Abstract One of the most prominent low-angle normal fault systems in the Basin and Range province... more Abstract One of the most prominent low-angle normal fault systems in the Basin and Range province of the western US is the~ 100 km-long South Virgin-White Hills detachment. Multiple low-temperature thermochronometers have been used to document rapid Miocene exhumation of the footwall of this detachment, most notably within the Gold Butte block of southern Nevada. Less attention has been paid to the actual faults that comprise the detachment system, one of which (the Salt Spring fault) is very well-exposed over an ...
A ~100 km long N-S belt of metamorphic core complexes is localized along the boundary between the... more A ~100 km long N-S belt of metamorphic core complexes is localized along the boundary between the Yazd and Tabas tectonic blocks of the central Iranian microcontinent, between the towns of Saghand and Posht-e-Badam. Amphibolite facies mylonitic gneisses are structurally overlain by east-tilted upper crustal stratified rocks including thick (>1 km), steeply dipping, nonmarine siliciclastic and volcanic strata. Near the detachment, the gneisses are generally overprinted by chlorite brecciation. Cross-cutting relationships along with U-Pb zircon and Ar/Ar age data indicate that migmatitization, mylonitic deformation, volcanism and sedimentation all occurred in the middle Eocene, between ~49 and 42 Ma. The westernmost portion of the Tabas block immediately east of the complexes is an east-tilted crustal section of Neoproterozoic/Cambrian crystalline rocks and metasedimentary strata >10 km thick. A horizontal transect of (U-Th)/He apatite and zircon ages through the section shows l...
The Hillsboro mining district is located in the Animas Mountains in Sierra County, New Mexico, ap... more The Hillsboro mining district is located in the Animas Mountains in Sierra County, New Mexico, approximately 180 miles south of Albuquerque. The Copper Flat porphyry is a Laramide copper deposit located near the center of the district. The porphyry has been exposed by limited mining, revealing local and distal zonation of minerals related to the emplacement of the porphyry and mineralization. In addition to the copper porphyry, skarns, jasperoids and precious metal veins are present at the site. Alteration assemblages within ...
Pacific-North American plate boundary motion is partially accommodated by the Eastern California ... more Pacific-North American plate boundary motion is partially accommodated by the Eastern California shear zone and Walker Lane. These NW-trending dextral shear zones are misaligned along strike and are linked in west-central Nevada in a region that is characterized by active ENE-striking sinistral faults and NW-SE extension. Within this region, a low-angle normal fault exposed in Tule Canyon, NV may form the central segment of a regionally-extensive detachment fault system with a strike length> 50 km. The low-angle ...
Abstract A variety of geochronological problems call for in-situ dating with high spatial resolut... more Abstract A variety of geochronological problems call for in-situ dating with high spatial resolution (ie, scales of tens of microns or less). Such techniques are particularly important for studies that focus on the relationships between intracrystalline age variations and petrologic or diagenetic processes. Precise, in-situ geochronology of particularly small grains or domains (ca. 1-5 microns) may be feasible with the Cameca NanoSIMS ion microprobe, which has a spatial resolution as good as tens of nanometers and sufficient ...
Geosphere, December 2019, v. 15, no. 6, 2057-2061, doi:10.1130/GES02182.1, Animation 2. Animated ... more Geosphere, December 2019, v. 15, no. 6, 2057-2061, doi:10.1130/GES02182.1, Animation 2. Animated reconstruction showing the last 60 m.y. of motion of the Australian plate using the global APWP of Besse and Courtillot (2002) applied to the base reconstruction file of Seton et al. (2012), with all plates moving with respect to Australia. Approximate position of New Guinea composite terrains after Davies et al. (1997).
Abstract Paleogeographic history of Laurentia during the Ediacaran (630-542 Ma) still remains a p... more Abstract Paleogeographic history of Laurentia during the Ediacaran (630-542 Ma) still remains a puzzling controversy. The existing paleomagnetic data for this period suggest that Laurentia was located at low latitudes at 615 Ma, drifted toward high southerly latitudes from 590-575 Ma, back to lower latitudes by 573 Ma, then back to higher latitudes by 561 Ma, and finally returned to an equatorial position after about 550 Ma. These multiple migrations imply extremely high velocities sometimes exceeding 45cm/yr. This strongly conflicts with an ...
Abstract The young continental collision of Arabia with Eurasia reflects the final demise of the ... more Abstract The young continental collision of Arabia with Eurasia reflects the final demise of the Neotethys Ocean through northward subduction of oceanic lithosphere under Eurasia. Several aspects of this collision still poorly understood include, the width of the Neotethys Ocean basin, the relationship between the demise of this ocean basin and voluminous magmatic arcs in Iran, age of collision between the Arabian and Iranian margins and the connection between the opening of the Red Sea and growth of the Zagros Mountains. We ...
