Petrologic re-evaluation of classic Tauern Window (Austria) rocks corroborates the first P-T path calculated from chemical zoning in garnet.

Abstract:

Selverstone et al. (1984; J. Pet.) presented the first P-T path calculated from thermodynamic inversion of chemical zoning in garnet. That path – exhumation with heating – radically changed metamorphic research in demonstrating that chemical zoning could be used to determine a rock’s P-T history that could then be linked to geodynamic models of overthrusting. Here, we revisited these rocks with updated approaches to assess the accuracy of the path, and how it is recorded by different methodologies. Similarities in calculated P-T paths using new and old approaches would build confidence in prior petrogenetic and tectonic interpretations, while major differences might recommend reevaluating numerous studies.

We applied conventional thermobarometry, Gibbs free energy minimization, and quartz-in-garnet (QuiG) geoba-Raman-try with updated calibrations and thermodynamic data. We focused on two samples kindly provided to us by Prof. Selverstone: FH-1M (the original P-T path) and Z3H. Z3H garnet has abundant quartz inclusions and is well-suited for QuiG, excepting a narrow, strongly zoned, inclusion-poor rim that represents ≤10% of the garnet volume.

For FH-1M, rim P-T conditions from thermobarometry are 635±25˚C, 7±1 kbar (new data) vs. 550±25˚C and 7±1 kbar (published). Differences in T reflect updated calibrations. Conditions calculated for the garnet interior are 610°C, 8.5 kbar (new pseudosection) vs. 530 °C, 10 kbar (published Gibbs method). The new results suggest FH-1M garnet grew over a narrower range of P and T compared to former calculations.

For Z3H, thermobarometry shows rim P-T conditions of 575±25 °C and 7.5±0.5 kbar. For the garnet interior, QuiG pressures are ~11±1 kbar (at assumed 500-600°C), independent of the radial position of inclusions. QuiG results support an exhumation with heating P-T path, but the position-independent estimates also suggest ≥90 % of Z3H garnet grew at essentially constant P.

Combining results from FH-1M and Z3H, the overall P-T path that we infer corroborates the original, published P-T path and its tectonic implications. However, our highest calculated pressure reflects QuiG barometry, not thermodynamically-based calculations. The small change in P and T for FH-1M – and most of the Z3H garnet – may point to overstepping of the garnet-in reactions in these rocks.