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Zircon shape is commonly reported during geochronology and geochemistry analyses of igneous, metamorphic, and sedimentary rocks, but the relationship of zircon shape to primary growth environmental conditions remains poorly constrained. Current models for the control on igneous zircon shape focus on the relative growth of crystal prisms and pyramids, which are not discernible in the imaging techniques used for rapid quantification of zircon shape in geochronology sample mounts. We model the relationship between whole‐rock composition and zircon 2D shape in mineral separates from 45 mafic to felsic igneous samples, representative of Archean and Proterozoic crust in Western Australia. Shape parameters are derived from semi‐automated measurement of photomicrographs of polished zircon crystals in epoxy resin mounts. Whole‐rock composition shows a statistically significant relationship to median magmatic zircon crystal area and mathematically defined "roundness." Zircon populations show reduced median area and increased median roundness as whole‐rock silica decreases. Phase equilibrium modeling based on whole‐rock composition, and automated electron microscopy mineral maps, indicates that the compositional predisposition of zircon shape is influenced by fundamentally different physical growth environments in mafic versus felsic melts. Specifically, influential factors that differ between mafic and felsic liquids include crystallization sequence and duration—which influence unconstrained growth space—and the potential for absorption/exsolution of zirconium from the accompanying mineral assemblage. We present quantitative, explanatory models for the relationship between zircon 2D shape and whole‐rock silica and demonstrate that the relationships are adhered to across a broad spectrum of whole‐rock compositions. Plain Language Summary: Zircon is a resilient mineral that can preserve chemical information over billions of years and is used to date different types of rocks. The 2D boundary shapes of polished zircon crystals are commonly reported during their analysis. However, it is challenging to interpret the geological meaning of the 2D shapes as little is known about how the crystals' growth environment affects them. Current understanding is mostly based on 3D crystal faces that are not easily interpreted from imaging techniques geared toward rapid measurements of large quantities of zircon grains, as used during typical isotopic analysis of this mineral. We investigate whether there is geological information in easily acquired 2D zircon boundary shape. We measured the 2D shapes of zircons from different rock types and model how the shape changes with rock composition. We then model how different rock types crystallize and compare the crystallization models to mineral growth relationships seen in the rocks. We find that the 2D shape of zircon is affected by growth space, growth duration, and competition for mineral components. These three factors vary with rock composition. Thus, zircon 2D shape varies with rock composition, hinting at potentially useful, currently untapped, additional information stored in zircon grains. Key Points: Parent magma silica content influences the median 2D shape of an igneous zircon populationMajor controls of median zircon 2D boundary shape appear extrinsic to the crystalPredominant zircon crystallization processes differ between mafic and felsic rocks [ABSTRACT FROM AUTHOR]

Throughout geological history, biodiversity trends have been punctuated by sharp declines coinciding with mass extinction events. Certain organisms—known as disaster forms—flourish during the extinction aftermath from a lack of ecological competition and predation. Microbialites (in particular, stromatolites) are known to increase in environmental diversity following these biotic crises. However, it remains important to identify whether individual microbialite occurrences are a result of globally driven competition reduction or favourable local conditions. Here, we reconsider stromatolites from the northern Perth Basin of Western Australia, previously reported as Smithian (late Olenekian) in age and part of a biotic rebound following the end-Permian mass extinction and re-evaluate their paleodepositional setting and age. Detailed mapping, macro-analysis and meso-analysis of the Perth Basin locality have identified a well-preserved and diverse morphological assemblage of stromatolites that are intimately associated with a coarse siliciclastic facies. The characteristics of, and relationships between the stromatolitic and coarse siliciclastic facies support a restricted aquatic paleodepositional environment with fluvial/alluvial influences. In the Perth Basin, such depositional environments occurred most commonly during the Permian (Guadalupian to Lopingian). The robustness of previous age constraints interpreted from overlying strata (Kockatea Shale) are questioned by the identification of a depositional hiatus and onlapping relationship of the Kockatea Shale on the stromatolitic sequence. Therefore, we suggest that the mid-Phanerozoic northern Perth Basin Stromatolitic Sequence cannot be unequivocally associated with the end-Permian mass extinction and may have instead thrived owing to a favourable paleodepositional setting. The mid-Phanerozoic northern Perth Basin stromatolites are not a unique case. A compilation of reported Permian and Triassic microbialite occurrences shows that stromatolites, although most common following the end-Permian mass extinction, also occur before and after the extinction event in a range of environmental settings. Ten microbialite forms were described and mapped out over 30 km in the northern Perth Basin. Perth Basin Stromatolitic Sequence formed in a restricted aquatic depositional setting with fluvial and/or alluvial influences. Stromatolites from the northern Perth Basin are unlikely to be associated with the end-Permian mass. [ABSTRACT FROM AUTHOR]

