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The magmatic arc represented by the crystalline core of the North Cascades (Cascades core) reached a crustal thickness of >55 km in the mid-Cretaceous. Eocene collapse of the arc was marked by migmatization, magmatism, and exhumation of deep-crustal (9-12 kb) rocks at the same time as subsidence and rapid deposition in nearby transtensional nonmarine basins. The largest region of deeply exhumed rocks, the migmatitic Skagit Gneiss Complex, consists primarily of leucocratic, biotite tonalite orthogneiss intruded between ca. 76-59 Ma and 50-45 Ma. Well-layered biotite gneiss is also widespread. U-Pb (isotope dilution-thermal ionization mass spectrometry) dating of zircon and monazite...

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