MID-CRETACEOUS PLUTONISM IN CHELAN MOUNTAINS TERRANE, NORTH CASCADES, WASHINGTON: RELATIONSHIP OF INTRUSIVE PLUTONS TO CHELAN MIGMATITE COMPLEX

A tilted crustal section along the Chelan Mountains terrane reveals a NW to SE depth spectrum from relatively shallow to progressively deeper exposures (<10 km to >20 km) through "mid-Cretaceous" (~90 Ma) tonalite-granodiorite plutons, to a huge migmatitic metaplutonic mass (Chelan Migmatite Complex, CMC) emplaced at ~25-30 km (Hopson, Mattinson & Mattinson, 2000). The plutons intrude Cretaceous supracrustal metamorphic rocks, chiefly remnants of an early Mesozoic arc-arc basin assemblage, whereas the migmatite complex deflects the supracrustals from their NW-SE regional trend and injects them with leucotrondhjemite-leucogranodiorite dikes in an aureole up to 3 km wide. Restoring these plutonic and metamorphic rocks to their approximate mid-Cretaceous positions by removing the intervening younger intrusive bodies reveals a narrow belt of upward-widening plutons aligned with antiformal CMC that plunges northwestward (and by inference continues) beneath them. The amphibolitic-metatonalitic-leucotrondhjemitic CMC (120 to <100 Ma), derived from partial melting of tonalitic basement, was emplaced into the Mesozoic arc supracrustals as heterogeneous crystal-melt mush at 7-9 kbar. This climaxed its rise from still deeper levels accompanied by repeated intrusion of mafic magma and its differentiates and by mixing/commingling of mafic and felsic melts (Hopson & Mattinson, 1994, 1996). The antiformal CMC was evidently parental to the overlying plutons. The following progressive stages are inferred: 1) anatexis of plutonic roots of the early Mesozoic volcanic arc at ~120-100 Ma, with repeated influx of mantle-derived basaltic magma, formed heterogeneous mafic-felsic mushy mixtures that rose from deeper crust as a partially molten antiformal mass (CMC); 2) upward segregation of melts during the mushy upwelling concentrated tonalitic neomagma (granodioritic melt + crystalline residuum) toward the antiform apex; 3) the arc strata, arched and extended above the upwelling anatectic basement, were invaded by mushy magma (the tonalitic/granodioritic plutons) expelled from the melt-enriched antiform apex; and 4) a last upward pulse of the hypersolidus CMC mafic/tonalitic restite pushed into the overlying metavolcanics as a migmatitic "gneiss dome", expelling residual melt (leucocratic dikes) in an injection aureole.