ArticlePDF Available

40Ar/39Ar geochronology of Ferrar Dolerite sills from the Transantarctic Mountains, Antarctica: Implications for the age and origin of the Ferrar magmatic province

Authors:

Abstract

The Ferrar Dolerite constitutes the hypabyssal phase of the tholeiitic Ferrar Group of Antarctica. Sills with compositions representing most of the range of geochemical variation of the Ferrar Dolerite, and separated by distances of as much as 1400 km, have been analyzed by the 40Ar/39Ar method on feldspar and biotite separates. The 40Ar/39Ar ages for five individual sills range from 176.2 to 177.2 Ma and show no significant difference. These ages reflect crystallization at 176.7 ± 1.8 Ma (where the uncertainty includes provision for systematic uncertainty in the age of the neutron-fluence monitor calibrated relative to MMhb-1 at 513.5 Ma). Combining data from these sills with previous determinations on coeval lavas and underlying pyroclastic units indicates an age of 176.6 ± 1.8 Ma for the Ferrar tholeiitic rocks as a whole. The duration of magmatic activity was less than approximately 1 m.y. By extension, other rocks in the Ferrar magmatic province, which occur from southeastern Australia, along the Transantarctic Mountains to the Theron Mountains, are inferred to have this age. The short duration of magmatic activity as well as the consistent pattern of geochemical variation and distinctiveness of the Ferrar rocks suggest that magmas were transported laterally by an extensive dike swarm which is inferred to have originated in the Weddell Sea sector of the province.
... The main aim of this paper is the petrological-geochemical comparison of the Ferrar Igneous Province with the simultaneous Karoo (South Africa) and Queen Maud Land (QML) large igneous provinces (Encarnacion et al., 1996;Duncan et al., 1997;Fleming et al., 1997;Minor and Mukasa, 1997;Riley and Knight, 2001) to provide insight into the evolution and specifics of the Karoo-Maud plume. The study was carried out using geochemical database on igneous rocks of the Ferrar Province compiled using available published data, reports of the Russian Antarctic expeditions, and the GEOROC international base (Antonini, et al., 1999;Brewer et al., 1992Brewer et al., , 1996Brotzu et al., 1988;Compston et al., 1968;Demarchi et al., 2001;Faure et al., 1974Faure et al., , 1982Fleck et al., 1977;Fleming et al., 1992Fleming et al., , 1997Foland et al., 1993;Ford and Kistler, 1980;Gunn, 1966;Heimann et al., 1994;Hoers et al., 1980Hoers et al., , 1989Hornig, 1993;Kyle, 1980;Melluso et al., 2014;Mensing et al., , 1996Molzahn et al., 1996;Riley et al., 2001;Semenov et al., 2014;Sidcrs and Elliot, 1985;Sung et al., 2019;Zavala et al., 2011;Zieg and Marsh, 2012). ...
... The main aim of this paper is the petrological-geochemical comparison of the Ferrar Igneous Province with the simultaneous Karoo (South Africa) and Queen Maud Land (QML) large igneous provinces (Encarnacion et al., 1996;Duncan et al., 1997;Fleming et al., 1997;Minor and Mukasa, 1997;Riley and Knight, 2001) to provide insight into the evolution and specifics of the Karoo-Maud plume. The study was carried out using geochemical database on igneous rocks of the Ferrar Province compiled using available published data, reports of the Russian Antarctic expeditions, and the GEOROC international base (Antonini, et al., 1999;Brewer et al., 1992Brewer et al., , 1996Brotzu et al., 1988;Compston et al., 1968;Demarchi et al., 2001;Faure et al., 1974Faure et al., , 1982Fleck et al., 1977;Fleming et al., 1992Fleming et al., , 1997Foland et al., 1993;Ford and Kistler, 1980;Gunn, 1966;Heimann et al., 1994;Hoers et al., 1980Hoers et al., , 1989Hornig, 1993;Kyle, 1980;Melluso et al., 2014;Mensing et al., , 1996Molzahn et al., 1996;Riley et al., 2001;Semenov et al., 2014;Sidcrs and Elliot, 1985;Sung et al., 2019;Zavala et al., 2011;Zieg and Marsh, 2012). In addition, we studied 28 samples from the collection of VNIIOkeanologiya (Tables 1 and 2, Supplementary). ...
