Abstract

Microanalysis of O isotope ratios has been made in traverses across two garnet crystals and one staurolite sample (all ~ 1 cm in diameter) from a metapelitic schist and across one garnet sample ~4.5 mm in diameter from an amphibolite from the Cordillera Darwin metamorphic complex, Tierra del Fuego, Chile. The metapelitic sample contains garnet + biotite + staurolite + plagioclase + muscovite + quartz + ilmenite + chlorite, and P-T conditions determined from mineral rims were approximately 575 °C and 7 kbar. O isotope analyses were collected with a spatial resolution of 0.5–1 mm using a laser extraction system. The 72 analyses of the two garnet crystals in the metapelite indicate a gradual increase in δ18O from ~3.5‰ (V-SMOW) in the core to ~4.0‰ at the rims. The 70 analyses of the staurolite grain show that it is generally unzoned, with an average value of 4.37 ± 0.08‰ (1σ), except for localized increases in δ18O toward some parts of the rim; these increases may be the result of late chlorite alteration. The garnet amphibolite sample contains garnet + biotite + plagioclase + quartz + chlorite + hornblende + cummingtonite, and P-T conditions were approximately 625 °C and 4.5 kbar. The 25 analyses of the garnet sample indicate that it is homogeneous, with an average isotopic value of 3.44 ± 0.05‰ (1σ), except for three analyses that average 3.26‰.The 22 analyses of the fine-grained matrix near the garnet indicate that the matrix is compositionally neady homogeneous, with an average value of 3.70 ± 0.15‰ (1σ).

With this P-T-t path, the reaction history, and no infiltration by a fluid with disequilibrium δ18O, the predicted prograde zonation for the metapelitic garnets is between +0.2 and +0.75‰ (rim to core), with a best estimate of +0.5‰. The zonation for staurolite is ~−0.05‰, and for the amphibolitic garnet it is between −0.01 and +0.06‰. The correspondence of the predictions and observations suggests that the rocks were not infiltrated by a fluid that is not in isotopic equilibrium during regional metamorphism. The data show that regional metamorphic garnet and staurolite can preserve prograde variations of O isotopes in amphibolite facies terranes.

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