Source of Detritus in Subducted Turbidites, Tectonic Mélange, Port Macquarie Block, Southern New England Orogen, Australia —A Geochemical Perspective Robin Offler* and Ron Boyd New South Wales Institute of Frontiers Geoscience, University of Newcastle, Callaghan, New South Wales 2308, Australia ABSTRACT A tectonic mélange containing blueschists and eclogites, Middle Ordovician mid-ocean ridge basalt, cherts, and clastic sediments occurs at Port Macquarie in the southern New England Orogen, Australia. The clastics are quartz- poor and are dominated by mafic volcanic and fragmented plagioclase clasts; felsic volcanic clasts are less common. They exhibit erosional bases, graded bedding, load structures, and lithologies ranging from laminated mudstones to pebbly sandstones. Based on these features, they are interpreted as turbidites. During subduction, these turbidites were deformed and metamorphosed under prehnite–pumpellyite and lower greenschist facies conditions. Geo- chemically, they have a calc-alkaline, intra-oceanic arc signature; show no recycling; and have been derived from a provenance dominated by mafic volcanic rocks of basaltic-andesite composition. Further, chemical index of alter- ation (44–69) and Index of Compositional Variability (0.8–2.4) data reveal they show little weathering and are im- mature. The lack of weathering of rocks in a location where tropical climatic conditions existed is attributed to extreme erosion associated with a dynamic setting resulting in rapid transportation of the sediments to the fore arc basin and subsequently to the trench. As a consequence, little time was available for weathering to take place. The detritus in the turbidites is thought to have been derived from Late Ordovician volcanics in the Macquarie Arc and fore arc basin sequences of the Murrawong Formation. The cherts with which they are associated record both a continental and oceanic arc geochemical signature. Online enhancements: appendix figure, appendix tables. Introduction The New England Orogen (NEO) is a major, north- trending structure along the eastern margin of Australia (fig. 1). It is the youngest and easternmost component of the Tasmanides, a collection of subduction-related orogens varying in age from Cambrian to Triassic that record the evolution of the eastern Gondwanan margin (Glen 2005; Ro- senbaum 2018). The southern part of the NEO is made up of a series of fault bounded blocks. One of these is the Port Macquarie Block (Scheibner and Basden 1996), which is made up of narrow belts delimited by north–northeast-striking faults. Rocks in these blocks vary in age from Cambrian to Early Triassic (Och et al. 2007a). The easternmost belt is made up of a variety of mélanges exposed as a result of exhumation before the Devonian. The belt contains remnants of subduction-related rocks such as blueschists, minor eclogite, and clastic sediments as well as mafic volcanic, plutonic rocks, and ribbon cherts. Apart from the eclogite that is Cambrian in age (Tamblyn et al. 2020), all other sub- duction related rocks are Ordovician (Barron et al. 1976; Fukui et al. 1995; Och et al. 2003, 2007a, 2007b; Buckman et al. 2015; Tamblyn et al. 2020); see figure 1. Clastic sediments, the subject of this study, occur as isolated outcrops in a serpentinite mélange at Nobbys, Windmill Hill, Rocky, Town, and Oxley Beaches (fig. 1). Together with basalts and ribbon cherts of similar age, they make up the Watonga Formation (Middle–Late Ordovician; Och et al. 2007b; Buckman et al. 2015). Manuscript received March 16, 2020; accepted October 19, 2020; electronically published April 23, 2021. * Author for correspondence; email: robin.offler@newcastle .edu.au. 49 [The Journal of Geology, 2021, volume 129, p. 49–62] q 2021 by The University of Chicago. All rights reserved. 0022-1376/2021/12901-0003$15.00. DOI: 10.1086/713684