A ~30 MILLION YEAR HISTORY OF MAFIC MAGMATISM AND PARTIAL MELTING IN THE CHELAN MIGMATITE COMPLEX, NORTH CASCADE RANGE, WASHINGTON, USA

The Chelan migmatite complex (CMC) records partial melting and multiple stages of mafic magmatism. Split-stream U-Pb + REE LA-ICPMS zircon analyses of 7 samples from the southern half of the CMC, interpreted as the most deeply exposed part, document Jurassic-Triassic inheritance and 90-117 Ma ages interpreted to record the time span the CMC was partially molten.

Four protolith-dominated samples contain abundant 160-170 Ma zircon, and a leucosome sample yields one 248 Ma analysis. These zircons occur as entire grains and as resorbed cores overgrown by Cretaceous rims, and are interpreted to record the igneous age of the CMC tonalite protolith. A 112-117 Ma population is observed in 4 samples distributed across the sampled area, and is always associated with inherited grains. This population includes oscillatory-zoned whole grains and rims on inherited cores. Two younger populations only occur in the leucosome and two mafic samples; mafic samples lack both the 112-117 Ma and older inherited populations. A 98-105 Ma population occurs as oscillatory- and sector-zoned whole grains and as rims on >112 Ma cores. A 90-98 Ma population only occurs as thin, oscillatory-zoned rims.

These results suggest that injection of mafic magma beneath and into Jurassic-Triassic protoliths began >117 Ma, causing heating and partial melting of the protolith, and variable resorbtion of inherited zircons. Initial cooling from peak temperature produced the widely distributed 112-117 Ma population, and thus ~117 Ma is taken as the time of the temperature peak. Escape of partial melt resulted in the samples with little or no younger zircon growth. Continued mafic magma intrusion resulted in the younger zircon populations observed in the leucosome and mafic samples. In these samples, variable trace element trends indicate open-system processes, consistent with field evidence for magma mixing, multiple stages of magma injection, and inferred migration of hybridized melts. The thin zircon rims as young as 90 Ma are interpreted to record crystallization of the last melt, consistent with the interpretation that an 89 Ma CMC-derived dike cutting the adjacent supracrustal metamorphic rocks marks final CMC emplacement, and that escaping partial melt may also have fed adjacent 89-92 Ma plutons.