The Copper Flat porphyry is a Laramide copper deposit in southwest New Mexico. High-altitude AVIR... more The Copper Flat porphyry is a Laramide copper deposit in southwest New Mexico. High-altitude AVIRIS data were acquired over the site in 1998. Limonite minerals, carbonates, some phyllosilicates, epidote, gypsum, and tremolite/talc have been preliminarily mapped based on AVIRIS data. The results indicate a fairly regular zonation of hydrothermal alteration minerals around the porphyry that is consistent with classic models of alteration from porphyry copper deposits. Verification of the preliminary results is on-going. X-ray diffraction analyses and laboratory spectral analyses will be performed on samples from the area in order to confirm the results of mineral mapping. 1.0 INTRODUCTION The Hillsboro mining district is located in the Animas Mountains in Sierra County, New Mexico, approximately 180 miles south of Albuquerque (Figure 1). The district lies on the eastern edge of the Laramide Arizona-Sonora-New Mexico porphyry-copper belt. The district contains six types of mineral depo...
Despite being one of the earliest applications of K-Ar geochronology, conflicting models still ex... more Despite being one of the earliest applications of K-Ar geochronology, conflicting models still exist for the interpretation of K-Ar and 40 Ar/39 Ar data from greenschist to sub-greenschist pelites. We conducted 40 Ar/39 Ar step-heating experiments on micron to submicron grain sizes of illite-and muscovite-rich Middle Cambrian pelites from the western US than range in metamorphic grade from the early diagenetic zone (zeolite facies) to epizone (greenschist facies). With increasing metamorphic grade, maximum ages reached on 40 Ar/39 Ar ...
Abstract Minerals which both preserve age information and indicate metamorphic conditions are par... more Abstract Minerals which both preserve age information and indicate metamorphic conditions are particularly useful in thermochronology. Variations in sub-greenschist facies metamorphism have traditionally been quantified in terms of the illite to muscovite transition, a transformation which involves the growth of crystallites of increasing thickness at higher metamorphic temperatures. Thickness variations may influence Ar retention within these K-rich minerals, both in nature and during neutron irradiation. Along a transect in the ...
Abstract Carbonate successions are principal geochemical archives of the extraordinary environmen... more Abstract Carbonate successions are principal geochemical archives of the extraordinary environmental changes that occurred during the Neoproterozoic Era. In this study, we present a stratigraphic record of carbonate rare earth element (REE) compositions from Neoproterozoic and Cambrian strata in central and southern Australia. The new REE results provide insight into the redox conditions of the Neoproterozoic-Cambrian ocean, the influence of glacial meltwater on carbonate precipitation, and Neoproterozoic-Cambrian paleoenvironmental changes. High Ce/Ce* and low Y/Ho suggest that the major Neoproterozoic and Cambrian carbonate accumulations of central and southern Australia formed in restricted marine environments that were largely dysoxic. Elevated ∑REE of mid-Cryogenian and early Ediacaran carbonate from the Amadeus Basin (central Australia) and King Island (Tasmania), respectively, are interpreted to reflect increased detrital mineral flux following periods of Cryogenian glaciation. REE data from the basal Ediacaran Cumberland Creek Dolostone of King Island suggest a transient episode of ocean oxidation in the wake of the terminal Cryogenian glaciation, but the balance of evidence suggests dominantly low oxygen ocean conditions during carbonate deposition in the remainder of the Neoproterozoic Era. An apparent mismatch in timing between carbonate deposition in central and southern Australia versus previously proposed mid-to-late-Ediacaran oxygenation events based on geochemical results from strata of South China highlights what may be important factors in reconciling varied geochemical records of Neoproterozoic-Paleozoic ocean oxygenation.
