An unusual occurrence of scolecite from the Antarctic Peninsula
We believe that the biotite ages were reset and reflect reheating due to intrusion of the younger quartz monzonite-granodiorite phase (compare Mehnert et al., 1975). Samples 3 and 4 are typical of the younger quartz monzonite-granodiorite phase of the Werner batholith. The age of this phase thus is considered to be about 100 million years old (table). Sample 5 also is granodiorite, but it occurs at the roof of the batholith adjacent to the older mafic phase (sample 2); detailed petrography and geochemistry demonstrate that during emplacement it was contaminated by partial assimilation of the Latady Formation and the older mafic phase (W. R. Vennum, unpublished data, 1976). Its older potassium-argon age may reflect partial assimilation of biotite of the older mafic phase. The Galan batholith, consisting mostly of granodiorite, is about 20 kilometers east of the northern Werner pluton. The Galan batholith intrudes the northern edge of the Grimminger stock, which consists of diorite and quartz diorite. The Grimminger stock was sampled because field evidence suggested that it might be the oldest intrusive rock in the Lassiter Coast and the southern Black Coast. Its biotite age (sample 7) must be considered a minimum age, for it may have been reset by intrusion of the Galan batholith (sample 6). The Rath stock consists of diorite and granodiorite and is 20 kilometers south of the Werner batholith. The biotite age of the Rath stock (sample 8) may have been reset, inasmuch as a small batholith, probably younger, occurs several kilometers north of the Rath pluton. The new ages are similar to previously determined ages from plutons in the southern Antarctic Peninsula. Ages of all plutonic rocks in the Lassiter Coast and southern Black Coast range from 119 to 95 million years (Mehnert et at., 1975; Rowley et al., 1975) and overlap the 109- to 102-millionyear dates on plutons (Halpern, 1967) in eastern Ellsworth Land, which is less than 200 kilometers west-southwest of the Lassiter Coast. Thus plutons in the southern Antarctic Peninsula have a relatively restricted time range in the late Cretaceous, in contrast to plutons in other parts of the Peninsula (Adie, 1972). This study was supported by National Science Foundation grant AG-187 and by a grant from the National Research Council of Canada. References Adie, R. J . 1955. The petrology of Graham Land. Part II, the Andean granite-gabbro intrusive suite. Falk/and Islands Dependencies Surve). Scient!/lc Report, 12. 39p.
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Adie, R. J . 1972. Recent advances in the geology of the Antarctic Peninsula. In: Antarctic Geology and Geophysics (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, Universitetsforlaget. Series (1): 121-124. Bateman, P. C. 1961. Granitic formations in the east-central Sierra Nevada near Bishop, California. Geological Society of America Bulletin, 72: 1521-1538. Halpern, M. 1967. Rubidium-strontium age measurements of plutonic igneous rocks in eastern Ellsworth Land and northern Antarctic Peninsula, Antarctica. Journal of Geophysical Research, 72(20): 5133-5142. Mehnert, H. H., P. D. Rowley, and D. L. Schmidt. 1975. K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica. USGS Journal of Research, 3(2): 233-236. Rowley, P. D. 1973. Geologic observations on the northern Lassiter Coast and southern Black Coast. Antarctic Journal of the U.S., VIII(4): 154-155. Rowley, P. D., and P. L. Williams. 1974. Plutonic rocks of the Lassiter Coast. Antarctic Journal of the U.S., TX(S): 225226. Rowley, P. D., P. L. Williams, D. L. Schmidt, R. L. Reynolds, A. B. Ford, A. H. Clark, E. Farrar, and S. L. McBride. 1975. Copper mineralization along the Lassiter Coast of the Antarctic Peninsula. Economic Geology, 70(5): 982-987. Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula—preliminary report. In: Antarctic Geology and Geophysics (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, Universitetsforlaget. Series B, (1): 143-148.
An unusual occurrence of scolecite from the Antarctic Peninsula WALTER R. VENNUM* and JOANNE L. BENTZ Department of Geology Cahfornia State College, Sonoma Rohnert Park, California 94928 White, fan-shaped aggregates of a fibrous zeolite as much as 5 millimeters long coat fracture surfaces in diorite on the northern slope of a small nunatak at 73°24S. 63°13'W. in the south western Dana Mountains of the northern Lassiter Coast. This location is about 10 kilometers southwest of Mount Barkow. X-ray power data (table) indicate that the mineral is either natrolite or scolecite. Negative elongation, small extinction angle on cleavage fragments, refractive indices above 1.505,
*Also: U.S. Geological Surve y , Menlo Park, California 94025.
