Middle Paleozoic magmatism and orogenesis in the Brooks Range, Alaska

Abstract

Felsic metaigneous rocks from the southern Brooks Range formerly thought to be of Cretaceous age have yielded Devonian and late Proterozoic U-Pb and Pb-Pb zircon ages. Eight slightly discordant U-Pb ages on euhedral zircons from south-central Brooks Range plutons and metavolcanic rocks define a chord that intersects concordia at 365 ±15 m.y. and 6 ± 120 m.y. Assuming episodic lead loss, the upper intercept is interpreted as the time of magmatic crystallization and the lower intercept as the time of cessation of lead loss due to metamorphism. The Middle Devonian crystallization date is supported by a Rb-Sr whole-rock isochron of 373 ± 25 m.y. Six zircon separates from the Ernie Lake and Sixtymile plutons of the south-central Brooks Range gave various late Proterozoic U-Pb and Pb-Pb ages. Because some of the zircons from these two plutons have rounded cores inherited from zircons in either the source rocks or the country rocks of the plutons, we are uncertain if the two plutons first crystallized during Proterozoic or Paleozoic time. From these data and other geologic criteria we hypothesize that (1) many Brooks Range plutons and some volcanic rocks were formed during middle Paleozoic time; (2) younger K-Ar mineral dates from these metaigneous rocks reflect a Mesozoic metamorphic event; (3) middle Paleozoic magmatic rocks together with adjacent coeval marine carbonate and clastic rocks and syngenetic ore deposits constituted an ensialic island arc or submerged continental margin magmatic belt; and (4) amphibolite-facies rocks around the plutons include early Paleozoic and late Proterozoic(?) crystalline basement to the middle Paleozoic magmatic belt.