Dissected caldera structures expose thick intracaldera tuff and, uncommonly, cogenetic shallow plutons, while remnants of correlative outflow tuffs deposited on the pre-eruption 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 east-central 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 fill. Sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) analyses of zircon from intracaldera tuff, granite porphyry, and outflow 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 SiO2 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 solidified crystal mush. A rare magmatic enclave (54% SiO2, arc geochemical signature) in the porphyry may be similar to parental magma and provides evidence of mafic 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 fill 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 outflow tuff at the west margin of the caldera structure and beneath welded outflow 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 km2 to ∼100 km2. 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 flowed into the paleovalley of the Middle Fork of the North Fork of the Fortymile River. To the northwest of the Middle Fork caldera, in the Mount Harper crustal block, mid-Cretaceous plutonic rocks are widely exposed, indicating greater total exhumation. To the southeast of the Middle Fork block, the Mount Veta block has been uplifted sufficiently to expose a ca. 68–66 Ma equigranular granitic pluton. Farther to the southeast, in the Kechumstuk block, the flat-lying outflow tuff remnant in Gold Creek and a regionally extensive high terrace indicate that the landscape there has been little modified since 70 Ma other than entrenchment of tributaries in response to post–2.7 Ma lowering of base level of the Yukon River associated with advance of the Cordilleran ice sheet.
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