ANTARCTIC JOURNAL
birefringence of 0.006, and noticeable dispersion confirm its identification as scolecite. Mode of occurrence severely restricts the amount of material available for thinsection examination; all optical properties, except birefringence, were determined by oil immersion. The diorite is the oldest and outermost unit of the composite Werner batholith—an elongate, concentrically zoned igneous body that extends 140 kilometers from an unnamed mountain range in the southern Black Coast south to the Hutton Mountains of the central Lassiter Coast (Rowley, 1973; Rowley and Williams, 1974). Potassiumargon dating shows that all intrusive phases of the batholith are Upper Cretaceous (Edward Farrar and S. L. McBride, Queens University, Ontario, written communication, 1975). The pluton intrudes black slate, siltstone, and sandstone of the Upper Jurassic Latady Formation. Thin septa and pods of diorite, as much as several hundred meters wide, locally occur along the western margin of this batholith in the southwestern Dana Mountains and along its eastern margin in the central Werner Mountains. At both localities, the diorite is intruded by younger granodiorite that grades into the quartz-monzonite core of the batholith. The intrusive contact between diorite and granodiorite is exposed 25 meters south of the zeolite locality. Detailed petrography of the rocks in the area (Vennum, unpublished data, 1976) confirms that both the diorite and thegranodiorite adjacent to the zeolite locality are hybrid rocks that have assimilated large amounts of the surrounding Latady Formation. Scolecite (CaAl2Si2O 19 . 3H20) is an uncommon member of the zeolite group; its chief mode of occurrence is in cavities of basaltic volcanic rocks. Two notable localities are the Tertiary basalts of Mull and Skye and at Berufjord, Iceland. Scolecite also has been reported in contact-metamorphosed calcareous rocks and as a hydrothermal mineral concentrated along fissures in other types of metamorphic rocks. Only rarely does it occur as a vein or joint filling in quartz-bearing rocks (summary in Deer et al., 1963). Textural relations and experimental data by others indicate that the scolecite on the Lassiter Coast is of hydrothermal origin. The mineral fibers embay and replace both plagioclase and quartz and, where in contact with biotite, the mica is extensively chloritized. Amphibole, however, is not replaced by the zeolite. Coombs et al. (1959) stated that sodium and calcium zeolites are unstable above 320°C, although they may be synthesized at temperatures below 450°C. Koizumi and Roy (1960) synthesized scolecite from a mixture of CaO Al203 • 3SiO2 in the presence of seed crystals of natural scolecite in the range from 230° to 285°C at 1,100 December 1976
kilograms per square centimeter water pressure; they noted that scolecite breaks down to yield anorthite and wairakite + H 20 at 300°C at the same water pressure. We conclude that the scolecite near Mount Barkow formed from hydrothermal alternation of the diorite by solutions emanating from the granodiorite during its emplacement. Stewart (1964) recorded scolecite in a list of minerals reported from Antarctica, but did not document its locality. This mineral has not previously been reported from the Antarctic Peninsula. Zeo lites associated with plutonic rocks have been found at only one other locality on the Antarctic Peninsula—the southern Bowman Coast, 500 kilometers north of the Mount Barkow area. Fraser and Grimley (1972) tentatively have identified stilbite and chabazite of primary magmatic origin from both tonalite and granodiorite in that region. This study was supported by National Science Foundation grant AG-187. References Coombs, D. S., A. D. Ellis, W. S. Fyfe, and A. M. Taylor. 1959. The zeolite facies with comments on the interpretation of hydrothermal synthesis. Geochimica et Cosmochzmzca Acta, 17: 53-107. Deer, W. A., R. A. Howie, and J. Zussman. 1963. Rock-Forming Minerals, 4, Framework Silicates. New York, John Wiley and Sons. 435p. Fraser, A. G., and P. M. Grimley. 1972. The geology of parts of the Bowman and Wilkins Coasts, Antarctic Peninsula. British Antarctic Survey Science Report, 67. 59p. Koizumi, M., and R. Roy. 1960. Zeolite studies, I. Synthesis and stability of the calcium zeolites. Journal of Geology, 68: 41-53. Rowley, P. D. 1973. Geologic observations on the northern Lassiter Coast and southern Black Coast. Antarctic Journal of the U.S., V1II(4): 154-155. Rowley, P. D., and P. L. Williams. 1974. Plutonic rocks of the Lassiter C)ast. Antarctic Journal of the U.S., IX(5): 225226. Stewart, D. 1964. Antarctic mineralogy. In: Antarctic Geology (Adie, R. J., editor). Amsterdam, North-Holland. 395-401. X-ray powder data from Mount Barkow zeolites. d (A) (obs)
I
hkl
6.645 5.896 4.756 4.412 4.230 3.663 3.230 3.095 2.894
7 9 3 8 1 1 2 1 10
001 111,310 040,400 131 311,420 510 060 600, 441 441
Copper-potassium alpha radiation, nickel-filtered, 35,000 volts, 18 milliamperes, powder camera diameter 114.6 millimeters.
