The transformation of tillite by migmatization at Mount Fitton, South Australia

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Abstract
Summary: This paper gives an account of the transformation of tillite by migmatization in the vicinity of Mount Fitton, South Australia. The mineralogical and chemical changes invplved are described and discussed. During an orogeny in the early Palaeozoic some of the tillites, slates and quartzites, which are stratigraphically part of the Sturtian section of the Adelaide System (late Pre-Cambrian), were altered to migmatites. Large granitic migmatite complexes were formed by granitization and small patches of epidiorite-like rocks were formed by basification. The tillite contains erratics of varying type, shape and size set in a semi-pelitic matrix made up essentially of quartz and biotite and with a composition generally falling within the subgreywacke range. The tillite at Mount Fitton is similar to late Pre-Cambrian tillites found in many parts of South Australia. The transformation of tillite by granitization involved an increase in the amount of potash-felspar, acid plagioclase, quartz (in the case of the more pelitic rocks) and in grain-size and a decrease in the amount of biotite. K and Na and, in most cases, Si increased and Fe, Mg, Ca, Ti, P and Mn decreased. Granitization altered the rocks by direct replacement, except in one area where the rocks were agmatized. The transformation of tillite by basification involved a considerable decrease in the amount of quartz, the disappearance of biotite and the development of actinolitic hornblende, labradorite and ilmenite as well as an increase in grain-size. Si and K decreased and Fe, Mg, Ca, Ti, Mn and Cr were introduced. These introduced constituents are the same as those expelled during the granitization of tillite and the processes of granitization and basification are considered to be complementary.

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