A wide number of genetic models have been proposed for volcanically transported ruby and sapphire... more A wide number of genetic models have been proposed for volcanically transported ruby and sapphire deposits around the world. In this contribution we compare the trace element chemistry, mineral and melt inclusions, and oxygen isotope ratios in blue to reddish-violet sapphires from Yogo Gulch, Montana, U.S.A., with rubies from the Chantaburi-Trat region of Thailand and the Pailin region of Cambodia. The similarities between Thai/Cambodian rubies and Yogo sapphires suggest a common origin for gem corundum from both deposits. Specifically, we advance a model whereby sapphires and rubies formed through a peritectic melting reaction when the lamprophyre or basalts that transported the gem corundum to the surface partially melted Al-rich lower crustal rocks. Furthermore, we suggest the protolith of the rubies and sapphires was an anorthosite or, in the case of Thai/Cambodian rubies, an anorthosite subjected to higher pressures and converted into a garnet-clinopyroxenite. In this model the rubies and sapphires are rightfully considered to be xenocrysts in their host basalts or lamprophyre; however, in this scenario they are not "accidental" xenocrysts but their formation is intimately and directly linked to the magmas that transported them to the surface. The similarities in these gem corundum deposits suggests that the partial melting, non-accidental xenocryst model may be more wide-reaching and globally important than previously realized. Importantly, in both cases the gem corundum has an ostensibly "metamorphic" trace element signature, whereas the presence of silicate melt (or magma) inclusions shows they ought to be considered to be "magmatic" rubies and sapphires. This discrepancy suggests that existing trace element discriminant diagrams intended to separate "metamorphic" from "magmatic" gem corundum ought to be used with caution.
The effect of the Permian onset of the Hunter-Bowen Orogeny on sedimentation patterns in the east... more The effect of the Permian onset of the Hunter-Bowen Orogeny on sedimentation patterns in the eastern Galilee Basin of Queensland was investigated through a comparison of age populations of detrital zircon grains in sandstones in the Cisuralian (early Permian) Joe Joe Group and the Lopingian (late Permian) Betts Creek beds and a potential shift in sediment provenance has been recognised. One key well (OEC Glue Pot Creek 1) was selected from the eastern Galilee Basin for detrital zircon analysis. The well intersected both Cisuralian and Lopingian strata. Nine sandstone samples were collected (three from the Cisuralian and six from Lopingian) and zircon grains were extracted. U-Pb isotopic data was gathered using the Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) technique. A total of 286 concordant ages were obtained from the nine samples. The Cisuralian samples have a varied age population range, with multiple peaks between 300 and 1200 Ma, suggesting numerous sources and orogenic recycling. In contrast, the Lopingian samples have a dominant peak of 250 to 300 Ma zircons, with a singular minor peak of 1500 Ma zircons, suggesting a transition in provenance between the Cisuralian and Lopingian.
The Cryogenian-Ediacaran boundary marks a profound change in the Earth environment from preceding... more The Cryogenian-Ediacaran boundary marks a profound change in the Earth environment from preceding glacial conditions to a subsequent period of shallow marine carbonate deposition. Earliest Ediacaran carbonate (i.e., "cap carbonate") serves as a record of the chemical and physical conditions of the world's ocean immediately following this transition. We present new step-leached trace element, rare earth element plus yttrium (REY), and Sr isotope (87 Sr/ 86 Sr) results from mid-Cryogenian, earliest Ediacaran, and late Ediacaran carbonate of the Amadeus Basin in central Australia that bracket the Cryogenian-Ediacaran boundary. Both REY and 87 Sr/ 86 Sr from a sample of mid-Cryogenian carbonate in the Amadeus Basin (the Ringwood Mbr. of the Aralka Fm.) are consistent with deposition in an anoxic marine environment. Subsequent basal Ediacaran cap carbonate (the Olympic cap) has REY compositions that document an up-section change from non-seawater to oxic marine conditions during cap dolostone deposition in the wake of Cryogenian deglaciation, presumably reflecting an important early contribution from glacial meltwater. Overlying cap limestone of the Olympic cap is marked by a strong negative Ce anomaly and, at early and intermediate leaching steps, elevated Y/Ho, attributes that point toward deposition in oxic seawater. Some step-leaching REY results from a sample of mid-Ediacaran carbonate of the Julie Fm. are also suggestive of a marine origin, though the remainder of leaching steps from this sample are consistent with pervasive "contamination" by detrital clay. Cumulatively, these data are consistent with key Cryogenian-Ediacaran transitions from largely anoxic Highlights 87 Sr/ 86 Sr and REE+Y data were collected from Neoproterozoic carbonate of the Amadeus Basin in central Australia. Step-leached 87 Sr/ 86 Sr and REY results complement each other. Mid-Cryogenian carbonate has REY compositions suggesting anoxic marine conditions. Basal Ediacaran cap carbonate has REY compositions indicative of a non-marine source. The Cryogenian-Ediacaran boundary marks a transition from largely anoxic to largely oxic marine conditions.
ABSTRACT Many sapphire and ruby occurrences are spatially linked with orogenic belts such as the ... more ABSTRACT 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 (δ18O) that span >45‰, reflecting the wide range of environments and conditions involved in corundum (Al2O3) formation. We compiled a global data base of sapphire and ruby δ18O, from which the following major groups of gemstones emerge: a dominant population of sapphires with δ18O 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 δ18O that are associated with skarn and pegmatite; rubies with relatively low δ18O of 0‰–7‰ that occur in hydrothermally altered ultramafic metamorphic rocks in collision zones; and rubies with high δ18O of 14‰–25‰ that are found, almost exclusively, in Himalayan marble. The spatial distribution of the δ18O 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.