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258
Adie, R. J . 1972. Recent advances in the geology of the Antarctic Peninsula. In: Antarctic Geology and Geophysics (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, Universitetsforlaget. Series (1): 121-124. Bateman, P. C. 1961. Granitic formations in the east-central Sierra Nevada near Bishop, California. Geological Society of America Bulletin, 72: 1521-1538. Halpern, M. 1967. Rubidium-strontium age measurements of plutonic igneous rocks in eastern Ellsworth Land and northern Antarctic Peninsula, Antarctica. Journal of Geophysical Research, 72(20): 5133-5142. Mehnert, H. H., P. D. Rowley, and D. L. Schmidt. 1975. K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica. USGS Journal of Research, 3(2): 233-236. Rowley, P. D. 1973. Geologic observations on the northern Lassiter Coast and southern Black Coast. Antarctic Journal of the U.S., VIII(4): 154-155. Rowley, P. D., and P. L. Williams. 1974. Plutonic rocks of the Lassiter Coast. Antarctic Journal of the U.S., TX(S): 225226. Rowley, P. D., P. L. Williams, D. L. Schmidt, R. L. Reynolds, A. B. Ford, A. H. Clark, E. Farrar, and S. L. McBride. 1975. Copper mineralization along the Lassiter Coast of the Antarctic Peninsula. Economic Geology, 70(5): 982-987. Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula—preliminary report. In: Antarctic Geology and Geophysics (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, Universitetsforlaget. Series B, (1): 143-148.
An unusual occurrence of scolecite from the Antarctic Peninsula WALTER R. VENNUM* and JOANNE L. BENTZ Department of Geology Cahfornia State College, Sonoma Rohnert Park, California 94928 White, fan-shaped aggregates of a fibrous zeolite as much as 5 millimeters long coat fracture surfaces in diorite on the northern slope of a small nunatak at 73°24S. 63°13'W. in the south western Dana Mountains of the northern Lassiter Coast. This location is about 10 kilometers southwest of Mount Barkow. X-ray power data (table) indicate that the mineral is either natrolite or scolecite. Negative elongation, small extinction angle on cleavage fragments, refractive indices above 1.505,
*Also: U.S. Geological Surve y , Menlo Park, California 94025.
