SHORT PAPERS AND NOTES 231

THE GEOLOGY OF BASE GABRIEL port the northern island is named Isla GONZALEZVIDELA, ANTARCTICA “Lomnitz” and the southern island, Isla “Dott”.Between the islandsand the Introduction mainland is a 90-metre channel, which is dry at low tide (Fig. 2). This channel TheChilean scientific base, Gabriel and the eastern shores of the islands are Gonzalez Videla, is located onthe Danco covered by till. Coast at the north end of the “Antarctic” Isla “Lomnitz” is composed of strat- Peninsula(Fig. 1). IslaUtil, a small ified andesite sills or flows intruded by island in the Gerlache Strait, lies 7 a quartz dioritepluton. Thin aplite miles north of the base.Field work dykesand veins cut the andesite and wasdone during the 1960-61 austral quartz diorite everywhere onthe island, summer,and 70 thinsections of the but are most abundant along the north- rockscollected were studied subse- eastern shore. Along the central part of quently at the University of Wisconsin. the western shoreline, a 6-metre section The chief features of the geology of of metamorphosed, interlaminated silt- the area are shown in Fig. 6. The base is stone and shalelies between andesite situated on two small islands (approx. sills or flows.

Fig. 1. Location map of base Gabriel Gonzalez TN 64’42% IT Videla and Isla Util.

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50’

0 ! 2 3 4Km.,\

0 1 2 3 4 Miles

63’00‘W , 56’ I 52‘ I 48’ , 62’44’W

18,000 square metres), which are sep- The andesite sheets extend southward arated by a very narrow channel partly andoccupy the northern half of Isla filled with morainal material.In this re- “Dott”. Approximately 5 metres below 232 SHORTAND PAPERS NOTES the uppermost sill or flow is a bed of suritized feldspar. Palegreen chlorite metaconglomerate, 4 metres thick. An and amphibole total 7 per cent of the intrusive body of porphyritic andesite rock, and lesser accessory minerals in- occupies the southern half of Isla “Dott” clude epidote, apatite, zircon, magnetite, where it is intruded by the andesite sill and ilmenite. Thematrix has a pro- rock. nounced flow structure and is composed

Fig. 2. Aerial view of base Gabriel Gonzales and neighboring Danco Coast.

The Igneous Complex mainly of microlites and altered micro- crystalline feldspar. 1. Porphyritic Andesite The porphyritic andesite occupies the 2. Andesite southern half of Isla “Dott” whereasthe The multiple andesite sills or flows southern tip of the island is cut by ir- of Islas “Lomnitz” and “Dott” are about regulardikes of fine-grained andesite 200 metres thick. Thickness of individu- less than 0.5 metre thick. al sheets is extremely difficult to deter- In hand specimen, about 45 per cent mine because of intense fracturing and of the rock is composed of light grey poor exposures due to snow and guano plagioclase feldspar phenocrysts as long cover. Where measurable sheets are 4 as 2 mm. in a fine-grained, dark grey to 7 metres thick and dip steeply south. matrix. The fine-grained andesite has a Conclusive evidence in favour of the chilled zone 2 to 3 mm. wide against andesite being a series of multiple sills porphyritic andesite, andthe fine-grain- rather than a series of flows is not pres- ed andesite penetrates micro-fractures ent. However, intrusion of the he- in the porphyry (Fig.3). grained andesite into the porphyritic The phenocrysts are of calcic andesine andesite andthe absence of rounded (as determined by the Michel-Lévy inclusions of the underlying rock in the method) and exhibit albite and Carls- overlying andesite rock suggests that bad-and-albite twinning. Rare crystals they may be multiple sills. are zoned. The fine-grained ground The non-porphyritic andesite is a mass includes 2 percent of quartz dark grey, fine-grained rock. Calcic myrmekitically intergrownwith saus- andesine comprises at least 75 per cent SHORT PAPERS AND NOTES 233 of the rock, and exhibits albite and to Isla “Lomnitz” and crops out as a large a less degree Carlsbad-and-albite twin- intrusive body on the mainland east of ning. Zoning is rare. The alteration of the islands. The quartz diorite is coarse the plagioclase ranges from negligible and even-grained, and is mottled green to complete saussuritization. About 5 and light to medium grey. It is separable per cent quartz is present, and some is into three facies on the basis of colour, intergrown myrmekitically with sodic composition, and grain size.

