceous granodiorite. Note columnar joints above base of tuff. Base of tuff indicates land surface at ca. 70 Ma. (D) Aerial view southeast showing western edge of welded outfl ow tuff (Kro) resting on mid-Cretaceous granodiorite at southwest end of northeast-trending ridge. (E) Welded outfl ow tuff outcrop ~70 m above Gold Creek valley fl oor (Fig. 3; photo by H.L. Foster at station 71AFr980: longitude 142°35.44′W, latitude 64°12.34′N). Dated sample 71AFr965 was obtained ~1100 m due west of photo location, on opposite side of Gold Creek. (F) Aerial view east over granite porphyry (Kgp) to high country consisting of densely welded intracaldera tuff and Early Jurassic monzonite of Mount Veta on south caldera boundary. Prior to erosion, Middle Fork caldera apparently had a resurgent structural dome, raised by intrusion of the granite porphyry in the western part of the caldera.

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is not exposed (compare Lipman, 1976a). One underlies outfl ow tuff and the tuff on its south- of map unit Kgp (Fig. 4). Differential erosion large marble block >100 m across (Fig. 5B), east is possible (but note faceted spurs and along widely spaced vertical joints has given shown as unit Kb on the geologic map, swims linear fault trace northwest of outfl ow tuff in this 600-m-long by 150-m-high cliff a fl uted in welded tuff near the southeast caldera bound- Fig. 2). Exposure appears suffi cient such that appearance that is visible from a great distance. ary. Other possible areas of megabreccia are foot traverses in this area might resolve the The porphyry intrusion is everywhere sur- conservatively indicated by Kb? on the map. geometry of map units and the important ques- rounded by intracaldera tuff except possibly at Because of poor exposure, other large areas tion of the nature of this contact. its poorly exposed northernmost limit (Fig. 4). of megabreccia could be present, such as that Between ~16 and ~23 km east-southeast of There do not appear to be exposures of porphyry presently shown as unit Pzm between the north- the caldera in the Kechumstuk block are expo- east of about longitude 143°07′, that is, within ern mapped extent of intracaldera tuff and the sures of welded tuff mapped by Foster (1976) approximately the eastern one-third of the cal- Middle Fork of the North Fork of the Fortymile in Gold Creek (Fig. 3). The preserved thickness dera fi ll. Calderas the size of the Middle Fork River (Fig. 4). of the tuff may be >70 m (Fig. 5E). All of the and larger commonly have a resurgent structural characteristics of the tuff in Gold Creek are con- dome in caldera fi ll that forms by intrusion of Outfl ow Tuff sistent with it being distal outfl ow tuff from the magma beneath the fl oor soon after caldera col- Areas of rhyolite welded tuff near the west Middle Fork caldera. No other potential source, lapse (Smith and Bailey, 1968). The resurgent edge of the geologic map (Figs. 2 and 4) are Tertiary or Cretaceous, is known in the region. dome can be eccentric to the caldera, such as interpreted as remnants of proximal outfl ow Pyro clastic fl ows that deposited this ignimbrite in the 0.78 Ma Long Valley caldera in Cali- (unit Kro) of the caldera-forming eruption. would have traversed a land surface of low fornia (Bailey et al., 1976). On the basis of its The tuff caps a northeast-trending ridge con- relief in comparison to today’s topography in map pattern, we infer that the granite porphyry tinuously for a distance of 4 km, as well as a the Mount Veta block between the caldera and within the Middle Fork caldera is the solidifi ed ridge top to the south and an isolated hill to the valley of Gold Creek. Exposure of the ca. intrusion that was responsible for producing a the northeast of the main exposure. The out- 68–66 Ma pluton north of Mount Veta (Fig. 4; resurgent dome that has since been eroded away. crop pattern indicates that the tuff has a sub- Day et al., 2014) and establishment of south- Hence, we refer to the porphyry as a resurgent horizontal base that represents the 70 Ma land to-north drainage in the uplifted Mount Veta intrusion. Resurgent intrusions are exposed at surface, a relation that is illustrated particularly block between major northeast-tending, high- eroded calderas elsewhere, such as the Lake well on the north-northwest–trending ridge angle faults (Fig. 3) postdates the Late Creta- City (Lipman, 1976b), Grizzly Peak (Fridrich south of the largest area of outfl ow tuff that ceous landscape. Survival of the welded tuff in et al., 1991), Turkey Creek (du Bray and Pal- displays columnar joints above its base (Fig. the valley of Gold Creek, which has a limited lister, 1991), Chegem (Lipman et al., 1993), 5C, leftmost outcrop). The sample from which catchment area and evidently was unaffected and Caetano (John et al., 2008) calderas. Com- Bacon and Lanphere (1996) reported a biotite by Pleistocene glaciation (Weber and Wilson, paction foliation of intra caldera tuff would be 40Ar/39Ar age of 69.10 ± 0.19 (1σ) Ma was 2012), requires protracted stability of the land- expected to dip steeply away from contacts with obtained from the west base of the northeast scape there over the 70 m.y. since the caldera- a resurgent intrusion, but we have not visited hill exposure. Densely welded basal tuff has forming eruption. a suffi cient number of appropriate outcrops of well-developed columnar joints (Fig. 5D) and Middle Fork intracaldera tuff to test this hypoth- evidently rests on mid-Cretaceous granodiorite Granite Porphyry esis. As will be seen below, the resurgent intru- (unit Kgd) and Paleozoic metamorphic rocks Much of the western and central parts sion interpretation is consistent with available (Pzm), although the actual contact is obscured of the area within the mapped extent of the petrographic, geo chrono logic, and whole-rock by talus, scree, and colluvium. Here, the tuff Middle Fork caldera are occupied by K-feld- geochemical data. has a preserved thickness of as much as 100 m. spar megacrystic granite porphyry (unit Kgp, Immediately east of these areas of outfl ow tuff Fig. 4). The porphyry appears to be the only Rock Textures are poorly exposed granodiorite (?) and prob- rock exposed within an ~7 km × 12 km area able intra caldera tuff, which together suggest and is easily recognized in fl oat by ubiquitous Intracaldera Tuff that the outfl ow tuff is a proximal deposit on skeletal quartz as large as 1 cm and alkali feld- The densely welded intracaldera tuff (Fig. what possibly is the original topographic cal- spar megacrysts typically as large as 1–2 cm 6A) contains phenocrysts diagnostic of its rhyo- dera rim. Existing geologic mapping is insuf- (rarely 4 cm) in a fi ne-grained groundmass litic composition. Typical intracaldera tuff is fi cient to clearly delineate the west bound- that weathers light tan. We interpret the por- crystal rich (Fig. 6B) and somewhat heteroge- ary of intra caldera tuff and the nature of the phyry as intruding intra caldera tuff, although neous in hand specimen owing to the presence of contact between grano diorite wall rock and we have not discovered a locality where the lithic fragments and collapsed pumice clasts in intra caldera tuff. On ridges southeast of the two rock types can be seen in contact. The por- a matrix of phenocrysts, crystal fragments, and mapped outfl ow tuff, outcrops interpreted phyry appears to have crystallized from rela- welded shards that initially were glass but are as intracaldera tuff are less densely welded tively uniform magma that forms a contigu- now fi nely crystalline. A collapsed pumice clast and locally contain centimeter-sized feldspar ous intrusive mass. No xenoliths were seen in that displays large intact feldspar crystals can megacrysts, characteristics that would be con- outcrops or amongst fl oat during foot traverses be seen in more representative intracaldera tuff sistent with proximity to the original west wall of ridges and only one mafi c enclave (sample in Figure 6C. Less densely welded tuff near the of the caldera. However, northeast projection 10ADb20) was found. The porphyry intrusion caldera margins locally contains 1–2 cm K-feld- of a regional northeast-trending fault shown by has maximum topographic relief of ~650 m spar megacrysts and pumice clasts to 6 cm. Foster (1976), part of the Mount Harper linea- (Fig. 2). The most continuous exposure may In thin section, intracaldera tuff specimens ment of Wilson et al. (1985), would pass just be the prominent cliff shown in Figure 5F east (Figs. 7A–7E) have embayed quartz and vari- southeast of the ridge-capping outfl ow tuff, so of the Middle Fork of the North Fork of the ably altered plagioclase and alkali feldspar that a fault contact between granodiorite that Fortymile River near the southernmost extent crystals, each as large as 2–4 mm. Plagioclase

1438 Geosphere, December 2014

A C 2 cm

lithic large small pumice fragment pumice clasts & matrix clast

small pumice clasts & matrix

lithic fragment D B 2 cm

1 cm

Figure 6. Welded tuff specimens. (A) Field photograph of densely welded intracaldera rhyolite tuff containing cm-size lithic fragments. Knife is 8 cm long. (B) Sawed slab of densely welded intracaldera rhyolite tuff showing abundant white plagioclase crystals (sample 10ADb08). (C) Sawed slab of densely welded intracaldera rhyolite tuff containing large collapsed pumice clast (sample 10ADb16). Note larger crystal size and greater fraction of gray groundmass in pumice clast. Typical intracaldera tuff (small pumice clasts and matrix) has broken crystals, concentrated by preferential transport of glass shards from eruption column and pyroclastic ﬂ ows. (D) Sawed slab of welded outﬂ ow tuff from valley of Gold Creek (sample 71AFr965). Note cm-sized white fíamme and abundant biotite books.

