Phase relations of calcic amphibole–orthoamphibole–chlorite- talc assemblages, with applications to ultramafic rocks from the KTB pilot hole, Bavaria

Abstract

Lenses of ultramafic rocks intercalated within a metagabbro–amphibolite sequence were encountered in the KTB pilot hole. A pervasive metamorphic overprint formed the dominating assemblage calcic amphibole–orthoamphibole–chlorite–talc. In this study the phase relations of ultramafic rocks are investigated in order to (a) constrain the stability field of this assemblage in general, and (b) define the equilibrium pressure–temperature (P–T) conditions of this assemblage in the ultramafic rocks from the KTB borehole. For that purpose, phase equilibria were calculated in the model systems CaO–MgO–Al2O3–SiO2-H2O (CMASH) and CaO–MgO–FeO–Al2O3–SiO2-H2O (CMFASH). Thereby, the continuous compositional change of solid solutions with pressure and temperature was modeled, including the Tschermak’s substitution and the MgFe-1 exchange. Based on these results, petrogenetic grids were constructed, revealing that calcic amphibole–orthoamphibole–chlorite–talc assemblages cover a stability field of <650–770 °C/1→ 14 kbar (CMASH) and <550–650 °C/1→14 kbar (CMFASH), respectively. This explains the widespread occurrence of the considered assemblage. Based on the bulk rock composition of the KTB samples, a special P–T diagram was constructed, limiting the stability field of the calcic amphibole–orthoamphibole–chlorite–talc assemblage. At 580 °C the stability field extends from 6 to 14 kbar pressure, and shrinks to 10–11 kbar at 630 °C. Conventional estimates using the mineral compositions of the KTB samples yield a temperature around 630 °C, at which the calculated stability field of calcic amphibole–orthoamphibole–chlorite–talc extends from 10 to 11 kbar.