Reply: Stratigraphy, facies architecture and tectonic implications of the Upper Devonian to Lower Carboniferous Campwyn Volcanics of the northern New England Fold Belt. [Discussion by Henderson RA, Fergusson C]

Henderson and Fergusson present several points of disagreement based on their earlier work (Fergusson et al. 1994) and emphasise the major differences between the two studies. Although not explicitly stated previously, we believe that paper to be fundamentally flawed in terms of basic lithological identification, stratigraphy and facies architecture. The main issue is one of contradictory volcanic rock-type identification, in which we maintain that many of the units described as mafic by Fergusson et al. (1994) are in fact silicic. We also found that at least some of the mafic to intermediate ‘volcanic’ components described belonged to younger intrusive dyke suites, some of which now have geochronologic control. These observational data underpin the respective interpretations on the volcanic sequence and architecture. A secondary ‘data’ issue arises in the sedimentology of the sequences; our more extensive palaeocurrent datasets are strongly at variance to those presented in Fergusson et al. (1994). Likewise, differences in tectonic models reflect these opposed datasets. A further issue raised, but which is not critical to the main thrust of the argument, is that of biostratigraphic control for the Campwyn Volcanics. However, we now have further constraints from U/Pb zircon geochronology (Bryan et al. in press)

Reply to: Stratigraphy, Facies Architecture and tectonic Implications of the Upper Devonian to Lower Carboniferous Campwyn Volcanics of the Northern New England Fold belt

Discusses stratigraphy, facies architecture and tectonic implications of the Upper Devonian to Lower Carboniferous campwyn volcanics of the northern New England Fold Belt. Ignimbrites and volcanic compositions; Lower and upper facies associations; Biostratigraphy and evidence for Middle Devonian rocks; Sedimentary provencance and plaeoccurent data; Supra-subduction zone magmatism

U-Pb zircon geochronology of Late Devonian to Early Carboniferous extension-related silicic volcanism in the northern New England Fold Belt

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of zircons confirm a Late Devonian to Early Carboniferous age ( ca 360–350 Ma) for silicic volcanic rocks of the Campwyn Volcanics and Yarrol terrane of the northern New England Fold Belt (Queensland). These rocks are coeval with silicic volcanism recorded elsewhere in the fold belt at this time (Connors Arch, Drummond Basin). The new U–Pb zircon ages, in combination with those from previous studies, show that silicic magmatism was both widespread across the northern New England Fold Belt (>250 000 km 2 and 500 km inboard of plate margin) and protracted, occurring over a period of 15 million years. Zircon inheritance is commonplace in the Late Devonian — Early Carboniferous volcanics, reflecting anatectic melting and considerable reworking of continental crust. Inherited zircon components range from ca 370 to ca 2050 Ma, with Middle Devonian (385–370 Ma) zircons being common to almost all dated units. Precambrian zircon components record either Precambrian crystalline crust or sedimentary accumulations that were present above or within the zone of magma formation. This contrasts with a lack of significant zircon inheritance in younger Permo-Carboniferous igneous rocks intruded through, and emplaced on top of, the Devonian-Carboniferous successions. The inheritance data and location of these volcanic rocks at the eastern margins of the northern New England Fold Belt, coupled with Sr–Nd, Pb isotopic data and depleted mantle model ages for Late Palaeozoic and Mesozoic magmatism, imply that Precambrian mafic and felsic crustal materials (potentially as old as 2050 Ma), or at the very least Lower Palaeozoic rocks derived from the reworking of Precambrian rocks, comprise basement to the eastern parts of the fold belt. This crustal basement architecture may be a relict from the Late Proterozoic breakup of the Rodinian supercontinent