Late Cretaceous Middle Fork caldera and its resurgent granite porphyry intrusion, east-central Alaska

Named for the Middle Fork of the North Fork of the Fortymile River, the Middle Fork caldera encompasses a 10 x 20 km area of rhyolite welded tuff and granite porphyry ~100 km west of the Yukon border. Intracaldera tuff has ≤4 mm quartz and feldspar phenocrysts and cm-sized fiamme; its maximum exposed thickness is 850 m. Less densely welded tuff near the caldera margins locally contains 1-2 cm K-feldspar megacrysts and pumice clasts to 6 cm. Zircon from intracaldera tuff yields a SHRIMP-RG U-Pb age of 68.7 ± 1.1 Ma (all ages 95% confidence). Granite porphyry occupies much of an 8 x 12 km area having 650 m of relief within the western part of the caldera fill. Zircon from the porphyry gives a SHRIMP-RG U-Pb age of 68.4 ± 1.0 Ma. These ages agree with a previous ⁴⁰Ar/³⁹Ar biotite age of 69.1 ± 0.5 Ma for proximal outflow tuff. The crystal-rich intracaldera tuff contains embayed quartz, plagioclase>K-feldspar, biotite, and Fe-Ti oxide phenocrysts in a very fine-grained crystalline groundmass. The porphyry carries 40-50% of larger phenocrysts of the same phases (skeletal quartz to 1 cm; K-feldspar to 2 cm, rarely to 4 cm) in a fine-grained groundmass characterized by abundant 50-100 μm quartz. Compositions of 3 tuff and 3 porphyry samples overlap, form a limited differentiation series at 69-72% SiO₂, have arc geochemical signatures, and yield subparallel chondrite-normalized rare earth element patterns with light REE enrichment, concave-upward heavy REE, and small negative Eu anomalies. Although their phenocrysts differ in size (owing to fragmentation of crystals in the tuff) and abundance, the similar mineralogy, composition (in spite of crystal concentration in the tuff), and indistinguishable ages of the tuff and porphyry indicate that the magmas were closely related. 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 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 cuts an arcuate valley into and around the caldera on the west and north, and may have cut down from an original caldera moat. Proximal outflow tuff, and thus the 69 Ma land surface, remains at the west margin of the caldera structure. The Middle Fork caldera lies within a region of Paleozoic metamorphic rocks and Mesozoic plutons bounded by northeast-trending faults. To the northwest, Cretaceous plutonic rocks are widely exposed, indicating greater exhumation. The Middle Fork is a relatively well preserved caldera within a broad region of Alaska and adjacent Yukon that contains Late Cretaceous plutons and, in the less deeply exhumed blocks, silicic volcanic rocks.