O ISOTOPE ZONING IN GARNET AND STAUROLITE - EVIDENCE FOR CLOSED-SYSTEM MINERAL GROWTH DURING REGIONAL METAMORPHISM

Microanalysis of O isotope ratios has been made in traverses across tw o garnet crystals and one staurolite sample (all approximately 1 cm in diameter) from a metapelitic schist and across one garnet sample appr oximately 4.5 mm in diameter from an amphibolite from the Cordillera D arwin metamorphic complex, Tierra del Fuego, Chile. The metapelitic sa mple contains garnet + biotite + staurolite + plagioclase + muscovite + quartz + ilmenite + chlorite, and P- T conditions determined from mi neral rims were approximately 575-degrees-C and 7 kbar. O isotope anal yses were collected with a spatial resolution of 0.5-1 mm using a lase r extraction system. The 72 analyses of the two garnet crystals in the metapelite indicate a gradual increase in deltaO-18 from approximatel y 3.5 parts per thousand (V-SMOW) in the core to approximately 4.0 par ts per thousand at the rims. The 70 analyses of the staurolite grain s how that it is generally unzoned, with an average value of 4.37 +/- 0. 08 parts per thousand (1sigma), except for localized increases in delt aO-18 toward some parts of the rim; these increases may be the result of late chlorite alteration. The garnet amphibolite sample contains ga rnet + biotite + plagioclase + quartz + chlorite + hornblende + cummin g-tonite, and P-T conditions were approximately 625-degrees-C and 4.5 kbar. The 25 analyses of the garnet sample indicate that it is homogen eous, with an average isotopic value of 3.44 +/- 0.05 parts per thousa nd (1sigma), except for three analyses that average 3.26 parts per tho usand. The 22 analyses of the fine-grained matrix near the garnet indi cate that the matrix is compositionally nearly homogeneous, with an av erage value of 3.70 +/- 0.15 parts per thousand (1sigma). With this P- T-t path, the reaction history, and no infiltration by a fluid with di sequilibrium deltaO-18, the predicted prograde zonation for the metape litic garnets is between +0.2 and +0.75 parts per thousand (rim to cor e), with a best estimate of +0.5%. The zonation for staurolite is appr oximately -0.05 parts per thousand, and for the amphibolite garnet it is between -0.01 and +0.06 parts per thousand. The correspondence of t he predictions and observations suggests that the rocks were not infil trated by a fluid that is not in isotopic equilibrium during regional metamorphism. The data show that regional metamorphic garnet and staur olite can preserve prograde variations of O isotopes in amphibolite fa cies terranes.