ANTARCTIC JOURNAL
birefringence of 0.006, and noticeable dispersion confirm its identification as scolecite. Mode of occurrence severely restricts the amount of material available for thinsection examination; all optical properties, except birefringence, were determined by oil immersion. The diorite is the oldest and outermost unit of the composite Werner batholith—an elongate, concentrically zoned igneous body that extends 140 kilometers from an unnamed mountain range in the southern Black Coast south to the Hutton Mountains of the central Lassiter Coast (Rowley, 1973; Rowley and Williams, 1974). Potassiumargon dating shows that all intrusive phases of the batholith are Upper Cretaceous (Edward Farrar and S. L. McBride, Queens University, Ontario, written communication, 1975). The pluton intrudes black slate, siltstone, and sandstone of the Upper Jurassic Latady Formation. Thin septa and pods of diorite, as much as several hundred meters wide, locally occur along the western margin of this batholith in the southwestern Dana Mountains and along its eastern margin in the central Werner Mountains. At both localities, the diorite is intruded by younger granodiorite that grades into the quartz-monzonite core of the batholith. The intrusive contact between diorite and granodiorite is exposed 25 meters south of the zeolite locality. Detailed petrography of the rocks in the area (Vennum, unpublished data, 1976) confirms that both the diorite and thegranodiorite adjacent to the zeolite locality are hybrid rocks that have assimilated large amounts of the surrounding Latady Formation. Scolecite (CaAl2Si2O 19 . 3H20) is an uncommon member of the zeolite group; its chief mode of occurrence is in cavities of basaltic volcanic rocks. Two notable localities are the Tertiary basalts of Mull and Skye and at Berufjord, Iceland. Scolecite also has been reported in contact-metamorphosed calcareous rocks and as a hydrothermal mineral concentrated along fissures in other types of metamorphic rocks. Only rarely does it occur as a vein or joint filling in quartz-bearing rocks (summary in Deer et al., 1963). Textural relations and experimental data by others indicate that the scolecite on the Lassiter Coast is of hydrothermal origin. The mineral fibers embay and replace both plagioclase and quartz and, where in contact with biotite, the mica is extensively chloritized. Amphibole, however, is not replaced by the zeolite. Coombs et al. (1959) stated that sodium and calcium zeolites are unstable above 320°C, although they may be synthesized at temperatures below 450°C. Koizumi and Roy (1960) synthesized scolecite from a mixture of CaO Al203 • 3SiO2 in the presence of seed crystals of natural scolecite in the range from 230° to 285°C at 1,100 December 1976
kilograms per square centimeter water pressure; they noted that scolecite breaks down to yield anorthite and wairakite + H 20 at 300°C at the same water pressure. We conclude that the scolecite near Mount Barkow formed from hydrothermal alternation of the diorite by solutions emanating from the granodiorite during its emplacement. Stewart (1964) recorded scolecite in a list of minerals reported from Antarctica, but did not document its locality. This mineral has not previously been reported from the Antarctic Peninsula. Zeo lites associated with plutonic rocks have been found at only one other locality on the Antarctic Peninsula—the southern Bowman Coast, 500 kilometers north of the Mount Barkow area. Fraser and Grimley (1972) tentatively have identified stilbite and chabazite of primary magmatic origin from both tonalite and granodiorite in that region. This study was supported by National Science Foundation grant AG-187. References Coombs, D. S., A. D. Ellis, W. S. Fyfe, and A. M. Taylor. 1959. The zeolite facies with comments on the interpretation of hydrothermal synthesis. Geochimica et Cosmochzmzca Acta, 17: 53-107. Deer, W. A., R. A. Howie, and J. Zussman. 1963. Rock-Forming Minerals, 4, Framework Silicates. New York, John Wiley and Sons. 435p. Fraser, A. G., and P. M. Grimley. 1972. The geology of parts of the Bowman and Wilkins Coasts, Antarctic Peninsula. British Antarctic Survey Science Report, 67. 59p. Koizumi, M., and R. Roy. 1960. Zeolite studies, I. Synthesis and stability of the calcium zeolites. Journal of Geology, 68: 41-53. Rowley, P. D. 1973. Geologic observations on the northern Lassiter Coast and southern Black Coast. Antarctic Journal of the U.S., V1II(4): 154-155. Rowley, P. D., and P. L. Williams. 1974. Plutonic rocks of the Lassiter C)ast. Antarctic Journal of the U.S., IX(5): 225226. Stewart, D. 1964. Antarctic mineralogy. In: Antarctic Geology (Adie, R. J., editor). Amsterdam, North-Holland. 395-401. X-ray powder data from Mount Barkow zeolites. d (A) (obs)
I
hkl
6.645 5.896 4.756 4.412 4.230 3.663 3.230 3.095 2.894
7 9 3 8 1 1 2 1 10
001 111,310 040,400 131 311,420 510 060 600, 441 441
Copper-potassium alpha radiation, nickel-filtered, 35,000 volts, 18 milliamperes, powder camera diameter 114.6 millimeters.
259