Neoproterozoic shallow-marine and carbonate- rich strata of the northern Amadeus Basin, central A... more Neoproterozoic shallow-marine and carbonate- rich strata of the northern Amadeus Basin, central Australia, comprise a geochemical record of Neoproterozoic seawater that facilitates correlation with other basins in Australia and worldwide. This study includes new carbon and oxygen isotope measurements of Cryogenian and Ediacaran carbonate in the northern Amadeus Basin that, in combination with previous data from Tonian strata of the Bitter Springs Formation, form a composite Amadeus Basin Neoproterozoic δC record. Key features of the Cryogenian-Ediacaran portion of this record include (1) negative δC excursions in cap carbonates overlying Cryogenian glacigenic deposits of the Areyonga and Olympic Formations (i.e., the Rasthof and Maieberg excursions, respectively); (2) a mid-Cryogenian negative excursion in the Ringwood Member of the Aralka Formation (seemingly correlative with the Taishir excursion of SW Mongolia); (3) a subsequent mid-Cryogenian positive excursion (the Keele Peak excursion) in the Limbla Member of the Aralka Formation; and (4) relatively low-magnitude excursions in the Ediacaran Julie Formation. In comparison with other Neoproterozoic stratigraphic successions, key portions of the overall Neoproterozoic δC record that appear to be absent from the Amadeus Basin based on existing data include large-magnitude negative excursions (Islay and Trezona excursions) that immediately preceded basaland terminal-Cryogenian (i.e., Sturtian and Marinoan, respectively) glaciations. These "missing" δC excursions are associated with significant unconformities in the Amadeus Basin. δC and δO from carbonate clasts in diamictite of the Areyonga and Olympic Formations suggest that deposition of Islay- and Trezona-equivalent strata was limited or nonexistent in the Amadeus Basin, as opposed to the possibility that these strata were deposited but subsequently removed by glacial erosion. Our new Amadeus Basin results refine Cryogenian and early Ediacaran correlations among the Amadeus Basin, Adelaide rift complex, and Namibia. The lower part of the Ringwood Member, which was deposited during early stages of the Cryogenian nonglacial interlude, includes two negative δC excursions that are also evident in previous results from the Tapley Hill Formation of the Adelaide rift complex. The upper part of the Ringwood Member contains abundant stromatolites and seems to correlate with Cryogenian stromatolite reefs of the Balcanoona Formation in the Adelaide rift complex. The overlying Limbla Member in the Amadeus Basin correlates with the Yankaninna, Etina, Amberoona, and Enorama Formations of the Adelaide rift complex, as well as carbonate from other continents that record the Keele Peak excursion. δC from the early Ediacaran Olympic cap of the Amadeus Basin is (1) comparable to previous results from the Maieberg and Elandshoek Formations in Namibia; and (2) consistent with diachronous deposition of Marinoan cap dolostone in the Amadeus Basin. Additionally, new δC from the Julie Formation of the Amadeus Basin is similar to previous results from the late Ediacaran Kuibis Subgroup in southern Namibia. These correlations underscore that Ediacaran strata of Namibia, as well as the Cryogenian nonglacial interlude strata of the Adelaide rift complex, are roughly an order of magnitude thicker than stratigraphic counterparts in the Amadeus Basin. The wide disparity in sedimentation rates implied by this finding is likely attributable to fundamental differences in the Neoproterozoic tectonic settings of the three locations.
ABSTRACT Deformation patterns of Paleozoic and Mesozoic strata in eastern Australia are evidence ... more ABSTRACT Deformation patterns of Paleozoic and Mesozoic strata in eastern Australia are evidence of a structural and tectonic history that included multiple periods of deformation with variable strain intensities and orientations. Detailed analysis of structural data from the Bowen Basin in northeastern Australia reveals previously undescribed, north–south elongate, Type-1 fold-interference patterns. The Bowen Basin structures have similar orientations to previously described interference patterns of equivalent scale in upper Paleozoic strata of the New England Orogen and Sydney Basin of eastern Australia. The east Australian folds with north–south-trending axes most likely formed during late stages of the Permian–Triassic Hunter–Bowen Orogeny, and they were subsequently refolded around east–west axes during post 30 Ma collision of the Indo-Australian plate with the Eurasian and Pacific plates. The younger, east–west-trending folds have orientations that are well aligned with the present-day horizontal stress field of much of eastern Australia, raising the possibility that they are active structures.
