CHEMISTRY OF MULTIPLE POPULATIONS OF MAGMATIC EPIDOTE IN TONALITE PEGMATITE, NORTH CASCADE MOUNTAINS, WASHINGTON

At least six textural groups of epidote are present in pegmatitic tonalite dikes in the North Cascade Mountains in Washington state. The epidote is mostly close to the Czo/Ep boundary (Fe³⁺ ~0.4-0.5 cpfu, on the basis of 12.5 O). The individual textural groups involve populations of crystals ranging in average size from a few tens of microns to 1-2 cm. The largest individual crystals are 13 cm long, with some boudinaged crystals attaining nearly 20 cm in total length. Texturally, there are clear overgrowth and rimming relationships, with the largest crystals displaying sharply defined euhedral oscillatory zoning.

Based on electron microprobe analyses, Mn is low (~0.04 cpfu), and SrO ranges from 0.35 to 0.92 weight percent. LA-ICP-MS data have been gathered on five of the populations, and using discriminant analysis, all appear to be distinct chemically. The earliest epidote has a fairly flat REE pattern, contrary to the LREE-enriched pattern of most epidote. The later generations are stranger still, showing strong LREE depletions, with chondrite-normalized LREE values as low as ~0.1.

We interpret at least four of the textural populations to be chemically distinct generations of magmatic epidote. Successive generations appear to track REE depletion of the magma, with early growth of REE-rich epidote or allanite leading to strong LREE depletion of the melt. The later generations of epidote, through strong depletions in Y followed by P and Ce, may reflect crystallization of garnet followed by apatite, followed by higher Mn, Y, and HREE contents consistent with garnet breakdown as pressure decreased during magma ascent. The latest magmatic epidote generation shows decreasing Ti, Mg, and Ba, probably a result of crystallization of magmatic muscovite.

The early growth of epidote rather than plagioclase, the rare presence of garnet partially replaced by epidote, and the lack of biotite, all suggest early high pressure crystallization of epidote, possibly at depths exceeding 50 km, and later crystallization of epidote at pressures exceeding 1 GPa, prior to final emplacement of the dikes at pressures of ~0.8 GPa.