I. Cenozoic Geology of Iran: An Integrated Study of Extensional Tectonics and Related Vulcanism. II. Ediacaran Stratigraphy of the North American Cordillera: New Observations from Eastern California and Northern Utah

I. The late Oligocene to Miocene collision of Arabia and Eurasia was preceded by ~175 My of subduction of Neotethyan oceanic crust. Associated magmatic activity includes late Triassic(?) to Jurassic plutons in the Sanandaj-Sirjan zone of southern Iran, limited Cretaceous magmatism in the Alborz Mountains of northern Iran, and widespread Eocene volcanism across central Iran. Metamorphic core complexes of Eocene age have recently been recognized in widely separated parts of Iran, suggesting that Tertiary volcanism was related to extension. Geochemical data indicate that Eocene volcanism was typical of continental arcs and was followed by less voluminous Oligocene basaltic volcanism of the type often associated with back-arc basins. This set of observations suggests that mid-Mesozoic plutons in southern Iran are the remnants of an original volcanic arc that was only weakly developed because of slow subduction rate. Magmatic activity largely ceased in southern and central Iran during the Cretaceous and shifted to the north, suggesting a period of flat slab subduction. Subsequent slab-rollback during the Eocene extended the overriding plate, forming metamorphic core complexes and inducing pressure-release melting of partially hydrated lithospheric mantle and upwelling of asthenosphere. II. The Ediacaran Period spans from the base of cap carbonates overlying glacial deposits of the Marinoan “Snowball Earth” event to the Precambrian-Cambrian boundary, ~635 to 542 Ma. Sediments deposited during the rifting of southwest Laurentia, which are now exposed in a relatively narrow belt in the western US, are one of the best records on earth of the geological, geochemical, and geobiological events that occurred during this period. Evidence for one of the most significant of these, the final oxygenation of the oceans, is found within the upper Johnnie Formation in the southern Great Basin. C isotope data from thick, basinal facies of the Johnnie Fm. in the Panamint Range provide a more complete record of ocean chemistry associated with this event than previously determined from thinner, platformal facies. Strata in northern Utah of roughly the same age include a rift-related basalt, providing some of the youngest geologic evidence for the rifting of western Laurentia.</p

Tectonic environments of sapphire and ruby revealed by a global oxygen isotope compilation

Many sapphire and ruby occurrences are spatially linked with orogenic belts such as the Pan-African Orogen, the Himalayas, and regions of active or former subduction along the western margin of the Pacific Ocean. These gemstones have oxygen isotope compositions (δO) that span >45‰, reflecting the wide range of environments and conditions involved in corundum (AlO) formation. We compiled a global data base of sapphire and ruby δO, from which the following major groups of gemstones emerge: a dominant population of sapphires with δO centred around 5.5‰ (the mantle value) that is spatially related to regions of former subduction; a lesser population of sapphires and rubies with slightly higher δO that are associated with skarn and pegmatite; rubies with relatively low δO of 0‰–7‰ that occur in hydrothermally altered ultramafic metamorphic rocks in collision zones; and rubies with high δO of 14‰–25‰ that are found, almost exclusively, in Himalayan marble. The spatial distribution of the δO groups relative to plate boundaries provides insight into the two major periods of continental collision involved in sapphire and ruby formation: the Ediacaran collision of East and West Gondwana (the East African Orogeny) and the Cenozoic collision of India and Asia

Low-temperature thermochronology of the northern Thomson Orogen: implications for exhumation of basement rocks in NE Australia

The Tasmanides of eastern Australia record much of the Phanerozoic tectonic development of the retreating Pacific-Australia plate boundary and are an oft-cited example of an orogen that has undergone "tectonic mode switching." To begin to constrain the timing of exhumation of basement rocks that are now exposed in portions of the NE Tasmanides, we measured apatite and zircon (U-Th)/He ages from the Thomson Orogen and overlying Paleozoic strata in the back-arc of the New England Orogen in NE Australia. Zircon (U-Th)/He ages from basement samples (including those recovered from boreholes at depths of up to 1.1 km) are characterized by large inter- and intra-sample variability and range from approximately 180 Ma (Early Jurassic) to 375 Ma (Late Devonian). (U-Th)/He zircon ages from several individual samples are negatively correlated with effective uranium (eU), a pattern that is also true of the dataset as a whole, suggesting that variations in U and Th zoning and radiation damage are partially responsible for the age variability. The oldest zircon (U-Th)/He cooling ages coincide with the formation of regionally extensive Late Devonian-early Carboniferous back-arc basins, suggesting that Late Devonian extension played a significant role in exhumation of parts of the northern Thomson Orogen. Apatite (U-Th)/He ages from a basement sample and a late Permian sandstone in the overlying Bowen Basin, which are also marked by intra-sample variability and age-eU correlations, span from the Early Cretaceous through Oligocene, in general agreement with previous apatite fission track data. In conjunction with observations of key geologic relationships and prior K-Ar and Ar/Ar data, our results suggest four overall phases in the thermal history of the northern Thomson Orogen: (1) Cambrian-early Silurian metamorphism during the Delamerian and Benambran Orogenies; (2) protracted cooling during the Late Devonian through mid-Permian that likely resulted from extensional exhumation; (3) Permian-Triassic reheating during burial beneath thick sedimentary basins; and (4) Cretaceous and Paleogene cooling during uplift and erosion

