The Mona Complex comprises a 'Bedded Succession' of late Proterozoic low-grade metasedimentary schists, a group of gneisses of uncertain age, and the Coedana Granite. New Rb-Sr whole-rock isochron data for gneisses from the Holland Arms area indicate a late Precambrian metamorphic episode at 595 ± 12 Ma and a possible Cambrian igneous episode represented by an age with relatively large errors of 562 ± 31 Ma obtained from orthogneisses. A Rb-Sr whole-rock isochron age for the Coedana granite is 603 ± 34 Ma. The relatively low initial 87Sr/86Sr ratio for the late Precambrian gneisses, 0.7061 ± 3, is considered to preclude a long crustal history for these rocks and they are identified as metamorphosed equivalents of part of the Bedded Succession. The initial 87Sr/86Sr ratios of the orthogneisses and the Coedana Granite (0.7081 ± 8 and 0.7086 ± 9, respectively) are identical within analytical error and are consistent with an origin for these igneous rocks either by crustal contamination of mantle-derived melts, or by melting of young crustal material such as the older gneisses.

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