Bachelor of Science (Honours)
School of Earth & Environmental Science
Stokes, Natalie, Relationships of mafic volcanic rocks and surrounding rocks in the inferred subduction complex, Batemans Bay District, Southeastern New South Wales, Australia, Bachelor of Science (Honours), School of Earth & Environmental Science, University of Wollongong, 2013.
The Cambro-Ordovican Wagonga Group rocks of the Narooma Accretionary Complex in the southeastern Lachlan Fold Belt, are well exposed at Melville Point and Barlings Head, 20 km south of Batemans Bay at Tomakin, on the New South Wales south coast. They are comprised of ocean floor turbidites and pelagic sedimentary rocks, and mafic volcanic rocks. Relationships between the volcanic rocks and the surrounding rocks are in need of more detailed examination. Detailed mapping, petrography (microscopy, XRD) and whole rock geochemistry (laboratory and hand-held XRF) of the rock units in the study area, have been used for detailed description of the lithologies. This study shows that the chaotic rocks at Barlings Head form a mélange that is not a stratigraphic sequence, like that at Melville Point. The overall assemblage at both localities is similar. An array of different facies occurs at Barlings Head, including a massive greenstone unit and several different types of mudstone, sandstone and chert units that are commonly mixed and highly deformed. The greenstone unit at Barlings Head is massive and highly altered (secondary clay minerals dominate the rock composition), and it injects along its contacts with adjacent chert and black mudstone units. At Melville Point, the greenstone occurs as a massive unit, pillow lavas and folded within the core of anticlines in the Narooma Chert. Geochemical findings indicate that greenstones at Barlings Head and Melville Point have different magmatic affinities. The Barlings Head greenstone has an ocean-island basalt affinity and is interpreted to be offscraped from a subducted seamount. The Melville Point greenstone has an island arc basalt affinity, derived from a depleted mantle source, and is interpreted to have erupted in a deep marine setting by its association with the Narooma Chert. At Barlings Head a mélange of black mudstone/sandstone is interpreted to have been diapirically injected along an early fault, with later injection of greenstone. The mélange in the centre of Barlings Head has been extensively faulted, contains out-of-sequence turbidite units and fragments of disrupted units. The mudstone may have been accreted unconsolidated sediment, diapirically injected along early faults/bedding. The mélange is interpreted to be the result of the complex interaction and superposition of tectonic processes such as progressive deformation in a thickening fault zone, and diapiric processes.