Abstract One of the most prominent low-angle normal fault systems in the Basin and Range province... more Abstract One of the most prominent low-angle normal fault systems in the Basin and Range province of the western US is the~ 100 km-long South Virgin-White Hills detachment. Multiple low-temperature thermochronometers have been used to document rapid Miocene exhumation of the footwall of this detachment, most notably within the Gold Butte block of southern Nevada. Less attention has been paid to the actual faults that comprise the detachment system, one of which (the Salt Spring fault) is very well-exposed over an ...
A ~100 km long N-S belt of metamorphic core complexes is localized along the boundary between the... more A ~100 km long N-S belt of metamorphic core complexes is localized along the boundary between the Yazd and Tabas tectonic blocks of the central Iranian microcontinent, between the towns of Saghand and Posht-e-Badam. Amphibolite facies mylonitic gneisses are structurally overlain by east-tilted upper crustal stratified rocks including thick (>1 km), steeply dipping, nonmarine siliciclastic and volcanic strata. Near the detachment, the gneisses are generally overprinted by chlorite brecciation. Cross-cutting relationships along with U-Pb zircon and Ar/Ar age data indicate that migmatitization, mylonitic deformation, volcanism and sedimentation all occurred in the middle Eocene, between ~49 and 42 Ma. The westernmost portion of the Tabas block immediately east of the complexes is an east-tilted crustal section of Neoproterozoic/Cambrian crystalline rocks and metasedimentary strata >10 km thick. A horizontal transect of (U-Th)/He apatite and zircon ages through the section shows l...
The Hillsboro mining district is located in the Animas Mountains in Sierra County, New Mexico, ap... more The Hillsboro mining district is located in the Animas Mountains in Sierra County, New Mexico, approximately 180 miles south of Albuquerque. The Copper Flat porphyry is a Laramide copper deposit located near the center of the district. The porphyry has been exposed by limited mining, revealing local and distal zonation of minerals related to the emplacement of the porphyry and mineralization. In addition to the copper porphyry, skarns, jasperoids and precious metal veins are present at the site. Alteration assemblages within ...
Pacific-North American plate boundary motion is partially accommodated by the Eastern California ... more Pacific-North American plate boundary motion is partially accommodated by the Eastern California shear zone and Walker Lane. These NW-trending dextral shear zones are misaligned along strike and are linked in west-central Nevada in a region that is characterized by active ENE-striking sinistral faults and NW-SE extension. Within this region, a low-angle normal fault exposed in Tule Canyon, NV may form the central segment of a regionally-extensive detachment fault system with a strike length> 50 km. The low-angle ...
Abstract A variety of geochronological problems call for in-situ dating with high spatial resolut... more Abstract A variety of geochronological problems call for in-situ dating with high spatial resolution (ie, scales of tens of microns or less). Such techniques are particularly important for studies that focus on the relationships between intracrystalline age variations and petrologic or diagenetic processes. Precise, in-situ geochronology of particularly small grains or domains (ca. 1-5 microns) may be feasible with the Cameca NanoSIMS ion microprobe, which has a spatial resolution as good as tens of nanometers and sufficient ...
Geosphere, December 2019, v. 15, no. 6, 2057-2061, doi:10.1130/GES02182.1, Animation 2. Animated ... more Geosphere, December 2019, v. 15, no. 6, 2057-2061, doi:10.1130/GES02182.1, Animation 2. Animated reconstruction showing the last 60 m.y. of motion of the Australian plate using the global APWP of Besse and Courtillot (2002) applied to the base reconstruction file of Seton et al. (2012), with all plates moving with respect to Australia. Approximate position of New Guinea composite terrains after Davies et al. (1997).
Abstract Paleogeographic history of Laurentia during the Ediacaran (630-542 Ma) still remains a p... more Abstract Paleogeographic history of Laurentia during the Ediacaran (630-542 Ma) still remains a puzzling controversy. The existing paleomagnetic data for this period suggest that Laurentia was located at low latitudes at 615 Ma, drifted toward high southerly latitudes from 590-575 Ma, back to lower latitudes by 573 Ma, then back to higher latitudes by 561 Ma, and finally returned to an equatorial position after about 550 Ma. These multiple migrations imply extremely high velocities sometimes exceeding 45cm/yr. This strongly conflicts with an ...
Abstract The young continental collision of Arabia with Eurasia reflects the final demise of the ... more Abstract The young continental collision of Arabia with Eurasia reflects the final demise of the Neotethys Ocean through northward subduction of oceanic lithosphere under Eurasia. Several aspects of this collision still poorly understood include, the width of the Neotethys Ocean basin, the relationship between the demise of this ocean basin and voluminous magmatic arcs in Iran, age of collision between the Arabian and Iranian margins and the connection between the opening of the Red Sea and growth of the Zagros Mountains. We ...