Figure 7 (on following page). Photomicrographs of thin sections of tuff. Crystal fragmentation and concentration by eruptive processes is evident in all tuff samples. (A) Densely welded intracaldera rhyolite tuff (sample 10ADb22, crossed nicols). Note skeletal quartz, brown biotite, and fragments of feldspar in cryptocrystalline matrix. Zircons from this sample yield a 70.0 ± 1.2 Ma crystallization age (Fig. 10A). (B, C) Densely welded intracaldera tuff with quartz-rich metamorphic lithic fragment at upper left (sample 10ADb21, stained yellow for K-feldspar; [B] plane polarized light; [C] crossed nicols). Brown biotite is partially chloritized. Very fi ne grained groundmass appears light brown in (B) because of intergrown quartz, stained alkali feldspar, and other phases. In (B), skeletal and resorbed quartz appears white, plagioclase cloudy gray, and alkali feldspar yellow. (D) Densely welded intracaldera tuff (sample 10ADb31A, stained yellow for K-feldspar; plane polarized light). Crystal-poor, light-brown area at left center is collapsed pumice clast that carries four touching fragments of resorbed quartz (center); larger quartz crystal with elongated dark inclusion is not in pumice clast. (E) Densely welded intracaldera tuff (sample 10ADb16, stained yellow for K-feldspar; plane polarized light). Upper por- tion of view is part of collapsed large pumice clast (see Fig. 6C); lower third is typical crystal-rich intracaldera tuff. Note large skeletal, highly resorbed quartz (upper left) and subrounded plagioclase cluster (upper right) and brown biotite in tan groundmass speckled with tiny quartz crystals in pumice clast. Groundmass of crystal-rich tuff is distinctly more uniform in appearance, and crystals are smaller than in pumice clast. (F) Welded outfl ow tuff from Gold Creek (sample 71AFr965; plane polarized light). Specimen has quartz and feldspar fragments and brown biotite similar to intracaldera tuff samples. Note eutaxitic texture and very fi ne grained crystalline groundmass. Relatively crystal-poor collapsed pumice clast is evident in lower left.

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A D

1 mm 1 mm

B E

1 mm 1 mm

C F

1 mm 1 mm

Figure 7.

1440 Geosphere, December 2014

is more abundant than alkali feldspar. Quartz a lesser fraction of crystals than intracaldera grained groundmass (Fig. 9D) is more coarsely and feldspar in intracaldera tuff and outfl ow or proximal outfl ow tuff. Petrographic features crystalline than those of the tuffs and is charac- tuff commonly are fragments that formed by are consistent with this extensive remnant of terized by abundant 50–100 μm quartz crystals. explosive vesiculation of melt inclusions or ash-fl ow tuff being outfl ow from the Middle intergranular melt in crystal aggregates (Best Fork caldera. Mafi c Magmatic Enclave and Christiansen, 1997). Biotite as large as The single mafi c magmatic enclave (sample 1.4 mm may be brown, variably altered, or Granite Porphyry 10ADb20; Fig. 8C), obtained from a porphyry chlori tized. Accessory minerals are Fe-Ti oxides, The porphyry carries 40–50 vol% of larger fl oat block, measures 8 cm × 12 cm. Although apatite, and rare zircon. Eutaxitic texture is phenocrysts of the same phases found in the the enclave is subangular, and therefore is a frag- well preserved in some specimens, which carry tuffs (Figs. 8A, 8B, and 9A–9C). Porphyry sam- ment of a larger object, its texture indicates that relatively crystal-poor fíamme to >1 cm. Lithic ples have skeletal or embayed quartz as large as it is a sample of mafi c magma that was incorpo- fragments observed in thin section range in size 8 mm (rarely to 1.2 cm), plagioclase to 3 mm, rated into cooler magma, presumably that of the from <1 to >20 mm and comprise foliated meta- and euhedral to subrounded alkali feldspar com- granite porphyry, and crystallized rapidly in an morphic rocks and probable felsic volcanic and monly 1–2 cm, rarely to 2 cm × 4 cm. Brown undercooled state (Walker and Skelhorn, 1966; porphyritic felsic plutonic rocks. biotite is the most abundant mafi c mineral, is as Vernon, 1984). Such enclaves are examples of much as 3 mm across, and commonly is chlo- less evolved magma that was coeval with the Outfl ow Tuff ritized. Sparse prismatic hornblende pseudo- host granitic magma. Outcrop-scale features, notably columnar morphs as long as 7 mm are locally visible in In thin section (Figs. 9E and 9F), the enclave jointing, distinguish proximal outfl ow tuff hand specimen. Opaque Fe-Ti oxides are as displays brown biotite as large as 2–4 mm and from intracaldera tuff more than texture and large as 0.4 mm. Apatite and zircon (Fig. 9A) more abundant green hornblende to 2 mm. Euhe- modal composition. Proximal outfl ow tuff is are ubiquitous accessory phases. The fi ne- dral plagioclase with clear rims and saussuritized crystal rich, containing ~50 vol% phenocrysts and fragments of mainly plagioclase and alkali feldspar as large as 3 mm and quartz, com- A monly embayed, as large as 2 mm. Plagioclase hbl may be partially replaced by sericite and carbonate minerals. Brown biotite phenocrysts are euhedral hexagonal books up to 2 mm across and 1 mm thick. The biotite commonly is intergrown with quartz ± plagioclase ± Fe-Ti oxide and contains apatite and rare zircon inclusions. hbl Opaque oxides and minor euhedral brown Figure 8. Specimens from the amphibole ≤0.5 mm long also are present, as resurgent intrusion. (A) Granite are minor subhedral, partially chloritized, bio- porphyry hand specimen (sam- tite xenocrysts. Lithic fragments as large as ple 10ADb30B). Alkali feldspar 6 cm are muscovite schist and altered biotite megacrysts as large as 2–4 cm granite. The devitrifi ed matrix has relict glass (e.g., white crystal in upper 2 cm shards. Collapsed pumice clasts have ~20 vol% right corner) can be found in crystals of the same phases, which may be rep- most exposures and fl oat blocks. B resentative of the bulk magma crystallinity as Impressions left by hornblende opposed to the crystal-enriched character of the crystals indicated by “hbl.” bulk welded tuff. (B) Wet sawed slab of granite Foster (1976) mapped welded tuff (Figs. 5E porphyry (sample 10ADb30A). and 6D) that appears locally >70 m thick in the Large white crystals are alkali valley of Gold Creek from ~16 km to ~23 km feldspar; smaller ones are pla- east-southeast of the Middle Fork caldera. She gioclase. Gray skeletal crys- reported embayed quartz, brown biotite, and tals are quartz; black miner- rare lithic fragments of quartzite and green- als are biotite and Fe-Ti oxide. stone. Thin sections of Foster’s three samples (C) Mafic enclave (sample reveal abundant quartz, plagioclase, probable 10ADb20) in granite porphyry. 2 cm Dark-gray splotches at left are alkali feldspar, and brown biotite, all as large C as 2 mm, together with minor Fe-Ti oxides lichen. Knife is 8 cm long. and apatite in a honey-colored groundmass (Fig. 7F). Quartz phenocrysts are embayed and contain melt inclusions. The groundmass originally was vitric but has crystallized to fi ne-grained salic minerals, zeolites (?), and clay (?). Fíamme are well preserved, and porphyritic rhyolitic lithic fragments are common. Although crystal rich, this tuff contains

