Petrology and petrogenesis of the Cardinal Peak pluton, North Cascades, Washington

The focus of this study is on the magmatic evolution of the 72 Ma Cardinal Peak pluton. This arc-type pluton is predominantly tonalitic, although its northwest and central sections contain significant quantities of mafic rocks. Field evidence and geochemical data indicate that both fractional crystallization and melting of a mafic lower crust were important processes in the evolution of the pluton, and that all sections of the pluton were constructed by numerous magma pulses. This study concludes that the following sequence of events formed the pluton: (1) Evolved mafic to felsic mantle-derived magmas were emplaced as the northwest and central Border Complex and the northwest Main Phase. (2) Heat from these mantle-derived magmas melted the lower crust and caused production of a tonalitic magma, which was emplaced as the central and southeast Main Phase. (3) Minor mafic magmatism continued during the emplacement of the central and southeast Main Phase.