The geotherm of the lithosphere beneath Qilin, SE China: a re-appraisal and implications for P-T estimation of Fe-rich pyroxenites

The Qilin geotherm established by Xu et al. [Xu, X.S., O'Reilly, S.Y., Zhou , X.M. and Griffin, W.L., 1996. A xenolith-derived geotherm and the crust-m antle boundary at Qilin, southeastern China. Lithos, 38. 41-62.] using the Ellis and Green [Ellis D.J, and Green D.H., 1979. An experimental study of the effect of Ca upon garnet-clinopyroxene Fe-Mg exchange equilibria. Contr ib. Mineral. Petrol., 71: 13-22]/Wood [Wood B.J., 1974. Solubility of alumi na in orthopyroxene coexisting with garnet. Contrib. Mineral. Petrol. 46: 1 -15] combination is in need of revision on the basis of re-evaluation of ge othermobarometers, comparison of calculated pressures and temperatures with experimentally determined phase relationships and geological/geophysical d ata. The invalid assumption that all iron is present as Fe2+ in minerals, a nd the thermal destruction of equilibrium Fe-Mg exchange between clinopyrox ene and garnet that may have resulted from heating of the Qilin xenoliths b y the host magma resulted in unrealistically high temperatures estimated by the Ellis and Green's thermometer. An important implication arising from t his study is that care must be taken when applying thermobarometers to Fe-r ich pyroxenites for the purpose of geotherm construction and a comprehensiv e analysis is often required. (C) 1999 Elsevier Science B.V. All rights res erved.