MULTIPLE POPULATIONS OF MUSCOVITE; TEXTURAL AND CHEMICAL INDICATORS OF PETROGENETIC ORIGIN

Muscovite is a common mineral in igneous rocks, occurring in peraluminous felsic intrusive rocks crystallized at high pressures. Discrimination between primary magmatic and secondary hydrothermal muscovite within peraluminous igneous rocks, which has been based upon textural and chemical characteristics, has commonly been inconclusive. Trondhjemite intrusions located within the North Cascade Mountains, Washington preserve eight textural populations of muscovite. This study illustrates that combined textural and chemical attributes of muscovite allows petrogenetic discrimination between magmatic and post-magmatic muscovite.

Textural attributes of muscovite of this study include coarse-grained euhedral to subhedral and fine-grained anhedral groundmass grains, relatively coarse-grained anhedral grains partially pseudomorphing magmatic epidote, fine-grained anhedral grains distally and proximally associated with alteration halos of magmatic epidote, overgrowths of fine-grained muscovite upon earlier muscovite, and muscovite after plagioclase. The textures indicate episodes of early magmatic, late magmatic, and subsolidus muscovite growth.

Major and minor element concentrations were obtained for all textural populations using electron microprobe. Bivariant diagrams have been used to discriminate between populations. These chemical indicators suggest that coarse-grained groundmass grains and coarse-grained muscovite grains within and around partially pseudomorphed epidote, fine-grained groundmass grains and fine-grained muscovite proximal to epidote alteration halos are late magmatic, and fine-grained muscovite distal to epidote alteration halos, fine-grained muscovite overgrowths, and muscovite after plagioclase are subsolidus. The studies shows that Ti and Na can be used as chemical indicators of magmatic origin for Ms, and in conjunction with the proper textural attributes can be used to indicate a magmatic origin.