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We present 40Ar/39Ar ages and boron isotopes of tourmalines from a quartz-tourmaline intrusion cross-cutting the Hadean detrital-zircon-bearing metasedimentary rocks from the Mt. Alfred and Brooking Hills localities of the Illaara Greenstone Belt, Western Australia. Concordant 40Ar/39Ar plateau ages on the tourmalines at Mt. Alfred give a weighted mean age of 2935 ± 9 Ma (2σ). These results are the first reported Archean 40Ar/39Ar ages directly obtained on tourmaline, and provide a minimum depositional age for this sequence, placing constraints on the depositional timing of detrital Hadean zircons to between 2935 ± 9 Ma and c.a. 3250 Ma (youngest detrital zircon age in sequence). This is the last known occurrence of abundant detrital Hadean zircons having been directly deposited in a sedimentary sequence, and thus suggests that most Hadean zircon sources have been exhausted by this time. The 2935 ± 9 Ma tourmalines occur within a quartz-tourmaline intrusion with associated stratiform layers and veins, inferred to have grown during a hydrothermal fluid circulation event. A younger generation of tourmalines from the Brooking Hills, 30 km south of Mt. Alfred yielded concordant 40Ar/39Ar plateau ages with a weighted mean age of 2624 ± 16 Ma. These tourmalines occur within and along the margins of post-kinematic quartz veins, parallel to the foliation fabric and marks the end stage of high temperature tectono-thermal events within the Illaara Greenstone Belt. The retention of 2935 ± 9 Ma tourmaline ages within the Illaara Greenstone Belt shows that the K/Ar system of tourmaline remained closed throughout repeated upper-greenschist metamorphic events (∼450 °C) between the ages of c.a. 2930 to 2630 Ma in contrast to muscovite recording plateau ages of about 2600 Ma. This is in agreement with an approximate Ar closure temperature between ∼534 and 628 °C experimentally determined in this study. [ABSTRACT FROM AUTHOR]

• 1560 Ma TTG suite and migmatised mafic source exposed in NE Australia. • 'High-pressure' TTG generated at a depth of 25–35 km. • Field observations and trace-element modelling point to fluid-fluxed melting. • TTG chemical diversity reflects variable hydration, not depth. Across the Archaean to Proterozoic transition, the composition of newly-formed felsic continental crust changed from tonalite–trondhjemite–granodiorite (TTG) to calc-alkaline granitoid, possibly coinciding with the emergence of plate tectonics. Nevertheless, TTG suites were sporadically produced in Proterozoic and Phanerozoic orogenic belts, and such occurrences may provide petrological and tectonic insights into the formation of ancient continents. Here we demonstrate that the ca 1560 Ma Forest Home TTG plutonic suite in the Georgetown Inlier, NE Australia, was derived from partial melting of spatially-associated mafic rocks in a post-collisional setting. The studied TTG rocks have a 'high-pressure' geochemical signature, with elevated Sr, low heavy rare earth element and low high field strength element contents. Established petrogenetic models suggest they were derived by partial melting either of hydrated basaltic crust at >70 km depth or enriched lithospheric mantle, or by fractionation of lower-pressure mafic magmas. Using phase equilibrium calculations and trace-element modelling, we show that the geochemical signature of the Georgetown TTG likely resulted from fluid-fluxed crustal melting at relatively shallow depths (25–35 km), consistent with field observations and the inferred metamorphic evolution of the inlier. Our results suggest that the chemical variability of TTGs can reflect the variable availability of fluids rather than depth of melting, which has implications for tectonic processes responsible for the formation of early continental crust. [ABSTRACT FROM AUTHOR]