Petrological constraints on the cooling history of high-temperature garnet peridotite massifs in lower Austria |
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| Authors: | Harry Becker |
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| Institution: | (1) Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, D-55020 Mainz, Germany, DE |
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| Abstract: | High-temperature peridotite massifs occur as lensoid bodies with high-pressure granulites in the southern Bohemian massif.
In lower Austria the peridotites comprise garnet lherzolites lacking primary spinel, rare garnet and garnet-spinel harzburgites,
and harzburgites containing Cr-rich primary spinel instead of garnet. These phase assemblages suggest initial high-pressure
equilibration and are consistent with results from garnet-orthopyroxene geobarometry indicating equilibration at around 3–3.5 GPa.
Maximum temperature estimates obtained on core compositions of coexisting minerals from the peridotites are not higher than
ca. 1100 °C. In contrast, pyroxene megacryst compositions, garnet exsolution textures in the garnet pyroxenites, and results
from geothermometry indicate much higher original equilibration temperatures in most of the pyroxenites (up to 1400 °C). High
temperatures, modal zoning, the occasional presence of Mg-rich garnetites and chemical evidence suggest that the pyroxenites
are cumulates which crystallized from low-degree melts derived from the sub-lithospheric mantle. Isothermal interpolation
of the high temperatures to an upper mantle adiabat suggests that the melts were derived from a minimum depth of 180–200 km.
The formation of small garnet II grains and garnet exsolution lamellae in the pyroxenites and pyroxene megacrysts may reflect
isobaric cooling of the cumulates from temperatures above 1400 °C to ca. 1100–1200 °C (at 3–3.5 GPa) to approach the ambient
lithospheric isotherm. This model differs from other models in which the formation of garnet II was explained by an increase
in pressure during cooling in a subduction zone. Isobaric cooling was followed by near-isothermal decompression from 3–3.5 GPa
to 1.5–2 GPa at 1000–1200 °C, as indicated by the increase of Al in pyroxenes near garnet. Further cooling in the spinel lherzolite
stability field is indicated by spinel exsolution lamellae in pyroxenes from lherzolites. The formation of symplectites and
kelyphites indicate sub-millimetre scale re-equilibration during exhumation in the course of the Carboniferous collision in
the Bohemian massif. The peridotite massifs represent fragments of normal (non-cratonic) lithospheric mantle from a Paleozoic
convergent plate margin.
Received: 22 July 1996 / Accepted 28 February 1997 |
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