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basalt sequence. Tree stumps in situ were noted at several places. Observations during this field season support the conclusion, tentatively drawn last season (Elliot et al. 1982), that there is no compelling evidence for major faulting at the head of the Rennick Glacier between Exposure Hill and Vantage Hills. Nevertheless, a fault probably separates Sheehan Mesa from outcrops to the west and significant faulting undoubtedly occurs to the north, along the Rennick Glacier. Petrographic examination and laboratory studies of the major and trace element composition of basalts and diabases, of the mineral chemistry, and of the isotopic composition of these rocks are in progress. Dating of the basalts by the argon-40/ argon-39 technique will also be attempted. The basalts and diabases will be used to determine a paleomagnetic pole position, to establish a record of magnetic polarity reversals, and to study the magnetic properties of the minerals. This study was supported by National Science Foundation grant DPP 80-21401.

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Location and geologic sketch map for the Mesa Range region of northern Victoria Land.

and Schulte (1968), was reexamined but no new fossil material was collected during the 1982-1983 field season. Interbeds composed of volcaniclastic debris occur at numerous localities throughout the Mesa Range, principally in the lower part of the

Petrogenesis of the Kirkpatrick Basalt, Solo Nunatak, northern Victoria Land TERESA M. MENSING, GUNTER FAURE, and KAREN S. TAYLOR Institute of Polar Studies The Ohio State University Columbus, Ohio 43210

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M. JONES

CONOCO Inc., Research and Development Department Ponca City, Oklahoma 74603

12

Ballance, P. F., and W. A. Watters. 1971. The Mawson Diamictite and Carapace Sandstone, formations of the Ferrar Group at Allan Hills and Carapace Nunatak, Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 14, 512-527. Elliot, D. H., M. Siders, G. Faure, and K. S. Taylor. 1982. The Kirkpatrick Basalt, Mesa Range, northern Victoria Land. Antarctic Journal of the U.S., 17(5), 19--20. Hall, B. A., J. F. Sutter, and H. W. Borns. 1982. The inception and duration of Mesozoic volcanism in the Allan Hills—Carapace Nunatak area, Victoria Land, Antarctica. In C. Craddock (Ed.), Antarctic geoscience, Madison: University of Wisconsin Press. Nathan, S., and F. J . Schulte. 1968. Geology and petrology of the Campbell-Aviator Divide, northern Victoria Land, Antarctica. New Zealand Journal of Geology and Geoph ysics, 11, 960-975.

Anomalously high initial strontium-87/strontium-86 (87Sr/86Sr) ratios (greater than 0.710) for the Jurassic tholeiites (approximately 170 million years old) of the Transantarctic Mountains have been well documented by Compston, McDougall, and Heier (1968); Faure (1981); Faure, Bowman, and Elliot (1979); Faure et al. (1972, 1974); Hoefs, Faure, and Elliot (1980); Kyle (1980); and Kyle, Elliot, and Sutter (1980). These rocks of the Ferrar Supergroup extend from northern Victoria Land to the Pensacola Mountains and are composed of dolerite intrusions (Ferrar Dolerite Group) and basalt lava flows (Kirkpatrick Basalt Group). Major element data for these rocks display normal differentiation trends when plotted on variation diagrams. That is, positive correlations exist between Si0 2 and Na2O, K20, Ti02, and P205 (Kyle 1980). Negative correlations are observed between S'02 and calcium oxide (CaO), magnesium oxide (MgO), and aluminum oxide (Al 20) (Kyle 1980). Two petrogenetic models have been proposed to explain the observed isotopic and chemical trends for these rocks: (1) contamination of mantle-derived melts by the assimilation of crustANTARCTIC JOURNAL

