Age and evolution of Western Brooks Range ophiolites, Alaska: Results from 40Ar/39Ar thermochronometry

Basalt, gabbro, and peridotite in the western Brooks Range structurally overlie Mississippian through Late Jurassic platform sediments that were deformed during Late Jurassic - Cretaceous time. 40Ar/39Ar step-heating experiments on hornblende, biotite, plagioclase, and potassium feldspar from gabbro, diorite, plagiogranite, and granite yielded complex release spectra. On isotope correlation diagrams, the incremental-heating data indicate that gabbro, diorite, and plagiogranite cooled below the closure-temperature for argon in amphibole between 187 and 184 Ma. Because the closure temperature of argon retention in amphibole is relatively high, these ages are interpreted to be the time since crystallization. Biotite and hornblende from metamorphic rock exposed along faults separating peridotite and gabbro from underlying basalt yielded plateau and isochron dates of 169 – 163 Ma; these dates are interpreted to record postmetamorphic cooling following intraoceanic detachment and transport of “hot” peridotite and gabbro over “cold” basalt and sediment. Detachment and faulting of the gabbros and peridotites is interpreted to have occurred approximately 20 Ma after crystallization, and is longer than the 5–10 Ma interval commonly observed in studies of other ophiolites. Fossils in flysch underlying the ophiolites indicate that the ophiolites were emplaced onto the continental margin during the Tithonian, approximately 15–20 Ma after initial detachment. The ages of feldspars (149 – 137 Ma) in the ophiolites and associated volcanic rocks are similar to the fossil ages of the underlying flysch and might record uplift during emplacement of the ophiolites onto the continental margin. A fission-track age of apatite implies that the ophiolitic rocks were overlain by >4 km of rock until ∼59 Ma ago, the time of regional uplift in the Brooks Range. Supplement Appendix 1 is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. document T92-005; $2.50. Payment must accompany order.