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Slab rollback during subduction plays a key role in controlling continental growth at convergent plate boundaries. The dynamics of currently subducting slabs can be precisely constrained using geophysical techniques. In contrast, ancient episodes of slab rollback can be difficult to constrain, yet are critical to unlocking the tectonic evolution of long‐lived orogens such as the Phanerozoic Australian Tasmanides of eastern Gondwana. Recognition of ancient slab rollback relies on the identification of the progressive migration of magmatic arcs. Here, we investigate the timing and isotopic variation of ∼90 km of the trans‐orogen migration of the Carboniferous Bathurst Batholith as a potential indicator of slab rollback. U–Pb–Hf isotopes, combined with a regional zircon Hf isotope data set, suggest that the eastward migration of the batholith over ∼18 Myr, from 340.1 to 322.4 Ma, records a maximum slab steepening rate of ∼0.6–1.2° Myr−1 in a relatively stable trench setting. These results provide a magmatic record of Carboniferous slab rollback and establish a missing link between the long‐lived Lachlan and New England orogens of the eastern Gondwanan Tasmanides. Plain Language Summary: Slab rollback occurs at convergent plate boundaries, which is key to understanding the evolution of subduction processes. However, for ancient subduction zones where the geometry of long since subducted slabs no longer exists, the evolution of slab dynamics remains unclear. Occasionally though, the progressive migration of magmatic trends is observable and provides constraints on slab rollback, which aid in reconstructing the tectonic evolution of long‐lived orogens, such as the Phanerozoic orogens of eastern Australia. Here, we focus on how the unique magmatism of the Carboniferous Bathurst Batholith progressively migrated in the direction of the trench and analyze and interpret the geochronological and geochemical evolution of the batholith to better constrain subduction dynamics. We find that the Bathurst Batholith records the Carboniferous slab rollback of eastern Gondwana and reveals the relationship between the two key orogens of eastern Australia. Thus, the migration of magmatism in convergent settings can be used to track the slab rollback processes and evolution of the long‐lived orogens. Key Points: The Bathurst Batholith is part of a Carboniferous continental arc formed as a result of westward subduction in eastern GondwanaEastward migration of the batholith over ∼18 Myr recorded a maximum slab steepening rate of ∼0.6–1.2° Myr−1 in a stable trench settingThe batholith as a trans‐orogen "missing link" reveals continental arc magmatism extended across the Lachlan and New England orogens [ABSTRACT FROM AUTHOR]

The Sanandaj-Sirjan Zone is a basement culmination northeast of the Neo-Tethys suture in Iran. In this zone near Azna, granite has a magmatic zircon U-Pb age of 568 ± 11 Ma, with 900-800, ca. 2400, and ca. 3600 Ma inherited cores. The ca. 3600 Ma inherited zircon is the oldest crustal component yet detected in Iran. Near Chadegan, orthogneiss has a magmatic zircon U-Pb age of 637 ± 15 Ma, and carries ca. 1000 and 2000 Ma inherited zircons. Inherited 900-1000 Ma zircons have juvenile initial [ζ.sub.Hf] values of ca. +8 to +9, whereas the younger 630 and 568 Ma magmatic zircons show lower initial [ζ.sub.Hf] values; however, the 3600 Ma core has initial [ζ.sub.Hf] = 0.0. A Neoproterozoic rim on the inherited 3600 Ma core has the most extreme initial [ζ.sub.Hf] value of -18. The Hf isotopic data indicates generation of the magmatic protoliths from a mixture of juvenile Neoproterozoic and Archean sources. Previous studies showed that in Turkey the Central Anatolian Crystalline Complex is underlain by Neo-Eoarchean rocks, the Menderes Massif contains Neoproterozoic granitoids, and that central Iran's basement and the northern Sanandaj-Sirjan Zone contain Neoproterozoic granitic rocks. This basement terrane is from Gondwana, and was transferred across Paleo-Tethys to dock against Eurasia's southern margin. Occurrence in Iran and Turkey of Eoarchean crust raises the possibility of sinistral migration of this terrane in the closure of Tethys because the nearest known early Archean crust occurs in northeast India. La zone Sanandaj-Sirjan constitue un bombement du socle au nord-est de la suture de l'ocean Neotethys en Iran. Dans cette zone a proximite d'Azna, le granite a un age magmatique (U-Pb sur zircon) de 568 ± 11 Ma, avec des noyaux herites de 900-800, ~2400 et ~3600 Ma. Le zircon herite d'environ 3600 Ma est le plus ancien composant de la croute detecte a ce jour en Iran. A proximite de Chadegan, un orthogneiss a un age magmatique (U-Pb sur zircon) de 637 ± 15 Ma et comporte des zircons herites d'environ 1000 et 2000 Ma. Les zircons herites ages de 900-1000 Ma ont des valeurs juveniles initiales [ζ.sub.Hf] d'environ +8 a +9, alors que les zircons magmatiques plus jeunes, 630 et de 568 Ma, montrent des valeurs [ζ.sub.Hf] initiales inferieures et que le noyau de 3600 Ma a une valeur initiale [ζ.sub.Hf] de 0,0. Une bordure neoproterozoique sur le noyau herite de 3600 Ma a la valeur initiale [ζ.sub.Hf] la plus extreme, soit de -18. Les donnees isotopiques Hf indiquent une generation de protolites magmatiques a partir d'un melange de sources juveniles proterozoiques et archeennes. Des etudes anterieures ont montre que le complexe cristallin de l'Anatolie centrale en Turquie repose sur des roches neo-eoarcheennes, que le massif Menderes contient des granitoides neoproterozoiques et que le socle du centre de l'Iran ainsi que la zone Sanandaj-Sirjan nord contiennent des roches granitiques neoproterozoiques. Ce terrane de socle provient de Gondwana et il a ete deplace a travers la Paleotethys pour s'amarrer a la bordure sud de l'Eurasie. La presence en Iran et en Turquie d'une croute eoarcheenne souleve la possibilite d'une migration senestre de ce terrane lors de la fermeture de l'ocean Tethys; la croute archeenne la plus proche se trouve en effet dans le nord-est de l'Inde. [Traduit par la Redaction], Introduction The cratons are the traditional hunting ground for early Precambrian crust, with most revealing histories dating back to 3.4 Ga or earlier (e.g., Nutman et al. 1996). However, serendipitous [...]

