Abstract

Dr A. T. V. Rothstein writes: In the paper concerning the isotopic evolution of the Lizard complex G. R. Davies (1984) writes, concerning the upper mantle peridotites occurring in this complex ‘…[the] spinel lherzolites are porphyroclastic…PT estimates for the cores of adjacent enstatite and diopside porphyroclasts are in the region of 19 kb and 1100°C…’, a ‘multi-stage melting event of garnet lherzolite is postulated’, and ‘It is suggested that the depleted trace element and isotopic characteristics of the spinel lherzolite reflect multiple, small percentage, melt extraction while in the garnet stability field’. Thus the geochemical data lead to the suggestion that rocks of spinel lherzolite facies evolved from the higher PT conditions of the garnet lherzolite facies. The P-T estimates given by Davies for the cores of the co-existing pyroxene porphyroclasts of the spinel lherzolite are comparable with the range published (Rothstein 1981) for the primary mineral assemblages of the spinel lherzolite, using both new analyses and those published previously (Green 1964). The P-T estimates given by Davies are particularly close to the set which can be estimated for the spinel lherzolites from locality H (Rothstein 1981) (Table 1).

These P-T estimates for locality H are from minerals which, by a detailed petrographic study, are seen to be part of well-preserved relics of primary textures resembling crescumulates. The deformation of these primary textures into recrystallized fabrics can be traced and mapped (Rothstein 1981Rothstein 1977). The deformed textures include porphyroclastic varieties.

I discussed (Rothstein 1981) various hypotheses concerning the

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