EUROPEAN OROGENIC GARNET PERIDOTITES: GEOTHERMOBAROMETRY, COOLING RATES, AND TECTONOTHERMAL IMPLICATIONS

The Caledonian, Variscan, and Alpine belts in Europe all include bodies of orogenic garnet peridotite, which represent fragments of mantle incorporated in these orogens. In general, the garnet peridotites fall into two distinct P/T groups, one along a high P/T field gradient and the other along a low P/T gradient, based on the garnet-olivine Fe-Mg exchange geothermometer and the Al-in-orthopyroxene geobarometer. The high P/T group was likely derived from subcontinental mantle, and the low P/T group, from suboceanic mantle, as inferred from regional geological relations and geochemical characteristics. Three representative samples of high P/T peridotite yield 935 ºC, 36.7 kbar (Almklovdalen, Caledonian, Norway), 1200 ºC, 52.4 kbar (Nové Dvory, Variscan, Czech Republic), and 1220 ºC, 63.8 kbar (Alpe Arami, Alpine, Switzerland), and two samples of low P/T peridotite yield 1170 ºC, 22.5 kbar (Mohelno, Variscan, Czech Republic), and 1140 ºC, 21.4 kbar (Ronda, Alpine, Spain).

Garnet in these peridotites has compositionally uniform cores, but shows an increase in Fe and decrease in Mg at contacts with olivine and pyroxene, typically on a scale of ~100-200 mm, which results from the arrested exchange of Fe and Mg between these phases during cooling. The zoning of garnet against olivine inclusions has been modeled by finite difference methods, using the diffusion equation and currently accepted diffusion coefficients. Good agreement between the observed and calculated compositional profiles is achieved with cooling rates of 1 ºC/m.y. for Nové Dvory; 5 ºC/m.y., Almklovdalen; and 500 ºC/m.y., Alpe Arami, which belong to the high P/T group of peridotites. In contrast, the low P/T peridotites require extremely high cooling rates to preserve the garnet core compositions, i.e., ~10,000 ºC/m.y. for both the Mohelno and Ronda peridotites.

The high P/T and low P/T peridotites evolved from two contrasting tectonothermal regimes. The high P/T peridotites were likely derived from a subcontinental mantle wedge, and their cooling rates may reflect cooling associated with exhumation. In contrast, the low P/T peridotites were probably derived from subducted, suboceanic mantle, and their high cooling rates may represent cooling related to tectonic juxtaposition of hot mantle with colder surrounding rocks, rather than exhumation.