Teresa M. Mensing

The geochemistry of Jurassic dolerites from Portal Peak, Antarctica

Geochemical and isotopic analyses have been performed on a suite of samples from a Jurassic quartz tholeiite sill of the Ferrar Group at Portal Peak, Queen Alexandra Range, near the Beardmore Glacier in Antarctica. The data include major and trace element (XRF and INAA) concentrations, as well as Sr and Nd isotopic compositions, and are combined with the results of other studies on samples from Antarctica. It is demonstrated that despite differences in the pre-intrusion (or eruption) evolution of the Ferrar Group magmas, the similarity in isotopic and chemical compositions for these rocks supports the existence of a remarkably uniform mantle source with unusual signature over a distance of thousands of kilometres. The favoured origin of this source involves the subduction of terrestrial sedimentary material into a depleted mantle reservoir.

Petrogenesis of the Kirkpatrick Basalt, Solo Nunatak, northern Victoria Land, Antarctica, based on isotopic compositions of strontium, oxygen and sulfur

Chemical and isotopic compositions of Jurassic tholeiites of the Kirkpatrick Basalt Group from Solo Nunatak, northern Victoria Land, indicate that these rocks are contaminated with crustal material. The basalts are fine grained and contain phenocrysts of augite, pigeonite, hypersthene and plagioclase. The flows on Solo Nunatak are chemically more similar to average tholeiite than flows from Mt. Falla and Storm Peak in the Central Transantarctic Mountains (TAM) which appear to be more highly differentiated. Initial 87Sr/86Sr ratios of the flows on Solo Nunatak are high (>0.710) and are similar to those reported for the Kirkpatrick Basalt in the Central TAM. Whole-rock δ18O values are also high, ranging from +6.0 to +9.3‰ and correlate positively with initial 87Sr/86Sr ratios, similar to the Kirkpatrick Basalt in the Central TAM. The correlation between initial 87Sr/86Sr ratios and δ18O values is explained as the result of simultaneous fractional crystallization and assimilation of a crustal contaminant. Sulfur isotope compositions vary between limits of δ34S= -4.01 to +3.41‰ Variations in (δ34S probably resulted from outgassing of SO2 under varying oxygen fugacities.

KAr dates and paleomagnetic evidence for Cretaceous alteration of Mesozoic basaltic lava flows, Mesa Range, northern Victoria Land, Antarctica☆

Abstract The Mesozoic tholeiite basalts in the Transantarctic Mountains are probably Jurassic in age based on a fragmentary fossil record, on their stratigraphic relation to the older sedimentary rocks of the Beacon Supergroup, and on a few 40 Ar 39 Ar and RbSr age determinations. However, the whole-rock KAr dates of these basalts vary widely between ∼ 100 and ∼ 200 Ma and do not constrain the age of these rocks. New whole-rock AAr dates of flows on the summit of Pain Mesa in the Mesa Range of northern Victoria range from 103 ± 4 to 174 ± 7 Ma and confirm previously published results. However, chemical analyses indicate that the K2O concentrations of these flows on Pain Mesa decrease with increasing volatile content (loss on ignition) and that most of the low-K flows in the Mesa Range yield low KAr dates. This evidence suggests that K was lost as a result of alteration of the flows by aqueous solutions and that the alteration was accompanied by even greater losses of radiogenic 40Ar. Accordingly, the new KAr dates of these rocks indicate that the age of alteration is 176 ± 8 Ma. The discordance of KAr dates of the lava flows of the Mesa Range, as well as those at Litell Rocks reported by other investigators, suggests that northern Victoria Land experienced a geologic event during the Cretaceous Period that caused low-grade thermal metamorphism and hydrothermal alteration of the flows. The occurrence of such an event is supported by paleomagnetic evidence indicating that the basalt flows of the Mesa Range and the Litell Rocks as well as sedimentary rocks of early Paleozoic age of the Bowers Terrane in northern Victoria Land, have anomalous virtual geomagnetic pole positions caused by remagnetization during the Cretaceous Period. The tectonothermal and hydrothermal activity in northern Victoria Land during the Cretaceous Period indicated by these results may have been caused by the reactivation of deep crustal faults in the area during the separation of Australia from Antarctica.

Identification and age of neoformed Paleozoic feldspar (adularia) in a Precambrian basement core from Scioto County, Ohio, USA

Fourteen core samples of Precambrian granitic gneisses from a well drilled in the Green Township, Scioto County, Ohio were studied to determine the origin of alkali feldspar in these rocks. The well intersected the basement at a depth of 1,700 m and penetrated 11.3m of Precambrian crystalline rocks. Petrographically the samples in the upper 6.4 m of the basement core show evidence of severe alteration by the presence of hematite, limonite and chlorite and by the absence of plagioclase. Alkali feldspars from this part of the core are turbid, have a low 2 V of about 10°, are highly enriched in K, have low Na and Rb concentrations, lack cathode luminescence, and form a straight line on a Rb-Sr isochron diagram yielding a date of 599±69 Ma. Core samples from below 6.4 m appear relatively fresh and unaltered. Alkali feldspar from this portion of the core is orthoclase, shows uniform blue luminescence and gives a Rb-Sr date of 1,162±11 Ma. These results indicate that feldspars in the lowest part of the core are primary minerals that crystallized during the Grenville Orogeny, whereas the K-feldspar in the top of the core is of low-temperature secondary origin. The formation of this feldspar is explained as a consequence of chemical weathering of primary feldspar during late Precambrian time to clay minerals that were later reconstituted under low-temperature hydrothermal conditions as K-feldspar (adularia) by reactions with brines derived from the overlying Mt. Simon Formation of Cambrian age.

