Plagioclase compositions and non-anatectic origin of migmatitic gneisses in Northern Cascade mountains of Washington State

Abstract

Regionally developed migmatitic gneisses make up most of the Skagit Gneiss, though there are some orthogneisses also, derived from pre- to late-metamorphic intrusives. The migmatites contain countless, though quantitatively subordinate, remnants of biotite schists, less abundant amphibolitic rocks, and minor varieties of metasediments. Biotite schists (fine-grained paragneisses) predominantly are plagioclase-rich, having the compositions of highly immature graywackes; quartz-rich varieties are minor, and alumina-excess rocks very rare. There are para- and ortho-amphibolites, the latter of basaltic-gabbroic parentage. Leucocratic hornblende schists are largely meta-sedimentary. Apart from subordinate varieties in which K-feldspar is a major constituent, the leucocratic migmatitic gneisses have (leuco-) trondhjemitic and, to a lesser extent, quartz-dioritic compositions.