METAMORPHIC AND TECTONIC HISTORY DEDUCED FROM GARNET COMPOSITIONS AND GEOCHRONOLOGY: NORTH CASCADES, WASHINGTON

Garnet compositional zoning in pelitic schist and gneiss from the Crystalline Core includes 4 types: I] simple growth zoning - concentric with Mn decrease, Mg# increase, and little or no variation in Ca; II] modified zoning - simple concentric with Mn decrease, Mg# increase in crystal cores, and Ca and Mg# decrease in narrow (<50) micron or broad (>100 micron) rim areas; III] complex growth zoned - concentric with Mn decrease, Mg# increase, and complex Ca variation - inner small scale ‘sector-type' and outer fine oscillatory zoning; and IV] complex zoning - concentric with Mn decrease, Mg# increase, and high Ca annuli or high Ca rims. Types I (common) and II (rare) are found in regional metamorphic (M1? & M3) rocks. The distribution of type III and IV garnet appears to be restricted to rocks within ca. 1 km of orthogneiss sheets and plutons, including the 96-91 Ma Mt. Stuart batholith.

Type I have Sm-Nd ages of 90 to 86 Ma (6 ages) and are interpreted to have grown subsequent to most plutonism during M3. Type II have not been dated and are interpreted to have a regional and/or contact metamorphic origin, but to have undergone significant alteration subsequent to growth. Type III have Sm-Nd ages of 91-90 Ma (3 ages) which are identical to U-Pb zircon ages from adjacent orthogneiss. Complex type III Ca zoning varies from patchy to systematic sector-like geometry. 3-d zoning patterns suggest two possible causes: growth sectors resulting from differential incorporation of Ca during growth, and post growth Ca exchange in areas adjacent to fractures and or inclusions. Sharp gradients in Ca, peak temperatures that did not exceed ca. 625 ºC, and Mg, Fe, and Mn growth zoning are most compatible with little or modification of garnet compositions after growth. Therefore, rapid M2 garnet growth in response to contact metamorphism was likely followed by low to moderate temperature M3 that did not significantly modify earlier zoning. Type IV garnet (M2?) is inferred to have originated from local consumption of Ca-bearing phases during prograde growth. However, pressure increases during growth of high Ca rims cannot be ruled out. M2 garnet locally grew with andalusite while M3 garnet grew with kyanite or sillimanite. P-T pseudosections, and compositional zoning of M3 garnet and adjacent plagioclase indicates near isobaric (6-8 kbar) temperature increases during garnet growth.