Byline: Allen P. Nutman (1,2,3), Vickie C. Bennett (1), Clark R. L. Friend (4), Kenji Horie (5,6), Hiroshi Hidaka (6) Keywords: Eoarchaean; Tonalites; Crustal evolution; Zircons; Greenland; Nd isotopes; Itsaq Gneiss Complex Abstract: The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually < 10 um) younger (3,660--3,590 Ma and Neoarchaean) shells of lower Th/U metamorphic zircon are present on these 3,850 Ma zircons. Several samples with this simple zircon population occur on islands near Akilia. In contrast, migmatites usually contain more complex zircon populations, with often more than one generation of igneous zircon present. Additional zircon dating of banded gneisses across the Complex shows that samples with c. 3,850 Ma igneous zircon are not just a phenomenon restricted to Akilia and adjacent islands. For example, migmatites from Itilleq (c. 65 km from Akilia) contain variable amounts of oscillatory-zoned 3,850 Ma and 3,650 Ma zircon, interpreted, respectively, as the rock age and the time of crustal melting under Eoarchaean metamorphism. With only 110--140 ppm Zr in the tonalites and likely magmatic temperatures of >850degC, zircon solubility--melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble. Combined with the cathodoluminesence imaging, this demonstrates that the c. 3,850 Ma oscillatory zoned zircon crystallised out of the melt and hence gives a magmatic age. Thus the rare well-preserved tonalites and palaeosome in migmatites testify that c. 3,850 Ma quartzo--feldspathic rocks are a widespread (but probably minor) component in the Itsaq Gneiss Complex. C. 3,850 Ma zircon with negative Eu anomalies (showing growth in felsic systems) also occurs as detrital grains in rare c. 3,800 Ma metaquartzites and as inherited grains in some 3,660 Ma granites (sensu stricto). These demonstrate that still more c. 3,850 Ma rocks were present, but were recycled into Eoarchaean sediments and crustally derived granites. The major and trace element characteristics (e.g. LREE enrichment, HREE depletion, low MgO) of the best-preserved c. 3,850 Ma rocks are typical of Archaean TTG suites, and thus argue for crust formation processes involving important contributions from melting of hydrated mafic crust to the earliest Archaean. Five c. 3,850 Ma tonalites were selected as the best preserved on the basis of field criteria and zircon petrology. Four of these samples have overlapping initial E.sub.Nd (3,850 Ma) values from +2.9 to +3.6+- 0.5, with the fourth having a slightly lower value of +0.6. These data provide additional evidence for a markedly LREE-depleted early terrestrial mantle reservoir. The role of c. 3,850 Ma crust should be considered in interpreting isotope signatures of the younger (3,800--3,600 Ma) rocks of the Itsaq Gneiss Complex. Author Affiliation: (1) Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia (2) Department of Earth and Marine Science, Australian National University, Canberra, ACT, 0200, Australia (3) Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037, People's Republic of China (4) 45 Stanway Road, Headington, Oxford, OX3 0BP, UK (5) Department of Earth and Planetary Systems Sciences, University of Hiroshima, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8526, Japan (6) Department of Science and Engineering, The National Science Museum, 3-23-1, Hyakunin-cho, Shinjuku-ku, Tokyo, 169-0073, Japan Article History: Registration Date: 30/03/2007 Received Date: 27/02/2006 Accepted Date: 27/03/2007 Online Date: 12/07/2007 Article note: Communicated by I. Parsons. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-007-0199-3) contains supplementary material, which is available to authorized users.