... In the present-day coordinates, the Ferrar Igneous Province extending from the Coats Land through the Transantarctic Mountains to the Victoria Land and Georg V Land and further into the Southeastern Australia and New Zealand (Kyle, 1980;Kyle et al., 1981;Fleming, 2004, 2008;Hergt et al., , 1991Mortimer et al., 1995) was formed 183 Ma practically simultaneously with magmatism in the Karoo (South Africa) and Queen Maud Land (Eastern Antarctica) provinces. This allows one to correlate these events with the impact of the Large Karoo Superplume on the southwestern Gondwana paleocontinent (Hei-mann et al., 1994;Encarnacion et al., 1996;Duncan et al., 1997;Fleming et al., 1997;Minor and Mukasa, 1997;Riley and Knight, 2001;Riley et al., 2005Riley et al., , 2006. Thereby, the Ferrar Province intersects regions with different tectonic structure and crustal age. ...
... The Karoo-Ferrar LIP is composed primarily of basaltic lavas, sills, and dikes, and was emplaced in the early Jurassic during breakup of Gondwana (e.g., Elliot, 2013;Elliot & Fleming, 2008;Fleming et al., 1997;Svensen et al., 2012). At present, the LIP extends over multiple continents, with the two primary portions of the LIP being the Karoo, which is found in South Africa, and the Ferrar, which crops out predominantly in Antarctica, with volumetrically subordinate intrusive rocks found in Australia, Tasmania, and New Zealand (Elliot & Fleming, 2004. ...
... For the Ferrar, initial dating efforts by U-Pb and 40 Ar/ 39 Ar chronometers constrained emplacement duration to between ~1 and 2 Ma (e.g., Duncan et al., 1997;Encarnación et al., 1996;Fleming et al., 1997;Foland et al., 1993;Heimann et al., 1994;Minor & Mukasa, 1997). Subsequent CA-ID-TIMS U-Pb zircon dates by significantly decreased this range, suggesting emplacement over 349 ± 49 kyr, with magmatism starting by 182.779 ± 0.033 Ma and persisting until at least 182.430 ± 0.036 Ma. ...
... This duration is consistent Burgess et al. (2017). Sources for other age data are Antonini (1998);Brewer et al. (1996); Duncan et al. (1997); Elliot et al. (1999); Encarnación et al. (1996);Fleming et al. (1997); Foland et al. (1993); Le Gall et al. (2002); Hargraves et al. (1997);Heimann et al. (1994); Ivanov et al. (2017); Jones et al. (2001); Jourdan et al. (2005Jourdan et al. ( , 2007Jourdan et al. ( , 2008; Minor and Mukasa (1997); Moulin et al. (2017); Riley et al. (2005); Sell et al. (2014); Svensen et al. (2012Svensen et al. ( , 2015; Ware and Jourdan (2018). ...
Chapter
Full-text available
There is an apparent temporal correlation between large igneous province (LIP) emplacement and global environmental crises, including mass extinctions. Advances in the precision and accuracy of geochronology in the past decade have significantly improved estimates of the timing and duration of LIP emplacement, mass extinction events, and global climate perturbations, and in general have supported a temporal link between them. In this chapter, we review available geochronology of LIPs and of global extinction or climate events. We begin with an overview of the methodological advances permitting improved precision and accuracy in LIP geochronology. We then review the characteristics and geochronology of 12 LIP/event couplets from the past 700 Ma of Earth history, comparing the relative timing of magmatism and global change, and assessing the chronologic support for LIPs playing a causal role in Earth's climatic and biotic crises. We find that (1) improved geochronology in the last decade has shown that nearly all well‐dated LIPs erupted in < 1 Ma, irrespective of tectonic setting; (2) for well‐dated LIPs with correspondingly well‐dated mass extinctions, the LIPs began several hundred ka prior to a relatively short duration extinction event; and (3) for LIPs with a convincing temporal connection to mass extinctions, there seems to be no single characteristic that makes a LIP deadly. Despite much progress, higher precision geochronology of both eruptive and intrusive LIP events and better chronologies from extinction and climate proxy records will be required to further understand how these catastrophic volcanic events have changed the course of our planet's surface evolution.
