2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 14
Presentation Time: 1:30 PM-5:30 PM


MERKEL, Ian S. and MAGLOUGHLIN, Jerry F., Department of Geosciences, Colorado State University, Fort Collins, CO 80523, ism@cnr.colostate.edu

Dikes of pegmatitic tonalite from the North Cascade Mountains, Washington, USA, contain clinozoisite/epidote (Czo/Ep), muscovite (Ms), quartz (Qtz), plagioclase (Pl), and rare garnet. The textures are complex and their interpretations offer the opportunity to better understand the ascent, evolution, and emplacement of deep-seated silicic magmas.

These dikes contain at least six textural groups of Czo/Ep and at least five textural groups of Ms. Czo/Ep crystals are up to 13 cm in length and often contain lamellar twins. Four Czo/Ep groups are most common and contain optically visible, intricate zoning, including euhedral oscillatory and patchy zoning as a result of primary crystal growth, and a diffusional type zoning. The diffusional zoning consists of higher iron areas along grain boundaries and intra-granular fractures. This zoning is likely the result of late migration of iron rich fluids along discontinuities. Some oscillatory-zoned crystals contain unzoned, chemically distinct cores. The dikes also contain large unzoned Fe-poorer Czo/Ep crystals, compared to the zoned Czo/Ep, and which have well developed diffusional zoning. Czo/Ep crystal groups locally rim these. Two final groups are mm-scale unzoned, unaltered anhedral Czo/Ep, and Czo after Pl. The largest Ms crystals are cm-scale and zoned in Ba as determined by electron microprobe. This zoning is locally detectable optically. There are subhedral to anhedral mm-scale primary Ms, mm-scale Ms with skeletal overgrowths and radial Ms associated with Czo/Ep alterations, micron-scale Ms crystal overgrowths on larger Ms crystals, and Ms after Pl. Statistical evaluations of reconnaissance trace element analyses indicate that many of these groups are also chemically distinct.

The coarsest-grained, magmatic Czo/Ep crystals have been partially replaced by Ms+Pl+Qtz+/-Ep in the form of crude pseudomorphs and alteration halos. In the pseudomorphs coarse-grained Ms replaces Czo/Ep. The well-formed habit of these Ms implies replacement prior to dike solidification. The halo alterations commonly consist of fine-grained intergrowths of Ms and Pl, suggesting subsolidus alteration.

These preliminary findings indicate at least four magmatic Czo/Ep textures and one metamorphic generation, enabling unique insights into the evolution of silicic magmas.