The origin of scapolite in the regionally metamorphosed rocks of Mary Kathleen,Queensland, Australia

Scapolite at Mary Kathleen (North-Western Queensland) occurs in calcareous and non-calcareous metapelites, acid and basic metavolcanics and metadolerites. Graphical treatment of the relationship between scapolite composition (Me%) and the host rock oxide ratios CaO/Na₂O and Al₂O₃/(CaO + Na₂O) reveals the following points:

1. The calcareous metapelites are also very sodic.
2. Scapolite in calcareous metapelites is more marialitic than that in low-calcium equivalents.
3. In graphs of Me% against CaO/Na₂O and Al₂O₃/(CaO + Na₂O) the metasediments and the metaigneous rocks show markedly different trends.

It is concluded that scapolite in the metasediments originated by isochemical metamorphism of shales and marls containing evaporitic halite. The local abundance of halite was the main control on the composition and distribution of the scapolite, but the relative abundance of CaO and Na₂O was a modifying factor. In the metaigneous rocks scapolite formed metasomatically during regional metamorphism by the introduction of volatile-rich fluids derived from the adjacent evaporitic sediments. The relative availability of CO₂ and Cl₂ again appears to have been the primary control on scapolite composition and may in turn have been controlled by bulk rock composition.