The Significance of Primary Scapolite in Granulitic Inclusions from Deep-seated Pipes

Sulphur-rich scapolite is described as a primary constituent of several granulite inclusions occurring, along with eclogitic and rare ultrabasic rocks, in the breccia- and basalt-filled pipes of eastern Australia. The wide distribution of such inclusions in space and time indicates that granulites, some scapolite-bearing, form an important part of the base of the crust of this region. Two inclusions with the assemblage orthopyroxene-clinopyroxene-plagioclase-scapolite, two with the assemblage clinopyroxene-garnet-plagioclase-scapolite, and one with the assemblage orthopyroxene-clinopyroxene-plagioclase, have been analysed and chemical and optical data on the constituent minerals are given. Since it is impossible to adequately separate scapolite from primary and secondary plagioclase, partial analyses of these minerals have been made with an A.R.L. electron probe X-ray microanalyser. From our data and that obtained from the literature, we suggest that scapolites with more than 10 mol per cent sulphur in the anion group are confined to rocks of the granulite meta-morphic facies. Within the granulite facies there appears to be a progressive increase in the sulphur content of scapolite in equilibrium with such sulphur-rich minerals as pyrrhotite, with increasing metamorphic grade.