Kirkpatrick Basalt Research Articles

Landslides in the Transantarctic Mountains: lower Jurassic and older strata displaced in late Mesozoic to late Cenozoic time

ABSTRACT Eight landslides, seven certain and one possible, have been identified in the central Transantarctic Mountains and one at Carapace Nunatak, south Victoria Land. Four consist of Kirkpatrick Basalt lavas alone, two comprise Kirkpatrick lavas with associated pyroclastic rocks, one consists of Hanson Formation beds and Kirkpatrick lavas, and one involves Fremouw Formation strata. One possible block, of uncertain origin, consists only of Hanson Formation beds. All rocks comprising the displaced blocks, except one, are Early Jurassic in age. The exception is the inferred slide involving the Triassic Fremouw beds. The locations of some landslides are consistent with emplacement on present-day topography, which has been little modified since the middle Miocene, but the time of emplacement of others is either Oligocene to pre-middle Miocene or pre-dates the onset of glaciation in Eocene/Oligocene time. The older landslides reflect fortuitous preservation of an ancient landscape not unlike that of today, one dominated by horizontal beds consisting of resistant dolerite sills and quartz-rich sandstones alternating with intervals of weak fine-grained sedimentary beds, and capped by basalt lavas. The landslides are interpreted to document three stages in landscape evolution: a pre-glaciation semi-arid landscape, an early warm-based glacial environment, and a late cold-based glacial setting.

Identification of Phyllosilicates in the Antarctic Environment Using ASTER Satellite Data: Case Study from the Mesa Range, Campbell and Priestley Glaciers, Northern Victoria Land

In Antarctica, spectral mapping of altered minerals is very challenging due to the remoteness and inaccessibility of poorly exposed outcrops. This investigation evaluates the capability of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite remote sensing imagery for mapping and discrimination of phyllosilicate mineral groups in the Antarctic environment of northern Victoria Land. The Mixture-Tuned Matched-Filtering (MTMF) and Constrained Energy Minimization (CEM) algorithms were used to detect the sub-pixel abundance of Al-rich, Fe3+-rich, Fe2+-rich and Mg-rich phyllosilicates using the visible and near-infrared (VNIR), short-wave infrared (SWIR) and thermal-infrared (TIR) bands of ASTER. Results indicate that Al-rich phyllosilicates are strongly detected in the exposed outcrops of the Granite Harbour granitoids, Wilson Metamorphic Complex and the Beacon Supergroup. The presence of the smectite mineral group derived from the Jurassic basaltic rocks (Ferrar Dolerite and Kirkpatrick Basalts) by weathering and decomposition processes implicates Fe3+-rich and Fe2+-rich phyllosilicates. Biotite (Fe2+-rich phyllosilicate) is detected associated with the Granite Harbour granitoids, Wilson Metamorphic Complex and Melbourne Volcanics. Mg-rich phyllosilicates are mostly mapped in the scree, glacial drift, moraine and crevasse fields derived from weathering and decomposition of the Kirkpatrick Basalt and Ferrar Dolerite. Chlorite (Mg-rich phyllosilicate) was generally mapped in the exposures of Granite Harbour granodiorite and granite and partially identified in the Ferrar Dolerite, the Kirkpatrick Basalt, the Priestley Formation and Priestley Schist and the scree, glacial drift and moraine. Statistical results indicate that Al-rich phyllosilicates class pixels are strongly discriminated, while the pixels attributed to Fe3+-rich class, Fe2+-rich and Mg-rich phyllosilicates classes contain some spectral mixing due to their subtle spectral differences in the VNIR+SWIR bands of ASTER. Results derived from TIR bands of ASTER show that a high level of confusion is associated with mafic phyllosilicates pixels (Fe3+-rich, Fe2+-rich and Mg-rich classes), whereas felsic phyllosilicates (Al-rich class) pixels are well mapped. Ground truth with detailed geological data, petrographic study and X-ray diffraction (XRD) analysis verified the remote sensing results. Consequently, ASTER image-map of phyllosilicate minerals is generated for the Mesa Range, Campbell and Priestley Glaciers, northern Victoria Land of Antarctica.

Paleomagnetism of the Jurassic Transantarctic Mountains revisited — Evidence for large dispersion of apparent polar wander within less than 3 Myr

The Transantarctic mountains predominantly consist of Jurassic continental flood basalts (Kirkpatrick) and sills (Ferrar) emplaced during the earliest phase of the break-up of Pangea. Published ages, based on a variety of geochronological methods all agree rather well and cluster around 180Ma suggesting emplacement during a rather short time interval of not more than 3Myr. Paleomagnetic studies, mostly carried out between the 60s and 90s of the last century, however, yield two well defined but significantly different groups of paleomagnetic pole positions plotting either in intermediate latitudes (A) or at latitudes exceeding 60°S (B). Pole positions belonging to group A are generally interpreted to be of primary origin whereas the significance of group B poles remains unclear. Here we report new data from the Kirkpatrick basalts of Northern Victoria Land, Antarctica, where 157 oriented paleomagnetic samples were taken, covering about 800m of stratigraphy and 23 volcanic flows in Gair Mesa (73.4666°S; 162.8666°E). After removal of a steep magnetic overprint with rather low coercivities straight linear segments of exclusively normal polarity trending toward the origin of projection are identified in 151 samples from 22 sites. Maximum unblocking temperatures do not exceed 580°C and maximum coercivities not 60mT. The resulting mean virtual geomagnetic pole (VGP) for the Gair Mesa plots at 66.4°S; 227.7°E (α95 of 6.2°, k=25.7) comparable to group B poles. Reflecting light microscopy reveals that the magnetic inventory of all samples analyzed is dominated by magnetite showing shrinking cracks and broad ilmenite lamellae, the latter being diagnostic for high temperature oxidation. Analysis of the distribution of the 22 pole positions obtained here shows that secular variation has successfully been averaged (Sb=15.9°) suggesting that the rocks studied here have recorded the time averaged geomagnetic field during early mid Jurassic times. Based on these new results we postulate that both clusters A and B of pole positions reflect primary magnetizations and that the Kirkpatrick basalts of Gair Mesa have been emplaced in a rather short normal polarity time interval of 3Myr. If our reasoning is correct, the apparent polar wander path for the latest early Jurassic might be more complex than previously thought. Whether this complexity is evidence for massif True Polar Wander remains speculative.

Cracking the lid: Sill-fed dikes are the likely feeders of flood basalt eruptions

Although subparallel swarms of dikes are thought to be the primary feeders to voluminous volcanic eruptions, increasing recognition of volumetrically significant sill complexes suggests that they too play an important role in magma ascent through the shallow crust. However, the extent to which sills and interconnected, sill-fed dikes actually transport magma to the earth's surface in many large igneous provinces (LIP) is presently unclear. By analyzing field relationships and dimensions of intrusions of the Ferrar LIP in South Victoria Land, Antarctica, we show that sill-fed dikes were the likely feeders for voluminous flood basalt eruptions. These intrusions are small but numerous, with cumulative dimensions equivalent to a feeder network 308,000 km long and 1.8 m wide. Due to the tremendous aerial extent of this intrusive network, each individual dike-feeder segment would only be required to actively feed magma for 2 to 3 days on average to erupt the 70,000 km3 of flood lavas represented by the Kirkpatrick basalts. The Ferrar intrusions form a broadly-distributed array of small, moderately dipping dikes (<2 km long, 1.8 m wide, 56° mean dip) exhibiting almost any orientation. This sill-fed dike network contrasts with dike swarms conventionally depicted to feed flood basalt provinces, and has the appearance of a variably “cracked lid” atop a sill complex. The cracked lid model may apply to a range of shallow feeder systems (<4 km depth) intruding sedimentary basins, where the effects of far-field tectonic stresses are negligible and sill intrusions exert the dominant control on dike orientations. We conclude that sill inflation, and resulting deformation of surrounding host rock, plays a critical role in the ascent of magma in shallow volcanic systems that span the full spectrum of eruptive volumes.

From the Transantarctic Basin to the Ferrar Large Igneous Province—New palynostratigraphic age constraints for Triassic–Jurassic sedimentation and magmatism in East Antarctica

We present new palynological data from the Transantarctic Mountains that clarify the timing of sedimentary and magmatic processes in the transition from continental deposition of the Beacon Supergroup to emplacement of the Ferrar Large Igneous Province. Samples were collected from twenty-three Triassic and Jurassic sections in the southern area of north Victoria Land (NVL), East Antarctica. Recovered palynomorph assemblages are correlated with the widely used, although informal palynostratigraphic framework established for eastern Australia by Price. The associated Late Triassic–earliest Jurassic zone, APT5, is modified here with a proposed new subdivision: Lower APT5 (“APT5L”; middle–late Norian), Middle APT5 (“APT5M”; Rhaetian), and Upper APT5 (“APT5U”; Hettangian–earliest Sinemurian). We further propose a modification unifying the relevant formal eastern Australian and New Zealand palynostratigraphic zones, with a new Polycingulatisporites crenulatus Association Zone (new zonal status) that includes the P. crenulatus Association Subzone (new subzone; equivalent to APT5L) and the following Foveosporites moretonensis Association Subzone (new subzonal status; equivalent to APT5M). Our palynostratigraphic dating of the NVL assemblages demonstrates that the onset of sedimentation was diachronous in this part of the Transantarctic Basin, ranging from at least the Rhaetian to, in places, early Sinemurian. By lack of evidence for rocks containing APT5U assemblages and by analogy with the few coeval sections in Australia, we infer that the Hettangian interval in NVL is probably consumed by unconformity. Deposition of ashes from distal silicic volcanism commenced in the early Sinemurian and reached a peak phase beginning in middle Pliensbachian (ca 187Ma), coinciding with the first major magmatic interval of the silicic Chon Aike Province in Patagonia and West Antarctica. Two major episodes of phreatomagmatic activity, driven by shallow-level sill intrusion into sandstone aquifers, occurred during the middle Pliensbachian and during the late Pliensbachian–early Toarcian. The latter episode was closely followed by the first pillow extrusion and local lava effusion. Contrary to some previous studies, we further conclude that all available palynological evidence is compatible with a short-lived emplacement of the plateau-forming Kirkpatrick Basalt at around 180Ma during the early Toarcian.

Evidence for a dioecious mating system in Early JurassicHardapestheria maxwelligen. et sp. nov. (Crustacea, Branchiopoda, Spinicaudata) from the Kalkrand Formation of Namibia

A new spinicaudatan (clam shrimp), Hardapestheria maxwelli gen. et sp. nov., is described from the Jurassic Kalkrand Formation of central Namibia. Specimens were collected from a sedimentary interbed within a succession of flood basalts. These are the first spinicaudatans to be described from the Jurassic of south-western Africa. The new taxon is assigned to the family Eosestheriidae based on the combination of punctae and radial ornamentation on the carapace. Ornamentation on the growth bands in H. maxwelli differs from other eosestheriid genera because the punctate ornamentation is not restricted to the dorsal region of the carapace. Instead, all growth bands include a proximal region with punctae even though the distal portion of each growth band may exhibit anastomosing lirae. Among well-documented Mesozoic spinicaudatan genera, Hardapestheria is most closely related to Carapacestheria from the contemporaneous Kirkpatrick Basalt of Antarctica and Yanjiestheria from the Early Cretaceous of China. Hardapestheria maxwelli displays sexual dimorphism, which can be definitively related to a dioecious mating system with discrete male and female individuals. Review of additional early members of the Eosestheriidae suggests that the maintenance of two discrete sexes was the ancestral state for the clade. The ability to discriminate sexual mating system type unambiguously is rare in fossils, and this new species sheds light on the relationship between environmental stability and mating system evolution.

Paleoenvironmental changes in the Black Sea region during the last glacial and deglaciation: a multi-proxy approach

We present new palynological data from the Transantarctic Mountains that clarify the timing of sedimentary and magmatic processes in the transition from continental deposition of the Beacon Supergroup to emplacement of the Ferrar Large Igneous Province. Samples were collected from twenty-three Triassic and Jurassic sections in the southern area of north Victoria Land (NVL), East Antarctica. Recovered palynomorph assemblages are correlated with the widely used, although informal palynostratigraphic framework established for eastern Australia by Price. The associated Late Triassic–earliest Jurassic zone, APT5, is modified here with a proposed new subdivision: Lower APT5 (“APT5L”; middle–late Norian), Middle APT5 (“APT5M”; Rhaetian), and Upper APT5 (“APT5U”; Hettangian–earliest Sinemurian). We further propose a modification unifying the relevant formal eastern Australian and New Zealand palynostratigraphic zones, with a new Polycingulatisporites crenulatus Association Zone (new zonal status) that includes the P. crenulatus Association Subzone (new subzone; equivalent to APT5L) and the following Foveosporites moretonensis Association Subzone (new subzonal status; equivalent to APT5M). Our palynostratigraphic dating of the NVL assemblages demonstrates that the onset of sedimentation was diachronous in this part of the Transantarctic Basin, ranging from at least the Rhaetian to, in places, early Sinemurian. By lack of evidence for rocks containing APT5U assemblages and by analogy with the few coeval sections in Australia, we infer that the Hettangian interval in NVL is probably consumed by unconformity. Deposition of ashes from distal silicic volcanism commenced in the early Sinemurian and reached a peak phase beginning in middle Pliensbachian (ca 187 Ma), coinciding with the first major magmatic interval of the silicic Chon Aike Province in Patagonia and West Antarctica. Two major episodes of phreatomagmatic activity, driven by shallow-level sill intrusion into sandstone aquifers, occurred during the middle Pliensbachian and during the late Pliensbachian–early Toarcian. The latter episode was closely followed by the first pillow extrusion and local lava effusion. Contrary to some previous studies, we further conclude that all available palynological evidence is compatible with a short-lived emplacement of the plateau-forming Kirkpatrick Basalt at around 180 Ma during the early Toarcian.

Taphonomy of Lacustrine Interbeds in the Kirkpatrick Basalt (Jurassic), Antarctica

The Kirkpatrick Basalt (Jurassic) of South Victoria Land and the Central Transantarctic Mountains, Antarctica, includes sedimentary interbeds representing shallow lakes and ephemeral ponds (some with microbial mat accumulations), deep permanent lakes, and lake-margin areas, especially vegetated wetlands. Fossil assemblages in these sedimentary interbeds are dominated by spinicaudatans (conchostracans), but ostracodes, insect nymphs, actinopterygian fish, and plants are locally abundant. Similar biotas in contrasting contemporaneous deposits allow the taphonomy of these organisms to be compared across lacustrine depositional settings. Spinicaudatan carapaces and fish remains are preserved primarily in calcium phosphate, whereas ostracode carapaces are preserved in calcium carbonate, reflecting the original skeletal composition of the animals. Where microbial mats are present, silica replacement of spinicaudatan carapaces occurs more extensively than in other deposits; microbial processes may have enhanced silicification. This study is the first well-documented example of microbial mat influence on preservation in high-latitude lacustrine systems.

Geological evolution of the Coombs–Allan Hills area, Ferrar large igneous province, Antarctica: Debris avalanches, mafic pyroclastic density currents, phreatocauldrons

The Jurassic Ferrar large igneous province of Antarctica comprises igneous intrusions, flood lavas, and mafic volcaniclastic deposits (now lithified). The latter rocks are particularly diverse and well-exposed in the Coombs–Allan Hills area of South Victoria Land, where they are assigned to the Mawson Formation. In this paper we use these rocks in conjunction with the pre-Ferrar sedimentary rocks (Beacon Supergroup) and the lavas themselves (Kirkpatrick Basalt) to reconstruct the geomorphological and geological evolution of the landscape. In the Early Jurassic, the surface of the region was an alluvial plain, with perhaps 1 km of mostly continental siliciclastic sediments underlying it. After the fall of silicic ash from an unknown but probably distal source, mafic magmatism of the Ferrar province began. The oldest record of this event at Allan Hills is a ≤ 180 m-thick debris-avalanche deposit (member m 1 of the Mawson Formation) which contains globular domains of mafic igneous rock. These domains are inferred to represent dismembered Ferrar intrusions emplaced in the source area of the debris avalanche; shallow emplacement of Ferrar magmas caused a slope failure that mobilized the uppermost Beacon Supergroup, and the silicic ash deposits, into a pre-existing valley or basin. The period which followed (‘Mawson time’) was the main stage for explosive eruptions in the Ferrar province, and several cubic kilometres of both new magma and sedimentary rock were fragmented over many years. Phreatomagmatic explosions were the dominant fragmentation mechanism, with magma–water interaction taking place in both sedimentary aquifers and existing vents filled by volcaniclastic debris. At Coombs Hills, a vent complex or ‘phreatocauldron’ was formed by coalescence of diatreme-like structures; at Allan Hills, member m 2 of the Mawson Formation consists mostly of thick, coarse-grained, poorly sorted layers inferred to represent the lithified deposits of pyroclastic density currents. Meanwhile at Carapace Nunatak, mafic clasts were mixed with detrital material to form the Carapace Sandstone in alluvial and eventually lacustrine environments. Eruptions then became largely effusive, producing hundreds of metres of flood lavas that covered the landscape (‘Kirkpatrick time’). In places, lava flowed into ephemeral lakes to form pillow-palagonite breccias (base of sequence, Carapace Nunatak) or pillow lavas (top of sequence, Coombs Hills). Several generations of Ferrar intrusions were emplaced during the course of these events; at least three can be distinguished based on field relations. New geochemical data indicates that for the Ferrar province, magma involved in the explosive eruptions had the same major element composition as that which produced shallow intrusions and lavas. We also note the possibility that flood lavas were fed by plugs cross-cutting the Mawson Formation at Coombs Hills, rather than by major dikes extending to the surface. Finally, we infer that eruption plumes were limited to the troposphere and that direct environmental impacts were thus likely restricted to the southern hemisphere.

Physical volcanology and geological relationships of the Jurassic Ferrar Large Igneous Province, Antarctica

The Ferrar Large Igneous Province forms a linear belt for 3500 km along the Transantarctic Mountains, and as a geochemical province extends into southeastern Australasia. The principal components of the Ferrar are: intrusive — Ferrar Dolerite sills and dikes, and Dufek intrusion; pyroclastic — the Prebble, Mawson and Exposure Hill Formations; effusive — the Kirkpatrick Basalt. In terms of the three dimensional architecture of the Ferrar, a range of “facies” can be recognized in each of the principal components. The Ferrar province was initiated with a major episode of phreatomagmatism leading to formation of tephra cones and associated deposits, and near-surface vent structures. Activity switched to predominantly quiet effusion of alternating thick flood basalt flows and thin pahoehoe lobes and flows. Intrusive bodies were emplaced early, given the occurrence of dolerite clasts in tuff–breccias, but most sills were probably intruded after accumulation of extrusive rocks. Pre-existing rift structures played a major role in controlling the transport and distribution of the Ferrar magmas and the apparent centers of extrusive activity. The associated paleohydrology controlled the eruption styles.

Jurassic volcaniclastic – basaltic andesite – dolerite sequence in Tasmania: new age constraints for fossil plants from Lune River

Jurassic plants excavated from a 12 × 5 m site, at Lune River, southern Tasmania, include an araucarian tree and numerous pteridophytes, belonging to the orders Osmundales, Filicales and Bennettitales. The fossils occur in 2 – 3 m of immature volcanilithic sandstone beds. The sandstone consists primarily of clasts from granitic basement rocks underlying much of southeast Tasmania and mafic clasts containing feldspathic microliths, and primary, phreatomagmatic quartz crystals. Detrital zircons from the sandstones are mostly Early Jurassic (Toarcian) in age (182 ± 4 Ma) with minor Triassic (226 Ma), Devonian (380 – 360 Ma) and Proterozoic populations. Basaltic andesite, hereafter referred to as andesite, caps the volcanilithic units and displays similar ratios of fluid-immobile trace elements (e.g. Zr/Nb, Ti/V), to the Jurassic dolerite found in Tasmania, indicative of a common source. The andesites are correlated with the Jurassic Kirkpatrick Basalts (Trans-Antarctic Mountains, Antarctica) based on their field relationships with bounding strata, age, and distinctive similarities in major-element composition and fluid-immobile trace-element ratios. The andesite is interpreted as an extrusive equivalent of the Tasmanian dolerite. Importantly, drillcore from Lune River contains stoped clasts of andesite in fine-grained dolerite, indicating that the andesite pre-dates the dolerite. Thermal alteration index of microfossils (3 – 3.3) and reflectance of organic material within the sediments (0.54 – 0.77 Ro) resulted from contact metamorphism associated with the emplacement of this basalt. The sedimentology and stratigraphy of the depositional environment, plus the presence of hydrophilic pteridophytes and gymnosperms, indicates that the Toarcian environment was temperate to warm and humid, with an abundant supply of water.

The “Preservation Paradox”: Microbes as a Key to Exceptional Fossil Preservation in the Kirkpatrick Basalt (Jurassic), Antarctica

Thin sedimentary interbeds within the Kirkpatrick Basalt (Jurassic) of Antarctica harbor exquisitely preserved ‘soft-bodied’ organisms that lived in and around shallow freshwater lakes of the time, some of which were hydrothermally influenced. In the hottest pools, associated with magmatic vents, unusual carbonates were apparently precipitated by Archaea extremophiles adapted to living near active lava flows. In cooler lakes more distant to the vents, fine siliciclastic sediments record other communities of spinicaudatan (“conchostracan”) and notostracan crustaceans, larval insects, fishes, and ostracodes much of which may have been sustained by microbial mats, which are also well preserved in the deposits. Along the lake-edge, carbonized leaves, peat or coal, and silicified logs are often found in association with paleosols. Notably, evidence of carnivory, a primary taphonomic filter, is essentially lacking in these sediments. Exceptional preservation of non-biomineralized or lightly skeletonized organisms, therefore, reflects the original diversity of non-biomineralized or lightly skeletonized organisms present.

Early Jurassic miospores from Ferrar Group of Carapace Nunatak, South Victoria Land, Antarctica

Along the Transantarctic Mountains, the siliciclastic foreland basin deposits of the Beacon Supergroup and the effusive rocks of the Ferrar Group overlay the pre-Devonian metamorphic basement. The Ferrar Group at Carapace Nunatak is composed of fluvial volcanic sandstones of the Carapace Sandstone Formation, which is overlain by the volcanic flows and breccias of the Kirkpatrick Basalt Formation. Fossiliferous horizons are present in both formations. Palynological analyses have been carried out on fifteen samples collected in marly–silty horizons within the Carapace Sandstone and Kirkpatrick Basalt formations. Samples from the Carapace Sandstone Formation contain a scarce but quite well preserved palynoflora, which is orange to light brown in colour. The spore–pollen assemblages lack dominant species and are very similar in composition. The stratigraphically significant taxa Corollina torosa and Podosporites variabilis are present. The two samples from the Kirkpatrick Basalt Formation contain a very rich microflora dominated by Corollina species (mainly C. torosa) and Exesipollenites tumulus. Some representatives of the genus Callialasporites are also present. Comparisons with previous Carapace Nunatak and Antarctic palynological studies are discussed. The Australian Jurassic biostratigraphic zonation suggests a Late Sinemurian to basal Pliensbachian age for the Carapace Sandstone Formation and a Pliensbachian age for the basal part of the Kirkpatrick Basalt Formation, thus implying a revision of the stratigraphy and correlation between the formations in the Transantarctic mountains.

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.

Hydrothermal alteration, contact metamorphism, and authigenesis in Ferrar Supergroup and Beacon Supergroup rocks, Carapace Nunatak, Allan Hills, and Coombs Hills, Victoria Land, Antarctica

Permian‐Triassic Beacon Supergroup (dominantly quartz‐rich sandstones with coal) and Jurassic Ferrar Supergroup (mafic intrusives, extrusives, and phreatomagmatic clastic rocks), in the region studied, are separated by 20 m.y. of uplift, erosion, and nondeposition. Probable pre‐Ferrar secondary minerals in Beacon rocks comprise overgrowths of quartz and K‐feldspar followed by a cement of fine‐grained sericitic mica and quartz. In Ferrar clastic rocks, some intrastratal dissolution of quartz and feldspar was followed by a cement of hydrothermal zeolite (heulandite/clinoptilolite‐stilbite) which fills pores and replaces fine matrix and detrital mineral grains. Later overgrowths of plagioclase, quartz, and potash feldspar all replace zeolite. Finally, calcite replaces all previous mineral phases. This paragenesis occurs in Carapace Sandstone (Ferrar Supergroup), but Mawson Formation breccias (Ferrar) and some Beacon sandstones probably follow the same sequence, though less completely. Silica needed to form the late phases may have been derived from mineral conversions deeper in the stratigraphic pile or in underlying basement rocks (e.g., sodic plagioclase to laumontite), whereas silica released by conversion of heulandite to late calcite may have provided the quartz‐chalcedony filling of vugs in Kirkpatrick Basalt. Pore‐filling calcite cement in some Beacon sandstones may be pre‐Ferrar, but a suitable Ca source is not known. Late replacement calcite may have utilised methane and CO2 from Beacon coals, and Ca from plagioclase and calcium zeolites in Ferrar rocks. Zeolite‐facies burial metamorphism is indicated by laumontite, but its rarity constrains temperatures to around or below 210°C, and there is no indication of a vertical temperature zonation. Thermal metamorphism adjacent to dikes and sills is generally minor, involving recrystallisation of matrix and loss of original fabric. Sandstone blocks within large dolerite intrusions have locally melted; tridymite and cristobalite probably formed at this time but inverted to quartz during cooling. Slow cooling of partial fusion melt produced spherulitic structures. Zeolite and calcite cements postdate thermal metamorphism. The main zeolite minerals are heulandite/clinoptilolite and stilbite, accompanied locally by one or more of laumontite, chabazite, analcime, and epistilbite.

Origin of widespread, exceptionally thick basaltic phreatomagmatic tuff breccia in the Middle Jurassic Prebble and Mawson Formations, Antarctica

Eruption of the Middle Jurassic Kirkpatrick Basalt flood lavas of Antarctica was preceded by a significant episode of basaltic pyroclastic activity. The resulting deposits include minor tuff and lapilli tuff of base-surge and fall origin, but consist dominantly of massive, poorly sorted tuff breccia in beds up to 170m thick, which form unbroken successions as much as 370m thick. Tuff breccia consists of clasts of igneous and sedimentary rock up to several meters long plus basaltic bombs and spatter (≤1m) dispersed in an ash-and lapilli-size matrix of basaltic pyroclasts and abundant, disaggregated sedimentary debris. Pyroclast characteristics indicate production by phreatomagmatic processes. Sedimentary particles were produced by explosive disruption of underlying strata; some clasts were derived from depths >1km. Tuff-breccia deposits are attributed to eruption as high concentration mixtures of basaltic tephra, accidental sedimentary debris and water; the ensuing lahars filled valleys and low-standing areas within a tectono-magmatic rift zone associated with Gondwana break-up.

Stratigraphy of Jurassic pyroclastic rocks in the Transantarctic Mountains

Jurassic pyroclastic rocks cap the Antarctic Gondwana sequence and document an important event in the evolution of the continent. The Hanson Formation, which crops out in the central Transantarctic Mountains, consists of ca 240 m of silicic tuffs, tuffaceous sandstones, and subordinate quartzose sandstones of probable Early Jurassic age. It is overlain by basaltic pyroclastic rocks of the Prebble Formation. Similar basaltic rocks also crop out in Victoria Land and constitute the Mawson and Exposure Hill Formations. These basaltic pyroclastic rocks and the overlying Kirkpatrick Basalt lavas are Middle Jurassic. The pyroclastic rocks comprise thick tuff-breccias with minor lapilli-tuffs and tuffs; sequences are as much as 400 m thick. These silicic and basaltic pyroclastic rocks record the transition from foreland basin fluvial sedimentation of the Antarctic Gondwana sequence to an extensional tectonic regime associated with flood basalts and Gondwana break-up.