Trace-element compositions of sapphire and ruby from the eastern Australian gemstone belt

Significant uncertainty surrounds the processes involved in the formation of basalt-hosted corundum, particularly the role that the mantle plays in corundum generation. Some previous studies have suggested that trace-element ratios (namely, Cr/Ga and Ga/Mg) are useful for distinguishing two types of corundum: “magmatic” and “metamorphic,” designations that include mantle and crustal processes. However, recent studies, including this one, have discovered transitional groups between these end-members that are difficult to classify. We used LA–ICP–MS to measure trace-element concentrations in sapphire and ruby crystals from eight alluvial deposits that span a significant length of the eastern Australian gemstone belt. Additionally, we collected LA–ICP–MS U–Pb and trace-element data from zircon megacrysts at Weldborough, Tasmania, which is also within the gemstone belt. Our sapphire and ruby results reveal a continuum in trace-element compositions, a finding that raises questions about previous classifications that ascribe corundum from basalt-hosted gemfields to either “magmatic” or “metamorphic” sources. The spatial association of basalt-related gemfields in eastern Australia with a long-lived convergent margin suggests a link between corundum formation and Al-enrichment of the mantle wedge during periods of subduction

Detrital zircon petrochronology of central Australia, and implications for the secular record of zircon trace element composition

Hafnium (Hf) isotope composition of zircon has been integrated with U-Pb age to form a long- term (>4 b.y.) record of the evolution of the crust. In contrast, trace element compositions of zircon are most commonly utilized in local- or regional-scale petrological studies, and the most noteworthy applications of trace element studies of detrital zircon have been in “fingerprinting” potential source lithologies. The extent to which zircon trace element compositions varied globally over geolog- ical time scales (as, for example, zircon U-Pb age abundance, O isotope composition, and Hf isotope composition seem to have varied) has been little explored, and it is a topic that is well suited to the large data sets produced by detrital zircon studies. In this study we present new detrital zircon U-Pb ages and trace element compositions from a conti- nent-scale basin system in Australia (the Centralian Superbasin) that bear directly on the Proterozoic history of Australia and which may be applicable to broader interpretations of plate-tectonic processes in other regions. U-Pb ages of detrital zircon in the Centralian Superbasin are dominated by pop- ulations of ca. 1800, 1600, 1200, and 600 Ma, and secular variations of zircon Hf isotope ratios are correlated with some trace element parameters between these major age populations. In particular, elevated εHf(i) (i.e., radiogenic “juvenile” Hf isotope composition) of detrital zircon in the Centralian Superbasin tends to correspond with relatively high values of Yb/U, Ce anomaly, and Lu/Nd (i.e., depletion of light rare earth elements). These cor- relations seem to be fundamentally governed by three related factors: elemental compatibility in the continental crust versus mantle, the thick- ness of continental crust, and the contributions of sediment to magmas. Similar trace element versus εHf(i) patterns among a global zircon data set suggest broad applicability. One particularly intriguing aspect of the global zircon data set is a late Neoproterozoic to Cambrian period during which both zircon εHf(i) and Yb/U reached minima, marking an era of anomalous zircon geochemistry that was related to significant contributions from old continental crust

Detrital zircon U-Pb geochronology of Permian strata in the Galilee Basin, Queensland, Australia