• New geochronological and stratigraphic data for the Edmund Basin. • Tripartite subdivision of the stratigraphy hosting the giant base metal Abra deposit. • 30 Ma window for the deposition of the low Edmund Basin units (Deposition Packages 1 and 2) and for the mineral precipitation (Depositional Package 2). • Detrital zircons used to investigate the sources of the Edmund Basin sediments in the Capricorn Orogen. Characterization of host rocks is an essential step towards understanding the genesis and history of sedimentary rock-hosted mineral deposits. Abra is a sedimentary rock-hosted polymetallic deposit, located within the Mesoproterozoic Edmund Basin, in the central Capricorn Orogen, Western Australia. Stratigraphic analysis of the Abra host rock, coupled with detrital zircon geochronology and geochemistry, revealed that the host comprises four distinct units that correlate with four formations of the Edmund Basin (Irregully Formation, Gooragoora Formation, Blue Billy Formation, Kiangi Creek Formation). Detrital zircon U-Pb geochronology constrains the maximum depositional age of the Irregully Formation to 1626 Ma, and 1484 Ma for the Kiangi Creek Formation. The detrital zircon ages compared with available geochronological data on the mineralization, highlight an age gap of c. 100 Ma between the deposition of the basal succession, which hosts the primary base metals deposit, and a second mineralization event in the topmost sedimentary succession. The lowest formations accumulated just after the 1680–1640 Ma Mangaroon Orogeny. Moreover, the Gooragoora and Blue Billy formations exhibit a distinct shift of sediment provenance from the west (Gascoyne Complex) to the south (Yilgarn Craton granites), coincident with extensional tectonic reactivation of the northern Yilgarn Craton margin. Tectonic reactivation is interpreted to have promoted basin deepening and local hydrothermal fluid circulation, which is inferred to have enhanced mineralization in the Abra polymetallic deposit. [ABSTRACT FROM AUTHOR]

Zircon grain shape is traditionally interpreted as a product of the physico-chemical conditions during crystal growth and may be modified during grain transport processes. The analysis of magmatic zircon grain shape has been proposed to inform on crystallization conditions, whereas detrital zircon grain shape has been proposed to complement traditional sediment provenance analysis. Shape parameters can be automatically measured from digital images of zircon mounts; however, this requires extraction of individual grain boundaries for measurement. Existing image segmentation software may require the use of proprietary languages, or knowledge of scripting to develop automated image segmentation routines, and is typically not tailored towards the geosciences. Furthermore, the separation of touching zircon grains in images remains a challenge for existing algorithms. To facilitate zircon grain shape analysis, we present AnalyZr, an open-source graphical Python application designed to segment reflected and transmitted light images of zircons mounted in resin. A new segmentation algorithm is implemented to improve the separation of touching zircon grains. Shape parameters are automatically measured from the segmented images and may be output to a.csv or.mdb file. Two case studies demonstrate the use of the application in resolving geologically relevant information in zircon grains sourced from: i) compositionally and age-distinct granite, diorite, and gabbro samples from across Western Australia, and ii) age-distinct detrital zircons from the Canning Basin, Western Australia. • Open-source software for segmentation and shape measurement of imaged mineral grains. • New grain separation algorithm for improved particle segmentation. • Chemically distinct host rocks show statistically significant differences in zircon shape. • Software to link shape measurements to isotopic analyses per mineral grain. [ABSTRACT FROM AUTHOR]