THE TECTONIC RECORD PRESERVED IN GROWTH AND DIFFUSION ZONING PATTERNS IN GARNET FROM MIGMATITIC PELITES OF THE EAST HUMBOLDT RANGE, NEVADA

Garnets in migmatitic metapelites from the East Humboldt Range metamorphic core complex in northeastern Nevada record a multiphase growth and resorption pattern in major and trace element zoning. These patterns are consistent with garnet growth prior to and during anatexis. While much of the metasedimentary section is significantly overprinted by (probably Eocene and younger) lower amphibolite facies metamorphism associated with extensional tectonism, rocks in the upper limb of the Winchell Lake fold nappe preserve original in situ leucosome­–melanosome textures. Relict staurolite and kyanite are present in melanosomes composed of grt + bt + sil + qtz + pl + graphite + tourmaline. Garnets adjacent to and in contact with leucosomes show embayed grain boundaries and partially preserved corona textures of pl + bt + sil replacing grt. These textures are interpreted to represent garnet resorption via a retrograde net transfer reaction (ReNTR), likely during melt crystallization. Plagioclase involved in this texture is significantly enriched in X_An (~0.40-0.45 vs. 0.30-0.35 in the matrix) due to garnet breakdown. Garnet porphyroblasts contain three preserved growth zones evidenced by X_Grs zoning: (1) low X_Grs (~ 0.08–0.09) inclusion-rich cores, (2) moderate to high X_Grs (~ 0.10–0.11) mantles, and (3) low X_Grs (~ 0.07–0.08) rims. Trace element zoning is consistent with these growth zones. Sc zoning is similar to Ca yet shows a trough inward from a step up at the rim. Y zoning is inverse of Ca, punctuated by a high Y core. Fe, Mg, and Mn show a different pattern that overlaps the growth zoning. A uniform core with a near-rim trough and then increase in Fe/(Fe + Mg) and Mn at the rim is observed. This pattern may reflect diffusional homogenization followed by continued growth of lower Fe/(Fe + Mg) peritectic garnet that is “frozen in” during rapid exhumation and cooling. The near-rim increase in Fe/(Fe + Mg) and Mn indicates garnet resorption during the corona­-producing ReNTR, probably beginning at supersolidus conditions. Zr–in–rutile thermometry yields temperatures of ~680°C for inclusions in garnet, consistent with garnet (2) growth at supersolidus temperatures. These results support a local in situ source for at least some of the leucogranites that are prevalent throughout the Ruby Mountain­–East Humboldt Range core complex.