... A K-Ar age of 174 ± 10 Ma has been reported for these rocks from the Mount Murchison quadrangle (Brotzu et al., 1989). Fleming et al. (1997) indicate an Ar-Ar age of 176.7 ± 8 Ma for the Ferrar tholeiitic rocks as a whole. Encarnación et al. (1996) provided more precise zircon and baddeleyite U-Pb ages for the Ferrar dolerite, of 183.6 ± 1.0 Ma. ...
Preprint
Full-text available
In this paper, we supply a geological map of the area between 76°-76°30 ′ S and 159°-163°E, that was the only missing portion to complete an entire coverage of Victoria Land, filling the gap between the GIGAMAP program (to the north) and the maps by the New Zealand Antarctic program (to the south). The mapped area encompasses an early Paleozoic basement, and a flat-lying cover of sedimentary and igneous rocks, Permo-Triassic to Jurassic in age. The basement consists of large bodies of the Granite Harbour Igneous Complex, a granitic complex linked to the Ross Orogeny. After the early Paleozoic Ross Orogeny, the area was uplifted and eroded, and the sandstones of the Beacon Supergroup were deposited on the resulting erosion surface. The Beacon Supergroup sandstones were in turn covered and in most cases incorporated into the volcanic and sub-volcanic rocks of the Jurassic Ferrar Group. ARTICLE HISTORY
... The Hanson Formation in the Central Transantarctic Mountains contains silicic tuffs (Elliot et al., 2016). In addition, a 300 m thick granophyre caps the mafic sequence in the Dufek intrusion, in the northern Pensacola Mountains (Ford, 1976;Fleming et al., 1997). The Red Hill intrusion of Tasmania/Tasman, Australia, considered to be associated with Ferrar LIP, contains a~180 m thick granophyre of dacitic composition (McDougall, 1962;Melluso et al., 2014). ...
Article
Full-text available
We present a detailed review of the petrological and geochemical aspects of rhyolite and associated silicic volcanic rocks (up to 20 vol% of all rocks) reported to date from twelve well known Phanerozoic continental mafic Large Igneous Provinces (LIPs). These typically spread over ≤10⁴ km² (rarely 10⁵ km² for Paraná-Etendeka) area and comprise ≤10⁴ km³ of extrusive silicic rocks, erupted either during or after the main basaltic eruption within <5 Myr, with some eruption(s) continuing for ≤30 Myr. These rhyolites and associated silicic volcanic rocks (60−81 wt.% of SiO2) are mostly metaluminous to peraluminous and are formed via (i) fractional crystallization of parental mafic magma with negligible crustal contamination, and (ii) melting of continental crust or assimilation and fractional crystallization (AFC) of mafic magma with significant crustal contribution. Rhyolites formed by extensive fractional crystallization are characterized by the presence of clinopyroxene phenocrysts, exhibit steep negative slopes in bivariate major oxides plots and weak to no Nb-Ta anomaly; these typically have temperature >900 °C. Rhyolites formed by significant crustal contribution are characterized by strong negative Nb-Ta anomalies, absence of clinopyroxene phenocrysts, and are likely to have a magma temperature <900 °C. Geochemical signatures suggest rhyolite melt generation in the plagioclase stability field with a minor fraction originating from lower crustal depths. A large part of the compositional variability in rhyolites, particularly the Sr-Nd-Pb-O isotope ratios, suggests a significant role of continental crust (upper crustal melting or AFC) in the evolution of these silicic rocks in the continental mafic LIPs.
