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Petrogenetic grid for ultrahigh‐pressure metamorphism in the model system CaO‐MgO‐SiO 2 ‐CO 2 ‐H 2 O

1995; Wiley; Volume: 4; Issue: 4 Linguagem: Inglês

10.1111/j.1440-1738.1995.tb00147.x

1440-1738

Yuki Ogasawara, J. G. Liou, R. Y. Zhang,

earthquake and tectonic studies

Abstract Petrogenetic grids for ultrahigh‐pressure (UHP) metamorphism were calculated at different X co2 conditions in the model system CaO‐MgO‐SiO 2 ‐CO 2 ‐H 2 O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P‐T grids at X co2 = 0.01 and 0.5 are described. In the calculated P‐T grid at X co2 = 0.01, four out of 10 stable invariant points, Co‐En‐Ms‐Tc, Co‐Di‐En‐Tc‐Tr, Co‐Di‐Ms‐Tc‐Tr and Di‐En‐Ms‐Tc‐Tr lie within the stability field of coesite. If the fluid phase has X co2 = 0.5, no invariant point is stable under UHP conditions. Some magnesite‐bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO 2 , Ms + Tr = Do + Fo + CO 2 + H 2 O and Ms + Tc = Fo+ CO 2 + H 2 O at X co2 = 0.01 and by reaction Ms + Tc = Fo + CO 2 + H 2 O together with these three at X co2 = 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low X co2 conditions include the following: Co‐Do‐Ms, Co‐Di‐Ms, Co‐Di‐Tc, Di‐Ms‐Tc, Di‐En‐Tc‐, Di‐En‐Ms, Co‐Di‐En, Di‐En‐Fo, Di‐Fo‐Ms, Di‐Do‐Fo. Among them, talc‐bearing assemblages are restricted to X co2 < 0.02 and their high‐P limit is 31.7 kb (749°C) at X co2 = 0.01. Dolomite‐magnesite‐silica assemblages have large P‐T stability fields even if X co2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite‐dolomite assemblage is restricted only to a calc‐silicate rock interlayered with marble where X co2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low X co2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of X co2 on the stability of coesite‐dolomite‐magnesite, diopside‐enstatite‐magnesite, diopside‐talc assemblages is examined and the occurrence of coesite‐dolomite, magnesite‐bearing and talc‐bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P‐T grids.