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cores is abundant (to 1.5 mm but mostly 0.2– yields 69.94 ± 0.52 Ma at 95% confi dence, in aeromagnetic survey was conducted in 1972 0.8 mm). Several examples of late-crystallized agreement with the U-Pb zircon data. with three-quarter–mile fl ight line spacing, at ~1 mm quartz and one 4 mm cluster of ≤3 mm The age determinations are consistent with 1000 feet above ground where possible; after sieved plagioclase are present. Plagioclase, the view that outfl ow and intracaldera tuff removing the regional fi eld, contours of mag- together with hornblende, biotite, and acces- formed during the same explosive eruption. The netic intensity were shown by Veach (1973). sory Fe-Ti oxides and apatite, give the enclave dated zircon crystals carried by erupted rhyo- Bacon et al. (1990) noted that the aeromagnetic an intergranular groundmass texture indicative lite tuff and intruded porphyry may have grown map has an elliptical ring of comparatively large of crystallization of phenocryst-poor melt. Very from the same or similar magmas prior to—per- amplitude magnetic lows surrounding a central fi ne grained matrix between groundmass crys- haps as much as a few hundred k.y.—eruption, magnetic high within the area of the Middle tals, such as in the center of Figure 9F, is the caldera formation, and resurgence (cf. Reid, Fork caldera and suggested that the lows are product of crystallization of residual liquid. 2008). Note that in the porphyry, at least some best explained by reversely magnetized rock. zircons are enclosed within large plagioclase Comparing the aeromagnetic map with new GEOCHRONOLOGY crystals that occur in clusters (Fig. 9A). Con- geologic mapping (Fig. 11), the distribution of cordance of the 40Ar/39Ar biotite age for outfl ow intracaldera tuff is coincident with magnetic Radiometric ages have been determined for tuff and the zircon ages indicates that the zircon lows with values of ~4240–4850 nT, the cen- samples of intracaldera and outfl ow tuff and data refl ect crystallization in the igneous system tral magnetic high over the porphyry reaches for resurgent granite porphyry using the Stan- that produced the caldera-forming eruption and nearly 5500 nT, and the western lobe of por- ford–U.S. Geological Survey (USGS) sensitive porphyry intrusion, even if there may be uncer- phyry reaches nearly 5300 nT. Terrain outside high-resolution ion microprobe with reverse tainty as to when in the magmatic history the of the area of intracaldera tuff has values gener- geometry (SHRIMP-RG) at Stanford Univer- dated zircon crystallized. At 70 Ma, precision ally >5000 nT, with highs near 5300 nT. More- sity. The primary ion beam extracted material of SHRIMP-RG 206Pb/238U ages is insuffi cient over, outfl ow tuff west of the caldera coincides from inclusion- and crack-free areas of interior to resolve potential differences between times with a magnetic trough that has values as low as mantles that display fi ne concentric oscillatory of zircon growth recorded within crystals and ~4000 nT. The outfl ow tuff in the valley of Gold zoning in cathodo luminescence (CL) (Fig. 10). times of rhyolite eruption or porphyry solidifi - Creek (Fig. 3) also is associated with a magnetic The analytical crater was ~25–30 µm in diameter cation that might be recorded by the outermost trough with minima of ~4700–4800 nT between and ~0.5–1 µm deep. Isotopic data were reduced micrometers of zircon in contact with melt (cf. highs of ~5000–5500 nT. using Squid 2 (Ludwig, 2009) and plotted using Chamberlain et al., 2014). The geochronologic Clearly, the tuff of the Middle Fork caldera Isoplot 3 (Ludwig, 2003). Calculated ages for data show that the caldera-forming eruption and is reversely magnetized. Reverse magnetiza- samples in this study are weighted averages of intrusion of the porphyry took place closely in tion is established by consistent strong mag- 14–15 selected individual 206Pb/238U ages. Uncer- time, in a geological sense at least, and that the netic lows over areas of tuff and by very high tainties are quoted at the 95% confi dence level. erupted and intruded magmas likely originated natural remanent magnetic intensity (NRM) Analytical methods are described in detail in the during the same episode of igneous activity. and relatively low magnetic susceptibility mea- Supplemental File1, and results of individual spot sured for two cores from a non-oriented sample analyses are tabulated in the Supplemental Table2. GEOMAGNETIC POLARITY of intracaldera tuff that result in unusually high Crystallization ages of zircon crystals are Koenigs berger ratios (Q) of remanent to induced 70.0 ± 1.2 Ma for intracaldera tuff (sample An aeromagnetic map of the Eagle 1° × 3° magnetic intensity (Table 1; cf. Bath et al., 10ADb22), 69.7 ± 1.2 Ma for granite porphyry quadrangle provides additional insight into the 1972). The porphyry either is normally magne- (sample 10ADb17), and 71.1 ± 0.5 Ma for distribution and ages of tuff and porphyry. The tized or contemporary induced magnetization welded tuff in the valley of Gold Creek (sample 71AFr965) (Fig. 10). These ages agree within analytical uncertainty. Bacon and Lanphere (1996) reported a Figure 9 (on following page). Photomicrographs of thin sections of samples of the resur- 40Ar/39Ar weighted mean age of 69.10 ± 0.19 gent intrusion. (A) Resurgent granite porphyry (sample 10ADb17; crossed nicols). Large (1σ) for fi ve total-fusion measurements on indi- crystals are plagioclase, biotite, and skeletal quartz. Plagioclase commonly occurs in sub- vidual biotite phenocrysts from rhyolite outfl ow rounded clusters. Crystalline groundmass has abundant small feldspar and quartz grains. tuff immediately west of the caldera (sample Red circles mark ≤200 µm zircon inclusions in plagioclase; zircon is diffi cult to resolve in 91ADb29). Recalculating the biotite age using this view but obvious under the microscope. Zircons from this sample yield a 69.7 ± 1.2 Ma 28.02 Ma (Renne et al., 1998) for the age of crystallization age (Fig. 10B). (B, C) Resurgent granite porphyry (sample 10ADb18, stained the Fish Canyon Tuff sanidine fl uence monitor yellow for K-feldspar; [B] plane polarized light; [C] crossed nicols). Alkali feldspar mega- cryst at left, resorbed quartz, plagioclase clusters, and biotite in fi ne-grained crystalline 1Supplemental File. U-Pb geochronology. If you groundmass. Compare with pumice clast in Figure 7E. (D) Granite porphyry groundmass are viewing the PDF of this paper or reading it offl ine, (sample 10ADb17, stained yellow for K-feldspar; plane polarized light). Equant white please visit http:// dx .doi .org /10 .1130 /GES01037 .S1 quartz and yellowish alkali feldspar form bulk of groundmass; plagioclase occurs as cloudy or the full-text article on www .gsapubs .org to view the Supplemental File. patches (e.g., lower right). Although indistinct in photos, alkali feldspar forms crystals com- 2Supplemental Table. SHRIMP-RG U-Th-Pb data parable in size to quartz. (E) Mafi c magmatic enclave (sample 10ADb20; plane polarized for zircon from rocks related to the Middle Fork cal- light). Zoned plagioclase has saussuritized cores and clear rims. Mafi c crystals are predomi- dera, east-central Alaska. If you are viewing the PDF nantly hornblende, but poikilitic to interstitial brown biotite also is present. Quartz occurs of this paper or reading it offl ine, please visit http:// locally in patches (as in lower right quarter) and interstitially. Accessory phases are Fe-Ti dx .doi .org /10 .1130 /GES01037 .S2 or the full-text article on www .gsapubs .org to view the Supplemental oxides and apatite. (F) Enlarged view of enclave sample. Intergranular matrix in center of Table. view is cryptocrystalline and probably represents late-crystallized melt.

1442 Geosphere, December 2014

A D

1 mm 0.25 mm

B E

1 mm 1 mm

C F

1 mm 0.25 mm

Figure 9.

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A 0.07 80 has overwhelmed its remanent magnetization. 10ADb22 The most recent version of the paleomagnetic

U 76 time scale (Ogg, 2012) shows normally polar- 238 72 ized chron C31n spanning 68.369–69.269 Ma

0.06 Pb/ 68

age (Ma) and reversely polarized chron C31r encom- 206 64 passing 69.269–71.449 Ma. Because the out- Pb 60 ﬂ ow tuff and the intracaldera tuff are reversely 206 Age = 70.0 ± 1.2 Ma magnetized, our independent geochronologic 0.05 (MSWD = 1.51)

Pb/ results require that the caldera-forming erup- 90 80 70 60 50 tion must have occurred during chron C31r. The 207 U-Pb zircon crystallization age for the porphyry, 0.04 though nominally 69.7 Ma, is within analytical uncertainty of the presently accepted age of 100 µm chron C31n. However, because studies of resur- 0.03 gent calderas indicate that it is likely that the 70 90 110 130 comagmatic porphyry was intruded relatively 238U/206Pb soon after caldera formation (e.g., Bailey et al., 1976; Hildreth, et al., 1984), it is important to B 0.07 80 consider induced magnetization as the source

10ADb17 U 76 of the magnetic highs associated with the por-

238 phyry. Magnetic measurements on cores from 72 two non-oriented porphyry samples yield low 0.06 Pb/ 68 age (Ma) NRM, high susceptibility, and thus low Q (Table 206 64 1) typical of felsic intrusive rocks (e.g., Ponce Pb 60 and Langenheim, 1993). It is therefore likely 206 0.05 Age = 69.7 ± 1.2 Ma that the aeromagnetic highs over granite por- (MSWD = 1.61) phyry simply refl ect magnetization induced by Pb/ 90 80 70 60 50 the present normal fi eld (57.4 × 103 nT when the 207 survey was fl own) and that the tuff and porphyry 0.04 did not capture the C31r-to-C31n reversal.

