Ferrar Dolerite Research Articles

Triassic and Jurassic strata at Coombs Hills, south Victoria Land: stratigraphy, petrology and cross-cutting breccia pipes

AbstractThe Triassic Lashly Formation occurs to the east of Mount Brooke at Coombs Hills. Previously established informal members B, C, and D of the Lashly Formation are now identified at Coombs Hills. Lashly Formation member D passes up into a poorly exposed interval of silicic shard-bearing fine-grained sandstone and tuff, which is correlated with the Jurassic Shafer Peak Formation of north Victoria Land and Hanson Formation of the Beardmore Glacier region. Lashly Formation members C and D are intruded by three phreatic explosion pipes, resulting from emplacement of Ferrar Dolerite intrusions at depth and associated explosive steam generation. These pipes, ranging up to 180 m in horizontal dimension, comprise sedimentary clasts in a sand matrix, most of which was locally derived. Pipe margins are mainly ill defined and adjacent country rock is commonly disaggregated or shattered, although retaining stratigraphic order. Locally, thin basalt intrusions have interacted with coal beds.

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.

Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt

Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt

Contact Partial Melting of Granitic Country Rock, Melt Segregation, and Re-injection as Dikes into Ferrar Dolerite Sills, McMurdo Dry Valleys, Antarctica

Numerous, interconnected, granitic dikes (530 cm in width and hundeds of meters in length) cut Ferrar dolerite sills of the McMurdo Dry Valleys, Antarctica. The source of the granitic dikes is partial melting of granitic country rock, which took place in the crust at a depth of about 2^3 km adjacent to contacts with dolerite sills. Sustained flow of doleritic magma through the sill generated a partial melting front that propagated into the granitic country rock. Granitic partial melts segregated and collected at the contact in a melt-rich, nearly crystal-free reservoir adjacent to the initial dolerite chilled margin. This dolerite chilled margin was subsequently fractured open in the fashion of a trapdoor by the granitic melt, evacuating the reservoir to form an extensive complex of granitic dikes within the dolerite sills. At the time of dike injection the dolerite was nearly solidified. Unusually complete exposures allow the full physical and chemical processes of partial melting, segregation, and dike formation to be examined in great detail.The compositions of the granitic dikes and the textures of partially melted granitic wall rock suggest that partial melting was characterized by disequilibrium mineral dissolution of dominantly quartz and alkali feldspar rather than by equilibrium melting. It is also unlikely that melting occurred under water-saturated conditions. The protolith granite contains only � 7 vol.% biotite and estimated contact temperatures of 900^9508C suggest that melting was possible in a dry system. Granite partial melting, under closed conditions, extended tens of meters away from the dolerite sill, yet melt segregation occurred only over less than one-half a meter from the dolerite chilled margin where the degree of partial melting was of the order of 50 vol.%. This segregation distance is consistent with calculated length scales expected in a compaction-driven process. We suggest that the driving force for compaction was differential stress generated by a combination of volume expansion as a result of granite partial melting, contraction during dolerite solidification, and relaxation of the overpressure driving dolerite emplacement. On a purely chemical basis, the extent of melt segregation necessary under fractional and batch melting to match the Rb concentrations between melt and parent rock is a maximum of 48 and 83 vol.% melt, respectively.

Crustal architecture of the Transantarctic Mountains between the Scott and Reedy Glacier region and South Pole from aerogeophysical data

Aerogeophysical data collected in transects between the South Pole and West Antarctica, crossing the Transantarctic Mountains at the 150°W meridian, are used to constrain the sub-ice topography, the sub-ice geology and the inland structure of the Transantarctic Mountains. Forward modeling of gravity data suggests slight crustal thickening of 5 km beneath the mountain front indicating partial isostatic compensation by thickened crust. New magnetic data help characterize the sub-ice geology inland of the Transantarctic Mountains with the observed magnetic anomaly field dominated by Granite Harbour Intrusives, similar to the magnetic field in Victoria Land. However, the typical pattern of anomalies caused by Jurassic tholeiitic magmatism elsewhere along the Transantarctic Mountains is not observed, nor is the mesa topography that is often associated with the Ferrar Dolerite. Together, these observations rule out the widespread presence of Ferrar Dolerite sills within the survey area. A pronounced magnetic lineament, herein named the South Pole Lineament, parallel to the 0°/180° longitudinal meridian, beneath the South Pole defines a previously unknown tectonic trend of the East Antarctic craton. The lineament suggests the presence of a lithospheric-scale structure beneath South Pole, projecting into a fault mapped from ice-penetrating radar data and extending to Shackleton Glacier, the site of a major geological boundary across the Transantarctic Mountains. Potentially, the lineament is the expression of the edge of the undeformed craton, an inherited structure created during assembly or breakup of Rodinia and Gondwana supercontinents; or an intracontinental transform.

Magnetic study of an Antarctic weathering profile on basalt: Implications for recent weathering on Mars

In order to better interpret the Martian surface weathering-related mineralogy, we focused on a relevant analogue of the Martian subsurface in terms of lithology and paleoclimate: the Jurassic-aged Ferrar dolerite (Priestley Glacier, Transantarctic Mountains), weathered in cold and dry climate. Together with chemical and mineralogical studies, rock magnetic properties were investigated and completed with Mössbauer measurements. Weathering of the decimetric block is evidenced by chemical profiles showing an increase in Fe content (from 10.5 in the core to 13 wt.% in the surface) and a decrease in Si (from 57 to 53 wt.%, respectively). According to mineralogical, thermomagnetic and hysteresis properties, the main opaque mineral is inherited titanomaghemite, with a concentration about 1%. Enhancement in low field magnetic susceptibility (from 4 · 10 − 6 to 10 · 10 − 6 m 3/kg, respectively) and saturation magnetization (from 0.44 to 0.96 Am 2/kg, respectively) indicates the neoformation of metastable maghemite. Neoformed red ferric (oxy)hydroxides are abundant in the surface. High field susceptibility normalized to iron concentration and Mössbauer spectra indicate a replacement of Fe 2+ from primary silicates (pyroxene) by poorly crystalline antiferromagnetic Fe 3+ (oxy)hydroxides. Thus, highly magnetic titanomaghemite and maghemite coexist with nanosized ferric (oxy)hydroxides and primary silicates, in accordance with Martian in situ observations. Therefore, this study supports the formation of the Martian regolith as resulting from a slow weathering process in near present day conditions.

The age and origin of the Labyrinth, western Dry Valleys, Antarctica: Evidence for extensive middle Miocene subglacial floods and freshwater discharge to the Southern Ocean

A 50! -km-long network of bedrock channels and scoured terrain occupies the ice-free portion of a major trough that crosses the Transantarctic Mountains in southern Victoria Land. The channels, collectively termed the Labyrinth, emerge from beneath the margin of the East Antarctic Ice Sheet (Wright Upper Glacier) and are incised into a 300-m-thick sill of Ferrar Dolerite at the head of Wright Valley. Upper- and intermediate-elevation erosion surfaces of the Labyrinth exhibit striations and molding characteristic of glacial erosion. Channels and canyons on the lower surface are as much as 600 m wide and 250 m deep, have longitudinal profiles with many reverse gradients, and contain potholes ! 35 m deep at tributary junctions. These characteristics are most consistent with incision from fast-flowing subglacial meltwater; estimated discharge is on the order of 1.6-2.2 10 6 m 3 s # 1 . Our 40 Ar/ 39 Ar analyses of volcanic tephra from the Labyrinth show that the chan- nels are relict, that major channel incision predates 12.4 Ma, and that the last major subglacial flood occurred sometime between 14.4 Ma and 12.4 Ma. The most plausible origin for the Labyrinth is erosion associated with episodic drainage of subglacial lakes in East Antarctica. One compelling possibility is that discharge of large volumes of sub- glacial meltwater to the Southern Ocean, and to the Ross Sea in particular, may have coincided with, and contributed to, oscillations in regional and/or global climate during the middle Miocene.

Meltwater features that suggest miocene ice‐sheet overriding of the transantarctic mountains in victoria land, antarctica

ABSTRACT. We illustrate here spectacular meltwater features associated with outburst floods beneath an ice sheet that overrode the Transantarctic Mountains in southern Victoria Land. Because of long‐term hyperarid polar climate, these features are part of an ancient landscape preserved for about 14 million years. Some channels are associated with areal scouring of basement rocks extending from sea level to as much as 1200–2100 m elevation in coastal regions. Scablands with scallops, potholes and plunge pools are cut in Beacon Super group sandstones and Ferrar Dolerite and cover wide areas of high western plateaus near the mountain crest. Subglacial channel systems commonly originate near divides and converge downhill toward the northeast. We argue that the landforms were created beneath a major Antarctic Ice Sheet that submerged the whole area, with the possible exception of the high peaks of the Royal Society Range, as it flowed northeastward toward the outer Antarctic continental shelf. Areal scouring, associated with warm‐based regimes, is restricted to the lower slopes close to the coast. In the higher terra in, meltwater channels and scabland alongside preserved patches of regolith are best explained by the breaching of cold‐based ice on the mountain rim by subglacial melt water outbursts. Melt from warm‐based ice, along with subglacial lakes trapped upstream of the mountain rim, are possible sources of the meltwater necessary to form the channel systems and scablands.

Cosmogenic 3He concentrations in ancient flood deposits from the Coombs Hills, northern Dry Valleys, East Antarctica: interpreting exposure ages and erosion rates

In situ produced cosmogenic 3He analyses provide independent support for the model of a stable, hyper-arid polar climate persisting in East Antarctica since the mid-Miocene and provide quantitative constraints on long-term rates of erosion within the Dry Valleys. In the Coombs Hills area, a series of cobble-size boulders form mega-ripples with wavelengths of approximately 50 m. Their topographic position and association with features characteristic of scabland, such as stripped, corrugated bedrock surfaces, indicate the boulders were deposited by subglacial floodwaters. Such outburst flooding could only have occurred during overriding of the northern Dry Valleys by a greatly expanded East Antarctic ice sheet. Timing of the overriding episode has been previously assigned to 14.8 to 13.6 Ma by correlation with volcanic ash deposits dated by 40Ar/39Ar in the Asgard Range of the Dry Valleys. Cosmogenic 3He concentrations in clinopyroxene from Ferrar dolerite boulders are consistent with 8.6 to 10.4 Ma exposure, calculated using scaling factors appropriate for Antarctica and assuming zero erosion. These are among the oldest surface exposure dates yet measured on Earth, but are not however consistent with the 40Ar/39Ar chronology used to define the age of the landscape due to unconstrained levels of erosion. Erosion rates of 0.03–0.06 m Ma−1 are necessary to have produced the measured boulder exposure age if they were deposited at 14.8 Ma. These are less than half the steady-state erosion rate derived from cosmogenic 3He in the nearby bedrock surfaces (0.17 m Ma−1) and testify to the extreme stability of the landscape.

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.

A Late Episode of Ferrar Magmatism? Geochemistry and Preliminary 40Ar/39Ar Results of High Fe (SPCT) Tholeiites from Southern Victoria Land, Antarctica

The Middle Jurassic Ferrar Magmatic Province extends along the proto-Pacific margin of Gondwana from Antarctica to Tasmania, Australia and New Zealand. This magmatism is part of the Mesozoic low-Ti Gondwana Province (Cox, 1988), but differs from the other Mesozoic Large Igneous Provinces, related to the break-up of Gondwana, by having high SiO2 content, LILE/HFSE ratios and Sr isotopic compositions (87Sr/86Sr > 0.708). In Antarctica, the Ferrar magmatism is represented by the mafic layered intrusions of Dufek, the sills and dikes of the Ferrar Dolerite and by the lava flows of the Kirkpatrick Basalt. Geochemical and isotopic studies on the Kirkpatrick basalts from the Mesa Range (Victoria Land) revealed the occurrence of two geochemical types: the Mount Fazio (low-Fe; MFCT) and the overlying Scarab Peak (high-Fe; SPCT; Fleming et al., 1995). Ferrar sills and lavas from Prince Albert Mountains have been investigated for major and trace elements and Sr-Nd isotopic compositions. The studied rocks mainly belong to the low-Fe tholeiites (MFCT) and consist of tholeiitic-andesitic basalts and andesites, while the high-Fe tholeiites (SPCT) have been found only at Brimstone Peak. High-Fe (SPCT) rocks are represented by evolved (rag# 23-18) andesites characterized by relatively high FeOt (>I3.0 wt.%) and TiO2 (>1.8 wt.%) and low SiO2 (56-59) contents. Relative to the high-Fe (SPCT) rocks, the low-Fe (MFCT) rocks cover a wider range of mg# (54-23) showing lower FeOt (<12.0 wt.%) and TiO2 (<1.4 wt.%) and, for similar rag# values, higher SiO2 (56-61) contents. The ~Nd and aSr values of the low-Fe (MFCT) tholeiites range from -4.8 to -5.7 and from 85.8 to 112.1, respectively. The ~Sr values are positively correlated with the degree of differentiation, (e.g. increase of SiO2 content and L R E E / H R E E ratios), suggesting that crustal contamination may operated during the magmatic differentiation of low-Fe (MFCT) tholeiites. The high-Fe (SPCT) rocks exhibit less em'iched Sr-Nd isotopic compositions (~Nd = -2.7 to -3.9; 8Sr =73.6 to 78.9), with respect to the high-Fe (MFCT) indicating that the SPCT rocks can not derive from the MFCT magmas by fractional crystallization. This suggests SPCT and MFCT experienced different evolutionary processes, and may require distinct parental magmas. Previous 4~ data suggested that Kirkpatrick basalts and Ferrar dolerites were emplaced in a short time interval, less than 1 Ma (e.g. Heimann et al., 1994; Fleming et al., 1997). We present new 4~ data on an high-Fe (SPCT) andesite. The plagioclase separate of this sample yields an 4~ plateau age of 175.1 _ 1.0 Ma (lcy analytical precision; Fig. 1). This plateau is defined by 21 contiguous steps and by c. 90% of released 39Ar. The isochron age (174.9 + 2.5 Ma; MSWD = 0.95; intercept 4~ =

Mutinaite, a new zeolite from Antarctica: The natural counterpart of ZSM-5

Mutinaite is the third new zeolite from Ferrar dolerites at Mt. Adamson (Northern Victoria Land, Antarctica). The mineral occurs as subspherical aggregates of tiny radiating lath-like fibres or as aggregates of transparent, colourless to pale-milky, tiny tabular crystals; it has vitreous luster, white streak and good 100 cleavage. Mutinaite is brittle with dmeas = 2.14(3) and dcalc = 2.17 g/cm3. Optically, it is biaxial negative with α = 1.485(2), β = 1.487(2) and γ = 1.488(2). The orientation is X = b, Y = a, Z = c. Mutinaite is orthorhombic with a = 20.223(7), b = 20.052(8), c = 13.491 (5)Å, space group Pnma. The strongest powder X-ray diffraction lines are (d(Å), I, hkl): 11.20, 84, 101, 011; 9.98, 35, 200,020; 3.85, 100, 501, 051; 3.75, 98, 303; 3.67, 27, 133; 3.00, 32, 503. The framework topology is that of the synthetic zeolite ZSM-5. The chemical formula is: (Na2.76K0.11Mg0.21Ca3.78)Σ = 6.86(Al11.20Si84.91)Σ=96.11O192 · 60H2O. The Si/Al ratio, equal to 7.6, is the highest found in a natural zeolite. Thermal stability and rehydration capacity are very high. The name is from Mutina, the ancient Latin name of the city of Modena.

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.

Terranovaite from Antarctica; a new "pentasil" zeolite

A new high-silica zeolite, terranovaite, was recently found in cavities of Ferrar dolerites at Mt. Adamson (Northern Victoria Land, Antarctica). The mineral [(Na4.2K0.2Mg0.2Ca3.7)S8.3(Al12.3Si67.7)S80.0O160 · . 29 H2O] occurs as globular masses that flake off in transparent lamellae; it has a vitreous luster, white streak, {010} perfect cleavage, and {001} distinct parting. The observed density is 2.13 6 0.02 g/cm 3 . Optically, it is biaxial positive, with 2V 5 658, a5 1.476, b5 1.478, g5 1.483 (all 6 0.002). The orientation is X 5 c, Y 5 a, and Z 5 b. Terranovaite is orthorhombic with a 5 9.747(1),

Magnetic anisotropy produced by magma flow: theoretical model and experimental data from Ferrar dolerite sills (Antarctica)

SUMMARY Volcanic rocks forming sills, dykes or lava flows may display a magnetic anisotropy derived from the viscous flow during their emplacement. We model a sill as a steadystate flow of a Bingham fluid, driven by a pressure gradient in a horizontal conduit. The magma velocity as a function of depth is calculated from the motion and constitutive equations. Vorticity and strain rate are determined for a reference system moving with the fluid. The angular velocity and the orientation of an ellipsoidal magnetic grain immersed in the fluid are calculated as functions of time or strain. Magnetic susceptibility is then calculated for a large number of grains with a uniform distribution of initial orientations. It is shown that the magnetic lineation oscillates in the vertical plane through the magma flow direction, and that the magnetic foliation plane changes periodically from horizontal to vertical. The results are compared with the magnetic fabric of Ferrar dolerite sills (Victoria Land, East Antarctica) derived from low-field susceptibility measurements.

Gottardiite, a new high-silica zeolite from Antarctica: the natural counterpart of synthetic NU-87

Gottardiite has been discovered in the Jurassic Ferrar dolerites of Mt. Adamson (Northern Victoria Land, Antarctica). The new zeolite occurs as subparallel aggregates of transparent pseudo-hexagonal lamellae. the mineral is orthorhombic, space group Cmca with a = 13.698(2), b = 25.213(3) and c = 22.660(2) A. The strongest X-ray diffraction lines are (in A): 11.34 (100), 4.37 (79), 4.01 (57), 3.282(68). The gottardiite framework topology is the same as that of synthetic NU-87. The chemical formula is: Na (sub 2.5) K (sub 0.2) Mg (sub 4.8) Al (sub 18.8) Si (sub 117.2) O 272 93H 2 O. The Si/Al ratio is 6.2 the highest found up to now in a natural zeolite. Gottardiite is optically biaxial (-) with alpha = 1.480(2), beta = 1.485(2), 2V60, where X = b, Y = a and Z = c. The density is 2.14(4) g/cm 3 (obs.), and 2.16 g/cm 3 (calc.). Both thermal stability and rehydration capacity are very high. The name is in honour of Glauco Gottardi (1928-1988).

Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas, north Victoria Land, Antarctica

Isotopic (Nd and Sr) and chemical compositions of the 177 Ma Kirkpatrick Basalt and Ferrar Dolerite from north Victoria Land, Antarctica, are examined in order to address the role of crustal assimilation and the characteristics of their mantle source. Results for the Scarab Peak chemical type (SPCT) that constitutes the flow unit capping the lava sequence [Mg-number, Mg/(Mg+Fe+2)=24, MgO=2.4%, SiO2=57.1%, initial87Sr/86Sr=0.7087–0.7097, (eNd=−4.3) conform previous reports that attribute variations in the concentrations of the more mobile elements and calculated initial87Sr/86Sr to mid-Cretaceous alteration and elevated δ18O to low-temperature interaction with meteoric water. The underlying lavas and the sills that are of the Mt. Fazio chemical type (MFCT) display a much wider range of both chemical and isotopic compositions (Mg-number=40–65, MgO=3.7 7.5%; SiO2=52.6–58.3%, initial87Sr/86Sr=0.7087–0.7117, eNd=−5.6 to −4.8). The effects of rock alteration on apparent initial87Sr/86Sr are demonstrated by large differences between the initial ratio of mineral separates or leached fractions and whole rocks. Cretaceous alteration produced Rb and Sr redistribution within the lava sequence that results in erroneous calculated initial87Sr/86Sr ratios. These effects are responsible for the large initial87Sr/86Sr variations previousl7 proposed which, combined with the large range in whole-rock δ18O, were purported to show very large degrees of crustal assimilation. The variations in eNd are restricted and indicate much smaller degrees of assimilation. The least altered of the MFCT rocks show good chemical and isotopic correlations that can be integrated into a model involving fractionation of pyroxene and plagioclase coupled with assimilation of material similar to early Paleozoic basement. The lower87Sr/86Sr and higher eNd of the SPCT suggest that they were derived by extensive fractionation of a more primitive, less contaminated, precursor of the MFCT. The most isotopically primitive Ferrar rocks from the region still have a high initial87Sr/86Sr and low initial143Nd/144Nd; this may reflect either earlier assimilation or an enriched source. The chemical and isotopic similarities, as well as the close geographic correspondence of the Ferrar Group to granitoids produced during the early Paleozoic Ross Orogeny suggest that in either case Ross-type material may have been involved in the development of the enriched isotopic signature.

Geochemistry of Ferrar Dolerite sills and dykes at Terra Cotta Mountain, south Victoria Land, Antarctica

At Terra Cotta Mountain, in the Taylor Glacier region of south Victoria Land, a 237 m thick Ferrar Dolerite sill is intruded along the unconformity between basement granitoids and overlying Beacon Supergroup sedimentary rocks. Numerous Ferrar Dolerite dykes intrude the Beacon Supergroup and represent later phases of intrusion. Major and trace element data indicate variation both within and between the separate intrusions. Crystal fractionation accounts for much of the geochemical variation between the intrusive events. However, poor correlations between many trace elements require the additional involvement of open system processes. Chromium is decoupled from highly incompatible elements consistent with behaviour predicted for a periodically replenished, tapped and fractionating magma chamber. Large ion lithophile element-enrichment and depletion in Nb, Sr, P and Ti suggests the addition of a crustal component or an enriched mantle source. The trace element characteristics of the Dolerites from Terra Cotta Mountain are similar to those of other Ferrar Group rocks from the central Transantarctic Mountains and north Victoria Land, as well as with the Tasmanian Dolerites. This supports current ideas that the trace element signature of the Ferrar Group is inherited from a uniformly enriched mantle source region.

Minimum 10Be exposure ages of early Pliocene for the Table Mountain plateau and the Sirius Group at Mount Fleming, Dry Valleys, Antarctica

Using accelerator mass spectrometry (AMS), we have measured 10Be and 26Al in quartz from granites and sandstones from Table Mountain and Mount Fleming, Antarctica. Our data show that the plateau surface at Table Mountain had formed by early Pliocene time at the latest. Granites fringing but within the Sirius Group at Table Mountain give a minimum exposure age of 2.6 Ma for this deposit. A sandstone clast on the Ferrar dolerite surface just outside and below, and thus postdating, the Sirius Group has a minimum age of 2.9 Ma. Two samples from the Sirius Group at Mount Fleming have 10Be concentrations that have reached secular equilibrium. This deposit is at least 4.8 m.y. old. The Sirius Group at Mount Fleming cannot have been deposited after 3.0–2.5 Ma, as implied by biostratigraphic data. Our dates contradict the hypothesis that in the Pliocene East Antarctica was deglaciated and the climate was significantly warmer and wetter. The preservation of these surfaces indicates a continuous cold desert in the dry valleys since the beginning of the Pliocene. The high 10Be concentrations we have measured cannot be reconciled with uplift of the Transantarctic Mountains at a rate of 1 km/m.y. during the past 3 m.y.

A short interval of Jurassic continental flood basalt volcanism in Antarctica as demonstrated by [formula omitted] 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 40 Ar 39 Ar 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.