T240 Garnet and Its Use in Unraveling Metamorphic and Tectonic Processes

Wednesday, 3 November 2010
8:00 AM - 12:00 PM
Mile High Ballroom 1CD

INTERPRETING SM-ND GEOCHRONOLOGY OF GARNET-BEARING ROCKS: AN EXAMPLE FROM THE ZERMATT-SAAS OPHIOLITE, WESTERN ALPS, ITALY

Paper number: T240-6
Program: Technical Sessions
Time: 9:20 AM – 9:40 AM

Authors

Abstract

Metamorphism in subduction zone environments is a protracted process that can span millions of years. Geochronology aimed at constraining the timing and duration subduction-related metamorphism requires that uncertainties in age determinations are smaller than the duration of the process and that ages can be related to specific periods of the metamorphic cycle. This is especially relevant to chronology involving garnet because of its potentially long growth history under prograde conditions (e.g. Lapen et al., 2003, EPSL, 215; Skora et al., 2009, EPSL, 287; Pollington and Baxter, 2010, EPSL, 293). It has recently been demonstrated that core-to-rim zoning of parent nuclides such as Lu and Sm in garnet can dramatically influence the measured bulk garnet Lu-Hf and Sm-Nd ages to be weighted to onset of garnet growth and late stages of garnet growth, respectively. In addition, the mineral growth rate and nucleation histories also have profound effects on whether measured ages are weighted toward initial versus late garnet growth (Lapen et al., 2003, EPSL, 215; Kylander-Clark et al., 2007, Chem. Geol., 242; Skora et al., 2009, EPSL, 287; Springer, 2010, UH Thesis).

Here we present Sm-Nd geochronology, garnet crystal size distributions (CSD), garnet core-to-rim trace element zoning, and garnet growth modeling of eclogite-facies garnet-blueschist from the Zermatt-Saas ophiolite, St. Marcel, western Alps, Italy. Two samples yield garnet + whole rock + clinozoisite + amphibole + white mica Sm-Nd isochron ages of 44.1±2.7 and 43.1±2.8 Ma. Chemical zoning and CSDs show that garnet growth at St. Marcel was likely controlled by a complex interplay between interface- and diffusion-limited growth mechanisms and based on these data, the ages represent the volume average of garnet, in contrast to that of Lago di Cignana where the Sm-Nd age of 40.6±2.6 is weighted strongly toward the garnet rim and more closely represents the age of peak metamorphism. The Sm distributions in garnet from St. Marcel and Lago di Cignana indicate that both measured ages are consistent with peak metamorphism at 38-40 Ma in both localities and highlights the requirement of combined trace element, CSD, and growth modeling in interpreting isochron ages involving garnet.