100 µm GEOCHEMISTRY 0.03 70 90 110 130 Chemical analyses of rock specimens charac- 238U/206Pb terize magmas and reveal genetic relationships between the tuff and porphyry. Major-element C 0.07 80 oxide and trace-element concentrations were

71AFr965 U 76 determined for one sample of proximal out-

238 ﬂ ow tuff, three of intracaldera tuff, three of 72

Pb/ granite porphyry, and one maﬁ c enclave by a

0.06 age (Ma) 68

206 combination of X-ray fl uorescence (XRF) and 64 inductively coupled plasma mass spectrometry Pb 60 (ICP-MS) at the GeoAnalytical Laboratory at 206 Age = 71.1 ± 0.5 Ma Washington State University (Table 2). In spite 0.05 (MSWD = 0.7) of likely crystal concentration in bulk tuff and Pb/ 90 80 70 60 50 possible melt loss from porphyry, the seven 207 Middle Fork caldera felsic rocks are composi- 0.04 tionally similar rhyodacite-rhyolite and granite. Although minor amounts of corundum (0.3%– 100 µm 1.3%) appear in CIPW norms calculated with 2+ 0.03 all Fe as Fe (FeO*), accounting for a fraction 70 90 110 130 of the Fe as Fe3+ indicates that the samples are 238 206 U/ Pb metaluminous. The felsic rocks have SiO2 con- tents that range from 69 wt% to 72 wt% (major- Figure 10. Cathodoluminescence images of representative zircon and Tera-Wasserburg con- element oxides recalculated to sum to 100% cordia plots of sensitive high-resolution ion microprobe–reverse geometry (SHRIMP–RG) volatile free). The composition of proximal out- U-Pb data. (A) Intracaldera tuff sample 10ADb22. (B) Granite porphyry sample 10ADb17. fl ow tuff sample 91ADb29 is virtually identical (C) Outfl ow tuff sample 71AFr965. Analytical data are given in Supplemental Table (see to that of intracaldera tuff sample 10ADb11. footnote 2). Uncertainties are quoted at the 95% confi dence level. MSWD—mean square of The seven felsic rock compositions defi ne a weighted deviates. limited differentiation series within the range of

1444 Geosphere, December 2014

143° 143° 15’W 00’WPzm

Pzm ortymile f the F Rive Kgd rk o r Fo Qu h rt o N Pzm e Jg th Pzm f o k r Kr 64° Kro o 64° 20’N F 20’N

e l

d Pzm

d i

M Kgp Kro Kr Kb?

Kb Kgd Qu Pzm Jg Kb? 64° 64° 15’N 15’N Pzm Pzm Kg D U Pzm 0 5 10 km Pzm Mt. Jg Pzm Veta 143° Jg 143° 15’W 00’W

EXPLANATION

Areas of outflow tuff 10 nT contour Magnetic low Limit of intracaldera tuff 20 nT contour Magnetic maximum Limit of resurgent porphyry 5200 100 nT contour Magnetic minimum 5000 500 nT contour Flight lines and directions

Figure 11. Magnetic intensity contours from the aeromagnetic map of the Eagle 1° × 3° quadrangle with the regional fi eld removed (Veach, 1973) superimposed on the geologic map of the Middle Fork caldera and vicinity (Fig. 4). Magnetic lows are associated with reversely magnetized intracaldera and outfl ow tuff. Magnetic highs over granite porphyry likely result from magnetization induced by the contemporary geomagnetic fi eld.

TABLE 1. MAGNETIC PROPERTIES OF GRANITE PORPHYRY AND WELDED TUFF FROM THE MIDDLE FORK CALDERA, ALASKA Laboratory Susceptibility NRM Sample number core Material (10–6 SI units) (A/m) Q 10ADb30A AKB001A Granite porphyry 98.76 0.044 0.061 10ADb18 AKB002A Granite porphyry 110.1 3.83 4.84 10ADb18 AKB002B Granite porphyry 116.8 4.05 4.76 10ADb22 AKB003A Welded tuff 78.07 29.4 52.3 10ADb22 AKB003B Welded tuff 75.36 33.0 60.9 Note: Susceptibility values are in SI units. Measurements performed in the U.S. Geological Survey, Menlo Park, California, paleomagnetic laboratory by J.T. Hagstrum. NRM—natural remanent magnetic intensity. Q—Koenigsberger ratio of remanent magnetic intensity/induced magnetic intensity calculated using International Geomagnetic Reference Field value of 57,400 nT for 1972 for Middle Fork caldera vicinity.

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TABLE 2. CHEMICAL ANALYSES OF ROCKS FROM THE MIDDLE FORK CALDERA, ALASKA Sample 91ADb29 10ADb31A 10ADb22 10ADb11 10ADb18 10ADb16 10ADb30A 10ADb20 North latitude 64.3314 64.3142 64.2961 64.2786 64.3133 64.3119 64.2850 64.3186 West longitude 143.3317 143.3697 143.1119 143.0142 143.1417 143.1450 143.2733 143.1556 X-ray ﬂ uorescence analyses (wt%), recalculated to sum to 100 (wt% volatile free)

SiO2 71.1 71.0 71.2 71.6 69.2 71.3 72.3 53.9 TiO2 0.358 0.342 0.336 0.323 0.424 0.327 0.285 1.25 Al2O3 15.1 15.1 14.8 15.0 15.4 15.0 14.6 17.7 FeO* 2.57 2.62 2.49 2.45 3.12 2.34 2.12 9.20 MnO 0.07 0.08 0.07 0.08 0.07 0.08 0.07 0.26 MgO 0.97 0.76 0.68 0.70 1.06 0.62 0.64 4.51 CaO 2.54 2.42 2.77 2.35 3.10 2.91 2.04 6.17

Na2O 2.95 3.45 3.23 3.18 3.19 3.06 3.30 3.63 K2O 4.24 4.14 4.28 4.21 4.18 4.30 4.52 3.06 P2O5 0.121 0.115 0.122 0.114 0.166 0.113 0.103 0.259 Original reported values (wt%) Loss on ignition 2.22 1.21 1.33 2.23 1.87 1.74 1.77 1.96 Original sum 96.65 97.93 98.17 97.13 97.85 97.53 96.59 97.70 X-ray ﬂ uorescence analyses (ppm) V 39 35 35 30 46 32 30 246 Cr 8 6 6 5 3 5 4 8 Ni 5 2 1 b.d. b.d. 1 b.d. 2 Cu 3 2 2 1 4 2 1 26 Zn 57 45 47 47 46 44 40 109 Ga 16 16 17 17 17 17 16 23 Inductively coupled plasma–mass spectrometric analyses (ppm) Rb 147 140 147 141 135 149 169 116 Cs 5.87 4.26 5.07 2.73 2.14 5.51 2.31 12.39 Sc 5.1 5.4 5.3 4.5 6.2 4.9 5.0 27.5 Sr 515 618 550 546 665 532 465 897 Ba 1621 1482 1478 1547 1864 1504 1247 1353 Pb 24.1 21.7 25.8 25.2 19.4 21.4 24.3 14.9 Y 16.1 16.6 17.1 16.1 17.3 16.6 18.5 29.5 Zr 168 170 157 166 183 162 137 117 Hf 4.48 4.65 4.32 4.50 4.78 4.43 4.03 3.63 Nb 13.9 14.8 15.3 14.4 14.2 14.9 18.3 11.1 Ta 1.02 1.10 1.15 1.09 1.01 1.11 1.52 0.63 Th 19.6 20.7 20.4 21.1 19.1 20.7 20.7 10.4 U 4.97 5.80 6.87 6.20 4.92 7.60 6.45 2.00 La 54.2 50.7 46.4 54.7 56.0 50.2 37.9 43.4 Ce 94.2 89.7 84.7 97.3 99.2 90.1 66.8 83.3 Pr 9.84 9.53 8.98 10.08 10.53 9.44 7.65 10.17 Nd 32.9 31.7 30.4 33.2 35.5 31.8 26.4 38.7 Sm 5.20 5.26 5.20 5.34 5.81 5.28 4.94 7.71 Eu 1.24 1.20 1.20 1.21 1.33 1.19 1.00 2.22 Gd 3.68 3.81 3.81 3.71 4.09 3.75 3.86 6.48 Tb 0.53 0.55 0.57 0.54 0.60 0.55 0.62 0.99 Dy 2.99 3.10 3.22 3.05 3.31 3.14 3.50 5.74 Ho 0.58 0.60 0.62 0.58 0.64 0.60 0.68 1.13 Er 1.54 1.65 1.70 1.59 1.72 1.64 1.78 3.04 Tm 0.24 0.25 0.25 0.24 0.26 0.24 0.27 0.43 Yb 1.53 1.64 1.69 1.59 1.64 1.62 1.77 2.66 Lu 0.26 0.27 0.27 0.26 0.27 0.27 0.28 0.42 Note: FeO* is total Fe reported as FeO. b.d.—below detection. Analyses performed at GeoAnalytical Laboratory at Washington State University.