The Copper Flat porphyry is a Laramide copper deposit in southwest New Mexico. High-altitude AVIR... more The Copper Flat porphyry is a Laramide copper deposit in southwest New Mexico. High-altitude AVIRIS data were acquired over the site in 1998. Limonite minerals, carbonates, some phyllosilicates, epidote, gypsum, and tremolite/talc have been preliminarily mapped based on AVIRIS data. The results indicate a fairly regular zonation of hydrothermal alteration minerals around the porphyry that is consistent with classic models of alteration from porphyry copper deposits. Verification of the preliminary results is on-going. X-ray diffraction analyses and laboratory spectral analyses will be performed on samples from the area in order to confirm the results of mineral mapping. 1.0 INTRODUCTION The Hillsboro mining district is located in the Animas Mountains in Sierra County, New Mexico, approximately 180 miles south of Albuquerque (Figure 1). The district lies on the eastern edge of the Laramide Arizona-Sonora-New Mexico porphyry-copper belt. The district contains six types of mineral depo...
Despite being one of the earliest applications of K-Ar geochronology, conflicting models still ex... more Despite being one of the earliest applications of K-Ar geochronology, conflicting models still exist for the interpretation of K-Ar and 40 Ar/39 Ar data from greenschist to sub-greenschist pelites. We conducted 40 Ar/39 Ar step-heating experiments on micron to submicron grain sizes of illite-and muscovite-rich Middle Cambrian pelites from the western US than range in metamorphic grade from the early diagenetic zone (zeolite facies) to epizone (greenschist facies). With increasing metamorphic grade, maximum ages reached on 40 Ar/39 Ar ...
Abstract Minerals which both preserve age information and indicate metamorphic conditions are par... more Abstract Minerals which both preserve age information and indicate metamorphic conditions are particularly useful in thermochronology. Variations in sub-greenschist facies metamorphism have traditionally been quantified in terms of the illite to muscovite transition, a transformation which involves the growth of crystallites of increasing thickness at higher metamorphic temperatures. Thickness variations may influence Ar retention within these K-rich minerals, both in nature and during neutron irradiation. Along a transect in the ...
Abstract Carbonate successions are principal geochemical archives of the extraordinary environmen... more Abstract Carbonate successions are principal geochemical archives of the extraordinary environmental changes that occurred during the Neoproterozoic Era. In this study, we present a stratigraphic record of carbonate rare earth element (REE) compositions from Neoproterozoic and Cambrian strata in central and southern Australia. The new REE results provide insight into the redox conditions of the Neoproterozoic-Cambrian ocean, the influence of glacial meltwater on carbonate precipitation, and Neoproterozoic-Cambrian paleoenvironmental changes. High Ce/Ce* and low Y/Ho suggest that the major Neoproterozoic and Cambrian carbonate accumulations of central and southern Australia formed in restricted marine environments that were largely dysoxic. Elevated ∑REE of mid-Cryogenian and early Ediacaran carbonate from the Amadeus Basin (central Australia) and King Island (Tasmania), respectively, are interpreted to reflect increased detrital mineral flux following periods of Cryogenian glaciation. REE data from the basal Ediacaran Cumberland Creek Dolostone of King Island suggest a transient episode of ocean oxidation in the wake of the terminal Cryogenian glaciation, but the balance of evidence suggests dominantly low oxygen ocean conditions during carbonate deposition in the remainder of the Neoproterozoic Era. An apparent mismatch in timing between carbonate deposition in central and southern Australia versus previously proposed mid-to-late-Ediacaran oxygenation events based on geochemical results from strata of South China highlights what may be important factors in reconciling varied geochemical records of Neoproterozoic-Paleozoic ocean oxygenation.