Cretaceous alteration of Jurassic volcanic rocks, Pain Mesa, northern Victoria Land, Antarctica

Abstract The Kirkpatrick basalt (Jurassic) of Pain Mesa in the Mesa Range of northern Victoria Land consists of low-Ti tholeiite basalt overlain by up to six flows of hypocrystalline, high-Ti tholeiite. The Rb and Sr concentrations and 87 Sr 86 Sr ratios of 28 specimens of high-Ti basalt from Pain Mesa yield an anomalously low errorchron date of 100 ± 15 Ma in contrast to whole-rock samples of the low-Ti tholeftes which scatter above and below a 175-Ma reference isochron. The apparent lowering of the RbSr date of the high-Ti flows is attributed to aqueous alteration of the rocks and coincides with the separation of Australia from Antarctica during the Cretaceous Period.

Effect of oxygen fugacity on sulfur isotope compositions and magnetite concentrations in the Kirkpatrick Basalt, Mount Falla, Queen Alexandra Range, Antarctica

Abstract The δ 34 S-values of total sulfur in the Jurassic tholeiite flows on Mt. Falla in Antarctica range from −1.45 to +11.73‰. The concentrations of sulfur range from 80 to 480 ppm, which is typical of subaerial lava flows that lose varying proportions of sulfur by out-gassing of SO 2 . The concentrations of magnetite range from less than 1% to more than 4% and appear to correlate inversely with the total Fe content of the flows. However, the five flows which are anomalously enriched in 34 S also have elevated magnetite concentrations. We suggest that the elevated magnetite concentrations and the 34 S enrichment were both caused by high oxygen fugacities ( f O 2 ) in the melt. The magnetite concentrations are affected by the fugacity of oxygen through equilibrium in the FMQ buffer whereas the enrichment of the flows in 34 S resulted from outgassing of SO 2 at f O 2 greater than ∼ 10 −8 atm. The dependence of δ 34 S and the magnetite concentrations of the flows on f O 2 is supported by the stratigraphic variation of these parameters and by their direct linear correlation.

From Rodinia to Gondwana

The standard model for the origin of the Transantarctic Mountains postulates that during the Neoproterozoic Era a supercontinent existed called Rodinia which included what later became the East Antarctic craton. This supercontinent split into two large continental fragments called Laurentia and Gondwana. The continental fragment we call Laurentia drifted away (was displaced) and eventually became the North American continent. Gondwana originally included the land areas of Australia, East Antarctica, and India. The rift valley that split Rodinia widened into an ocean which was bordered by passive rift margins that formed the coasts of East Antarctica and of Laurentia. Along both coasts turbidite sequences composed of alternating layers of graywacke and shale were deposited. At the end of the Neoproterozoic the passive rift margin of East Antarctica was transformed into an active subduction zone along which the siliciclastic turbidites and locally developed platform carbonates and volcanic rocks were folded by compression against the mainland and, in the process, were regionally metamorphosed and intruded by granitic magma of anatectic origin (Stump 1995). The details of this process are the subject of Chapters 3–8 of this book.

Kirwan Volcanics, Queen Maud Land

Western Queen (Dronning) Maud Land in Fig. 14.1 is not considered to be part of the Transantarctic Mountains because the basement rocks were not deformed and regionally metamorphosed by the Ross Orogeny. Nevertheless, some of the nunataks that project through the East Antarctic Ice Sheet in the area outlined in Fig. 14.1 and enlarged in Fig. 14.2 consist of Jurassic tholeiite basalt of the Kirwan Volcanics that superficially resemble the Kirkpatrick Basalt in the Transantarctic Mountains. In addition, flat-lying sedimentary rocks of the Permian Amelang Formation resemble the sandstones and shales of the Beacon Supergroup. The groups of nunataks in Fig. 14.2 where the Kirwan Volcanics are exposed include Vestfjella (including Plogen and Basen), Fossilryggen, Mannefallknausane, Heimefrontfjella, Bjőrnnutane, Sembberget, and the Kirwan Escarpment. The first information about the geology of the Kirwan Volcanics was published by Brunn (1964) and Blundell (1964). The latter collected oriented samples of dolerite in the southern part of Vestfjella for a study of paleomagnetism (see also Lovlie 1979, 1988). These samples were later described by Brown (1967) and dated by Rex (1967).

The Beacon Supergroup

The geologists who accompanied Robert Scott and Ernest Shackleton to Antarctica were amazed when they entered the mountains of southern Victoria Land and discovered the ice-free valleys such as Wright Valley in Fig. 10.1 (Sections 1.4, 2.3, and 3.1). The modern traveler entering the ice-free valleys by helicopter from McMurdo is similarly affected because the wide U-shaped valleys present a familiar view of sandy plains dotted with lakes and ponds filled with liquid water and of meltwater streams that flow from alpine glaciers descending from the high mountain ranges that frame these valleys.

Antarctica: The Continent

The area of Antarctica is 13.97 × 106 km2 making it the fifth largest of the seven continents (Stonehouse 2002). It is conventionally oriented on maps as shown in Fig.2.1 and is subdivided into East Antarctica, West Antarctica, the Antarctic Peninsula, and certain islands that rise more than 500 m above sea level (i.e., Alexander, Bear, Berkner, Roosevelt, Ross, and Thurston). In addition, Antarctica is surrounded by the Ross, Ronne, Filchner, Riiser-Larsen, Fimbul, and Amery floating ice shelves as well as by the Larsen ice shelf located along the east coast of the Antarctic Peninsula. Except for the northernmost tip of the Antarctic Peninsula, the continent lies within the Antarctic Circle at latitudes greater than 62.5° south.