The Early Devonian Boggy Plain Supersuite is a belt of I-type granitic and volcanic rocks extending for over 500 km in the central Lachlan Fold Belt. It has a distinctive composition and origin. Compared with other Lachlan Fold Belt I-types, rocks of the supersuite are high in Cu and incompatible elements (K, Ba, Sr, Rb, La, Ce, U, and Th). The source for the magmas is interpreted to have been an incompatible element-rich gabbroic layer, underplated at the base of the crust in the Ordovician. This layer is inferred to correspond to a belt of Ordovician shoshonitic basaltic volcanic rocks that has a strikingly similar geographical distribution to the supersuite. Compositional variation in the supersuite is ascribed to fractional crystallization, in contrast to most other I-type magmas of the Lachlan Fold Belt which owe most of their compositional variation to fractionation by restite unmixing. This contrast resulted from differences in source rocks, with the Boggy Plain Supersuite being derived from basaltic sources at high temperatures, and the restite-bearing I-types being derived from more felsic sources at lower temperatures. The proposed crustal source for the supersuite magmas is consistent with the orogenic evolution in the Lachlan Fold Belt being mainly related to reworking of pre-existing crust, and has no analogy in the evolution of modern continental margin orogenic zones. Fractional crystallization of Boggy Plain Supersuite magmas produced rock types ranging from pyroxene-rich and plagioclase-rich gabbroic cumulates, through pyroxene-bearing quartz monzodiorites and granodiorites to high-silica granites. The latter represent the fractionated liquid removed during the crystallization of the more mafic cumulates. These liquids vary due to additional fractionation. The supersuite is particularly prospective for Cu and Au mineralization associated with the more mafic intrusives, and W and Bi in the most fractionated felsic bodies.
- Fractional crystallization
- Palaeozoic tectonics