Article
Full-text available
Mars exploration is focused on seeking evidence of habitable environments and microbial life. Terrestrial glassy basalts may be the closest Mars‐surface weathering analog and observations increasingly indicate their potential to preserve biogeochemical records. The textures, major and trace element geochemistry, and N concentrations and isotopic compositions of subaerial, subglacial and continental lacustrine hyaloclastites from Antarctica, Iceland, and Oregon, respectively, were studied using micro‐imaging and chemical methods, including gas‐source mass spectrometry. Alteration by meteoric‐sourced waters occurred in circum‐neutral, increasingly alkaline low‐temperature conditions of ∼60°C–100°C (Iceland) and ∼60°C–170°C (Antarctica). Incompatible large ion lithophile element (LILE) enrichments compared to mid‐ocean ridge basalt (MORB) are consistent with more advanced alteration in Antarctic breccias consisting of heulandite‐clinoptilolite, calcite, erionite, quartz, and fluorapophyllite. Granular and tubular alteration textures and radial apatite represent possible microbial traces. Most samples contain more N than fresh MORB or ocean island basalt reflecting enrichment beyond concentrations attributable to igneous processes. Antarctic samples contain 52–1,143 ppm N and have δ¹⁵Νair values of −20.8‰ to −7.1‰. Iceland‐Oregon basalts contain 1.6–172 ppm N with δ¹⁵Ν of −6.7‰ to +7.3‰. Correlations between alteration extents, N concentrations, and concentrations of K2O, other LILEs, and Li and B, reflect the siting of secondary N likely as NH4⁺ replacing K⁺ and potentially as N2 in phyllosilicates and zeolites. Although much of the N enrichment and isotope fractionation presented here is not definitively biogenic, given several unknown factors, we suggest that a combination of textures, major and trace element alteration and N and other isotope geochemical compositions could constitute a compelling biosignature in samples from Mars' surface/near‐surface.
Chapter
The surface of Mars is universally thought to have experienced widespread cold and dry environmental conditions for at least the last half of its geologic history, with more modern studies suggesting relatively cold and dry conditions early in its geologic history as well. However, the paucity of liquid water and mean annual temperatures well below the freezing point of water do not necessarily mean a complete cessation of all water-related geologic activity at the Martian surface. Over the past several decades, investigations in the McMurdo Dry Valleys (MDV) of Antarctica have revealed a dynamic geological, environmental, and ecological system resulting from locally optimized conditions operating over repeated, albeit brief, intervals during summer months. In this chapter, we compare the hyper-arid and hypo-thermal environments of the MDV and the modern Martian surface and discuss three unique enigmas that demonstrate how the Antarctic is a valuable analog to better understand processes on Mars.
Article
Full-text available
The Lower Jurassic Ferrar Large Igneous Province consists predominantly of intrusive rocks, which crop out over a distance of 3500 km. In comparison, extrusive rocks are more restricted geographically. Geochemically, the province is divided into the Mount Fazio Chemical Type, forming more than 99% of the exposed province, and the Scarab Peak Chemical Type, which in the Ross Sea sector is restricted to the uppermost lava. The former exhibits a range of compositions (SiO 2 = 52–59%; MgO = 9.2–2.6%; Zr = 60–175 ppm; Sr i = 0.7081–0.7138; ε Nd = −6.0 to −3.8), whereas the latter has a restricted composition (SiO 2 = c. 58%; MgO = c. 2.3%; Zr = c. 230 ppm; Sr i = 0.7090–0.7097; ε Nd = −4.4 to −4.1). Both chemical types are characterized by enriched initial isotope compositions of neodymium and strontium, low abundances of high field strength elements, and crust-like trace element patterns. The most basic rocks, olivine-bearing dolerites, indicate that these geochemical characteristics were inherited from a mantle source modified by subduction processes, possibly the incorporation of sediment. In one model, magmas were derived from a linear source having multiple sites of generation each of which evolved to yield, in sum, the province-wide coherent geochemistry. The preferred interpretation is that the remarkably coherent geochemistry and short duration of emplacement demonstrate derivation from a single source inferred to have been located in the proto-Weddell Sea region. The spatial variation in geochemical characteristics of the lavas suggests distinct magma batches erupted at the surface, whereas no clear geographical pattern is evident for intrusive rocks.