ca. 68 Ma to 66 Ma felsic intrusive rocks from the upper mantle, provides an approximation to elements on the right. Relative depletions in P the Fortymile area just south of the Middle Fork the parent or the agent of melting for the felsic and Ti reﬂ ect fractionation of apatite and titano- caldera on variation diagrams (Fig. 12). The magmas. The widely used Ta-Yb tectonic dis- magnetite. The companion rare-earth element maﬁ c enclave from the porphyry contains 54 crimination diagram (Fig. 14), originally devel- (REE) pattern for the enclave normalized to CI

wt% SiO2 and is basaltic trachyandesite on the oped for granitic rocks (Pearce et al., 1984), chondritic meteorites (Fig. 13B; McDonough basis of the silica versus total alkalis classifi ca- shows the felsic rocks of the caldera in the vol- and Sun, 1995) is smooth and has a gentle nega- tion scheme (Le Bas et al., 1986; not shown). canic arc fi eld near the boundary with syncol- tive slope from light to heavy REE. Lack of a Trace-element concentrations of the felsic lisional granites. negative Eu anomaly implies that the enclave samples are similar to those of 68–66 Ma fel- The mafi c enclave has a multi-element pattern magma did not experience signifi cant plagio- sic intrusive rocks from nearby in the Fortymile (Fig. 13A) typical of moderately evolved, sub- clase fractionation. The enclave composition area (Fig. 13). Multi-element patterns relative to duction-related basaltic andesite in which nota- plots well within calc-alkalic arc fi elds on the primitive mantle composition (McDonough and ble relative depletion in Nb and Ta and enrich- Th-Hf-Ta (Wood, 1980) and La-Y-Nb (Cabanis Sun, 1995) have the familiar shape characteristic ment in Pb are superimposed on a negatively and Lecolle, 1989) tectonic discrimination dia- of evolved magmas that formed in convergent sloping curve from high values for elements grams (not shown). plate margins. Importantly, the mafi c enclave, strongly incompatible in mantle minerals on the In comparison to the mafi c enclave, the felsic itself differentiated relative to a primary melt of left to lower values for moderately incompatible samples have multi-element and REE patterns

1446 Geosphere, December 2014

Figure 12. Silica variation diagrams for 1.5 18 eight samples from the Middle Fork caldera MFC intracaldera tuff (Table 2) and 68–66 Ma felsic igneous rocks MFC outflow tuff nearby in the Fortymile area. Analyses MFC granite porphyry 17 1.0 MFC enclave in porphyry recalculated to sum to 100% volatile free. 3

2 66–68 Ma intrusive rocks O Blue square for enclave of mafic magma 2 16 TiO Al (54% SiO2) in resurgent granite porphyry may represent maﬁ c input to Middle Fork 0.5 magmatic system. Compositions of felsic 15 rocks are consistent with crystallization dif-

ferentiation with or without assimilation of 0.0 14 evolved crustal rocks or partial melts. All 10 5 rock analyses by GeoAnalytical Laboratory, Washington State University. 8 4

6 3 FeO* MgO that show enrichment in the more incompatible 4 2 elements and depletion in elements compati- ble in feldspars (negative Eu anomaly), horn- 2 1 blende (middle to heavy REE), and accessory phases (P and Ti) that are qualitatively con- 0 0 sistent with derivation of the felsic magmas 7 5.0 from enclave-like magma by crystallization 6 differentiation. Trace-element abundances also 4.5 may be consistent with assimilation or partial 5 melting of crustal rocks. Data for the enclave 4 4.0 O 2

provide evidence of maﬁ c magma and thermal CaO input for either crystallization differentiation or 3 Na 3.5 partial melting scenarios. The small composi- 2 tional range of the tuff samples possibly could 3.0 1 be ascribed to varying degrees of syneruptive crystal concentration owing to selective loss 0 2.5 of glass shards to the eruption column (Smith, 6 0.3 1960). Similarly, the small variation in porphyry compositions may be related to accumu- 5 lation of crystals brought about by migration of 0.2 O 5

residual melt. 2 K O

4 2 OTHER CRETACEOUS–EARLY P TERTIARY FELSIC VOLCANICS 0.1 3 Several areas of felsic volcanic and hypabyssal intrusive rocks within ~50 km of the Middle 2 0.0 Fork caldera, as well as the area of the caldera 50 55 60 65 70 75 80 50 55 60 65 70 75 80 SiO SiO itself, were mapped as unit Tf in the Eagle (Fos- 2 2 ter, 1976) and Big Delta (Weber et al., 1978) 1° × 3° quadrangles (Fig. 15) and, lacking radiometric dates, were inferred to be Tertiary (?) in total area of Tf rock is greater than shown on the coincides with northeast-trending, high-angle age. Map patterns and archived thin sections of geologic maps. faults of the Black Mountain tectonic zone (Day specimens collected during reconnaissance geo- The ~60 km2 area of rhyolite lava and tuff et al., 2007; O’Neill et al., 2010). Although this logic mapping by Foster and coworkers allow us at the east edge of the Big Delta quadrangle rhyolite fi eld has dimensions similar to small to suggest the mode of occurrence of unit Tf in (Fig. 15, loc. 1) is the only dated Tf occurrence calderas, the depression in which it lies evi- most of these areas (Table 3). It should be borne other than those with a known Middle Fork dently is tectonic. Local vitrophyre high on the in mind that exposures typically are limited to caldera source. The nominal ~5 m.y. age dif- domes and preservation of a pyroclastic apron ridge tops, valley fl oors and gentle slopes are ference between dome clusters may be real, as indicate that this volcanic fi eld has not been obscured by surfi cial deposits and vegetation, the lavas have different mafi c phenocrysts. The greatly eroded. and not every ridge in the 48 1:63,360 scale northeastern Tf patch may be an intracanyon The largest area mapped by Foster (1976) quadrangles (12 of which are shown in Fig. 15) lava fl ow that delineates a Paleocene valley. as unit Tf in the Eagle quadrangle, apart from was traversed on foot. It thus is likely that the The southeastern boundary of the volcanic fi eld the Middle Fork caldera, is a 4–8 km × 18 km

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Figure 13. Multi-element (spider) diagram 1,000 (A) and rare-earth element (REE) diagram A (B) for eight samples from the Middle Fork MFC intracaldera tuff caldera. Gray ﬁ eld indicates range of com- MFC outflow tuff positions of analyzed 68–66 Ma felsic igne- MFC granite porphyry ous rocks in the Fortymile area having 100 MFC enclave in porphyry

67%–71% SiO2. Blue line represents enclave

of maﬁ c magma (54% SiO2) in resurgent granite porphyry. Note arc geochemical signature of all samples, close compositional similarity of Middle Fork caldera tuff and 10 porphyry to each other and to the 68–66 Ma rocks, and elevated middle and heavy rare- earth elements in enclave consistent with its hornblende + plagioclase mineralogy. Rock / Primitive Mantle 1 66–68 Ma felsic intrusive rocks

cluster of rhyolite porphyry domes or intrusions bisected by the Charley River (Fig. 15, loc. 2). Groundmass textures verging on granophyric 0.1 and ubiquitous sericitization of plagioclase Cs Ba U Ta La Pb P Sm Hf Ti Tb Y Er Yb are suggestive of intrusions or deeply eroded Rb Th Nb K Ce Sr Nd Zr Eu Gd Dy Ho Tm Lu domes. According to Weber and Wilson (2012), 1,000 glaciers of the early (?) Pleistocene Charley River advance and, to a lesser extent, the B MFC intracaldera tuff middle (?) Pleistocene Mount Harper and early MFC outflow tuff (?) Wisconsin Eagle advances occupied the val- MFC granite porphyry leys here, consistent with relatively intense ero- MFC enclave in porphyry sion. An isolated 0.3 km2 patch of Tf (Fig. 15, loc. 3) also would have been affected by multi- 100 ple glacial advances. In contrast, the 2 km × 3 km cluster of fi ve or more domes and adjacent clastic sedimentary rocks downdropped in a wedge-shaped graben along Ruby Creek (Fig. 15, loc. 4) escaped erosive destruction. Weber and Wilson (2012) show deposits of the Charley Rock / Chondrite River glaciation in the valley of Ruby Creek 10 west and north of the domes, approximately the eastern limit (generalized in Fig. 15) of major 66–68 Ma felsic ice streams of that age. intrusive rocks Northeast of the caldera is an ~20 km2 area of Tf with maximum relief of 500 m (Fig. 15, loc. 5). The only available thin section is of latite vitrophyre from breccia at the confl uence of 1 Manila Creek and the Middle Fork of the North La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Fork of the Fortymile River, ~60 m below early Pleistocene terrace deposits (Foster, 1976; Weber and Wilson, 2012). Because of probable eroded bimodal volcanic fi eld likely of early that source. Six small areas of Tf are mapped sluggish hydration of volcanic glass in a sub- Tertiary age. (Foster, 1976) south of the Middle Fork caldera arctic environment, the high water content (7.4 Near Montana Creek, ~24 km east of the cal- (Fig. 15, locs. 9–14). Of these, occurrences of

wt% H2Ototal) of the vitrophyre suggests that it dera, are fi ve small occurrences of unit Tf that Tf in a valley bottom (loc. 10) and on a broad is signifi cantly older than Pleistocene. Unit Tf intrude or rest on Paleozoic metasedimentary topographic high (loc. 13) are candidates wor- also is mapped 13–17 km east of the caldera rocks (Fig. 15, loc. 8). Werdon et al. (2000) thy of investigation as potential outfl ow tuff (Fig. 15, loc. 6), but no samples or fi eld notes described the tiny hilltop Tf patch south of from the Middle Fork source. Although the are available. Close association of rhyolite and the head of Montana Creek as biotite-bearing, topographic high of locality 13 is within an alkali basalt (Werdon et al., 2000) in the four crystal-lithic tuff and reported compositions extensive relative low on the aeromagnetic map ~3–10 km2 areas of unit Tf near Bullion Creek that broadly resemble those of welded tuffs of (Veach, 1973), no striking magnetic lows that 26–33 km east-northeast of the caldera (Fig. the Middle Fork caldera; thus, it is conceivable might be additional large areas of outfl ow tuff 15, loc. 7) suggest this area is a moderately that this locality preserves outfl ow tuff from are evident.

