Backarc basin and ocean island basalts in the Narooma Accretionary Complex, Australia: setting, geochemistry and tectonics

Abstract

The Cambrian-Ordovician Wagonga Group contains basalts at Melville Point and Barlings Beach, 20 km south of Batemans Bay, New South Wales. At Melville Point, the succession has basal altered basalts overlain by chert and interbedded siliceous mudstone of the Wagonga Group, in turn overlain by turbidites and chert of the Adaminaby Group with a latest Cambrian to earliest Ordovician age. By contrast, at Barlings Beach, basalt is associated with highly disrupted chert (tectonic mélange), various slivers of mudstone and turbidites, and turbidites of the Adaminaby Group. Immobile elements in the basalts show consistent patterns that allow the magmatic affinity and tectonic setting to be determined in spite of pervasive hydrothermal alteration and subsequent lower greenschist facies metamorphism that accompanied strong folding and multiple foliation development. The Melville Point basalts show Ti/V ratios transitional between arc and MORB and therefore may have formed in either a forearc or backarc basin setting. However, these rocks have higher Ti/V ratios, LREE, Th and Nb than found in forearc basalts and are therefore considered to have formed in a backarc basin setting. In contrast to Melville Point, most basalts at Barlings Beach have a geochemical signature distinctive of ocean island settings like those reported from elsewhere in the Wagonga Group. We believe these rocks developed in a Cambrian backarc basin setting. In the Early to Middle Ordovician, much of the ocean basin was inundated by quartzose turbidites followed by basin destruction with accretion/underplating at a Late Ordovician-early Silurian Benambran subduction zone and formation of the Narooma Accretionary Complex. The Cambrian-Ordovician Wagonga Group contains basalts at Melville Point and Barlings Beach, 20 km south of Batemans Bay, New South Wales. At Melville Point, the succession has basal altered basalts overlain by chert and interbedded siliceous mudstone of the Wagonga Group, in turn overlain by turbidites and chert of the Adaminaby Group with a latest Cambrian to earliest Ordovician age. By contrast, at Barlings Beach, basalt is associated with highly disrupted chert (tectonic mélange), various slivers of mudstone and turbidites, and turbidites of the Adaminaby Group. Immobile elements in the basalts show consistent patterns that allow the magmatic affinity and tectonic setting to be determined in spite of pervasive hydrothermal alteration and subsequent lower greenschist facies metamorphism that accompanied strong folding and multiple foliation development. The Melville Point basalts show Ti/V ratios transitional between arc and MORB and therefore may have formed in either a forearc or backarc basin setting. However, these rocks have higher Ti/V ratios, LREE, Th and Nb than found in forearc basalts and are therefore considered to have formed in a backarc basin setting. In contrast to Melville Point, most basalts at Barlings Beach have a geochemical signature distinctive of ocean island settings like those reported from elsewher...

Ordovician Macquarie Arc and turbidite fan relationships, Lachlan Orogen, southeastern Australia: stratigraphic and tectonic problems

“…Thus the pattern on the New South Wales South Coast and continental slope is similar to that in the Heathcote Greenstone Belt but opposite to that of the Howqua River belt and was considered evidence for the Lachlan Orocline of Cayley (2015) by Fergusson (2017, p. 33). Our interpretation is that the backarc basin basalts form basement to the Ordovician turbidites as documented in the Heathcote Greenstone Belt, Howqua River belt and at Melville Point (Stokes et al, 2015;VandenBerg et al, 2000) as was also argued by Packham & Hubble (2016). The boninitic volcanics form a distinct island arc that persisted as a topographic ridge adjacent to the basin containing the Ordovician quartz turbidites and did not form basement to them as given in other models (e.g.…”

“…(Spaggiari, Gray, & Foster, 2004). On the New South Wales South Coast backarc basin basalts are found at Melville Point (Figure 1) (Stokes, Fergusson, & Offler, 2015) and boninites are dredged from the continental slope to the east (Packham & Hubble, 2016). Thus the pattern on the New South Wales South Coast and continental slope is similar to that in the Heathcote Greenstone Belt but opposite to that of the Howqua River belt and was considered evidence for the Lachlan Orocline of Cayley (2015) by Fergusson (2017, p. 33).…”

“…In Victoria and on the New South Wales South Coast, the basal Ordovician turbidite succession overlies bedded chert and underlying mafic volcanic rocks with a backarc magmatic affinity (Glen, 2013;Stokes, Fergusson, & Offler, 2015;VandenBerg et al, 2000).…”

Mid to late Paleozoic shortening pulses in the Lachlan Orogen, southeastern Australia: a review

“…This pattern can be accounted for by linking the Bendigo and Tabberabbera zones in the proposed orocline of Cayley (2012). A similar pattern is evident from the South Coast of NSW, where boninites of inferred Cambrian age have been dredged from the continental slope (Packham & Hubble, 2016), whereas backarc basalts have been found in coastal exposures at Melville Point (Stokes, Fergusson, & Offler, 2015). Thus, the Cambrian boninitic volcanic rocks seem to form a marker horizon that can be traced around the inferred oroclines.…”

Different styles of modern and ancient non-collisional orogens and implications for crustal growth: a Gondwanaland perspective

“…Melville Point (MP) from personal observation and I. Stewart, personal communication. Burrawarra Point (Burr) from Bischoff and Prendergast (1987) and Stokes et al (2015); offshore Packham and Hubble (2016). Peel Manning Fault System (PMFS) and Murrawong area, New England Orogen from Cawood (1983), Leitch and Cawood (1987), and Stewart (1995).…”