Experimental constraints on the formation of high-P/high-T granulites in the Southern Bohemian Massif

Abstract

Large bodies of felsic high- P /high- T granulites with the assemblage quartz + ternary feldspar (mesoperthite) + garnet + rutile ± kyanite occur in the Southern Bohemian Massif. They are thought to have formed during the Variscan orogeny in a Carboniferous subduction setting, at 950–1050°C and 1.5–1.9 GPa, from granitic protoliths. Deformation and retrograde reactions during subsequent, near-isothermal, exhumation lead to considerable difficulties in the precise reconstruction of the original peak P-T phase relations and mineral compositions, and hence the processes and timing of granulite formation in the Southern Bohemian Massif are presently not fully understood and still quite controversially discussed. In order to assess the processes that occurred close to the P-T peak, fluid-absent piston cylinder experiments were conducted. We used a granitic gneiss as starting material with the assemblage K-feldspar + plagioclase + quartz + biotite + muscovite, whose chemical composition almost perfectly matches the main granulite type of the Southern Bohemian Massif. The experimental conditions were chosen to simulate the metamorphic P-T path determined for the granulites, with runs at 750°C/1.6 GPa, 800°C/1.6 GPa, 850°C/1.6 GPa, 900°C/1.6 GP and 1000°C/1.6 GPa (prograde path) and 950°C/1.4 GPa, 900°C/1.2 GPa and 800°C/1.2 GPa (retrograde path). The experiments in the temperature range of 850–1000°C all yielded the typical granulite assemblage garnet + ternary feldspar + quartz ± kyanite ± rutile. At 1000°C/1.6 GPa roughly 30–40 vol.% partial melt was present, and large melt volumes persisted also on the retrograde path (>40 vol. % at 950°C/1.4 GPa, ~25 vol. % at 900°C/1.2 GPa). The melt-forming reaction observed in the experiments is: biotite + plagioclase + quartz = garnet + ternary feldspar + melt. At pressures of 1.6 GPa, this reaction commences at temperatures >750°C and goes to completion between 800°C and 850°C. The melts obtained are strongly peraluminous granitic in composition with A/CNK ranging from 1.11 to 1.43. K-rich feldspars are close to Ab 28 Or 72 An 0 at 750°C/1.6 GPa and change to Ab 48 Or 47 An 5 at 1000°C/1.6 GPa. Coexisting Na-rich feldspars are stable up to 850°C/1.6 GPa and change from Ab 92 Or 2 An 6 at 750°C/1.6 GPa to Ab 63 Or 30 An 7 at 850°C/1.6 GPa. Garnets are almandine-pyrope solid solutions with high Ti contents ranging from ~0.2 wt. % TiO 2 at 750°C/1.6 GPa to ~1.8 wt. % TiO 2 at 1000°C/1.6 GPa, thus indicating a strong increase of the Ti content with temperature, whereas only a slight correlation with pressure was found in the experiments at 800°C. Notably, the TiO 2 contents of the natural granulite garnets are generally low ( 2 contents in the range of ca. 0.3 wt. %, which is two to three times the whole rock content. As a consequence, almost no rutile formed in the runs at T>900°C. This leads to the important conclusion that the big (0.2-1 mm) euhedral accessory rutiles, which are very characteristic for the granulites of the Southern Bohemian Massif, did not form at peak P-T conditions, but most likely during the decompression and cooling stage, in the presence of melt. This observation supports previous numerical models according to which accessory minerals like zircon, monazite or rutile form in melt-bearing granulites after the P-T peak and date cooling histories rather than the metamorphic climax.