CHEMICAL ZONING IN GARNET AS EVIDENCE FOR THE RAPID EXHUMATION OF UHP ROCKS (Invited Presentation)

Careful petrographic analysis, perceptive interpretation of chemical zoning in garnet, and development of methods for chemical dating of monazite were hallmarks of much of Bob Tracy’s work. He maintained a lifelong interest in the generation of Al-rich emeries during ultrahigh-temperature metamorphism, and had recently developed a fascination with the petrology of ultrahigh-pressure (UHP) metamorphic rocks. Here we focus on a coesite-bearing sample from the Dora Maira UHP Alpine massif that preserves a remarkably well-constrained record of its late-prograde to peak pressure history. The major phases are garnet, kyanite, Mg-phengite, phlogopite, Na-phlogopite, quartz/coesite, talc and rutile, and thermodynamic modeling suggests that their abundance and compositions are consistent with equilibration at ~3.5 GPa, 775 ˚C.

Major element zoning in garnet records an unusually detailed picture of the path taken to reach maximum Pand T. Asymmetric, high-Ca growth bands mimic the shape of their host crystals but have X_gro contents approximately three times higher. These bands are sharply defined at their inner margin but decay gradually to a low-Ca rim and can be interpreted as recording the introduction of a finite amount of Ca into the intergranular medium during progressive garnet growth and subsequent sequestration of this Ca during continued garnet growth. Thermodynamic constraints strongly indicate that the precursor host for this Ca was lawsonite, which is now absent from the sample but helps to constrain the early heating history. Diffusion modeling of this garnet zoning yields best-fit timescales of ~5 Myrs, consistent with rapid exhumation previously inferred from isotope geochronology. The constrained pressure-temperature history, including rapid exhumation and an almost complete lack of retrogressive reaction (other than slight garnet breakdown to Mg-rich talc and kyanite), indicates possible mechanisms for burial, heating and exhumation. Thermodynamic models suggest a density of ~3.2 g/cm³ at 110 km depth, implying that despite the relatively small volume of preserved whiteschist material, buoyant exhumation back along a subduction channel remains a feasible mechanism. Likely exhumation rates approach rapid plate tectonic velocities, indicating that erosion was not a major controlling factor.