The relationship of Palaeozoic metamorphism and S-type magmatism on the paleo-Pacific Gondwana margin James Scott a, b, ⁎, Janet Muhling c , Ian Fletcher d , Marco Billia a , J. Michael Palin a , Tim Elliot a , Christina Günter b a Geology Department, University of Otago, Leith Street, PO Box 56, Dunedin, New Zealand b Insitut für Erd- und Umweltwissenschaften, Universität Potsdam, Karl-Leibknecht-Straβe 24, 14476 Potsdam, Germany c Centre for Microscopy, Characterisation and Analysis, University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia d Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Western Australia abstract article info Article history: Received 25 April 2011 Accepted 6 September 2011 Available online 17 September 2011 Keywords: Geothermobarometric P–T Monazite growth Palaeozoic metamorphism New Zealand Gondwana A massive pulse of granitic magma was rapidly emplaced into the once contiguous West Antarctic and New Zealand segments of the palaeo-Pacific margin of the Gondwana supercontinent at ~ 371 Ma. In New Zealand, these Late Devonian S-type granitoids cover an areal extent of N 3400 km 2 , but the tectonic setting for crustal partial melting has remained unclear because most of the exposure represents either emplacement-level, or rocks that have been reworked during Cretaceous orogenesis. New petrologic data indicate that aluminous paragneisses and orthogneisses in the Bonar Range represent a rare portion of Devonian middle crust that preserves evidence for the initiation of crustal melting. The investigated rocks outline the tail of a clockwise P–T path that involved partial melting at peak conditions (~670 °C, 5.1 kb), deformation during the immedi- ately following near-isothermal decompression, and then partial re-equilibration under static conditions. Syn- to post-kinematic growth of zoned monazite establishes the timing of recrystallisation to a ~ 16 Ma period that began at 373.4 ± 4.1 Ma. This age overlaps with the initiation of regional Karamea S-type granitic magmatism. Although estimated metamorphic conditions were insufficient for large amounts of melt to have been produced from Bonar Range pelites (calculated melt volumes are b 10%), they do provide evidence con- sistent with widespread heating and partial melting in the deeper crust. This heating episode was contempo- raneous with partial melting in Fiordland (New Zealand) and West Antarctica, although Mesozoic thermal and deformational events complicate the Palaeozoic record in both those areas. Nevertheless, the apparent 1000 s km of along-strike crustal partial melting indicates that a continental-scale tectonic plate margin re-organisation took place at this time. The cause in the New Zealand segment was most likely, but not unequivocally, an extensional tectonic regime with an elevated geothermal gradient caused by conduc- tive heating from a shallowed lithospheric mantle. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In terranes where S-type granites and migmatites are present, the crustal P–T conditions must have been perturbed because typical geo- thermal gradients are seldom hot enough to melt continental crust (e.g. Brown, 2010; Clemens, 2003; Craven et al., 2011; Thompson, 2001). End-member causes of melting commonly appealed to are advective heating by emplacement of large volumes of hot plutonic rocks in continental arcs (e.g., Annen et al., 2006) or rapidly exhumed lower crust (e.g., O'Brien, 2000); near-isothermal decompression of the crust (e.g., Ring et al., 1999; Whitney et al., 2004); or conductive heating caused by upwelling of asthenosphere during continental extension (e.g., Collins and Richards, 2008). A recent synthesis of Palaeozoic plutonism in New Zealand has established that S-type granite magma was emplaced over ~3400 km 2 of the Westland portion of the New Zealand Gondwana margin be- tween 371 and c. 340 Ma (Fig. 1)(Tulloch et al., 2009b). The timing and nature of crustal partial melting is strikingly similar to magmatism in West Antarctica, which was contiguous with the New Zealand micro- continent prior to Cretaceous separation (Bradshaw et al., 1997). In New Zealand, coeval mafic rocks that could be indicators of heating by basaltic intra- or under-plating are not abundant (Muir et al., 1996; Tulloch et al., 2009b) and neither Palaeozoic decompressional fabrics nor evidence for juxtaposition against hot orogenic crust has yet been identified. However, these mechanisms cannot yet be ruled out because the exposed Westland geology is dominated by low metamorphic grade turbidites and relatively undeformed upper crustal granitoids (Cooper, 1989; Cooper and Tulloch, 1992), and the rare areas of exhumed middle crust that have been investigated have mostly been reworked in the Cretaceous (e.g., Hiess et al., 2010; Ireland and Gibson, 1998; Klepeis Lithos 127 (2011) 522–534 ⁎ Corresponding author at: Geology Department, University of Otago, Leith Street, PO Box 56, Dunedin, New Zealand. E-mail address: james.scott@otago.ac.nz (J. Scott). 0024-4937/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2011.09.008 Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos