PORT LOCKROY 69 



PORT LOCKROY, WIENCKE ISLAND 



Port Lockroy is a small harbour on the west coast of Wiencke Island, opening out on to the 

 Neumayer Channel which separates the large Anvers Island from Wiencke Island. Rocks from 

 Wiencke Island and Doumer Island, as well as from the islands in the Neumayer Channel, and on the 

 south and west of Wiencke Island, have been collected by several expeditions. Thus Pelikan^ 

 described quartz-diorite and gabbro, the former cut by diorite-porphyry and diabase dikes. Gourdon^ 

 described quartz-mica-pyroxene-diorite, quartz-diorite, and micro-diorite, with numerous ' labradorite ' 

 (hornblende-andesite) dikes penetrating the quartz-diorite massif. Ferguson wrote: 'Wiencke Island 

 is bounded on the side facing Neumayer Channel by almost vertical walls of sedimentary rocks in- 

 cluding bluish black mudstone ; it is, however, largely formed of gray diorite, which is the only rock 

 present in Doumer Island and the Cairn Islands.'^ From Ferguson's collection the writer described 

 tonalite, igneous breccias, and a siliceous mudstone.^ 



The most recent work on the petrography of this part of the Palmer Archipelago is that of T. Barth 

 and P. Holmsen.* They described eucrite and anorthosite (with chemical analyses) from an islet 

 near Victor Hugo Island (west of Wiencke Island). The Joubin Islands, also west of Wiencke Island, 

 consist mainly of igneous breccias, and an analysis is given of a prehnitized rock fragment from these 

 breccias. From Port Lockroy, Barth and Holmsen described quartz-diorite and adamellite, with 

 analyses. They remark that the whole region from Port Lockroy westward to the Joubin Islands and 

 Victor Hugo Island is penetrated by ' diabase ' dikes. The general picture of the geology of this region 

 is then that of an ancient basement consisting of sediments and igneous breccias, cut by plutonic 

 intrusions of tonalite and adamellite, the whole being penetrated by numerous dikes, especially 

 ' diabase '. 



Dr Mackintosh collected two rock specimens from an island in Port Lockroy harbour. Both consist 

 of tonalite identical with that described by me from Ferguson's collection, but the larger specimen 

 shows a sharp contact of tonalite with a dike of fine-grained grey micro-porphyritic rock which is a 

 porphyritic micro-tonalite. In thin section the tonalite shows biotite, hornblende, and magnetite as 

 mafic constituents, with very abundant euhedral plagioclase (andesine, Anjo), all of which are embedded 

 in a coarse ground-mass consisting of interlocking crystals of quartz with subordinate orthoclase. 

 Biotite and hornblende are present in roughly equal amounts. The hornblende is variegated in 

 shades of green, the larger crystals breaking up into aggregates of smaller, diflterently coloured 



grains. 



The dike rock shows numerous phenocrysts of andesine with heavy mechanical zoning, and 

 somewhat fewer phenocrysts of a fibrous, pale green hornblende, enclosed in a very fine-grained 

 equigranular ground-mass consisting of quartz, orthoclase, andesine, hornblende, biotite passing 

 into chlorite, and cubes of magnetite. It is a quartz-diorite porphyry or tonalite-porphyry ; or, if 

 it be desirable not to use the ambiguous term ' porphyry ', it may be designated as porphyritic micro- 

 tonalite. 



1 A. Pelikan, ' Petrographische Untersuchungen der Gesteinsproben ', Resultats du Voyage de S.Y. 'Belgica', Exped. 

 Antarctique Beige; Geologie, Anvers, 1909. 



" E. Gourdon, 'Geographic physique, Glaciologie, Petrographie', Exped. Antarctique Frattfaise, 1903-5, Paris, 1908. 



3 D. Ferguson, 'Geological Observations in the South Shetlands, the Palmer Archipelago, and Graham Land, Antarctica', 

 Trans. Roy. Sac. Edin. Liii, 1921, p. 49. 



« G. W. Tyrrell, 'A Contribution to the Petrography of the South Shetland Islands, the Palmer Archipelago, and the 

 Danco Land Coast, Graham Land, Antarctica', ibid. pp. 59, 73, 74. 



^ ' Rocks from the Antarctandes and the Southern Antilles ', Scientific Results of the Nonvegian Antarctic Expeditions 1927-28 

 and 1928-29, instituted and financed by Consul Lars Christensen, No. i8, Norske Vidensk.-Akad., Oslo, 1939, pp. 17-33- 



