The greening of western australian landscapes: The phanerozoic plant record

Western Australian terrestrial foras frst appeared in the Middle Ordovician (c. 460 Ma) and developed Gondwanan afnities in the Permian. During the Mesozoic, these foras transitioned to acquire a distinctly austral character in response to further changes in the continent’s palaeolatitude and its increasing isolation from other parts of Gondwana. This synthesis of landscape evolution is based on palaeobotanical and palynological evidence mostly assembled during the last 60 years. The composition of the plant communities and the structure of vegetation changed markedly through the Phanerozoic. The Middle Ordovician – Middle Devonian was characterised by diminutive vegetation in low-diversity communities. An increase in plant size is inferred from the Devonian record, particularly from that of the Late Devonian when a signifcant part of the fora was arborescent. Changes in plant growth-forms accompanied a major expansion of vegetation cover to episodically or permanently fooded lowland setings and, from the latest Mississippian onwards, to dry hinterland environments. Weter conditions during the Permian yielded waterlogged environments with complex swamp communities dominated by Glossopteris. In response to the Permian–Triassic extinction event, a transitional vegetation characterised by herbaceous lycopsids became dominant but was largely replaced by the Middle Triassic with seed ferns and shrubs or trees atributed to Dicroidium. Another foristic turnover at the Triassic–Jurassic boundary introduced precursors of Australia’s modern vegetation and other southern hemisphere regions. Most importantly, fowering plants gained ascendancy during the Late Cretaceous. Characteristics of the state’s modern vegetation, such as sclerophylly and xeromorphy, arose during the Late Cretaceous and Paleogene. The vegetation progressively developed its present-day structure and composition in response to the increasing aridity during the Neogene–Quaternary

The greening of Western Australian landscapes: the Phanerozoic plant record

Western Australian terrestrial floras first appeared in the Middle Ordovician (c. 460 Ma) and developed Gondwanan affinities in the Permian. During the Mesozoic, these floras transitioned to acquire a distinctly austral character in response to further changes in the continent’s palaeolatitude and its increasing isolation from other parts of Gondwana. This synthesis of landscape evolution is based on palaeobotanical and palynological evidence mostly assembled during the last 60 years. The composition of the plant communities and the structure of vegetation changed markedly through the Phanerozoic. The Middle Ordovician –Middle Devonian was characterised by diminutive vegetation in low-diversity communities. An increase in plant size is inferred from the Devonian record, particularly from that of the Late Devonian when a significant part of the flora was arborescent. Changes in plant growth-forms accompanied a major expansion of vegetation cover to episodically or permanently flooded lowland settings and, from the latest Mississippian onwards, to dry hinterland environments. Wetter conditions during the Permian yielded waterlogged environments with complex swamp communities dominated by Glossopteris. In response to the Permian–Triassic extinction event, a transitional vegetation characterised by herbaceous lycopsids became dominant but was largely replaced by the Middle Triassic with seed ferns and shrubs or trees attributed to Dicroidium. Another floristic turnover at the Triassic–Jurassic boundary introduced precursors of Australia’s modern vegetation and other southern hemisphere regions. Most importantly, flowering plants gained ascendancy during the Late Cretaceous. Characteristics of the state’s modern vegetation, such as sclerophylly and xeromorphy, arose during the Late Cretaceous and Paleogene. The vegetation progressively developed its present-day structure and composition in response to the increasing aridity during the Neogene–Quaternary.Also funded by US National Science Foundation (project #1636625); Spanish AEI/FEDER, UE Grant CGL2017-84419; RJC is funded by the ARC via Greg Jordan (University of Tasmania) and Bob Hill (University of Adelaide); LAM and CLM appreciate the support of Vimy Resources Ltd.</p

Shocked titanite records Chicxulub hydrothermal alteration and impact age

© 2020 Elsevier Ltd Hydrothermal activity is a common phenomenon in the wake of impact events, yet identifying and dating impact hydrothermal systems can be challenging. This study provides the first detailed assessment of the effects of shock microstructures and impact-related alteration on the U-Pb systematics and trace elements of titanite (CaTiSiO5), focusing on shocked granite target rocks from the peak ring of the Chicxulub impact structure, Mexico. A > 1 mm long, shock-twinned titanite grain preserves a dense network of irregular microcracks, some of which exploit shock twin interfaces. Secondary microcrystalline anatase and pyrite are heterogeneously distributed along some microcracks. In situ laser ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICPMS) analysis reveals a mixture of three end-member Pb components. The Pb components are: 1) common Pb, consistent with the Pb isotopic signature of adjacent alkali feldspar; 2) radiogenic Pb accumulated since magmatic crystallization; and 3) a secondary, younger Pb signature due to impact-related complete radiogenic Pb loss. The youngest derived ages define a regression from common Pb that intersects Concordia at 67 ± 4 Ma, in agreement with the established age of 66.04 ± 0.05 Ma for the Chicxulub impact event. Contour maps of LA-MC-ICPMS data reveal that the young ages are spatially restricted to microstructurally-complex domains that correlate with significant depletion in trace elements (REE-Y-Zr-Nb-Mo-Sn-Th) and reduction in magnitude of the Eu/Eu* anomaly. Mapping by time-of-flight secondary ion mass spectrometry (ToF-SIMS) show that patterns of localised element depletion in titanite are spatially related to microcracks, which are enriched in Al. The spatial correlation of ages and trace element abundance is consistent with localised removal of Pb and other trace elements from a pervasive network of fast fluid pathways in fractured domains via a fluid-mediated element transport process associated with the impact event. Here we interpret the 67 ± 4 Ma U-Pb age to represent hydrothermal Pb-loss in the Chicxulub peak ring in the wake of the impact event. These results highlight the potential of our analytical approach using titanite geochronology and geochemistry for dating post-impact hydrothermal activity in impact structures elsewhere