1448 Geosphere, December 2014

sion probably domed the caldera fi ll during a MFC intracaldera tuff geologically short time after the caldera-form- MFC outflow tuff ing eruption. The resurgent structural dome was west of the caldera center, similar to the Pleisto- MFC granite porphyry cene Long Valley caldera in California (Bailey 66–68 Ma felsic intrusive rocks et al., 1976). Between the topographic caldera walls and the resurgent dome, a moat, such as described by Smith and Bailey (1968), likely guided subsequent erosion along the course of the present Middle Fork of the North Fork of Within-plate the Fortymile River, although the river probably has migrated somewhat over the past 70 m.y. Syncollisional Geochronology, petrography, and geochemistry tie the intracaldera tuff, outfl ow tuff, and the resurgent intrusion to one igneous system Ocean ridge that, by analogy with Cenozoic calderas, probably was active for at least a few hundred thou- Volcanic arc sand years ca. 70 Ma. The caldera-forming eruption and intrusion of the porphyry may have occurred very closely in time because the intracaldera tuff apparently contains clasts of compositionally and mineralogically similar porphyry magma. Moreover, ubiquitous resorption of quartz and feldspar in tuff and intrusive Figure 14. Tectonic discrimination diagram (Pearce et al., 1984) for porphyry suggests that these magmas resided in seven felsic samples from the Middle Fork caldera and 68–66 Ma the upper crust as a body of crystal mush that felsic igneous rocks nearby in the Fortymile area. Data for felsic was rejuvenated shortly before the caldera- rocks plot in volcanic arc fi eld and into syncollisional fi eld. forming eruption. Resorption of crystals may have resulted from advection of heat or gas transfer associated with subjacent infusion of DISCUSSION Lipman, 1976a). Armed with that framework mafi c magma, such as proposed for the Fish and modern geochronological and geochemi- Canyon Tuff of southern Colorado (Lipman Caldera cal tools, we can now interpret the geology et al., 1997; Bachmann et al., 2002). Alterna- of the Middle Fork caldera with much greater tively, heating and remobilization of the mush Calderas are signifi cant for their formation confi dence. may have been a consequence of addition of the during catastrophic explosive volcanic erup- The elliptical map pattern of intracaldera most recent of many batches of less-evolved tions, favorable structures and thermal histories tuff (Fig. 4), reinforced by a ring of magnetic silicic magma from a deeper source (Moore for localization of economic mineral deposits, lows (Fig. 11) over reversely magnetized tuff, and Sisson, 2008). The felsic magma that accu- and association with cogenetic plutons. The defi nes the limits of the foundered cauldron mulated as crystal mush formed by processes Middle Fork caldera is notable because its degree block of the Middle Fork caldera. The block related to plate convergence through mantle of exhumation is suffi cient to reveal a comag- may extend farther north and (or) east, if vege- melting, crystallization differentiation, and matic granite porphyry intrusion, yet remnants tated and poorly exposed metamorphic rocks interaction with continental crust. Physical evi- of the outfl ow tuff sheet are locally preserved. mapped at low elevations actually are blocks dence for a mantle-derived component is pres- These characteristics provide evidence for the in megabreccia. The original topographic walls ent as a rare basaltic trachyandesite enclave in history of landscape evolution relative to verti- of the caldera may have been located out- the porphyry that has a clear subduction-related cal motion on throughgoing northeast-trending board of the present area of intracaldera tuff, geochemical signature. In comparison to many faults. The Middle Fork also is signifi cant in except on the west, because it is well known large caldera systems (e.g., Timber Mountain, being the only preserved Late Cretaceous cal- from studies of younger calderas that landslid- Christiansen et al., 1977; Cerro Galan, Sparks dera in a region of east-central Alaska where ing into a caldera during syneruptive collapse et al., 1985), the Middle Fork appears to have the distribution of plutons (Foster et al., 1994) enlarges the topographic basin (Lipman, 1997). had a relatively simple magmatic and eruptive suggests the possibility that other “supervol- By analogy with deeply eroded calderas, the history that represents a single cycle of caldera canoes” have been lost to uplift and erosion. original thickness of caldera fi ll, consisting formation, resurgent intrusion, and cooling Geologic mapping by Foster and coworkers in of in-falling juvenile pyroclasts and landslide (e.g., Creede, Steven and Lipman, 1976; Long the late 1960s and early 1970s documented the breccia from the failing walls, may have been Valley, Bailey et al., 1976; Calabozos, Hildreth felsic volcanic rocks, and their fi eld notes hinted as much as ~3–5 km (Lipman, 1997). Some et al., 1984). Beneath the present level of at the porphyry intrusion, at a time when stud- of this thickness of intracaldera tuff has been exposure, there is likely a related equigranular ies of younger and better exposed calderas that eroded away to expose the granite porphyry granodiorite-granite pluton of areal dimensions provided a conceptual framework for interpreta- intrusion. By analogy with Quaternary resur- at least as large as those of the caldera fi ll and tion of map patterns were only starting to appear gent calderas (e.g., Bailey et al., 1976; Hildreth possibly contiguous with the 68–66 Ma intru- in the literature (e.g., Smith and Bailey, 1968; et al., 1984; Kennedy et al., 2012), this intrusion to the south (Fig. 3).

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Bacon et al.

1 42°W

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64° Late Cretaceous (?)–Early Tertiary felsic volcanic and shallow intrusive rocks in pink; terrace deposits in yellow. Circled num Circled in pink; terrace deposits yellow. felsic volcanic and shallow intrusive rocks Tertiary (?)–Early Late Cretaceous Table 3; red circled letters and ages refer to samples indicating the timing of exhumation based on apatite ﬁ letters and ages refer circled 3; red Table into blocks of contrasting ex high-angle faults and fault zones, shown by solid dashed heavy lines, divide region trending, et al. (1979) and Day (2007 Foster Ages of rhyolite domes at locality 1 from et al. (1978), and Day (2007). Weber U.S. Geological Survey 10-m digital elevation data. (2012). Base from Wilson and Weber deposits after Figure 15. Shaded relief map of southwestern Eagle and adjacent part Big Delta 1 15. Shaded relief Figure

1450 Geosphere, December 2014

TABLE 3. CRETACEOUS (?)–EARLY TERTIARY FELSIC VOLCANIC ROCKS WITHIN ~50 KM OF THE MIDDLE FORK CALDERA, ALASKA Locality Composition Phenocrysts

number (SiO2 wt%) (vol%) Mode of occurrence Comment 1 Rhyolite Qz Sa Pl Bt Hbl Ox Lava domes, tuff Southeastern loc. 1; 57.2 ± 0.6 Ma U-Pb Zrn (Day et al., 2007) from 250-m-high dome; vitrophyre high on domes; tuff present southwestern loc. 1. 1 Rhyolite (72) Qz Sa Pl Bt ± Hyp Lava domes, Northern loc. 1; 61.6 ± 2 Ma K-Ar Sa (Foster et al., 1979); coarsely porphyritic; vitrophyre Ox (25–40) intracanyon fl ow high on domes; domes as high as 300 m; chemical analysis from Bacon et al. (1990). 2 High-silica rhyolite Qz Sa Pl altered Plugs or lava dome Porphyritic; groundmass nearly granophyric; Pl sericitized; local welded tuff and fl ow- mafi cs and/or fl ow remnants banded lava at southwestern limit. 3 Rhyolite Qz Small plug or lava fl ow Vapor-phase altered, aphyric to porphyritic. remnant 4 High-silica rhyolite Qz Sa Pl altered Lava domes, clastic Southwestern dome phenocryst poor, northern domes porphyritic; perlitic on northeastern mafi cs apron, dike fl ank; clastic apron south and east of domes; north- northeast-trending dike southwest of domes; domes as much as 320 m high.

