UHP–UHT peak conditions and near-adiabatic exhumation path of diamond-bearing garnet–clinopyroxene rocks from the Eger Crystalline Complex, North Bohemian Massif

Abstract

Intermediate garnet–clinopyroxene rocks from the Eger Crystalline Complex, North Bohemian Massif, contain microdiamonds enclosed in garnet and zircon. The variable mineral assemblage of these rocks allows for an evaluation of the P–T evolution using numerous univariant equilibria and thermodynamic modelling, in addition to the ternary feldspar solvus, Ti-in-garnet, Zr-in-rutile and Ti-in-zircon thermometry. Zircon mantle domains with diamond inclusions contain 111–189ppm Ti, reflecting temperatures of 1037–1117°C. The peak pressure consistent with diamond stability corresponds to c. 4.5–5.0GPa. Ti-in-garnet thermometry using the Ti content of diamond-bearing garnet core yielded temperatures of 993–1039°C at c. 5.0GPa. An omphacite inclusion in garnet (reflecting c. 2.3–2.4GPa at c. 1050°C) and metastably preserved kyanite represent relics of eclogite-facies conditions. The dominant high-pressure granulite-facies mineral assemblage of low-Ca garnet, diopsidic clinopyroxene, antiperthitic feldspar and quartz equilibrated at 1.8–2.1GPa and c. 1050°C, based on the XGrs isopleth of the garnet mantle, garnet–feldspar–kyanite–quartz univariant equilibria and ternary feldspar solvus. Our thermodynamic modelling shows that a steep decrease of XGrs from a maximum core value of 0.32 to 0.17 at the rim as well as a rimward XMg increase (from 0.42 to 0.50) are consistent with significant decompression without heating. The latter is related to omphacite and kyanite breakdown reactions producing garnet and plagioclase. The Ti content in the rim zone of zircon (13–42ppm), exsolved plagioclase and K-feldspar associated with matrix diopside and garnet rim, and late biotite reflect temperatures of c. 830–900°C at c. 1.4GPa. A similar temperature is recorded by matrix rutile grains, containing 2028–4390ppm Zr and representing a relatively homogeneous population in contrast to rutile enclosed in garnet with variable Zr content. Our results show that the garnet–clinopyroxene rocks from North Bohemia are UHP–UHT rocks which were extensively overprinted under HP granulite-facies conditions during rapid exhumation along a near-adiabatic P–T path. The UHT peak and UHT–HT exhumation distinguish this area from the other UHP terrains worldwide. We demonstrate that Ti-in-zircon thermometry can provide robust temperature estimates in the rocks exhumed at HT, where the UHP–UHT mineral assemblage has not been preserved. In addition, the calculated UHP–UHT conditions are similar to those determined for the associated garnet peridotites, providing evidence for juxtaposition of these crustal and mantle rocks during deep Variscan subduction.