We believe that the biotite ages were reset and re- Adie, R. J . 1972. Recent advances in the geology of the Ant- flect reheating due to intrusion of the younger arctic Peninsula. In: Antarctic Geology and Geophysics quartz monzonite-granodiorite phase (compare (Adie, R. J . , editor). International Union of Geological Mehnert et al., 1975). Sciences. Oslo, Universitetsforlaget. Series (1): 121-124. Bateman, P. C. 1961. Granitic formations in the east-central Samples 3 and 4 are typical of the younger quartz Sierra Nevada near Bishop, California. Geological Society monzonite-granodiorite phase of the Werner of America Bulletin, 72: 1521-1538. batholith. The age of this phase thus is considered Halpern, M. 1967. Rubidium-strontium age measurements of to be about 100 million years old (table). Sample 5 plutonic igneous rocks in eastern Ellsworth Land and north- ern Antarctic Peninsula, Antarctica. Journal of Geophysical also is granodiorite, but it occurs at the roof of the Research, 72(20): 5133-5142. batholith adjacent to the older mafic phase (sample Mehnert, H. H., P. D. Rowley, and D. L. Schmidt. 1975. K-Ar 2); detailed petrography and geochemistry demon- ages of plutonic rocks in the Lassiter Coast area, Antarctica. strate that during emplacement it was contami- USGS Journal of Research, 3(2): 233-236. nated by partial assimilation of the Latady Forma- Rowley, P. D. 1973. Geologic observations on the northern Lassi- ter Coast and southern Black Coast. Antarctic Journal of tion and the older mafic phase (W. R. Vennum, the U.S., VIII(4): 154-155. unpublished data, 1976). Its older potassium-argon Rowley, P. D., and P. L. Williams. 1974. Plutonic rocks of the age may reflect partial assimilation of biotite of the Lassiter Coast. Antarctic Journal of the U.S., TX(S): 225- older mafic phase. 226. Rowley, P. D., P. L. Williams, D. L. Schmidt, R. L. Reynolds, The Galan batholith, consisting mostly of grano- A. B. Ford, A. H. Clark, E. Farrar, and S. L. McBride. 1975. diorite, is about 20 kilometers east of the north- Copper mineralization along the Lassiter Coast of the Ant- ern Werner pluton. The Galan batholith intrudes arctic Peninsula. Economic Geology, 70(5): 982-987. the northern edge of the Grimminger stock, which Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. consists of diorite and quartz diorite. The Grim- 1972. Geology of the Lassiter Coast area, Antarctic Penin- sula—preliminary report. In: Antarctic Geology and Geo- minger stock was sampled because field evidence physics (Adie, R. J . , editor). International Union of Geological suggested that it might be the oldest intrusive rock Sciences. Oslo, Universitetsforlaget. Series B, (1): 143-148. in the Lassiter Coast and the southern Black Coast. Its biotite age (sample 7) must be considered a mini- mum 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 kilo- meters south of the Werner batholith. The biotite age of the Rath stock (sample 8) may have been re- set, inasmuch as a small batholith, probably An unusual occurrence of younger, occurs several kilometers north of the scolecite from the Antarctic Rath pluton. Peninsula The new ages are similar to previously deter- mined 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 WALTER R. VENNUM and JOANNE L. BENTZ et al., 1975) and overlap the 109- to 102-million- Department of Geology year dates on plutons (Halpern, 1967) in eastern Cahfornia State College, Sonoma Ellsworth Land, which is less than 200 kilometers Rohnert Park, California 94928 west-southwest of the Lassiter Coast. Thus plutons in the southern Antarctic Peninsula have a rela- tively restricted time range in the late Cretaceous, White, fan-shaped aggregates of a fibrous zeolite in contrast to plutons in other parts of the Penin- as much as 5 millimeters long coat fracture sur- sula (Adie, 1972). faces in diorite on the northern slope of a small nunatak at 73°24S. 63°13 W. in the south- This study was supported by National Science western Dana Mountains of the northern Lassiter Foundation grant AG-187 and by a grant from the Coast. This location is about 10 kilometers south- National Research Council of Canada. west of Mount Barkow. X-ray power data (table) indicate that the mineral is either natrolite or scole- cite. Negative elongation, small extinction angle on References cleavage fragments, refractive indices above 1.505,
Adie, R. J . 1955. The petrology of Graham Land. Part II, the Andean granite-gabbro intrusive suite. Falk/and Islands De- pendencies Surve). Scient!/lc Report, 12. 39p. Also: U.S. Geological Surve y , Menlo Park, California 94025.