A wide number of genetic models have been proposed for volcanically transported ruby and sapphire... more A wide number of genetic models have been proposed for volcanically transported ruby and sapphire deposits around the world. In this contribution we compare the trace element chemistry, mineral and melt inclusions, and oxygen isotope ratios in blue to reddish-violet sapphires from Yogo Gulch, Montana, U.S.A., with rubies from the Chantaburi-Trat region of Thailand and the Pailin region of Cambodia. The similarities between Thai/Cambodian rubies and Yogo sapphires suggest a common origin for gem corundum from both deposits. Specifically, we advance a model whereby sapphires and rubies formed through a peritectic melting reaction when the lamprophyre or basalts that transported the gem corundum to the surface partially melted Al-rich lower crustal rocks. Furthermore, we suggest the protolith of the rubies and sapphires was an anorthosite or, in the case of Thai/Cambodian rubies, an anorthosite subjected to higher pressures and converted into a garnet-clinopyroxenite. In this model the rubies and sapphires are rightfully considered to be xenocrysts in their host basalts or lamprophyre; however, in this scenario they are not "accidental" xenocrysts but their formation is intimately and directly linked to the magmas that transported them to the surface. The similarities in these gem corundum deposits suggests that the partial melting, non-accidental xenocryst model may be more wide-reaching and globally important than previously realized. Importantly, in both cases the gem corundum has an ostensibly "metamorphic" trace element signature, whereas the presence of silicate melt (or magma) inclusions shows they ought to be considered to be "magmatic" rubies and sapphires. This discrepancy suggests that existing trace element discriminant diagrams intended to separate "metamorphic" from "magmatic" gem corundum ought to be used with caution.
The effect of the Permian onset of the Hunter-Bowen Orogeny on sedimentation patterns in the east... more The effect of the Permian onset of the Hunter-Bowen Orogeny on sedimentation patterns in the eastern Galilee Basin of Queensland was investigated through a comparison of age populations of detrital zircon grains in sandstones in the Cisuralian (early Permian) Joe Joe Group and the Lopingian (late Permian) Betts Creek beds and a potential shift in sediment provenance has been recognised. One key well (OEC Glue Pot Creek 1) was selected from the eastern Galilee Basin for detrital zircon analysis. The well intersected both Cisuralian and Lopingian strata. Nine sandstone samples were collected (three from the Cisuralian and six from Lopingian) and zircon grains were extracted. U-Pb isotopic data was gathered using the Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) technique. A total of 286 concordant ages were obtained from the nine samples. The Cisuralian samples have a varied age population range, with multiple peaks between 300 and 1200 Ma, suggesting numerous sources and orogenic recycling. In contrast, the Lopingian samples have a dominant peak of 250 to 300 Ma zircons, with a singular minor peak of 1500 Ma zircons, suggesting a transition in provenance between the Cisuralian and Lopingian.
The Cryogenian-Ediacaran boundary marks a profound change in the Earth environment from preceding... more The Cryogenian-Ediacaran boundary marks a profound change in the Earth environment from preceding glacial conditions to a subsequent period of shallow marine carbonate deposition. Earliest Ediacaran carbonate (i.e., "cap carbonate") serves as a record of the chemical and physical conditions of the world's ocean immediately following this transition. We present new step-leached trace element, rare earth element plus yttrium (REY), and Sr isotope (87 Sr/ 86 Sr) results from mid-Cryogenian, earliest Ediacaran, and late Ediacaran carbonate of the Amadeus Basin in central Australia that bracket the Cryogenian-Ediacaran boundary. Both REY and 87 Sr/ 86 Sr from a sample of mid-Cryogenian carbonate in the Amadeus Basin (the Ringwood Mbr. of the Aralka Fm.) are consistent with deposition in an anoxic marine environment. Subsequent basal Ediacaran cap carbonate (the Olympic cap) has REY compositions that document an up-section change from non-seawater to oxic marine conditions during cap dolostone deposition in the wake of Cryogenian deglaciation, presumably reflecting an important early contribution from glacial meltwater. Overlying cap limestone of the Olympic cap is marked by a strong negative Ce anomaly and, at early and intermediate leaching steps, elevated Y/Ho, attributes that point toward deposition in oxic seawater. Some step-leaching REY results from a sample of mid-Ediacaran carbonate of the Julie Fm. are also suggestive of a marine origin, though the remainder of leaching steps from this sample are consistent with pervasive "contamination" by detrital clay. Cumulatively, these data are consistent with key Cryogenian-Ediacaran transitions from largely anoxic Highlights 87 Sr/ 86 Sr and REE+Y data were collected from Neoproterozoic carbonate of the Amadeus Basin in central Australia. Step-leached 87 Sr/ 86 Sr and REY results complement each other. Mid-Cryogenian carbonate has REY compositions suggesting anoxic marine conditions. Basal Ediacaran cap carbonate has REY compositions indicative of a non-marine source. The Cryogenian-Ediacaran boundary marks a transition from largely anoxic to largely oxic marine conditions.
ABSTRACT Many sapphire and ruby occurrences are spatially linked with orogenic belts such as the ... more ABSTRACT 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 (δ18O) that span >45‰, reflecting the wide range of environments and conditions involved in corundum (Al2O3) formation. We compiled a global data base of sapphire and ruby δ18O, from which the following major groups of gemstones emerge: a dominant population of sapphires with δ18O 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 δ18O that are associated with skarn and pegmatite; rubies with relatively low δ18O of 0‰–7‰ that occur in hydrothermally altered ultramafic metamorphic rocks in collision zones; and rubies with high δ18O of 14‰–25‰ that are found, almost exclusively, in Himalayan marble. The spatial distribution of the δ18O 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.
