We explore the formation conditions and inheritance probability of zircon in impact melts and the implications of using zircon geochronology to investigate planetary impact histories. By modeling the occurrence and crystallization temperature spectrum for zircon in simulated impact melts, we predict the presence of such grains within impactites. We also report U-Pb geochronology of sieve-textured, possibly poikilitic, zircon identified in the pseudotachylyte and granophyre units present within the largest known terrestrial impact crater (Vredefort, South Africa) to explore the accuracy of these grains in dating impact events at an impact structure of known age. Zircons with similar textures have been recently interpreted as growing in an impact melt in lunar meteorite SaU 169 and used to determine the age of the Imbrium impact. Modeling in simulated lunar melt compositions predicts crystallization of zircon in merely ~2% of melting events, in this case via impact. The modeled crystallization temperature spectrum is significantly below Ti-in-zircon crystallization temperatures reported from lunar samples. Zircon formation within an impact melt is dictated by saturation of [Zr] and requires a high abundance for lunar melt compositions. This essentially rules out the possibility of zircon growing in equilibrium with lunar meteorites. Poikilitic textures may be inherited from the lunar crust, presumably due to rapid decompression and/or resorption into an undersaturated magma, as previously recognized in plagioclase. Although either scenario could be due to an impact, endogenic processes cannot be ruled out, and thus lunar poikilitic zircons may not be recording impact melting events. Secondary ion mass spectrometry U-Pb analysis of zircon with similar textures from Vredefort clearly shows that these grains are inherited from the Archean target rocks, with varying degrees of Pb loss, and consequently cannot be used to accurately identify the age of the Vredefort impact structure. Further understanding of the growth and isotopic effects on zircon of shock and heating associated with large impacts could provide another tool that can be used to probe planetary impact histories.

Abstract We begin our journey through the Mesozoic Sierran arc with an examination of the Guadalupe Igneous Complex, a layered Jurassic pluton that intrudes into largely oceanic materials in the Foothills Terrane of the Western Metamorphic belt (Figs. 1-1 and 1-2). The Guadalupe Igneous Complex is an intuitively pleasing target to begin with because of its outboard (western) location and because it consists of some of the most mafic (>8% MgO gabbros) and felsic (high-silica and high-K 2 O granophyres and rhyolites) igneous units that we will see on this trip and thus raises some longstanding petrologic questions about the connections between mafic and felsic granitoids in arcs. It is also an exciting objective because of the preservation of its likely feeder zone (the Hornitos pluton), internal layering, and capping volcanics (Fig. 1-2)…