Inconsistencies between cpx-grt geothermometry and field observations: example of the peraluminous eclogites from Beni Bousera (North Morocco)

Large variations in clinopyroxene-garnet (cpx-grt) temperatures are recorded in thin peraluminous eclogite layers from Beni Bousera urtramafic massif (Morocco): cpx-grt temperatures in the core of layers < 20 cm thick are higher by up to 300 degrees C than in the margins. The cpx-grt temperatures are correlated to both Al-concentration in cpx and Ca-concentration in grt, the two latter parameters being themselves closely intercorrelated. It is demonstrated that the regular interaction parameter Delta w(Ca)(grt) calculated from the Beni Bousera natural data is very close to Delta w(Ca)(grt) calculated from experimental data analysis for Ca-rich garnet. It is concluded that: (i) Al-concentration in cpx has no significant effect on the distribution of re and Mg between cpx and grt; and (ii) cpx-grt temperature variations observed across Beni Bousera peraluminous eclogite layers do not correspond to real thermal gradients, but merely result from the inadequacy of the thermometric equations in Ca-rich systems (X-Ca(grt) > 0.15).