Middle Jurassic U-Pb Ages for Brooks Range Ophiolites, Alaska
Moore, T E (1), Aleinikoff, J N (2), Wirth, K R (3)
(1)USGS, MS 901, Menlo Park, CA 94025 United States; (2) USGS, MS 963, Denver, CO 80225 United States; (3) Macalester College, St. Paul, MN 55105 United States
Abstract:
Isolated klippen of mafic and ultramafic rocks are present along most of the length of the Brooks Range and form its highest structural units. These klippen are erosional remnants of once extensive allochthons of ophiolitic rocks of supra-subduction zone type that were obducted northward onto Devonian to Jurassic metabasalt and structurally underlying passive-continental margin deposits in the latest Jurassic or early Neocomian. Sparse metamorphic amphibolite from the sole of the ophiolitic allochthons has previously yielded K-Ar and 40Ar-39Ar emplacement ages of 171-153 Ma; gabbroic rocks and plagiogranite from the ophiolite yield 40Ar-39Ar crystallization ages of 197-162 Ma.
We report here U-Pb ages from plagiogranite for three of the ophiolitic klippen. Five fractions of subhedral, tan zircon from the plagiogranite in the Siniktanneyak Mountain ophiolite yield slightly discordant U-Pb ages with an upper intercept age of 170 ± 3 Ma and three fractions of tan, euhedral zircon from plagiogranite intruding high-level gabbro at Misheguk Mountain are concordant at 165±1 Ma (weighted average of 296Pb/238U ages). Seven fraction of subhedraltan zircon from quartz diorite from the Asik Mountain klippe have high common Pb contents, requiring a 3-d plot (Ludwig, 1998) to resolve the age of 162 ± 1 Ma. Three grains of monazite from a granitic dike that intrudes the ultramafic part of the Siniktanneyak Mountain have an age of 163 ± 1 Ma (weighted average of 207Pb/235U ages. This dike is interpreted to represent a plutonic episode that followed ophiolite crystallization and to be related to ophiolite emplacement. Biotite from a similar dike exposed in the Avan Hills yielded an 40Ar-39Ar age of 164 Ma, indistinguishable from the age of the metamorphic sole (169-163 Ma).
The U-Pb data indicate that the Siniktanneyak Mountain ophiolite and probably all of the Brooks Range ophiolites crystallized in the Middle Jurassic (170-162 Ma) and were emplaced by thrusting (169-163 Ma) onto mafic accretionary prism rocks shortly after crystallization. The U-Pb data from the Asik Mountain body are significantly younger than the published amphibole 40Ar-39Ar ages (187-184 Ma) from high-level gabbro from the same body. Only two of the twenty-five amphibole samples studied yielded "plateaus" (169-163 Ma). However, several amphibole samples yielded relatively straight-forward isochrons that indicate an older crystallization age (187-184 Ma). The low K compositions (K/Ca = 0.005-0.16) of the gabbroic amphiboles and complex release spectra may limit the utility of Ar-Ar dating of these, and other similar amphiboles, from ophiolites. Alternately, the granitic rocks studied using U-Pb techniques may be significantly younger than the gabbroic rocks that comprise the main layered portions of the ophiolites; cross-cutting relations in the field support this interpretation, but the magnitude of this age difference is not known. Additional U-Pb and Ar-Ar studies of the gabbroic portions of the ophiolites are needed to resolve this apparent inconsistency.