258 ANTARCTIC JOURNAL
birefringence of 0.006, and noticeable dispersion kilograms per square centimeter water pressure; confirm its identification as scolecite. Mode of oc- they noted that scolecite breaks down to yield anor- currence severely restricts the amount of material thite and wairakite + H 20 at 300°C at the same water available for thinsection examination; all optical pressure. We conclude that the scolecite near properties, except birefringence, were determined Mount Barkow formed from hydrothermal alter- by oil immersion. nation of the diorite by solutions emanating from The diorite is the oldest and outermost unit of the granodiorite during its emplacement. the composite Werner batholith—an elongate, con- Stewart (1964) recorded scolecite in a list of min- centrically zoned igneous body that extends 140 erals reported from Antarctica, but did not docu- kilometers from an unnamed mountain range in ment its locality. This mineral has not previously the southern Black Coast south to the Hutton been reported from the Antarctic Peninsula. Zeo Mountains of the central Lassiter Coast (Rowley, lites associated with plutonic rocks have been found 1973; Rowley and Williams, 1974). Potassium- at only one other locality on the Antarctic Penin- argon dating shows that all intrusive phases of the sula—the southern Bowman Coast, 500 kilometers batholith are Upper Cretaceous (Edward Farrar north of the Mount Barkow area. Fraser and Grim- and S. L. McBride, Queens University, Ontario, ley (1972) tentatively have identified stilbite and written communication, 1975). The pluton in- chabazite of primary magmatic origin from both trudes black slate, siltstone, and sandstone of the tonalite and granodiorite in that region. Upper Jurassic Latady Formation. Thin septa and This study was supported by National Science pods of diorite, as much as several hundred meters Foundation grant AG-187. wide, locally occur along the western margin of this batholith in the southwestern Dana Mountains and along its eastern margin in the central Werner References Mountains. At both localities, the diorite is intruded Coombs, D. S., A. D. Ellis, W. S. Fyfe, and A. M. Taylor. 1959. by younger granodiorite that grades into the The zeolite facies with comments on the interpretation of quartz-monzonite core of the batholith. The intru- hydrothermal synthesis. Geochimica et Cosmochzmzca Acta, 17: sive contact between diorite and granodiorite is 53-107. Deer, W. A., R. A. Howie, and J. Zussman. 1963. Rock-Forming exposed 25 meters south of the zeolite locality. De- Minerals, 4, Framework Silicates. New York, John Wiley tailed petrography of the rocks in the area (Ven- and Sons. 435p. num, unpublished data, 1976) confirms that both Fraser, A. G., and P. M. Grimley. 1972. The geology of parts the diorite and thegranodiorite adjacent to the of the Bowman and Wilkins Coasts, Antarctic Peninsula. zeolite locality are hybrid rocks that have assimi- British Antarctic Survey Science Report, 67. 59p. Koizumi, M., and R. Roy. 1960. Zeolite studies, I. Synthesis lated large amounts of the surrounding Latady and stability of the calcium zeolites. Journal of Geology, Formation. 68: 41-53. Scolecite (CaAl2Si2O19 . 3H20) is an uncommon Rowley, P. D. 1973. Geologic observations on the northern member of the zeolite group; its chief mode of oc- Lassiter Coast and southern Black Coast. Antarctic Journal of the U.S., V1II(4): 154-155. currence is in cavities of basaltic volcanic rocks. Two Rowley, P. D., and P. L. Williams. 1974. Plutonic rocks of the notable localities are the Tertiary basalts of Mull Lassiter C)ast. Antarctic Journal of the U.S., IX(5): 225- and Skye and at Berufjord, Iceland. Scolecite also 226. has been reported in contact-metamorphosed cal- Stewart, D. 1964. Antarctic mineralogy. In: Antarctic Geology careous rocks and as a hydrothermal mineral con- (Adie, R. J., editor). Amsterdam, North-Holland. 395-401. centrated along fissures in other types of meta- morphic rocks. Only rarely does it occur as a vein X-ray powder data from Mount Barkow zeolites. or joint filling in quartz-bearing rocks (summary in I hkl Deer et al., 1963). d (A) (obs) Textural relations and experimental data by 6.645 7 001 others indicate that the scolecite on the Lassiter 5.896 9 111,310 Coast is of hydrothermal origin. The mineral fibers 4.756 3 040,400 embay and replace both plagioclase and quartz and, 4.412 8 131 where in contact with biotite, the mica is extensively 4.230 1 311,420 chloritized. Amphibole, however, is not replaced by 3.663 1 510 the zeolite. Coombs et al. (1959) stated that 3.230 2 060 sodium and calcium zeolites are unstable above 3.095 1 600, 441 320°C, although they may be synthesized at tem- 2.894 10 441 peratures below 450°C. Koizumi and Roy (1960) synthesized scolecite from a mixture of CaO Al203 Copper-potassium alpha radiation, nickel-filtered, 35,000 • 3SiO2 in the presence of seed crystals of natural volts, 18 milliamperes, powder camera diameter 114.6 milli- scolecite in the range from 230° to 285°C at 1,100 meters. 259 December 1976