David H. Elliot

Interpretation of discordant 40Ar/39Ar age-spectra of mesozoic tholeiites from antarctica

Conventional K-Ar ages of tholeiitic basalts of the Ferrar Group in the central Transantarctic Mountains indicate significant loss of radiogenic 40Ar from this unit over much of its outcrop area. Argon loss varies inversely with amount of devitrified matrix in the basalts, which have not been thermally or tectonically disturbed since extrusion. 40Ar/19Ar age-spectra of these tholeiites are generally discordant and indicate significant inhomogeneity in the distribution of radiogenic 40Ar with respect to 39Ar, but are distinctly different from release patterns of thermally disturbed samples. Amounts of argon redistribution vary directly with amounts of devitrification and are reflected in progressive modification of the age spectra. A model of redistribution of radiogenic 40Ar by devitrification of originally glassy matrix is suggested that is consistent with disturbance of the conventional K-Ar systematics as well as the 40Ar/39Ar age-spectra. Samples with substantial redistribution but minor loss of radiogenic argon yield age spectra whose apparent ages decrease from low-temperature to high-temperature steps, similar to those reported for some lunar basalts, breccias, and soils. Modification of all the age spectra is attributed to redistribution of radiogenic 40Ar during progressive devitrification, although 39Ar-recoil effects suggested by Turner and Cadogan (1974) may be a factor in some cases. Where devitrification involves most potassium sites within the basalt, 40Ar/39Ar age-plateaux may be formed that have no geologic significance.

Synchronous emplacement of Ferrar and Karoo dolerites and the early breakup of Gondwana

Constraints on the timing of Karoo and Ferrar continental flood-basalt magmatism in Africa and Antarctica, respectively, are critical to understanding the relationship of the Karoo and Ferrar to mantle plumes, subduction, and the initial breakup of Gondwana. Although recent work has shown that Ferrar magmas were emplaced over a short interval (<1 m.y.), the timing of magmatism within the Karoo and its relationship to the Ferrar have been problematic. New zircon and baddeleyite U-Pb ages on Ferrar (183.6 ± 1.0 Ma) and southern Karoo (183.7 ± 0.6 Ma) dolerites demonstrate that part of Karoo magmatism occurred during the rapid emplacement of Ferrar magmas. A mantle plume is thought to have been important in the genesis of the Karoo province, whereas lithospheric extension, perhaps related to subduction, has been invoked for Ferrar magmatism. The new ages now suggest that Ferrar and southern Karoo magmatism were related to a single mantle thermal anomaly and rifting event. This event may have produced local rift basins and caused rotation of microblocks in west Antarctica several million years before the breakup of east and west Gondwana.

The Scotia Arc and Antarctic Margin

The Scotia Arc is the name generally applied to the largely submarine physiographic feature that joins southern South America to the Antarctic Peninsula. More correctly called the Scotia Ridge, it consists of a system of submarine ridges and associated troughs, forming a loop that extends for 1500 km east from Tierra del Fuego and the tip of the Antarctic Peninsula into the South Atlantic Ocean (Fig. 1). The arc encloses the Scotia Sea just as the ridge/trench system of the Greater and Lesser Antilles encloses the Caribbean Sea. In fact, the islands of the South Sandwich Ridge, closing off the eastern end of the Scotia Arc, have frequently been referred to as the Southern Antilles. Like the Lesser Antilles they are volcanically active and are situated on the concave western side of an arcuate, seismically active trench. The physiography of the two arc systems is similar, except that the Caribbean Sea is closed to the west by the Central American isthmus and the associated Middle American Trench on the Pacific side (Fig. 2).

A short interval of Jurassic continental flood basalt volcanism in Antarctica as demonstrated by 40Ar39Ar geochronology

A continental flood basalt province, the Ferrar Group (Kirkpatrick Basalt and Ferrar Dolerite), crops out along 3000 km of the Transantarctic Mountains in Antarctica and is temporally related to the break-up of Gondwanaland. Although a wide range of dates, between 90 and 193 Ma, have been published for the Kirkpatrick Basalt, it is now recognized that the young dates reflect non-ideal behavior of Ar in the matrix. In order to refine the geochronology, feldspar separates have been analyzed by the 40Ar39Ar incremental heating method. The main objectives are to constrain the duration of extrusive activity and the timing of volcanism along the outcrop belt. Basalt samples have been studied from the three principal outcrop areas, yielding the following apparent ages: central Transantarctic Mountains 176.8 ± 0.5 Ma; south Victoria Land 176.4 ± 0.4 Ma; north Victoria Land 176.6 ± 0.7 Ma. Ages from different stratigraphic levels within each area and from the three different areas are not analytically distinct. The data imply that the eruptive activity which produced the Kirkpatrick Basalt occurred within a short interval of less than about 1 m.y. at 176.6 ± 1.8 Ma, over an area which included more than 1200 km of the Transantarctic Mountains. The Jurassic volcanism in Antarctica represents a short episode of magmatism, comparable in duration with other well dated continental flood basalt provinces. The linearly extensive outcrop of the Ferrar Province and the rapid eruption of the lavas suggests that lithospheric stretching exerted a major control on magmatism. The poorly constrained age of the Bajocian-Bathonian boundary makes the previously suggested connection between Ferrar volcanism and an extinction event at that boundary uncertain.