Article
The Orestes Melt Zone (OMZ) is a massive contact melt zone (∼20 m thick by several kilometers long), located in the McMurdo Dry Valleys of Antarctica. The OMZ formed at shallow crustal depths by melting of the A-type Orestes Granite owing to intrusion of the underlying, doleritic Basement Sill. The OMZ can be divided broadly into two melting facies. The upper melting facies is distal from the contact and formed by melting at low temperature and water-saturated, or near water-saturated, conditions. The lower melting facies is proximal to the contact and formed by melting at high temperature and water-undersaturated conditions. Separate melting reactions occurred in both of the melting facies, resulting in distinct textures and melt compositions. Melting in the distal facies generated melts with compositions that plot near a predicted eutectic composition. Melting in the proximal facies was accomplished in part by replacement reactions in restitic feldspars. These reactions resulted in the development of plagioclase mantles on both restitic plagioclase and K-feldspar, and melt compositions that diverged from predicted minimum melt along an unexpected path, towards enrichment in orthoclase component. Thermal modeling indicates that this melt zone was active for a minimum of ∼150 years, with a contact temperature of ∼900 °C. Upon cooling, recrystallization generated ocellar textures around restitic quartz, as well as faceted albite as a late-stage product. Observations of the OMZ, combined with thermal modeling, provide new insights into the origin of rapakivi and albite granites. This study has implications for the origin of these two associated granite types in other geological settings.
Article
Petrographic and geochemical data for Late Permian coals and carbonaceous shales from the Transantarctic Mountains, Antarctica (source: Polar Rock Repository, PRR), were used to evaluate maturation levels and assess the effects of contact metamorphism. Coals were evaluated for locations in the Southern Transantarctic Mountains, the Central Transantarctic Mountains, and South Victoria Land, including samples from the Buckley, Mt. Glossopteris, and Queen Maud formations and the Weller Coal Measures. These formations have been intruded by sills and dikes of the Jurassic Ferrar Group (177–183 Ma) associated with the breakup of Gondwanaland. Proximate (129 samples), total sulfur (69) analyses, vitrinite reflectance analysis (92), and petrographic composition (34) were determined. One third of the samples have >50% ash yields (dry basis). A subset of samples (87) with <50% ash (dry basis) was treated with 6 N HCl to remove any carbonates formed during the intrusive events. Acid treatment did not significantly reduce ash, suggesting silicates are a major component; sulfur contents of 1–2% (dry basis) decreased to <0.8% reflecting gypsum dissolution (as confirmed by XRD). Volatile matter (VM) contents (dry, ash-free or daf basis) for samples with <50% ash range from 3 to 43%. Based on VM, samples range from high volatile bituminous to anthracite; however, reflectance analysis indicates anthracite to meta-anthracite, with some reflectances >7%. Thus, VM does not reveal true rank of the Antarctic coals. Vitrinite reflectance (Rr) typically surpasses that of inertinite. The VM–Rr relationship for these coals does not follow that of coals matured by normal burial maturation, but more closely follows that of intruded coals. Coke textures, including isotropic coke and anisotropic mosaics, vacuoles, pyrolytic carbon, and coked bitumen are observed, indicating alteration by contact metamorphism and providing insights to the rank of the coal at the time of intrusion. Coarse-grained circular and fine-grained lenticular mosaic textures suggest coal rank at the time of intrusion was medium volatile bituminous coal (maximum vitrinite reflectance ~1.2%). This would imply a burial depth by time of intrusion of ~5–5.5 km (assuming 25 °C/km) or ~ 4 km (assuming 34 °C/km). Modern-day background reflectance levels of ~2.5% Rr indicate continued post-intrusion maturation, possibly due to exposure to higher regional heat flow. Coals and carbonaceous shales from the Polar Rock Repository (PRR) can provide reliable petrographic and maturation data (using reflectance) to help decipher the burial history for various parts of the Transantarctic Mountains. However, geochemical data must be used with caution due to the high original inorganic content, and possible formation of gypsum and changes in VM during long-term storage. HCl-treatment removes some of the neoformed minerals, but samples should be treated extensively with HCl-HF to remove all silicate minerals prior to proximate and ultimate analysis to ensure more reliable data.