Neoproterozoic shallow-marine and carbonate- rich strata of the northern Amadeus Basin, central A... more Neoproterozoic shallow-marine and carbonate- rich strata of the northern Amadeus Basin, central Australia, comprise a geochemical record of Neoproterozoic seawater that facilitates correlation with other basins in Australia and worldwide. This study includes new carbon and oxygen isotope measurements of Cryogenian and Ediacaran carbonate in the northern Amadeus Basin that, in combination with previous data from Tonian strata of the Bitter Springs Formation, form a composite Amadeus Basin Neoproterozoic δC record. Key features of the Cryogenian-Ediacaran portion of this record include (1) negative δC excursions in cap carbonates overlying Cryogenian glacigenic deposits of the Areyonga and Olympic Formations (i.e., the Rasthof and Maieberg excursions, respectively); (2) a mid-Cryogenian negative excursion in the Ringwood Member of the Aralka Formation (seemingly correlative with the Taishir excursion of SW Mongolia); (3) a subsequent mid-Cryogenian positive excursion (the Keele Peak excursion) in the Limbla Member of the Aralka Formation; and (4) relatively low-magnitude excursions in the Ediacaran Julie Formation. In comparison with other Neoproterozoic stratigraphic successions, key portions of the overall Neoproterozoic δC record that appear to be absent from the Amadeus Basin based on existing data include large-magnitude negative excursions (Islay and Trezona excursions) that immediately preceded basaland terminal-Cryogenian (i.e., Sturtian and Marinoan, respectively) glaciations. These "missing" δC excursions are associated with significant unconformities in the Amadeus Basin. δC and δO from carbonate clasts in diamictite of the Areyonga and Olympic Formations suggest that deposition of Islay- and Trezona-equivalent strata was limited or nonexistent in the Amadeus Basin, as opposed to the possibility that these strata were deposited but subsequently removed by glacial erosion. Our new Amadeus Basin results refine Cryogenian and early Ediacaran correlations among the Amadeus Basin, Adelaide rift complex, and Namibia. The lower part of the Ringwood Member, which was deposited during early stages of the Cryogenian nonglacial interlude, includes two negative δC excursions that are also evident in previous results from the Tapley Hill Formation of the Adelaide rift complex. The upper part of the Ringwood Member contains abundant stromatolites and seems to correlate with Cryogenian stromatolite reefs of the Balcanoona Formation in the Adelaide rift complex. The overlying Limbla Member in the Amadeus Basin correlates with the Yankaninna, Etina, Amberoona, and Enorama Formations of the Adelaide rift complex, as well as carbonate from other continents that record the Keele Peak excursion. δC from the early Ediacaran Olympic cap of the Amadeus Basin is (1) comparable to previous results from the Maieberg and Elandshoek Formations in Namibia; and (2) consistent with diachronous deposition of Marinoan cap dolostone in the Amadeus Basin. Additionally, new δC from the Julie Formation of the Amadeus Basin is similar to previous results from the late Ediacaran Kuibis Subgroup in southern Namibia. These correlations underscore that Ediacaran strata of Namibia, as well as the Cryogenian nonglacial interlude strata of the Adelaide rift complex, are roughly an order of magnitude thicker than stratigraphic counterparts in the Amadeus Basin. The wide disparity in sedimentation rates implied by this finding is likely attributable to fundamental differences in the Neoproterozoic tectonic settings of the three locations.
ABSTRACT Deformation patterns of Paleozoic and Mesozoic strata in eastern Australia are evidence ... more ABSTRACT Deformation patterns of Paleozoic and Mesozoic strata in eastern Australia are evidence of a structural and tectonic history that included multiple periods of deformation with variable strain intensities and orientations. Detailed analysis of structural data from the Bowen Basin in northeastern Australia reveals previously undescribed, north–south elongate, Type-1 fold-interference patterns. The Bowen Basin structures have similar orientations to previously described interference patterns of equivalent scale in upper Paleozoic strata of the New England Orogen and Sydney Basin of eastern Australia. The east Australian folds with north–south-trending axes most likely formed during late stages of the Permian–Triassic Hunter–Bowen Orogeny, and they were subsequently refolded around east–west axes during post 30 Ma collision of the Indo-Australian plate with the Eurasian and Pacific plates. The younger, east–west-trending folds have orientations that are well aligned with the present-day horizontal stress field of much of eastern Australia, raising the possibility that they are active structures.
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Papers by Charles Verdel