al material (Faure 1981; Faure et al. 1972, 1974, 1979; Faure, Pace, and Elliot 1982; Hoefs et al. 1980) and (2) generation of tholeiites in a heterogeneous mantle (Kyle 1980). Faure (1981) and Faure et al. (1974, 1978) recognized systematic stratigraphic variations of the initial 87SrI86Sr ratios in the sequences of basalt lava flows at Storm Peak and Mount Falla in the Queen Alexandra Range. They also recognized significant correlations between major oxide components and initial 87 Sr/86 Sr ratios and concluded that a crustal contaminant is present in these rocks. They calculated that 20 to 40 percent of contaminant may have been assimilated to produce the observed isotopic and chemical variations. Kyle (1980) rejected the contamination model in favor of the heterogeneous-mantle model. He noted that there is a significant gap between the observed 87 Sr/ 86 Sr ratios of the antarctic tholeiites and the upper limit of 0.706 for the suboceanic mantle. He suggested that the absence of initial 87 Sr/ 86 Sr ratios below 0.7085 indicates that the subcontinental mantle sources of these rocks may be significantly different from suboceanic mantle. Kyle (1980) also observed that there is no strong correlation between initial 87 Sr/ 86 5r ratios and major element oxides for the Ferrar Group rocks as a whole. In addition, he found it difficult to accept the contamination model, because it involves such a high degree of contamination over such a widespread area. In an attempt to resolve this controversy, Hoefs et al. (1980) measured 8180 values for several samples on Mount Falla. They showed that a significant positive correlation exists between initial 87 Sr/ 86 Sr ratios and 6 18 0 values and concluded that these results reflect the assimilation of 18 0-rich crustal material by the basalt magma. They calculated that the basaltic component must have had a high initial 87 Sr/ 86 Sr ratio of approximately 0. 7093 assuming that it had 8180 value of 5.5 parts per thousand. Their results indicate that both previously described models are necessary to explain the petrogenesis of these basalts. That is, the parent magma was generated from a mantle source enriched in radiogenic 117 Sr and was subsequently contaminated by the assimilation of crustal material. The present study extends the work on the Kirkpatrick Basalt to northern Victoria Land where a thick sequence (greater than 700 meters) of tholeiitic lava flows are exposed in the Mesa Range and surrounding nunataks. Two field seasons have been devoted to measuring several complete sections and collecting samples for petrographic, geochemical, and isotopic analyses. A preliminary investigation of the 23 flows (constituting more than 300 meters) on Solo Nunatak has been undertaken. One sample from each flow has been analyzed for major elements, rubidium and strontium concentrations and for strontium isotopes. The results are similar to those obtained for the Ferrar Supergroup elsewhere in the Transantarctic Mountains. Normal differentiation trends as well as elevated initial 87Sr/56Sr ratios are observed on variation diagrams for the Solo Nunatak samples. The average initial 87 Sr/86 Sr ratio was calculated from 24 samples to be 0.71083 ± 0.00010 (i(r). A positive correlation between Si0 2 and initial 87 Sr/'Sr ratios indicates that silica-rich crustal material with a high 87Sr/16Sr ratio may have been assimilated by the basalt magma (figure). It is also interesting to note that no initial 87 Sr/ 16 Sr ratios below 0.709 occur in the Solo Nunatak samples as was also noted earlier by Kyle (1980) for Jurassic tholeiites throughout the Transantarctic Mountains. The results obtained thus far are consistent with the hypothesis of Faure (1981) and Hoefs et al. (1980) that primary magma, generated from a mantle source with an elevated 57 Sr/'Sr ratio 1983 REVIEW

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Plot of initial 81 Sr/ 86 Sr ratios (170 million years ago) versus silica for whole-rock samples from Solo Nunatak. ("wt." denotes weight.)

(0.7093), was subsequently contaminated by assimilating crustal material. The authors wish to express their thanks to David Buchanan for his assistance during the 1982-1983 field season. The support of CONOCO, Inc., through the use of its facilities is greatly appreciated. We are also grateful to the pilots and crew of vxE-6 for logistical support. This research was supported by National Science Foundation grant DPP 80-21401.

References Compston, W., I. McDougall, and K. S. Heier. 11968. Geochemical comparison of the Mesozoic basaltic rocks of Antarctica, South Africa, South America and Tasmania. Geochimica et Cosmochiinica Acta, 32, 129-149. Faure, G. 1981. Strontium isotope composition of volcanic rocks: Evidence for contamination of the Kirkpatrick Basalt, Anarctica. In R. J. O'Connell, and W. S. Fyfe (Eds.) Ecolution of the Earth, Geodynamics series, Vol. 5, Washington, D.C.: American Geophysical Union. Faure, G., J. R. Bowman and D. H. Elliot. 1979. The initial 17 Sr/8 Sr ratios of the Kirwan volcanics of Dronning Maud Land: Comparison with the Kirkpatrick Basalt, Transantarctic Mountains. Chemical Geology, 26, 77-90. Faure, G., J . R. Bowman, D. H. Elliot, and L. M. Jones. 1974. Strontium isotope composition and petrogenesis of the Kirkpatrick Basalt, Queen Alexandra Range, Antarctica. Contributions to Mineralogy and Petrology, 48, 153-169. Faure, G., R. L. Hill, L. M. Jones, and D. H. Elliot. 1972. Isotope composition of strontium and silica content of Mesozoic basalt and dolerite from Antarctica. In R. J . Adie (Ed.), Antarctic geology and geophysics symposium on Antarctic and Solid Earth geophysics. Oslo: Universitetsfor laget. Faure, G., K. K. Pace, and D. H. Elliot. 1982. Systematic variations of "Sr/"Sr ratios and major element concentrations in the Kirkpatrick Basalt of Mt. Falla, Queen Alexandra Range, Transantarctic Mountains. In C. Craddock (Ed.), Antarctic geosciences. Madison: University of Wisconsin Press. Hoefs, J . , G. Faure, and D. H. Elliot. 1980. Correlation of b'O and initial 'Sr/'"Sr ratios in Kirkpatrick Basalt on Mt. Falla, Transantarctic Mountains. Contributions to Mineralogy and Petrology, 75, 199-203. Kyle, P. R. 1980. Development of heterogeneities in the subcontinental 13