Occurrence and Dispersal of Magmas in the Jurassic Ferrar Large Igneous Province, Antarctica

Tholeiitic rocks of the Ferrar Large Igneous Province (FLIP) occur in a linear belt from the Theron Mountains to Horn Bluff in the Transantarctic Mountains and extend into southeastern Australasia. The FLIP was emplaced during the initial stages of Gondwana break-up from a source suggested to be in the proto-Weddell Sea region. Magma transport from its source (Weddell triple junction) was controlled by an Early Jurassic zone of extension. The FLIP comprises the Dufek intrusion, Ferrar Dolerite sills and dykes (sheet intrusions), and extrusive rocks consisting of pyroclastic strata overlain by Kirkpatrick Basalt lavas. The Dufek intrusion occurs in deformed supracrustal rocks of the foldbelt along the paleo-Pacific Gondwana margin. A few sills were emplaced in basement rocks, but the majority of the sheet intrusions occur in flat-lying Devonian to Triassic Beacon strata. Only in the central Transantarctic Mountains (CTM) and south and north Victoria Land (SVL, NVL) are extrusive rocks preserved overlying Beacon strata. The greatest cumulative thicknesses of magmatic rocks (ca. 2 km) occur in areas where lavas are preserved (CTM and SVL). Sheet intrusions have complex relationships. Dyke swarms (sensu stricto) are unknown and dykes cutting basement rocks are uncommon. Nevertheless, these dykes, including a 30-m-wide dyke in SVL, suggest that some magmas locally migrated up through basement rocks. In CTM and NVL the outcrop belt has a width of about 160 km. Sills originally extended farther toward the plate margin but have been cut out by erosion and Cenozoic faulting, most clearly in CTM; geophysical data suggest extension under the East Antarctic ice sheet for at least 100 km. Although Early Jurassic extension is documented in CTM, major rift-bounding faults have not been observed. Models for magma emplacement include transport along the axis of the Transantarctic Mountains and off-axis transport from major rift-bounding faults. Contrasts in geochemistry between lavas of NVL (MgO=67%) and CTM (MgO=24%) and the presence of massive dolerite bodies (CTM, SVL) suggest discrete episodes and locations of magma emplacement, and that there was no long range interconnection along the mountain range in supracrustal rocks.

Long-distance transport of magmas in the Jurassic Ferrar Large Igneous Province, Antarctica

Abstract The youngest preserved lava flows of the Jurassic Kirkpatrick Basalt of the Ferrar Large Igneous Province are unusually distinctive. These flows, recognized in three geographic areas spread over 1600 km, have identical stratigraphic positions and geochemical characteristics. The lavas have an evolved, Fe-rich tholeiitic composition with lithospheric trace element ratios and enriched 87Sr/86Sr and 143Nd/144Nd isotope ratios. Except for the highly mobile elements, the major, trace and isotopic compositions of lavas from all these localities lie within, or near, analytical precision of each other, and are distinct from other Ferrar rocks. Moreover, the ages of the flows at all localities are indistinguishable. The unique characteristics of these capping lavas suggest that they were derived from a single batch of magma. Magma dispersal from a single reservoir through dike swarms at middle to upper crustal levels is considered the most probable mechanism for large-scale transport that extended for more than 3000 km.

Weddell triple junction: The principal focus of Ferrar and Karoo magmatism during initial breakup of Gondwana

Middle Jurassic Ferrar tholeiites of Antarctica were emplaced during a short time interval (<1 m.y.) into an active rift system initiated in the Early Jurassic. Ferrar magmas were dispersed into the Antarctic sector of Gondwana from a source in the Weddell Sea region, within the thermal anomaly that also gave rise to the Karoo basaltic rocks of southern Africa. The Golden Gate lavas, part of the Karoo central area low-Ti tholeiites, show geochemical similarities to Ferrar rocks. Tectonic and geochemical relationships of the Ferrar and Karoo low-Ti magmas, which constitute a major part of the Jurassic Gondwana large igneous province, suggest that they were derived from a single source associated with a triple junction in the proto-Weddell Sea region.