Book
Full-text available
Volcanotectonics is comparatively new scientific field that combines various methods and techniques of geology and physics so as to understand the structure and behaviour of polygenetic (central) volcanoes and the conditions for their eruptions. More specifically, volcanotectonics uses the techniques and methods of tectonics, structural geology, geophysics, and physics to collect data on volcanoes, as well as to analyse and interpret the physical processes that generate those data. The focus is on processes responsible for periods of volcanic unrest, caldera collapses, and eruptions. For basic science, one principal aim of volcanotectonics is to develop methods for reliable forecasting of eruptions. Accurate forecasting as regards the location, time, and magnitude of eruptions has long been a major goal in volcanology. Volcanotectonics provides a theoretical framework and understanding of the physical processes that take place inside volcanoes prior to eruptions, thereby offering methods and techniques that allow us to use data obtained during unrest periods to forecast eruptions. For applied science and human society, one principal aim of volcanotectonics is to develop methods for preventing very large eruptions. This second aim – namely methods that allow us to prevent very large eruptions - may come as a surprise to some, but is of fundamental importance for the future of human civilisation. Very large eruptions, whose eruptive volumes may be of the order of hundreds or thousands of cubic kilometres, provide existential threat to human civilisation. The purpose of this book is to provide an overview of the scientific field of volcanotectonics. The book is primarily aimed at, first, undergraduate and graduate students in geology, geophysics, and geochemistry and, second, civil authorities, scientists, engineers, and other professionals who deal with volcanoes and the associated hazards in their work. The book has be designed so that it can be used (1) for an independent study, (2) as a textbook for a course on volcanotectonics, and (3) as a supplementary text for general courses on volcanology, structural geology, geology, geophysics, geothermics, and natural hazards. Each chapter begins with an overview of the aims and ends with a summary of the main topics discussed. In addition there is a list of symbols used in the chapter. Important concepts and conclusions are in bold face. In volcanotectonics the focus is on quantitative results. This is reflected in the 68 worked examples (solved probems) most of which include calculations. In addition there are 253 exercises (supplementary problems), many of which also require calculations. The examples and exercises are meant to provide a deeper understanding of the basic principles of volcanotectonics and their use for understanding the formation of volcanoes, the physical processes that maintain their activities, and providing reliable eruption forecasts. While volcanic activity cannot be understood or forecasted without basic knowledge of the relevant physics, the physics presented in the book is mostly elementary and explained in detail. The only exception is part of Chapter 10, where more advanced physics is introduced to explain the propagation paths of magma-driven fractures. I have taught much of the material in the book at various universities over the past 20 years to earth-science students in Norway, Germany, and England. In particular, many of the chapters form the basis of an undergraduate course on volcanology which I have taught in the past six years in England. Based on this experience, most of the material in the book should be suitable for earth-science students with a very modest knowledge of mathematics and physics. Contents Chapter 1. Introduction Chapter 2. Volcanotectonic structures Chapter 3. Volcanotectonic deformation Chapter 4. Volcanic earthquakes Chapter 5. Volcanotectonic processes Chapter 6. Formation and dynamics of magma chambers and reservoirs Chapter 7. Magma movement through the crust: dike paths Chapter 8. Dynamics of volcanic eruptions Chapter 9. Formation and evolution of volcanoes Chapter 10. Understanding unrest and forecasting eruptions Appendix A. Units, dimensions, and prefixes Appendix B. The Greek alphabet Appendix C. Some mathematical and physical constants Appendix D. Elastic constants Appendix E. Properties of some common crustal materials Appendix F. Physical properties of lavas and magmas
Article
Full-text available
