IAN W. D. DALZIEL Lamont-Doherty Geological Observatory, Columbia University, Palisades, New Yor{ 10964 K-Ar Dating of Rocks from Elephant Island, South Scotia Ridge ABSTRACT ? late Paleozoic sedimentary rocks; and (5) ? Paleozoic or older metamorphic rocks. K-Ar whole-rock ages ranging from 78 to 88 Elephant, Clarence, Gibbs, Cornwallis, Asp- m.y. have been determined from four samples land and O'Brien Islands at the northeastern of quartz-calcite-amphibole schist obtained end of the South Shetland group consist of from Elephant Island in the South Shetland highly deformed metamorphic rocks. The only Island group on the southern limb of the other rock types reported from these islands Scotia Arc. The lithology and structural history are sheets of dunite and serpentinite on Gibbs of the rocks forming Elephant Island indicate Island (Tyrrell, 1945; Dalziel and others, 1970) that they are part of the pre-Late Jurassic, and and white biotite granite on Cornwallis Island probably Paleozoic, metamorphic rocks of the (unpub. prelim, rept. of U.K. Joint Services region. The ages are likely to reflect a regional Expedition to Elephant Island, 1971). The thermal event in the Antarctandes during the presence of Acritarcha in schist from Clarence late Mesozoic-early Cenozoic Andean orogeny. Island, collected by G. E. Grikurov of the Soviet Research Institute of the Geology of the INTRODUCTION Arctic, has been reported by Htchenko (1972), Elephant Island (61°10' S., 55°14' W.) is the who tentatively correlates these rocks with most northerly island of the South Shetland middle-upper Proterozoic (Pre-Vandian) and group at the western end of the South Scotia Vandian sections in the Soviet Union. Ridge (Figs. 1 and 2). The South Shetland The rocks for which K-Ar ages are reported Islands are part of the Antarctandean Cordillera here were collected during ten days of field (Fig. 1). Elephant Island, 40 km long and a work on southwestern Elephant Island in maximum of 25 km wide, is mountainous and January 1970. The work was part of a con- highly glaciated (Fig. 3). Originally called Sea tinuing program to study the tectonic history Elephant Island, it is best known as the refuge of the Scotia Arc region. of Sir Ernest Shackleton's Imperial Trans- Antarctic Expedition after the ship Endurance PETROLOGY, METAMORPHISM, AND was beset by ice and sank in the Weddell Sea STRUCTURAL HISTORY in 1915. The first geologic studies on the island Wordie (1921), Ferguson (1921), Tilley were undertaken by James Wordie, the (1935) and Tyrrell (1945) point out that geologist of Shackleton's expedition (Wordie, Elephant Island (Fig. 3a) consists of relatively 1921). high-grade garnet-hornblende-albite-mica schist The islands of the North and South Scotia in the south around Cape Lookout, and Ridges (Figs. 1 and 2) are formed almost en- phyllite (quartz-albite-chlorite-sericite) on the tirely of continental rocks that can be cor- north coast at Cape Lindsey and Point Wild. related with those on the Antarctic Peninsula The rocks exposed along the part of the south- and southernmost South America. They are: western coast studied in detail by the author (1) Cenozoic sedimentary and volcanic rocks; and his associates (Fig. 3) are mainly light- (2) late Mesozoic-Cenozoic "Andean" batho- colored porphyroblastic albite schist with nar- lithic intrusive rocks; (3) late Jurassic-Cre- row bands of quartz-calcite-amphibole schist taceous sedimentary and volcanic rocks; (4) and quartz-garnet rocks. Geological Society of America Bulletin, v. 83, p. 1887-1894, 3 figs., June 1972 1887 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/83/6/1887/3428766/i0016-7606-83-6-1887.pdf by guest on 28 September 2021 1888 I.W.D. DALZIEL TECTONIC PROVINCES j | CORDILLERA EXTRA-CORDILLERAN BASINS PACIFIC "HINTERLAND" OF CORDILLERA 1000m SUBMARINE CONTOUR FOR SOUTH AMERICA, BLACK AND SHAG ROCKS.AND SOUTH GEORGIA 1000m SUBMARINE CONTOUR FOR ANTARCTIC PENINSULA AND SOUTH ORKNEY ISLANDS 40"W / 30«W DRAKE PASSAGE ELEPHANT ISLAND South , Orkney Islands South Sandwich Islands f'n Figure 1. Tectonic subdivisions of the southern- the Antarctandes (from Dalziel and Elliot, 1972). most Andes, the North and South Scotia Ridges, and The analyzed rocks (Table 1) are quartz- others, 1970) developed during the second of calcite-amphibole schist. They occur as sharply three recognizable deformation phases. defined pale to dark gray-green resistant ribs up to 20 cm thick interlayered with por- DISCUSSION OF K-AR AGES phyroblastic albite schist that crops out on the K-Ar analyses and calculated ages of the beach at the northern end of Animal Bay (Fig. specimens analyzed are shown in Table 1. The 3b). The specimens were all collected from the apparent ages of the four samples and geologic same large outcrop, but not from the same considerations indicate that they probably re- layer. Modal analysis of one of the analyzed flect a post-tectonic thermal event. specimens (E62-5, see Table 1) gave: 53 per- The rocks on Elephant Island are not in cent quartz, 28 percent calcite, 11 percent visible contact with any of the other rocks of amphibole, 4 percent mica, and 1 percent the Scotia Arc region. Apart from the Acri- chlorite and garnet (Dalziel and Price, in tarcha recorded by Iltchenko (1972) from prep.). The structure of the schists on the phyllite on Clarence Island and considered by southwest coast of the island is dominated by her to be Precambrian, the only geologic evi- folds with planar limbs and narrow hinge dence for the age of the metamorphic rocks in zones (Araya and Herve, 1966; Dalziel and the Elephant Island subgroup is based on com- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/83/6/1887/3428766/i0016-7606-83-6-1887.pdf by guest on 28 September 2021 K-AR DATING OF ROCKS FROM ELEPHANT ISLAND, SOUTH SCOTIA RIDGE 1889 Late Paleozoic sedimentary rocks Metamorphic rocks 50"W / Figure 2. Geologic map of the Antarctic Peninsula phant Island; CL-Clarence Island; CO-Coronation and the islands on the South Scotia Ridge (modified Island; G-Gibbs Island; H-Hope Bay; L-Livingston after Adie, 1964 and 1969, on the basis of work by the Island; LA-Laurie Island. Box indicates area of Institute Antartico Chileno and by Dalziel). E-Ele- Figure 3. parison with rocks elsewhere in the region Madre de Dios Basin in the islands off the west (Table 2). coast of southern Chile at 50° S. (Fig. 1; The metamorphic rocks of Elephant and Dalziel, 1972), and in comparison with the Clarence Islands are lithically similar to those Trinity Peninsula Series of the Antarctic of the South Orkney Islands (Wordie, 1921; Peninsula (Adie, 1964; Dalziel, 1972) from Ferguson, 1921; Tilley, 1935; Tyrrell, 1945). which Carboniferous spores have been re- Recent work in the South Orkney Islands ported by Grikurov and Dibner (1968). (Dalziel, 1971) has shown that they are also The best evidence for the age of the Elephant similar in structural history and style. On this Island metamorphic rocks comes from their basis, and on the basis of geophysical data metamorphic and structural history. At the tip (Harrington and others, 1972; Walters, 1972), of the Antarctic Peninsula and on the South the South Orkney Islands are placed close to Orkney Islands, Jurassic and Cretaceous sedi- the Elephant Island subgroup in recent recon- mentary and volcanic rocks are gently tilted, structions of the South Scotia Ridge (Barker but are unmetamorphosed and show no sign of and Griffiths, 1972; Dalziel and Elliot, 1971, penetrative deformation. In southernmost 1972). South America, the Jurassic rocks rest uncon- Fragments of schist and garnet reported in formably on metamorphic rocks that are dated the graywacke and shale of the South Orkney as Paleozoic or older in Central Chile (Gon- Islands (Tilley, 1935) suggest, but do not zalez-Bonarino, 1971) and in cores from the prove, that the schist there is older than the extra-Cordilleran Magallanes Basin (Halpern, sedimentary sequence. The sedimentary rocks 1972; Halpern and Carlin, 1971). are of uncertain age, although they are prob- Thus, although no direct evidence exists of ably late Paleozoic based on comparison of the age of the Elephant Island metamorphic lithology and structural features with Upper rocks, they were almost certainly metamor- Pennsylvanian-Lower Permian rocks in the phosed before Middle- to Late-Jurassic time, Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/83/6/1887/3428766/i0016-7606-83-6-1887.pdf by guest on 28 September 2021 1890 I.W.D. DALZIEL TABLE 1. ANALYZED SAMPLES* Isotopic . sec Ar»d/gm K Kad^Total Sample No. Age (m.y.)T Wt% 79.3 5.3598 x 10"' 0.317 0.16s, E 62-1 86.2 5.8398 x 10"' 0.258 0.16e Whole Rock 82.7 ± 3.5 0.16, 77.6 4.018 x 10"' o.n. 0.130, E 62-4 77.5 4.013 x 10"' O.lSj 0.12s Whole Rock 77.6 ± 1.6 0.12. s 88.9 1.3761 x 10" 0.46S 0.37,, E 62-5 87.7 1.3563 x 10"' 0.47, 0.38j Whole Rock 88.3 ± 1.6 0.38, 79.8 5.997 x 10"' 0.242 0.18], E 62-8 82.9 6.238 x 10"' 0.39s 0.189 Whole Rock 81.4 ± 1.6 0.185 *flnalyzed by Teledyne Isotopes, hfestwood Laboratories, New Jersey. trfle constants used for the age calculation are: W - 4.72 x 10 Xe = 0.585 x and K*° = 1.19 x atom percent of natural potassium. and probably in the Paleozoic. The K-Ar dates southernmost Andes, in South Georgia, and in reported here probably reflect a regional the Antarctandes.