Timing of subduction and exhumation along the Cambrian East Gondwana margin, and the formation of Paleozoic backarc basins David A. Foster† Department of Geological Sciences, PO Box 112120, University of Florida, Gainesville, Florida 32611, USA David R. Gray School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia Catherine Spaggiari Department of Applied Geology, Curtin University of Technology, Perth, Western Australia 6845, Australia ABSTRACT in Tasmania and southeast Australia. This al., 1993; Meffre et al., 2000). Ophiolite obduc- continental rift magmatism and extension tion resulted in high-pressure metamorphism of The inversion of the Neoproterozoic-Cam- were caused by west-dipping subduction the turbidite wedge (Meffre et al., 2000; Reed brian passive margin of East Gondwana under the Australian margin of Gondwana et al., 2002) and was followed shortly thereaf- occurred during the early Paleozoic Delame- after the collisional phase of the Delamerian- ter by postcollision continental extension and rian-Ross orogeny. We present 40Ar/39Ar and Ross orogeny. Rollback of subduction in the magmatism at 505–495 Ma (Meffre et al., 2000; structural data from deformed and metamor- Australian sector of the margin between ca. Perkins and Walsh, 1993; Black et al., 1997; phosed Neoproterozoic clastic rocks beneath 508 and 460 Ma produced a backarc basin Foster et al., 1998). Extension thinned and the Tasmanian ophiolite and the footwall of >1000 km wide that became the basement rifted the underthrust passive margin along with a high-pressure metamorphic complex in for the Lachlan orogen turbidites. Similar the ophiolite and ultimately formed an oceanic northern Tasmania. These data reveal the amounts of subduction rollback seem not backarc basin that became the depocenter for timing of accretionary deformation and the to have occurred in Antarctica at this time the Lachlan orogen turbidites (Foster and Gray, initiation of backarc extension along the (unless the record is lost), suggesting sig- 2000). The transition from obduction and short- Australian margin of Gondwana. 40Ar/39Ar nifi cant along-strike differences in the early ening to oceanic backarc basin extension started analyses of muscovite from lower greenschist Paleozoic geodynamics of the Delamerian- a cycle of alternating convergent and divergent facies fault slices bounding the Forth meta- Ross orogenic system. dynamics along the Pacifi c margin of Australia morphic complex give plateau ages of 521.4 that formed the enigmatic Lachlan orogen by ± 2.5 and 520.7 ± 1.6 Ma. These data suggest Keywords: 40Ar/39Ar dating, Gondwana, the middle Paleozoic, the New England orogen that deformation within an accretionary Tasmania, obduction, exhumation, Lachlan by the end of the Paleozoic, and continues to the prism off the margin of Tasmania, and pos- orogen, Delamerian-Ross orogen, Australia. present (Foster and Gray, 2000; Collins, 2002). sibly ocean arc collision, were under way by In this paper we present Ar-Ar geochrono- ca. 521 Ma. Muscovite from upper amphibo- INTRODUCTION logical data and summarize structural data from lite and upper greenschist facies rocks in fi ve Neoproterozoic clastic rocks that were deformed locations of the Forth metamorphic complex, The Neoproterozoic-Cambrian passive beneath the Tasmanian ophiolite during obduc- including retrograde shear zones, give 40Ar/ margin of East Gondwana underwent a long tion and are now exposed in the footwall of a 39Ar cooling ages that average 508.1 ± 2.6 Ma. progression of accretionary orogenic events high-pressure metamorphic complex in northern Identical muscovite cooling ages from rocks after the initiation of a convergent plate bound- Tasmania (Figs. 2 and 3). These data bear on originally at very different metamorphic P-T ary between Gondwana and the paleo-Pacifi c (1) the timing of deformation in the Delamerian- conditions suggest rapid cooling of the Forth by early Cambrian time. Inversion of the East Ross orogeny, (2) the nature of exhumation of complex at ca. 508 Ma, due to the juxtapo- Gondwana passive margin took place during the high-grade metamorphic complexes in this and sition of higher-grade against lower-grade latter stages of the suturing of Gondwana along other orogens, and (3) the initiation of back- rocks. Rapid cooling is also indicated by and within Pan-African orogenic belts, which arc extension along the Australian margin of concordant 40Ar/ 39Ar ages of hornblende and culminated at ca. 530–510 Ma (e.g., Meert, Gondwana. On a larger scale, these data further muscovite in the high-grade core. Cooling 2003; Boger and Miller, 2004). The Cambrian constrain the timing of orogenic intervals within was probably due to rapid exhumation along passive margin in Tasmania, which