Apatite and monazite: an effective duo to unravel superimposed fluid-flow and deformation events in reactivated shear zones

Mylonitic shear zones crosscutting homogenous granitoids can retain evidence of fluid-driven metasomatic retrogression and reactivation. However, the relationships between fluid-rock interaction, retrogression, deformation and mylonitisation, and the timing thereof, are often cryptically recorded. This study focuses on the granulite-facies Boothby Orthogneiss from the Reynolds Range, central Australia, which contains a large scale mylonitic shear zone with an apparent record of structural inheritance, fluid infiltration and reactivation. The chosen site provides an ideal natural laboratory in which to investigate the timing of deformation, associated fluid flow and mass transport. Usingle bondPb isotope analyses of monazite indicate an average Pb recrystallization age of c. 1560 Ma, demonstrating that the orthogneiss fabric developed during the Mesoproterozoic Chewings Orogeny (1590–1550 Ma). Structural mapping suggests that this shear zone represents a Riedel branch of larger structures that were subsequently reactivated during the Paleozoic Alice Springs Orogeny (450–300 Ma). The timing of reactivation and fluid flow is constrained by Usingle bondPb dating of apatite, which is present as a stable U-bearing mineral in both orthogneiss and mylonite. Modelling of apatite radiogenic-Pb retention ages, accounting for a wide potential range in common Pb compositions, demonstrates at least some growth and/or recrystallization at c. 1500 Ma and c. 400 Ma, confirming apatite precipitation during Alice Springs shearing and the reactivation of Chewings-age structures. In addition, Alice Springs-aged apatite is found along pre-existing fabrics in the orthogneiss in the vicinity of the shear zone, indicating pre-kinematic fluid flow across the shear zone boundary and into country rock that was otherwise largely unaffected. The combined datasets demonstrates that integrated apatite and monazite Usingle bondPb geochronology is an effective method to unravel the record of superimposed fluid-flow and deformation events. This includes the detection of an ‘inverse younging relationship’, where younger ages are preferentially recorded in the wall rock as rather than in the reactivated shear zone. Such effects are potentially common where deformation is driven by pre-kinematic fluid-rock interaction, with subsequent deformation enhancing the removal of replacement assemblages in more deformed rocks and favouring their preservation in less deformed rocks.Alexander M.Prent, Andreas Beinlich, Tom Raimondo, Christopher L.Kirkland, Noreen J.Evans, Andrew Putni

In situ U-Pb geochronology and geochemistry of a 1.13 Ga mafic dyke suite at Bunger Hills, East Antarctica: The end of the Albany-Fraser Orogeny

Antarctica contains continental fragments of Australian, Indian and African affinities, and is one of the key elements in the reconstruction of Nuna, Rodinia and Gondwana. The Bunger Hills region in East Antarctica is widely interpreted as a remnant of the Mesoproterozoic Albany–Fraser Orogen, which formed during collision between the West Australian and Mawson cratons and is linked with the assembly of Rodinia. Previous studies have suggested that several generations of mafic dyke suites are present at Bunger Hills but an understanding of their origin and tectonic context is limited by the lack of precise age constraints. New in situ SHRIMP U-Pb zircon and baddeleyite dates of, respectively, 1134 ± 9 Ma and 1131 ± 16 Ma confirm an earlier Rb-Sr whole-rock age estimate of ca. 1140 Ma for emplacement of a major mafic dyke suite in the area. Existing and new geochemical data suggest that the source of the dyke involved an EMORB-like source reservoir that was contaminated by a lower crust-like component. The new age constraint indicates that the dykes post-date the last known phase of plutonism at Bunger Hills by ca. 20 million years and were emplaced at the end of Stage 2 of the Albany-Fraser Orogeny. In current models, post-orogenic uplift and progressive tectonic thinning of the lithosphere were associated with melting and reworking of lower and middle crust that produced abundant plutonic rocks at Bunger Hills. A major episode of mafic dyke emplacement following uplift, cooling, and plutonic activity with increasing mantle input, suggests that the dykes mark the end of a prolonged interval of thermal weakening of the lithosphere that may have been associated with continued mafic underplating during orogenic collapse. If the undated olivine gabbro dykes with similar trend, geochemistry and petrology at Windmill Islands are coeval with the ca. 1134 Ma dyke at Bunger Hills, this would suggest the presence of a major dyke swarm at least 400 km in extent. In such case, the dykes could have been emplaced laterally from a much more distant mantle source, possibly a plume, and interacted with the locally heterogeneous and variably metasomatised lithosphere