The late Carboniferous to Triassic tectonic history of eastern Australia includes important periods of regional-scale crustal extension and contraction. Evidence for these periods of tectonism is recorded by the extensive Pennsylvanian (late Carboniferous) to Triassic basin system of eastern Australia. In this study, we investigate the use of U–Pb dating of detrital zircons in reconstructing the tectonic development of one of these basins, the eastern Galilee Basin of Queensland. U–Pb detrital zircon ages were obtained from samples of stratigraphically well-constrained Cisuralian and Lopingian (early and late Permian, respectively) sandstone in the Galilee Basin. Detrital zircons in these sandstones are dominated by a population with ages in the range of 300–250 Ma, and ages from the youngest detrital zircons closely approximate depositional ages. We attribute these two fundamental findings to (1) appreciable derivation of detrital zircons in the Galilee Basin from the New England Orogen of easternmost Australia and (2) syndepositional magmatism. Furthermore, Cisuralian sandstone of the Galilee Basin contains significantly more >300 Ma detrital zircons than Lopingian sandstone. The transition in detrital zircon population, which is bracketed between 296 and 252 Ma based on previous high-precision U–Pb zircon ages from Permian ash beds in the Galilee Basin, corresponds with the Hunter–Bowen Orogeny and reflects a change in the Galilee Basin from an earlier extensional setting to a later foreland basin environment. During the Lopingian foreland basin phase, the individual depocentres of the Galilee and Bowen basins were linked to form a single and enormous foreland basin that covered >300 000 km2 in central and eastern Queensland

The Shuram and subsequent Ediacaran carbon isotope excursions from southwest Laurentia, and implications for environmental stability during the metazoan radiation

Current understanding of secular changes in the carbon isotopic composition of mid- to late Ediacaran carbonates suggests a relatively long, steady recovery of the global ocean from the Shuram negative excursion, followed by a smaller negative excursion at the Precambrian-Cambrian boundary. New radiometric, stratigraphic, and carbon isotope data from thick exposures of the upper Johnnie Formation in the Panamint Range of eastern California, combined with data from carbonate-rich facies of the Stirling Quartzite in the Funeral Mountains, confirm an Ediacaran age for these strata and provide a more complete record of isotopic variations during this time interval than previously determined from SW Laurentia and other key sections around the globe. A siltstone in the lower part of the Johnnie Formation yielded a detrital zircon grain with an age of 640.33 ± 0.09 Ma, lowering the maximum radiometric age constraint on the Johnnie Formation by >400 m.y., consistent with an Ediacaran age based on chemo- and biostratigraphic data. In contrast to previous C isotope compilations from this region, which were generally based on relatively thin portions of the Cordilleran miogeocline near its depositional hinge, the more basinward exposures exhibit a recovery from values near –12‰ to 0‰ within the upper part of the Johnnie Formation. Details in the shape of the chemostratigraphic profile through the upper Johnnie Formation closely match those in profiles through the Wonoka Formation in South Australia (which lies above the basal Ediacaran global stratotype section and point) and the Shuram-Buah interval in Oman, confirming temporal correlation and suggesting genesis through changes in the isotopic composition of the global ocean. The Shuram excursion in SW Laurentia is followed by at least three smaller Ediacaran to earliest Cambrian isotopic excursions recorded within, from oldest to youngest, the uppermost Johnnie Formation, the middle Stirling Quartzite, and the lower Wood Canyon Formation. These data indicate that the negative excursion associated with the base of the Cambrian is not a unique post-Shuram event, and that post-Shuram, pre-Cambrian animal evolution occurred in an environment of repeated large-magnitude fluctuations in the carbon isotopic composition of the global ocean

Animated reconstructions of the late cretaceous to cenozoic northward migration of Australia, and implications for the generation of east Australian mafic magmatism

Details of the Late Cretaceous-Cenozoic migration of the Australian continent have been sources of contention since the 1960s. Two types of apparent polar wander paths (APWPs) have emerged from previous paleomagnetic studies: one group based on sedimentary and lateritic data that includes relatively linear northward motion of Australia away from Antarctica, and a second group, based on basaltic and lateritic data, that includes significant longitudinal movement of the Australian continent. This study compares the migration and evolution of the Australian plate over the past 100 m.y. using these two competing paths. Our animated reconstructions illustrate the relative motion of the Australian plate, the formation of Cenozoic volcanic provinces in eastern Australia, the opening of the Coral and Tasman Seas, and the docking of the Ontong Java Plateau with the Solomon Islands. The reconstructions incorporate new 40Ar/39Ar and previously published geochronology data from Late Cretaceous to Cenozoic east Australian mafic to felsic volcanism in order to evaluate potential relationships between volcanism, changes in the motion of the Australian plate, and the opening of the Tasman and Coral Seas. We conclude that the APWP that includes significant longitudinal movement is more compatible than the linear path with both observable geological features (such as volcanic tracks) and the global moving hotspot reference frame. Our reconstructions reveal little correspondence between opening of the Tasman and Coral Seas and eruption of east Australian lava fields. However, the reconstructions and new 40Ar/39Ar geochronology illustrate that the formation of east Australian Late Cretaceous to Cenozoic central volcanoes and lava fields were closely linked, both temporally and spatially, and we suggest that edgedriven convection was an important process in the generation of both types of east Australian volcanic provinces