5 Latite (61) Pl Hyp Aug (7) Lava ﬂ ow ± domes or Maximum relief 500 m; hydrated vitrophyre (7.4 wt% H2Ototal) in river exposure is only plugs sample*; unknown if latite sample is representative of entire area mapped as Tf. 6 Felsic n.a. Extrusive or intrusive No samples or notes available; contacts with Paleozoic quartzite and schist (Foster, 1976) could be consistent with extrusive or intrusive relationship. 7 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% SiO2) associated with basalt (52.3 wt% SiO2); southwestern beneath basalt (50.5 wt% SiO2); northwestern possibly intrusive; chemical analyses from Werdon et al. (2000).

7 High-silica rhyolite Qz Tuff Southeastern exposures associated with alkali basalt (50.0 wt% SiO2); chemical analyses (78) from Werdon et al. (2000). 8 High-silica rhyolite Qz Sa Pl Dome or plug Southwestern body is 220 m high.

8 Rhyolite (72–73.5) Bt bearing Crystal-lithic tuff Southeastern exposure samples have 71.8 and 73.5 wt% SiO2, Werdon et al. (2000); compositions of these altered or weathered samples broadly resemble those of welded tuffs from the Middle Fork caldera.

8 High-silica rhyolite n.a. Probably intrusive Northeastern three exposures; 77.2 and 77.7 wt% SiO2, south and north bodies, (77–78) respectively (Werdon et al., 2000); possibly a single dike. 9 Felsic n.a. Probably intrusive No samples or notes available 10 Felsic n.a. Lava or tuff? No samples or notes available; location suggests possibly tuff as in Gold Creek. 11 Dacite; andesite Pl Bt; Pl Hyp Aug Probably lava Dacite on northwest hill top; andesite by stream on southeast. 12 Felsic n.a. Probably intrusive No samples or notes available. 13 Felsic n.a. Possibly tuff No samples or notes available; mode of occurrence unknown but location suggests possibly tuff from Middle Fork caldera. 14 Felsic n.a. Dike No samples or notes available. Note: Locality number is circled number in Figure 15.

SiO2 wt% for analysis recalculated to sum to 100% volatile free. Phenocryst phase abbreviations: Aug—augite; Bt—biotite; Hbl—hornblende; Hyp—hypersthene; Ox—Fe-Ti oxide; Pl—plagioclase; Qz—quartz; Sa—sanidine; Zrn—zircon. Vol% phenocrysts estimated for thin section(s). n.a.—not available.

*Rapid rock chemical analysis (U.S. Geological Survey, 1971, for H.L. Foster) of sample 70AFr311, locality 5 (wt%): SiO2, 56.0; Al2O3, 14.1; Fe2O3, 3.0; FeO, 6.0; MgO, 1.1; + − CaO, 2.2; Na2O, 2.7; K2O, 5.4; H2O , 6.1; H2O , 1.3; TiO2, 1.8; P2O5, 0.24; MnO, 0.07; CO2, <0.05; total 100.0.

Landscape Evolution calderas once associated with Mount Harper The crustal block southeast of the Mount Veta block plutons have been removed by uplift and block across the northeast-trending Kechumstuk The region between the Denali and Tintina erosion, in contrast to mid-Cretaceous calderas fault zone, here called the Kechumstuk block, faults in east-central Alaska known as the in the Tanacross quadrangle to the south that has Late Triassic and Early Jurassic plutons but Yukon–Tanana Upland is characterized by mod- were downdropped across additional northeast- no exposed Cretaceous plutons (Fig. 3). Impor- erate to low relief (Fig. 1). Evidence of Pleis- trending fault zones (Bacon et al., 1990). Proxi- tantly for understanding landscape evolution tocene glacial erosion is subtle except in the mal outfl ow tuff from the Middle Fork caldera in this region, the Kechumstuk block contains higher elevation, alpine portions (Weber, 1986). rests on the ca. 70 Ma land surface immediately the extensive remnant of rhyolitic welded ash- Preservation of the Middle Fork caldera and west of the caldera fi ll (Figs. 4, 5C, and 5D), fl ow tuff, here considered distal outfl ow from correlated outfl ow tuff, as well as of Late Cre- although it presently is unclear if that surface the Middle Fork caldera, in the valley of Gold taceous–Paleocene lava domes and intracanyon is the northwestern limit of the Middle Fork Creek (Fig. 3). The tuff locally is at least 70 m fl ows, constrains interpretation of the evolution block or the southeastern limit of the Mount thick (Foster, 1976) and originally must have of this landscape and the degree of exhumation Harper block. The Mount Veta block contains been considerably thicker, having ponded in the in blocks bounded by northeast-trending, high- Early Jurassic plutons but also ca. 68–66 Ma paleovalley of Gold Creek (cf. Loma Seca Tuff, angle faults. The Middle Fork caldera occupies equigranular granodiorite-granite composition- Hildreth et al., 1984). This occurrence requires a crustal block, here termed the Middle Fork ally akin to the Middle Fork caldera tuffs and stability of low-lying areas of the Kechumstuk block, between the Mount Harper block to porphyry (Day et al., 2014), much as the ca. block landscape and, in contrast, signifi cant dif- the west and the Mount Veta block to the east 23 Ma Rio Hondo pluton is exposed adjacent to ferential uplift and erosion of the Mount Veta (Fig. 15). Whereas ca. 70 Ma intracaldera tuff the 25.4 Ma Questa caldera fi ll (Lipman, 1988; block in order to expose a ca. 68–66 Ma pluton and the resurgent intrusion are exposed in the Zimmerer and McIntosh, 2012). The Mount between Gold Creek and the caldera. Middle Fork block, the Mount Harper block Veta block therefore has been uplifted and Consistent with Kechumstuk block stability, contains mid-Cretaceous equigranular, and exhumed post–70 Ma to a somewhat greater the high terrace along the Mosquito Fork of the apparently no younger, plutonic rocks. Any extent than the Middle Fork block. Fortymile River (to the south and east) is cor-

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related by Weber and Wilson (2012) with the terrace on Gold Creek (a tributary of the Mos- A quito Fork) that is immediately downstream ANDERSONANDERSON from the outfl ow tuff (Fig. 15). The tuff evi- PPORCUPINEORCUPINE RIVERRIVER dently is preserved on either side of Gold Creek RIVERRIVER BASINBASIN because long-term local base level was that BASINBASIN of the regional high terrace. The high terrace deposits are shown by Weber and Wilson (2012) KKWIKHPAKWIKHPAK as both predating the early (?) Pleistocene Char- RIVERRIVER ley River glacial advance and including Charley BASINBASIN River–age outwash deposits. Remnants of the terrace were identifi ed by Weber and Wilson (2012) in all of the major drainages southeast of BBELLELL RIVERRIVER the Mount Harper lineament in the Eagle quad- YUKONYUKON RIVERRIVER BASINBASIN 500 km rangle, including the Middle Fork of the North BASINBASIN Fork of the Fortymile River as far upstream as Gulf of Alaska the northwest quadrant of the caldera fi ll and granite porphyry (Fig. 15). LATE PALEOCENE TO PLIOCENE RIVER BASINS The high terrace is evidence of regional lowering of base level that has driven cutting of the present stream channels following a long period B of stability that ended with deposition of Charley River–age outwash gravels. Although down- Diversion caused by cutting of the Fortymile and other rivers in east- Laurentide Glaciation central Alaska might be attributed to tectonic (MIS 2) uplift (e.g., Weber, 1986), it has become clear that climatic factors likely played the principal YYUKONUKON role. As a tributary of the Yukon River, the his- RIVERRIVER tory of the Fortymile River is tied to adjustments BASINBASIN in grade of the Yukon, and it is signifi cant that the youngest fl uvial deposits on the high terrace date from the Charley River glaciation. Lower- Diversion ing of base level and regional downcutting of the caused by MMACKENZIEACKENZIE first glaciation Yukon and its tributaries in east-central Alaska 500 km (MIS G6) RIVERRIVER began with the earliest glaciation—the Charley BASINBASIN River glaciation in the Fortymile area. Dur- Gulf of Alaska ing initial regional glaciation, the Cordi lleran ice sheet forced capture of the south-fl owing PLIOCENE TO PRESENT-DAY RIVER BASINS ancestral Yukon by the north-fl owing Kwikhpak River, establishing the modern Yukon River Figure 16. (A) Late Paleocene and (B) present-day drainages of drainage network (Fig. 16; Tempelman-Kluit, Alaska and northwestern Canada. Yellow star shows location 1980; Duk-Rodkin et al., 2001, 2010). Hence, of Middle Fork caldera. Adapted from Duk-Rodkin et al. (2010, the high terrace on the Fortymile River pre- Fig. 2) © Canadian Science Publishing or its licensors. MIS— serves a long-established paleovalley system marine isotope stage. that has been entrenched by many tens of meters since the onset of Cordilleran glaciation. The beginning of full glacial conditions in the Yukon Creeks and probably also the Charley River Mount Veta block, which we have argued was Territory occurred in the Gauss normal chron cluster (Fig. 15, locs. 1, 4, and 2, respectively). uplifted and exhumed to a greater extent post– >2.58 Ma (Froese et al., 2000) and probably The Slate Creek and Charley River domes are 70–66 Ma than the adjacent Middle Fork block. ca. 2.7–2.6 Ma (Haug et al., 2005; Miller et al., immediately northwest of the northeast-trending This higher standing ground provided less pro- 2010). The Middle Fork outfl ow tuff in Gold Black Mountain tectonic zone identifi ed by Day tection from erosive forces than basins that host Creek demonstrates that low-lying elements of et al. (2007) and O’Neill et al. (2010), northwest the domes near Slate and Ruby Creeks and the the landscape in the Kechumstuk block were of the relatively deeply exhumed Mount Harper Charley River. established by 70 Ma and were preserved with block. A large remnant of an intracanyon lava The high terrace on the Middle Fork of the little modifi cation until ca. 2.7 Ma. fl ow preserved near Slate Creek provides fur- North Fork of the Fortymile River apparently is Cretaceous–Early Tertiary lava domes within ther evidence of only minor downcutting since fl oored by felsic lava near Manila Creek (Fig. ~50 km of the Middle Fork caldera commonly the Paleocene. The evidently less completely 15, loc. 5), northeast of the caldera. Curiously, retain elements of their original constructional preserved felsic volcanic landforms northeast latite vitrophyre crops out at the confl uence, morphology. Some clearly occupy local down- of the Middle Fork caldera (Fig. 15, locs. 5–8) which is diffi cult to reconcile with Pleistocene faulted basins, such as near Slate and Ruby are largely, if not entirely, within the projected downcutting of the terrace if the vitrophyre

1452 Geosphere, December 2014

represents the front of a lava fl ow. A plausible (Fig. 15, locs. B and C) have AFT age distri- ca. 70 Ma age of the tuffs are consistent with explanation may lie in the change in character butions that suggest rapid cooling at ca. 40 Ma eruption and cooling during chron C31r. of the valley of the Middle Fork of the North and fi nal cooling after ca. 40 Ma, respectively Northeast-trending faults divide the land- Fork of the Fortymile River 5 km northeast, near (Dusel-Bacon and Murphy, 2001). The wide- scape into a set of crustal blocks, some of which Portage Creek (Fig. 15), where the river leaves a spread geographic distribution of ca. 40 Ma are relatively deeply exhumed and exhibit more broad alluviated valley to enter a narrow canyon AFT ages suggests that differential exhumation rugged terrain and others that have been stable downstream that traverses Triassic granodiorite on the order of 2–4 km of the sampled material since Late Cretaceous or Early Tertiary time. yet preserves antecedent meanders. The terrace and uplift across northeast-trending faults, such Like mid-Cretaceous calderas to the south, the surface has virtually the same ~650 m eleva- as of the Mount Veta block, postdates cooling Middle Fork caldera is preserved in a crustal tion at Manila Creek as downstream where the through the PAZ. Reconciliation of AFT results block that is downdropped along regional north- Middle Fork of the North Fork joins the North with preservation of Middle Fork outfl ow tuff east-trending, high-angle faults within a wide Fork proper (Fig. 15) but the modern river in the valley of Gold Creek and Paleocene region of low to moderate topographic relief. descends from ~600 m to ~550 m. Possibly, lava domes in local basins may require appeal- Uplift and exhumation have been greater for the pre-Pleistocene downfaulting west of the grano- ing to (1) Cretaceous establishment of canyon adjacent Mount Veta block to the south in which diorite along a cryptic fault raised local base fl oors and base level within a high-relief land- ca. 68–66 Ma equigranular plutonic rocks level and caused aggradation upstream to bury scape (e.g., Eocene Nevada, Henry, 2008), fol- are now exposed. At the west edge of the cal- the lava fl ow front; subsequently, the low gra- lowed by (2) rapid late Paleocene–early Eocene dera, proximal outfl ow tuff rests on the 70 Ma dient represented by the present terrace became degradation of interfl uves during that interval ground surface. Farther southeast, the northern established and persisted until early Pleistocene of warm and wet high-latitude climate (Jahren Kechumstuk block has been vertically stable regional base-level lowering resulted in renewed and Sternberg, 2003; Moran et al., 2006; Green- such that low-lying elements of the Late Cre- downcutting, terrace formation, and exhumation wood et al., 2010) to account for exhumation taceous landscape are preserved as evidenced of the vitrophyre, itself an intracanyon lava fl ow and rapid cooling of AFT-sampled rock bodies by presence of distal outfl ow tuff in the valley emplaced much earlier. On balance, the mor- (now exposed on relatively high ground), suc- of Gold Creek. Elsewhere in the region, Creta- phologies and topographic positions of the fel- ceeded by (3) slower adjustment of interfl uves ceous (?)–Paleocene rhyolitic lava domes and an sic volcanics are consistent with a long period and slopes to produce the modern subdued intracanyon lava fl ow are present in local basins, of stability of at least the low-lying parts of the landscape. downdropped adjacent to northeast-trending landscape, as demonstrated by the Middle Fork fault zones, and as isolated remnants of lava, outfl ow tuff in the valley of Gold Creek and the CONCLUSIONS tuff, and small hypabyssal intrusions. Preser- regional high terrace on Yukon River tributaries. vation of Middle Fork outfl ow tuff in Gold Further constraints on uplift and exhuma- The Middle Fork caldera serves as a clear Creek and of rhyolite domes and intra canyon tion in the Fortymile region are provided by example of voluminous eruption of rhyolitic lavas shows that many paleovalleys are little apatite fi ssion-track (AFT) data. Dusel-Bacon magma that was comagmatic with a spatially modifi ed since Late Cretaceous–Early Tertiary and Murphy (2001) presented weighted-mean associated K-feldspar megacrystic granite por- time except by comparatively recent incision ages along with single-grain and confi ned track- phyry intrusion. The caldera structure is defi ned that produced a regional terrace on tributaries length AFT results for fi ve plutonic and seven by a 10 km × 20 km area of intracaldera rhyolite of the Yukon River. Formation of this regional metamorphic rocks from the Yukon–Tanana tuff. The 8 km × 12 km porphyry intrusion was terrace apparently was driven by rapid lower- Upland. The majority of samples indicate rapid responsible for asymmetric resurgent doming of ing of Yukon River base level ca. 2.7 Ma when cooling at ca. 40 Ma for 3–5 m.y. through the caldera fi ll. Erosion has destroyed the original the Cordilleran ice sheet induced capture of the apatite partial annealing zone (PAZ; ~110– topographic caldera, removed perhaps 1–3 km south-fl owing ancestral Yukon by the Kwikhpak 60 °C). These results were interpreted to refl ect of intracaldera tuff, and exposed the porphyry. River to form the modern north-fl owing Yukon heating during Eocene extensional magmatism Radiometric ages of tuffs and porphyry are ana- River system. and exhumation from >3.8 km to 2.7 km depth, lytically indistinguishable at ca. 70 Ma. Despite ACKNOWLEDGMENTS assuming geothermal gradients of 32–45 °C/km. its Late Cretaceous age, outfl ow tuff is preserved Samples that yield weighted mean AFT ages at the west margin of the caldera and 16–23 km Our work on the Middle Fork caldera, the resur- >50 Ma have confi ned track lengths and single- east of the caldera in the valley of Gold Creek. gent intrusion, and felsic volcanic rocks in the region grain age distributions that suggest incomplete Similar whole-rock chemical compositions and has benefi ted greatly from access to the original fi eld Eocene annealing from temperatures ≤110 °C phenocryst mineralogy indicate that the tuffs notes, map sheets, and thin sections of H.L. Foster and coworkers. Renee Pillers separated zircon for U-Pb at depths of 3.3–2.0 km; these samples repre- and porphyry are products of one magmatic geochronology. Kate Gans and Joel Robinson helped sent rock bodies that were closer to the Eocene system that produced a single caldera-forming with geographic information system preparation of ground surface. In the Fortymile region, one eruption and resurgent intrusion, likely from the geologic map of the caldera. Jon Hagstrum kindly sample with a Late Cretaceous (ca. 76 ± 7 Ma; thermally rejuvenated crystal mush. Geochemi- measured magnetic properties of rocks. We appreciate σ constructive reviews by Joe Colgan (for USGS), Ryan 1 uncertainty) weighted mean AFT age (Dusel- cal data for rhyolite tuffs, granite, and a mafi c Mills, an anonymous reviewer, and Associate Editor Bacon and Murphy, 2001) is from the Mount enclave are consistent with continental arc mag- Lang Farmer that led to clarifi cation and improvement Harper block immediately west of the Mount matism. An aeromagnetic survey (Veach, 1973) of the manuscript. 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