The Marron Formation is a sequence of early Tertiary lavas associated with which are a number of intrusions, referred to as the Coryell Intrusions, which are thought to be consanguineous. Both lavas and intrusions probably extend beyond the area indicated by the map (Fig. 22), including southwards into the United States. The Marron Formation lavas include pyroxene andesites, trachyandesites, sodic trachytes and phonolites with a maximum thickness of about 2000 m. Only the lowermost part of the sequence is alkaline with a basal biotite-pyroxene phonolite containing anorthoclase rhombs, primary analcime phenocrysts up to several millimetres in diameter, as well as aegirine, nepheline and a little olivine. These rocks were referred to as 'shackanite' by Daly (1912) and their genesis is discussed by Church (1978) and Edgar (1979). The Coryell Intrusions range from granites through syenites to monzonite. Most of the rocks include alkali feldspar, andesine-labradorite, a pyroxene, apparently varying from augite to aegirine, hornblende, biotite, altered olivine and possible altered nepheline. The petrology has not been studied thoroughly, and other intrusions in the area not included as 'Coryell Intrusions' may prove to be part of the alkaline suite. A study of K, U and Th in six Coryell Intrusions has been made by Burwash and Berndt (1982).
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