was adjacent the >20,000-km-long Paleozoic active margin extensional shear zones within a regional to mainland Australia and North Victoria Land, of Gondwana and will help determine: (1) the extensional setting that also produced the Antarctica (Fig. 1), was overthrust between possible plate tectonic settings before and dur- Mount Read–Mount Stavely volcanic com- 520 and 505 Ma by a mafi c-ultramafi c ophio- ing the Cambrian orogeny; (2) the age of onset plexes (505–495 Ma) along with rift basins lite complex along with turbidites during the of subduction, which may have varied along the early convergent stage of the Delamerian-Ross margin, (3) the location of promontories and †E-mail: [email protected]fl .edu. orogeny (Berry and Crawford, 1988; Elliott et reentrants on the Neoproterozoic rifted margin, GSA Bulletin; January/February 2005; v. 117; no. 1/2; p. 105–116; doi: 10.1130/B25481.1; 9 fi gures; 1 table; Data Repository item 2005023. For permission to copy, contact [email protected] © 2005 Geological Society of America 105 FOSTER et al. (4) the importance of intraorogen rifting and oce- anic backarc basin formation, and (5) the nature and importance of continental blocks derived SoutheastSoutheast from Gondwana or other continents. Gawler AustraliaAustralia Craton GEOLOGICAL SETTING Lachlan Delamerian Orogen The western Tasmania terrane (Fig. 2) Orogen Mount formed as an attenuated Meso- and Neopro- Stavely Jamieson- Volcanic terozoic continental block that lay along or Licola complex Volcanics outboard of Gondwana (Meffre et al., 2000). Mesoproterozoic continental basement in Tas- mania (Berry and Burrett, 2002) does not have a direct counterpart exposed in mainland Austra- lia or Antarctica (Elliott and Gray, 1992). This Tyennan Lord Howe Rise block of continental crust was probably either Orogen a promontory along the Gondwana margin or a Ross continental ribbon rifted during the breakup of East Orogen Rodinia. It records details of the inversion of the Antarctic Neoproterozoic–Early Cambrian margin that Craton are not well expressed in eastern Australia or Antarctica (Berry and Crawford, 1988). North Mount ictoria The oldest rocks in Tasmania are quartzite V Read Volcanic and pelitic schist that contain detrital zircons complex with 207Pb/206Pb ages older than 1200 Ma (Black Land Takaka Bowers Tuhua Terrane et al., 1997; Turner et al., 1998). The rocks were 300 km Terrane metamorphosed and deformed probably during Orogen the late Mesoproterozoic, based on relatively imprecise electron microprobe dates of mona- Cambrian-Ordovician sedimentary rocks SSouthouth zite (Berry and Burrett, 2002), and intruded by deformed in mid-late Paleozoic IslandIsland shallow-level granitoids and mildly deformed at Cambrian volcanic and sedimentary rocks NewNew 780–760 Ma (Turner et al., 1998). Neoprotero- Ross / Delamerian Cambrian granitoids ZealandZealand zoic continental-derived turbidite (Oonah and Late Proterozoic- Cambrian sedimentary Burnie Formations) intercalated with basalt and rocks (foliation trend where deformed) intruded by syndepositional dolerite is in fault Pre-700 Ma basement contact with the late Mesoproterozoic metasedi- mentary basement (Fig. 2). These mafi c rocks Figure 1. Reconstruction of part of the Phanerozoic East Gondwana margin—including have alkaline affi nities suggesting a rift environ- southeast Australia, North Victoria Land, Antarctica, New Zealand, and the Lord Howe ment (Crawford and Berry, 1992) and give K-Ar Rise—at ca. 120 Ma showing major orogenic belts and Cambrian volcanic arcs (modifi ed biotite dates of 725 ± 25 Ma (Crook, 1979). from Gibson and Ireland, 1996). The late Neoproterozoic successions record extension and ultimately breakup of the Aus- tralia-Antarctic margin of Gondwana (Crawford and Berry, 1992) during the fragmentation of margin, and west- or south-directed obduc- schist facies rocks, and therefore the high P/T Rodinia (Powell et al., 1994). After Neoprotero- tion of an ophiolite interpreted to be the roots ratio assemblages are only locally preserved. zoic extension and eventual breakup, Tasmania of an oceanic island arc (Berry and Crawford, Metamorphic complexes in Tasmania produced may have been separated from Gondwana (or 1988; Meffre et al., 2000; Turner and Bot- and exhumed during the Cambrian Delamerian Laurentia) as an oceanic plateau, or may have trill, 2001). The leading edge of the western orogeny include the Port Davey, Franklin, remained attached as an attenuated promontory. Tasmania terrane was subducted to depths of Mersey River, Arthur, Forth, and Badger Head The Neoproterozoic-Cambrian continen- at least 40–50 km as documented by eclogite complexes (Meffre et al., 2000; Reed et al., tal rift to passive margin strata were highly slivers