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

Abstract

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

Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes

“…70 Ma Middle Fork caldera (Fig. 3) forms a 10 × 20 km area of rhyolitic welded tuff and granite porphyry (Bacon and Lanphere, 1996;Bacon et al, 2014). Altered rhyolite porphyry collected from drill core at the Fish mineral prospect (Table 1), located ~12 km east of the Middle Fork caldera, yielded a SHRIMP U-Pb zircon age of 70.5 ± 1.1 Ma (30 in Fig.…”

“…The Mount Harper batholith is cut by the steep, northeast-trending, Mount Harper fault that accommodated uplift of the Mount Harper block (Newberry et al, 1998a;Dusel-Bacon and Murphy, 2001;Dusel-Bacon et al, 2002;Bacon et al, 2014). The contact between the Lake George and Fortymile River assemblages is not exposed but is interpreted to be a fault based on the tectonic relationship of these assemblages in the eastern part of the Eagle quadrangle (Hansen and Dusel-Bacon, 1998) and on the occurrence of a northwest-trending high-angle fault that we infer continues along Molly Creek and coincides with the contact between these two assemblages ( Figs.…”

“…The 6 × 8 × 12 cm mafic enclave is a fine-grained hornblende-plagioclase-biotite diorite with a texture that shows that the enclave magma crystallized rapidly in undercooled conditions. Bacon et al (2014) suggested that the enclave may be similar to parental magma of the caldera and indicates an input of higher temperature mafic magma.…”

“…4-Laser ablation-inductively coupled plasma-mass spectrometry zircon age from Allan et al (2013). 5-SHRIMP zircon ages for samples 10ADb17 and 10ADb22 from Bacon et al (2014). Pb mantle curve (Doe and Zartman, 1979) Shale curve (Godwin and Sinclair,1982) average crustal curve (Stacey and Kramers, 1975) Devonian-Mississippian metaigneous rocks Shale curve (Godwin and Sinclair,1982) average crustal curve (Stacey and Kramers, 1975) mantle curve (Doe and Zartman, 1979) Table 4.…”

Landscape antiquity and Cenozoic drainage development of southern Yukon, through restoration modeling of the Tintina Fault

“…The Yukon plateaus represent a low-relief erosional surface that truncates polydeformed metamorphic rocks and intrusions of the Yukon-Tanana Terrane. The surface is locally overlain by mid-and Lower Cretaceous rhyolitic volcanic rocks (Gordey 1988(Gordey , 2013Bacon et al 1990Bacon et al , 2014, and early Cenozoic magmatism at 50-60 Ma produced felsic and mafic intrusive and extrusive rocks, sporadically distributed in central and east-central Alaska and adjacent western Yukon (Jackson et al 1986;Dusel-Bacon et al 2016, and references therein). In southern Yukon, the surface of the Yukon plateaus is best preserved beyond the limits of regional glaciation; principally in the Klondike Plateau where only limited alpine glaciation occurred during the late Pliocene or earliest Pleistocene (Lowey 2000;Nelson and Jackson 2003;Jackson et al 2009).…”