Paper No. 7
Presentation Time: 9:35 AM


DRAGOVIC, Besim, Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061, BAXTER, Ethan F., Earth and Environmental Science, Boston College, 140 Commonwealth Ave, Boston, MA 02467 and CADDICK, Mark J., Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061,

Coupling zoned geochronology with thermodynamic modeling of garnet elucidates the nature and scale of progressive metamorphism and dehydration during subduction of a quartzofeldspathic lithology from Sifnos, Greece. Zoned garnet geochronology is used to determine the timing and duration of garnet growth (used as a proxy for associated dehydration). Two adjacent, large garnet porphyroblasts (~5cm and ~3.5cm) were microdrilled based on major element chemical contours. Ten and nine distinct growth zones of garnet, respectively, were separated for Sm-Nd geochronology. Garnet separates were acid-cleansed, yielding very low Nd concentrations (as low as 0.02 ppm) and very high 147Sm/143Nd (as high as 9.82), and analyzed with TIMS using a NdO+ with Ta2O5 method. Acid-cleansed garnet/inclusion “powders” from each zone were also used, when appropriately fitted on an isochron. Thermodynamic modeling, along a series of prescribed P-T paths, reveals the causes and characteristics of garnet growth. Direct comparison of observed garnet growth timing with modeled growth allows for determination of the subduction P-T path. We chose eight possible P-T paths and tested mineral and fluid evolution along each. Combining multi-point garnet-powder-matrix isochron ages with thermodynamic modeling, three distinct stages of garnet growth were defined: initiation of garnet growth at 53.4 ± 2.6 Ma (~0.75 GPa and ~300°C); a period of slow to no growth until a second stage, centered at 47.27 ± 0.16 Ma; a final growth pulse, incorporating the majority of garnet growth, between 45.30 ± 1.00 Ma and 45.49 ± 0.19 Ma (2.13-2.19 GPa and 490-550°C). This amounts to a > 2 order of magnitude increase in volumetric growth rate from early core growth to the final growth pulse. This final pulse occurred rapidly (< 0.83 My) during a period of relatively isobaric heating, at a corresponding heating rate > 75°C/My. Garnet growth appears to have terminated at roughly the maximum temperature attained. Bulk rock dehydration during garnet growth amounts to 0.55 wt.% H2O loss, with the majority of H2O release (0.4 wt.%) occurring during the final rapid growth pulse. This represents a rapid, focused pulse of metamorphism along the otherwise continuous process of subduction, and provides strong evidence for short (< 1 My) timescales of dehydration during subduction.