Nicholas J. Gardiner

Earth’s first stable continents did not form by subduction

The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35u2009kilometres thick), ancient (more than 3.5 billion years old) basaltic crust that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust. Thus subduction was not required to produce TTGs in the early Archaean eon.

The metallogenic provinces of myanmar

Abstract Myanmar contains important deposits of tin, tungsten, copper, gold, gemstones, zinc, lead, nickel and silver. It has one of the most diverse and richly endowed collections of natural resources in Southeast Asia, largely reflecting a geological history stretching from the Late Triassic to the Miocene. At least three world class deposits include Bawdwin (lead–zinc–silver), Monywa (copper) and Mawchi (tin–tungsten). Myanmar can be divided into three principal metallotects: the Wuntho-Popa Arc, comprising subduction-related granites with associated porphyry-type copper-gold and epithermal gold mineralisation; the Mogok-Mandalay-Mergui Belt hosting both significant tin–tungsten mineralisation associated with crustal melt granites, and key orogenic gold resources; and the Shan Plateau with massive sulphide-type lead–zinc deposits. Myanmar as a jurisdiction remains poorly understood and underdeveloped with regards its natural resources. We have built a Geographic Information System database of known Myanmar deposits, outcrops and mineral occurrences as a tool for exploration targeting.

The Juvenile Hafnium Isotope Signal as a Record of Supercontinent Cycles

Hf isotope ratios measured in igneous zircon are controlled by magmatic source, which may be linked to tectonic setting. Over the 200–500u2009Myr periodicity of the supercontinent cycle - the principal geological phenomenon controlling prevailing global tectonic styleu2009-u2009juvenile Hf signals, i.e. most radiogenic, are typically measured in zircon from granites formed in arc settings (crustal growth), and evolved zircon Hf signals in granites formed in continent-collision settings (crustal reworking). Interrogations of Hf datasets for excursions related to Earth events commonly use the median value, however this may be equivocal due to magma mixing. The most juvenile part of the Hf signal is less influenced by crustal in-mixing, and arguably a more sensitive archive of Earth’s geodynamic state. We analyze the global Hf dataset for this juvenile signal, statistically correlating supercontinent amalgamation intervals with evolved Hf episodes, and breakup leading to re-assembly with juvenile Hf episodes. The juvenile Hf signal is more sensitive to Pangaea and Rodinia assembly, its amplitude increasing with successive cycles to a maximum with Gondwana assembly which may reflect enhanced subduction-erosion. We demonstrate that the juvenile Hf signal carries important information on prevailing global magmatic style, and thus tectonic processes.

Corrigendum: Earth’s first stable continents did not form by subduction

This corrects the article DOI: 10.1038/nature21383

An impact melt origin for Earth’s oldest known evolved rocks

Earth’s oldest evolved (felsic) rocks, the 4.02-billion-year-old Idiwhaa gneisses of the Acasta Gneiss Complex, northwest Canada, have compositions that are distinct from the felsic rocks that typify Earth’s ancient continental nuclei, implying that they formed through a different process. Using phase equilibria and trace element modelling, we show that the Idiwhaa gneisses were produced by partial melting of iron-rich hydrated basaltic rocks (amphibolites) at very low pressures, equating to the uppermost ~3u2009km of a Hadean crust that was dominantly mafic in composition. The heat required for partial melting at such shallow levels is most easily explained through meteorite impacts. Hydrodynamic impact modelling shows not only that this scenario is physically plausible, but also that the region of shallow partial melting appropriate to formation of the Idiwhaa gneisses would have been widespread. Given the predicted high flux of meteorites in the late Hadean, impact melting may have been the predominant mechanism that generated Hadean felsic rocks.Earth’s oldest known felsic rocks formed by partial melting at low pressures and high temperatures caused by impact melting of mafic Hadean crust, according to phase equilibria and trace element modelling.

Contrasting Cu–Au and Sn–W granite metallogeny through the zircon record

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Zircons and granite metallogeny

N. J. Gardiner, C. J. Hawkesworth, L. J. Robb, A. I. S. Kemp, H. Delavault and B. Dhuime Centre for Exploration Targeting – Curtin Node, Department of Applied Geology, Curtin University, Perth, Australia; School of Earth Sciences, University of Bristol, Bristol, UK; School of Earth Sciences, University of Oxford, Oxford, UK; School of Earth and Environment, University of Western Australia, Crawley, Australia

Myanmar: Tethyan tectonics and metallogeny

Naumov, E., Mizerny, A., Seltmann, R., KovalevK., and Izokh, A. 2013. Mineralization style and geochronology of the Sekisovka gold deposit, eastern Kazakhstan, in Mineral depostits for a high-tech world: Proceedings of the 12th SGA Biennial Meeting 2013 (Uppsala, Sweden, 12–15 August 2013); Vol. 3, 1164–1167; Uppsala, Sveriges Geologiska Undersokning. Rafailovich, M. S. 2009. Gold deposits of Kazakhstan: geology, metallogeny, exploration models. Almaty. English translation by CERCAMS NHM London, 2012.