Strontium isotope composition and petrogenesis of the Kirkpatrick basalt, Queen Alexandra Range, Antarctica

The initial 87Sr/86Sr ratios of twelve basalt flows of Jurassic age on Storm Peak in the Queen Alexandra Range are anomalously high and range from 0.7094–0.7133. The average value is 0.7112±0.0013 (1σ). The concentrations of rubidium and strontium have arithmetic means of 60.6±19.4 ppm and 128.8±11.9 ppm, respectively. The corresponding average Rb/Sr ratio is 0.47 which is also anomalously high for rocks of basaltic composition. In addition, these rocks have high concentrations of SiO2 (56.50%) and K2O (1.29%) and are depleted in Al2O3 (12.92%), MgO (3.44%) and CaO (7.91%) compared to average continental tholeiites. They are nevertheless classified as basalts on the basis of the composition of microphenocrysts.The initial 87Sr/86Sr ratios and all of the chemical parameters of the flows exhibit systematic stratigraphic variations. These are interpreted as indicating the occurrence of four eruptive cycles. In a typical cycle the initial 87Sr/86Sr ratios of successive flows and their concentrations of SiO2, FeO (total iron), Na2O, K2O, P2O5, Rb and Sr decrease in ascending stratigraphic sequence while the concentrations of TiO2, Al2O3, MgO, CaO and MnO increase upward. The initial 87Sr/86Sr ratios of the flows show a strong positive correlation with the strontium concentration. Similar correlations are observed between the initial 87Sr/86Sr ratios and all of the major oxide components. These relationships are incompatible with the hypothesis that these flows are the products of crystal fractionation of a-34 magma at depth under closed-system conditions. It is suggested that the flows resulted from the hybridization of a normal tholeiite basalt magma by assimilation of varying amounts of granitic rocks in the Precambrian basement which underlies the entire Transantarctic Mountain chain.Mixtures of two components having different 87Sr/86Sr ratios and differing strontium concentrations are related to each other by hyperbolic mixing equation. Such an equation was fitted by least squares regression of data points to a straight line in coordinates of initial 87Sr/86Sr and the reciprocals of the concentrations of strontium. This equation and plots of strontium versus other oxides were then used to estimate the chemical composition of the parent basalt magma and of the granitic contaminant by substituting reasonable estimates of their 87Sr/86Sr ratios. The chemical composition of the parent basalt (87Sr/86Sr=0.706) is generally compatible with that of average continental tholeiite, but is distinctive by having a low concentration of strontium (117 ppm). The chemical composition of the contaminant (87Sr/86Sr=0.720) is enriched in strontium (173 ppm), SiO2, FeO (total iron) and the alkalies but is depleted in Al2O3, MgO and CaO. The data for strontium indicate that the lava flows on Storm Peak contain between 20 and 40% of this granitic contaminant. The contamination of basalt magma is not a local event but is characteristic of the Jurassic basalt flows and diabase sills throughout the Transantarctic Mountains and in Tasmania.

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

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 40 Ar/ 39 Ar method on feldspar and biotite separates. The 40 Ar/ 39 Ar 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.

Jurassic magmatism and tectonism associated with Gondwanaland break-up: an Antarctic perspective

Abstract Magmatic and tectonic activity in Antarctica associated with the early stages of continental rifting and break-up of Gondwanaland culminated with tholeiitic magmatism at about 175–180 Ma. In the Ross Sea sector of the Transantarctic Mountains, Jurassic igneous rocks, comprising pyroclastic rocks and Ferrar Group tholeiitic basalts, overlie fluvial strata of the Permian-Upper Triassic Gondwana sequence. Petrological, structural and volcanological data suggest that the Jurassic pyroclastic rocks and overlying flood basalts were erupted into a volcano-tectonic rift system associated with lithospheric extension and decompression melting. Geochemically the Ferrar tholeiites form part of the Gondwana low-Ti province, but they exhibit marked differences in initial 87Sr/86Sr ratios and in high field strength element abundances in comparison with other parts of the province. Three Early to Middle Jurassic tectono-magmatic terrains were present in this part of Gondwanaland: a plate margin magmatic arc; a belt of silicic within-plate igneous rocks inboard of the arc; and a continental flood basalt province (the Gondwana low-Ti province). Silicic volcanism mainly preceeded emplacement of Ferrar tholeiites, whereas major silicic activity elsewhere either accompanied basaltic magmatism or, though spatially separated preceded it. Younger magmatic episodes have obscured much of the plate margin record. Lack of reliable age determinations makes details of relations between tectonism and magmatism difficult to assess.