Ages of Zircons from the Northern Cascade Mountains, Washington

Abstract

Pre-Late Cretaceous crystalline rocks in the core of the Northern Cascade Mountains of Washington are predominantly heterogeneous supracrustal gneiss and schist, and dioritic, quartz dioritic, and trondhjemitic plutons and bodies of orthogneiss. Migmatite derived from plutonic and supracrustal rocks is also widespread. Isotopic age determinations of zircons indicate that, of the supracrustal gneiss and schist, the Swakane Gneiss and possibly the Skagit Gneiss may have been deposited > 1,650 m.y. ago. Alternatively, some of these rocks may be younger but were derived at least in part from a >l,650-m.y.-old source terrane. A second group of supracrustal rocks, the Younger gneissic rocks of the Holden area, includes a metavolcanic unit about 265 m.y. old. Plutonic rocks and orthogneisses comprise four main age groups: 1,452 to 2,000 m.y., pyroxene gneiss of the Yellow Aster Complex; about 460 m.y., the Turtleback Complex of the San Juan Islands and younger orthogneiss of the Yellow Aster Complex; 220 m.y., plutonic and gneissic rocks of the Marblemount belt (Marblemount Meta-Quartz Diorite, Dumbell Mountain plutons); and 92 m.y., the Eldorado Orthogneiss. Strongly metamorphosed rocks of the Chelan Complex, chiefly quartz dioritic, have zircon Pb 206 /U 238 ages ranging from 100 to 183 m.y. These ages appear to be discordant, and the Chelan Complex may represent remobilized rocks from the 220-m.y.-old Marblemount belt. Ages of metamorphic minerals, zircons from pegmatitic material associated with migmatite, and zircons from synkinematic intrusive rocks indicate two major episodes of metamorphism in the Northern Cascade terrane, one about 415 m.y. ago and the other, 60 to 90 m.y. ago. Both episodes culminated in metamorphism to amphibolite facies grade—and in the case of the older episode, metamorphism to granulite facies grade—and both resulted in extensive migmatization of pre-existing rocks.