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The Rathjen Gneiss is the oldest and structurally most complex of the granitic intrusives in the southern Adelaide Fold-Thrust Belt and therefore provides an important constraint on the timing of the Delamerian Orogen. Zircons in the Rathjen Gneiss show a complex growth history, reflecting inheritance, magmatic crystallisation and metamorphism. Both single zircon evaporation ('Kober' technique) and SHRIMP analysis yield best estimates of igneous crystallisation of 514 Ϯ 5 Ma, substantially older than other known felsic intrusive ages in the southern Adelaide Fold-Thrust Belt. This age places an older limit on the start of the Delamerian metamorphism and is compatible with known stratigraphic constraints suggesting the Early Cambrian Kanmantoo Group was deposited, buried and heated in less than 20 million years. High-U overgrowths on zircons were formed during subsequent metamorphism and yield a 206 Pb/ 238 U age of 503 Ϯ 7 Ma. The Delamerian Orogeny lasted no more than 35 million years. The emplacement of the Rathjen Gneiss as a pre-or early syntectonic granite is emphasised by its geochemical characteristics, which show affiliations with within-plate or anorogenic granites. In contrast, younger syntectonic granites in the southern Adelaide Fold-Thrust Belt have geochemical characteristics more typical of granites in convergent orogens. The Early Ordovician post-tectonic granites then mark a return to anorogenic compositions. The sensitivity of granite chemistry to changes in tectonic processes is remarkable and clearly reflects changes in the contribution of crust and mantle sources.

One of the currently popular theories on magma ascent is that it mainly occurs by propagating hydrofractures (dykes) and that magma viscosity is the primary rate‐controlling factor. This theory is based on mathematical models for single hydrofractures under idealised conditions. We simulated magma ascent with air ascending through gelatine and observed that the air ascended in batches, following paths made by their predecessors. Multiple batches accumulate at obstacles along the path. Although magma viscosity may control ascent rate during movement, obstacles ultimately control the size and average ascent velocity of ascending batches. We propose that step‐wise movement of magma batches is the mechanism of primary accumulation and ascent from the partially molten source rock of a magma to its first emplacement site and therefore the main ascent mechanism for granitic magmas. ‘Classical’ dyking is the mechanism for secondary ascent from a magma chamber.

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