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New K-Ar ages from allochthonous mafic and ultramafic complexes of the western Brooks Range (Brooks Range "ophiolite") show that igneous rocks yield ages nearly identical to those of underlying metamorphic aureole rocks. Dated rocks of the Misheguk igneous sequence from Tumit Creek consist of (1) hornblende gabbro with minor greenschist and lower grade alteration, hornblende age 147.2 ± 4.4 Ma; and (2) hornblende-bearing diorite, also slightly altered, age 155.8 ± 4.7 Ma. Both samples come from presumed higher levels of the Misheguk sequence. Dated samples of metamorphic aureole rocks come from outcrops near Kismilot Creek and lie structurally beneath the Iyikrok Mountain peridotite body. The rocks consist of amphibolite and garnet-bearing biotite-hornblende gneiss considere to be metamorphosed Copter igneous sequence and related sedimentary rocks. Hornblende ages are 154.2 ± 4.6 Ma and 153.2 ± 4.6 Ma. Metamorphism is clearly related to the structurally overlying peridotite, as the degree of alteration decreases downward.

We suggest that the K-Ar ages of these rocks represent the effects of thermal metamorphism post-dating igneous crystallization and are related to tectonic emplacement of the complex. Earlier K-Ar data on igneous rocks give similar ages and have been interpreted as reflecting tectonothermal events. The age of igneous crystallization of the mafic and ultramafic rocks of the Misheguk igneous sequence remains uncertain.

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