Apophyllite 40Ar/39Ar and Rb‐Sr geochronology: Potential utility and application to the timing of secondary mineralization of the Kirkpatrick Basalt, Antarctica

Apophyllite (KCa4Si8O20(F, OH)·8H2O), which formed as a low‐temperature secondary mineral in the Kirkpatrick Basalt, has been examined to evaluate its potential for geochronology using the K‐Ar (by the 40Ar/39Ar technique) and Rb‐Sr methods. The 40Ar/39Ar total‐gas ages for apophyllites from the Kirkpatrick Basalt are 114–133 Ma for the central Transantarctic Mountains, 95–114 Ma for south Victoria Land, and 76–100 Ma for north Victoria Land. Within individual hand samples, apophyllite 40Ar/39Ar apparent ages show variations of up to 24 m.y., and within single large (up to ∼2 cm) crystals, differences of up to 14 m.y. are observed. The 40Ar/39Ar incremental‐heating spectra are generally flat but have lowand high‐temperature discordances attributed to 39Ar recoil. Rb‐Sr model ages for the apophyllites range from 94 to 144 Ma and vary from concordant with the 40Ar/39Ar dates to as much as 14 m.y. older. The dates are consistent with other temporal indicators of low‐temperature alteration in the basalts and are interpreted to broadly reflect the time of apophyllite precipitation. They document a geographically widespread early to middle Cretaceous secondary mineralization which is inferred to be caused by a major period of groundwater movement associated with the initiation of uplift and denudation of the Transantarctic Mountains. The results suggest that apophyllite can produce geologically meaningful ages by both Rb‐Sr and K‐Ar methods, although further work is required to fully understand the variations in age that may be attributed to Ar loss, alkali mobility, or an extended history of mineral precipitation.

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

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.

Provenance, geochemistry and grain-sizes of glacigene sediments, including the Sirius Group, and Late Genozoic Glaciol history of the southern Prince Albert Mountains, Victoria Land, Antarctica

The southern Prince Albert Mountains, between David and Mawson Glaciers (75°30' to 76°S) in Victoria Land, Antarctica, comprise a series of nunataks with elevations ranging from 800m near the coast to 2300 m ~130 km inland. Geochemical and grain-size analyses of tills from these nunataks reveal three major groups of deposits: (1) coarse to medium sandy tills, found on Glaciolly streamlined summit plateaus of Kirk-pat rick Basalt above 2000 ma.s.l.., with geochemical compositions very similar to those of the underlying jurassic Kirkpatrick Basalt; (2) bimodal silty and sanely tills of the Sirius Group with Ferrar/Beacon-dominated geochemical compositions, at elevations of 1300-1600 m a.s.l. on striated summit plateaus and high-elevation terraces; (3) fine-grained tills with high SiO2 contents from ice-cored moraines at the lee sides of large nunataks. The geochemical composition of sandy tills from the highest summit plateaus suggests that valleys had not yet cut through the Kirkpatrick Basalt and into Beacon and Ferrar rocks at the time of deposition. These tills represent a phase of temperate glaciation prior to deposition of diatom-bearing Sirius Group tills. The latter were deposited after a first phase of landscape dissection as inferred from geochemical data. The fine-grained ice-cored moraines are late-Pleistocene basal tills. The presence of pre-Pliocene Glacial deposits on high mountain summits in the Prince Albert Mountains has implications for the interpretation of high-elevation Sirius Group sediments in other areas of the Transantarctic Mountains. It is possible that the “Sirius debate” has its origin in interpretations of both thin, barren pre-Pliocene deposits on high mountain summits and thick sequences of diatom-bearing deposits in valleys elsewhere in the Transantarctic Mountains. Both types of deposits are associated with the Sirius Group, but they belong to separate Glacial episodes.