Fig. 3. Contactbetween fine-grained andesite sill or flowrock intrudingporphyritic andesite.PA-porphyritic andesite; A-andesite sill orflow rock. X 8, diffusedlight. oligoclase. Greento brownish-green Facies 1. Thequartz diorite of the chlorite comprises 10 per cent of the Danco Coast is medium grey witha rock. Accessory minerals are zircon and greenish tinge and has ahigh mafic con- apatite; sphene and biotite are rare. The tent (15 per cent). Cores of many of the biotite is present as minute flakes, which plagioclose crystalsin this facies are are commonly altered to palegreen saussuritized. Isla Util, which lies to chlorite and clinozoisite. Opaque mine- the north of the base, is composed of rals (2 per cent) are ilmenite and mag- quartzdiorite identical to that of the netite. Danco Coast facies. Three xenoliths of The lowermost 30 metres of andesite, porphyritic andesite were found on the near the northern shore of Isla “Lom- south-central coast of the island in the nitz”, is brecciated and consists of frag- quartz diorite. This andesite is quite ments 0.2 mm. to 15 mm. in diameter in different from that of Gonzalez Videla, a finely crystallinematrix. The cause but may be related to the same period of of brecciation was not determined. igneous activity. Facies 2. Along the east-central coast 3. Quartz Diorite of Isla “Lomnitz”, where thequartz Quartzdiorite intrudes the andesite diorite is near the andesites, it is pink sills or flows along the eastern coast of and green. A decrease in the amount of 234 NOTES AND PAPERS SHORT mafics to 8 per cent and a small increase zircon, dark brownish-greenhorn- in alkali feldspar accounts for this col- blende, which is slightly pleochroic, our change. Plagioclase is much less and rare flakes of dark brown biotite. altered than that of Facies 1. The opaque minerals (2 per cent) are magnetite andilmenite commonly alter- Facies 3. Immediately south of the ed to leucoxene. scientific office building, thequartz All facies contain xenoliths (0.1 to 1 diorite changes to a fine-grained facies. metre in diameter) of the finely crys- Orthoclase and plagioclase each com- talline andesite rock and are intruded prise 30 per cent of the rock, with quartz by veinlets of aplite. also comprising about 30 per cent of the rock. Because of the increase in alkali A minor fault cuts the quartz diorite feldspar, this small area(approx. 45 at the northern end of the Danco Coast metres x 20 metres) can be considered crop. Slickensides are horizontal. a microadamellite. Thequartz diorite of base Gabriel

Fig. 4. Quartz dioriteof base Gabriel Gonzales Videla (facies 1). Note altered plagioclase feldspar and characteristic myrmekitictexture. X 8, polarized light.

On microscopic examination, all facies Gonzalez Videla is possibly correlative generally display myrmekitic texture as with the post-Jurassic “Andean intru- an intergrowth of andesine (50 per sives” of the Trinity Peninsula. cent), orthoclase (15 percent) and quartz (15 percent) (Fig. 4). Chlorite 4. Aplite Dykes and amphibole, up to 20 per cent, are Throughout Isla “Lomnitz” and along present as light green, tabular crystals. the eastern shoreline of Islas “Lomnitz” The accessory minerals include: apatite,and “Dott” aplite dykes ranging in SH OR T PAPERS AND NOTES AND PAPERS SHORT 235 width from a few mm. to 0.5 metre in- sheets is very sharp and irregular.Load trudethe andesite sheets, porphyritic casts, microfaulting, and delicate cross andesite, and quartz diorite. The dykes laminae are preserved. The rare cross are offset slightly along minor normal lamination demonstratesa paleaocur- and reverse faults. The apliteis pink and rent direction from the east-northeast. contains 55 per cent of microcline feld- Quartzgrains are recrystallized and spar and 40 per cent of quartz. Zircon form a mosaic around chlorite and ser- is the main accessory mineral. Apatite, icite flakes. rutile and magnetite are rare. 2. Argillaceous Metaconglomerate Contact Metamorphic Rocks On Isla “Dott”, approximately 5 metres below the uppermost andesite 1. Argillite sheet, a 4-metre bed of metaconglomer- On the central western shoreline of ate crops out. The metaconglomerate Isla “Lomnitz” a 6-metre section of is composed of roundedgranules and metamorphosed, finely interlaminated pebbles of quartz and subordinate sand- silstone and shale crops out. The beds stone and quartz diorite in a metamor- are near vertical and tops, as indicated phosed siltymatrix. The matrix is by fine-scale cross-laminae, are to the carbonaceous and hasflowed around the south. Laminae are 1 mm. to 1.5cm. rock fragments. In places, where the thick andhave very sharp contacts. ratio of matrix to clasts is very high, Micrograded bedding is present in the the rock is a phyllite. darker laminae (Fig. 5). The contact Accessory minerals include garnet between the metasediments andthe (?) occurring as idioblasts in the ma- overlying andunderlying andesite trix; magnetite occurring in the matrix