The Prince Albert Mountains are part of the Transantarctic Mountains (TAM) chain in Central Victoria Land. The Transantarctic Mountains are the uplifted western shoulder associated with the Cenozoic Ross Sea Rift system. The Prince Albert Mountains differ from the regions to the N and S by its lower elevations, subdued morphology as well as geophysical aspects. Tilting and uplift appears to be smaller than elsewhere in the TAM. These features make this area particularly suited, geologically and logistically, to study Jurassic igneous rocks (Kirkpatrick lavas, Exposure Hill Formation and Ferrar sills) which are well exposed in a complete section. The field aspects of the Jurassic igneous sequence are described. -from Author
Article
Sandstone, mudstone and tuff of the Triassic Fremouw and Falla formations display diagenetic and zeolite-facies mineral assemblages that were controlled by parent material composition, fluid chemistry, permeability and temperature. Diagenetic reactions between sediments and ground water at near-surface temperature and pressure resulted in formation of smectite, chlorite, quartz, heulandite, mordenite and analcime cements and grain replacements. -from Author
Article
The 10 km-long Butcher Ridge was mapped on a scale of 1:5000; four sections were measured and 400 samples collected. An ⁴⁰Ar/³⁹Ar age shows the body to be 175 m.y. old. Lithologies are diverse and are described in detail: they include dolerite, massive andesite and rhyolite. The complex may represent mixing in a hypabyssal setting of a silicic magma derived from the melting of crustal rocks and a basaltic magma derived from the mantle. -G.R.M.
Article
Samples from 15 Kirkpatrick Basalt Group lava flows at Gorgon Peak, David Glacier, have been analysed for major and trace elements and strontium and oxygen isotope composition. They show many of the unusual features characteristic of Ferrar Supergroup rocks elsewhere, including relative enrichment of lithophile elements such as Rb and high initial 87Sr/86Sr ratios (mostly 0.710-0.712). The petrogenesis of the Ferrar Supergroup probably involves some crustal contamination. However the main features of the geochemistry are believed to reflect a heterogeneous mantle source. -from Authors
Article
K-Ar determinations on two igneous units from North Victoria Land, Antarctica, representative of the Ferrar Dolerite and Kirkpatrick Basalt, yield dates ranging from 144 to 180 Ma. The oldest date is considered to represent the age of the magmatic activity which produced the two suites; younger dates are attributed to argon loss subsequent to cooling. The chemical compositions of the rocks range from basaltic andesite to dacite. Major-element mass-balance, trace-element modelling, isotopic and geothermometric data, all imply a closed-system differentiation history from basaltic andesite to dacite. -from Authors
Article
New structural mapping in the southern Victoria Land and Beardmore Glacier regions of the Transantarctic Mountains, Antarctica, has documented the regional development of Jurassic normal faulting and consistent Jurassic Ferrar dolerite dike trends. The parallelism between the Jurassic structural trends within the Transantarctic Mountains and offshore basins in the Weddell and Ross embayments suggests that these basins may have initially formed in the Jurassic, within a zone of failed rifting that transected the Antarctic sector of Gondwana. Time relations between faults and Ferrar dolerites indicate that crustal extension occurred along the Transantarctic trend before and during the Ferrar magmatism. Similar time relations and orientation patterns of Jurassic structures in adjacent sectors of Gondwana indicate that a regional tensional stress field was present within the lithosphere of the supercontinent prior to breakup. Crustal extension followed by magmatism is consistent with recent models in which rifts form where extending lithosphere encounteres a region of anomalously hot mantle, possibly associated with mantle plumes. -from Author
Article
Comparisons of the magnetic and subglacial topographic profiles derived from a combined aeromagnetic and radio echo ice-sounding survey of the Dufek layered mafic intrusion in Antarctica illustrate the usefulness of this combination of methods in the study of bedrock geology beneath ice cover. Theoretical magnetic anomalies, computed from models based on the subice topography fitted to the highest-amplitude observed magnetic anomalies, required normal and reversed magnetizations ranging from 0.0001 to 0.01 emu/cu cm having directions and magnetizations consistent with measurements previously made on oriented samples. This result is interpreted as an indication that the Dufek intrusion cooled through the Curie isotherm during one or several reversals of the earth's magnetic field.