mantle: Evidence from the Ferrar Group, Antarctica. Contributions to Mineralogy and Petrology, 73, 89-104.

Kyle, P. R., D. H. Elliot, and J. F. Sutter. 1980. Jurassic Ferrar Supergroup tholeiites from the Transantarctic Mountains, Antarctica, and

Jurassic trees engulfed by lavas of the Kirkpatrick Basalt Group, northern Victoria Land TIMOTHY H. JEFFERSON*

their relationship to the initial fragmentation of Gondwana. In M. Cresswell and P. Vella (Eds.), Gondwana V. Proceedings of the Fifth International Gondwana Symposium, Wellington, New Zealand, 1980.

up, of the cell wall. Because of subsequent ordering of the silica and oxidation of the organic material, it often appears that the cell wall is replaced by silica. In rare cases, cells were filled before cell walls were completely silicified and oxidation has left silica casts of individual cells. These cell casts may be dissociated by weathering which produces a material resembling sawdust. In some chalcedonic fossil wood (which may be greer or orange) most areas are poorly preserved. Silica was no

Department of Botany The Ohio State University Columbus, Ohio 43210.

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MARY A. SIDERS and MARTA A. HABAN .:

Institute of Polar Studies The Ohio State University Columbus, Ohio 43210.

Fossil trees preserved in growth position in lavas of the Kirkpatrick Basalt Group were discovered in the Mesa Range area of northern Victoria Land during the 1981-1982 and 1982-1983 field seasons (Elliot et al. 1982, Antarctic Journal, this issue). These lavas represent the extrusive phase of the Lower Jurassic Ferrar Supergroup. Potassium/argon (K/Ar) dating of the Ferrar rocks yields an average age of 179 ± 5 million years (Kyle, Elliot, and Sutter 1981). The wood of the trees from the Kirkpatrick Basalt provides an important record of structurally preserved plant material and of paleoclimate in Antarctica during the Early Jurassic. Silicified wood is commonly associated with pillow lavas and pahoehoe toes and was described briefly by Gair (1967). At several localities trees are in growth position with roots within the organic-rich tops of sedimentary interbeds (figure 1). Trunks range from 0.5 to 1 meter in diameter and up to 4 meters or more in height. Despite the inundation of the trees by hot lava, charring appears to have been limited (block b of figure 1). This is probably because of rapid cooling of lavas (perhaps due to marshy ground) and insulation of the internal wood by the outer parts of the trunks. In well-preserved wood, the cell wall is composed largely of silica but may be defined by small quantities of dark, carbonaceous material representing remnants of the original wall. Scanning electron microscope observations suggest that silica grew onto helical, fibrillar structures within the cell wall (block d of figure 2). These structures were probably formed by early fungal delignification, and consequent opening

* Doctor Timothy H. Jefferson was killed in an avalanche on 12 September 1983, while working on a glaciology and climatology expedition in the Cordillera Blanca, Peru. 14

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Figure 1. Trees preserved in growth position. (a) Tree more than 4 meters high and 1 meter broad engulfed by a lava flow at Mount Fazio (locality 82-4). (Photograph by D. H. Elliot.) (b) Part of a tree about 2 meters high and 20 centimeters in diameter in a 31-meter thick flow at Mount Fazio (locality 82-4). Note apparent charring. (c) Tree stump 82 centimeters in diameter buried by pahoehoe toes at the base of a 44-meter thick lava flow at Tobin Mesa (locality 82-3). ANTARCTIC JOURNAL