Fig. 5. Interlaminatedsiltstone and shaleshowing micrograded bedding in the dark laminae. X 8, diffused light. 236 SHORT PAPERS AND NOTES and as inclusions in the quartz; minor Peninsula region to providethe pebbles amounts of dark brownbiotite; and found in the metaconglomerate of base zircon. Gabriel Gonzalez Videla. It is possible that Adie’s “basement complex” served Geologic Evolution as the source for these pebbles. Preceding the folding of the sediments Adie (ref. 1, p. 1) states that: “Rocks and intrusion of the quartz diorite, the of GrahamLand are allcrystalline- andesitesheets were emplaced. Adie2 differentiationproducts of acommon points out that the Jurassic period was parentalmagma and form anormal one of extensive extrusion of andesite calc-alkaline series”. Examinationof the flows in the Antarctic Peninsula. It is composition of the feldspar of the Gon- possible that during this period the por- zalez Videla dominant rock types and phyrite andesite and fine grained ande- their intrusive relations tends to sup- sitesheets were emplaced. Following port Adie’s thesis. A continual decrease the intrusion of the quartz diorite body, in Ca content with a corresponding in- aplite dykes and veinlets marking the crease in alkaline content, through time, most acid phase of the possible common tends to suggest acommon parent mag- parentalmagma, intruded the igneous ma (Table 1). and metamorphic complex.

Table 1. Composition of feldspar suggesting magmatic differentiation.

Rock type Relative Feldspar age - A plite Microline youngest Microline Aplite Quartz diorite Ab70An30 to AbSSAn45 Andesite Ab60An40 to AbSOAn50 Porphyritic andesite Ab60An40 oldestto Ab50An50

From the observed field relationships, Though generally referred to a Cret- the geologic history was as follows: aceous-early Tertiary age, the dating Youngest 3) Intrusion of the quartz di- of Antarctic Peninsula diorites remains oritepluton and aplite uncertain. A sample from Gabriel Gon- dykescoupled with tec- zalez Videla is now being analysed for tonicdisturbances in the absolute age. Antarctic Peninsula. 2) Intrusion and/or extru- Acknowledgements sion of the andesitesills and/or flows. The government of Chile and the Na- Oldest 1) Deposition of the arenace- tional Science Foundationof the United ous and conglomeratic States supported the co-operative scien- sediments. tific program. Dr. R. H. Dott, Jr. of the Adie2 believesthat old, possibly Pre- University of Wisconsin critically read cambrian or earlyPaleaozoic, granitic this paper. David A. Link acted as field crystallinerocks crop out along the assistantand aided in the drafting of west-central coastof the Antarctic Pen- the maps. insula (in the vicinity of Adelaide Is- MARTINHALPERN” landand northeastern Alexander I Land). Clearly,older (thanthe “An- *Department of Geology,University of dean”intrusives) granitic rocks must Wisconsin, Geophysical and Polar Research havebeen exposed inthe Antarctic Center,Madison, Wis., U.S.A. SHORT PAPERS ANDNOTES 237

’ I) , 2.0 , 40 , 60 , SO . 190 METRES

rlIce, snow, guano cover Quartz Diorite-facies 1

(:an:* Andesite sheets

Aplite dykes LKl Porphyritic Andesite Quartz Diorite-facies 3 .- lm]Argillaceous Metaconglomerate sT: 1x1Quartz Diorite-facies 2 w1 Argillite ”-/Fault 780 Strike andDip of Metosediments

Fig. 6. Geological map of base Gabriel Gonzales Videla, Antarctica.

‘Adie, R. J. 1955. The petrology of Graham ZAdie, R. J. 1954. The petrology of Graham Land. 11. The Andean granite-gabbro in- Land. I. The basement complex; early trusive suite. Falkland Islands Depend- Palaeozoic plutonic, and volcanic rocks. encies Surv. Sci. Rept. No. 12, 39 pp. Falkland Islands Dependencies Surv. Sci. Rept., No. 11, 22pp.