SECTOR AND OSCILLATORY ZONING IN METAMORPHIC ALUMINUM GARNET: A NEWLY REPORTED OCCURRENCE AND INFERRED CHANGES IN CRYSTAL MORPHOLOGY, NORTH CASCADES, WA

Metamorphic garnets with sector zoning in the cores and oscillatory zoning in the rims have been identified in the Chiwaukum Schist from the North Cascades. Compositional sector zoning in garnet is uncommon – one reported occurrence worldwide. However, oscillatory zoning is common and has been observed in several areas in the Chiwaukum Schist. Garnets were collected ca. 250 meters from a ca. 90.6±2.4 Ma [U-Pb zircon] gneissic granodiorite body. These garnets yield a Sm-Nd age of 91.1±2.3Ma and are interpreted to have grown rapidly during contact metamorphism. Ca X-ray intensity maps for two garnet crystals and quantitative analyses across one of the crystals show that garnet is characterized by core sector and rim oscillatory zoning. Almandine and pyrope increase while spessartine decreases from core to rim in a typical concentric bell-shaped profile. The oscillatory Ca zoning near the rim has a periodicity of tens of micrometers and an amplitude of 0.006 mole fraction. Ca sector zoning reveals evolution of crystal growth. Crystal centers are dominated by dodecahedral faces {110}, but the outer crystals is dominated by trapezohedral faces {211}. This implies that the initial growth is dominated by fast-growing {211} that preferentially incorporated Ca over slow-growing {110} faces. During this growth, {211} dominated garnet morphology. An apparent change in growth rates, caused {211} to grow slower than the {110} faces until {211} dominated. This evolution resulted in crystal cores that have star-like sector zoning with vertices representing the final growth of {110} faces. The last growth, near the rims, is characterized by {211} which is obvious in the outer oscillatory zoning of Ca. Preliminary interpretation of garnet morphology from Ca zoning and external morphology suggests a morphological transition from crystals dominated by dodecahedral to trapezohedral faces. This transition is closely related to the two chemical zoning patterns and may reflect the roles of local growth dynamics and external forcing mechanisms affecting growth of garnet.