- Email: [email protected]
K-Ar geochronology of the South Shetland Islands, Lesser Antarctica: apparent lateral migration of Jurassic to Quaternary island arc volcanism
214
Earth and Planetary Science Letters, 66 (1983) 214-222 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
[21
K-Ar geochronology of the South Shetland Islands, Lesser Antarctica: apparent lateral migration of Jurassic to Quaternary island arc volcanism R.J. Pankhurst 1 and J.L. Smellie 2 I British Antarctic Survey, Natural Environment Research Council, c / o Institute of Geological Sciences, 64- 78 Gray's Inn Road, London W C 1 X 8NG (U.K.) 2 Institute of Geological Sciences, Natural Environment Research Council, Murchison House, West Mains Road, Edinburgh EH9 3LA (U.K.)
Received June 5, 1983 Revised version accepted August 24, 1983
Approximately 70 K-Ar whole-rock ages for low-K tholeiitic and andesitic volcanic and intrusive rocks from the South Shetland Islands, Antarctica, including about 50 not previously published, are reviewed. Activity mainly spanned the range 130 to 30 Ma (Jurassic/Cretaceous to Oligocene/Miocene) with a very recent ( - 2 Ma), more alkaline, renewal. Throughout the main period of activity magmatism (or, perhaps, its cessation) migrated continuously northeastwards along the length of the island chain.
T h e South Shetland Islands, a northeasterly trending archipelago some 450 km in length, are situated about 100 km north of the Antarctic Peninsula (Fig. 1). The majority of the islands are formed of volcanic and hypabyssal rocks, with interbedded sediments, erupted subaerially in part on a continental basement of which only a few metasedimentary representatives are exposed [1-3]. Stratigraphical, petrological and geochemical studies suggest that the volcanism was related to the Mesozoic/Cenozoic igneous evolution of the Antarctic Peninsula, controlled by subduction of the Pacific Ocean floor beneath Lesser Antarctica [4-6]. Compositions are fairly restricted (mainly basalt andesite) and show low contents of lithophile elements in comparison with the calc-alkaline rocks of the peninsula, in keeping with closer proximity to the South Shetland Islands trench. Subduction virtually ceased about 4 Ma ago [7], since when the South Shetland Islands have separated from the mainland due to spreading in Bransfield Strait, a very young marginal 0012-821X/83/$03.00
© 1983 Elsevier Science Publishers B.V.
basin [6,8]. Here we report the results of the first comprehensive radiometric chronology of South Shetland Islands' volcanic rocks, which shows that the recorded age of magmatism migrated progressively northeastwards along the length of the archipelago from ca. 130 Ma to 30 Ma (with a slight renewal of magmatism in Pliocene-to-Recent times) although the mechanism responsible for this migration has not been identified. During two summer field seasons between 1974 and 1976 scientists of the British Antarctic Survey and the University of Birmingham carried out a comprehensive geological survey of the South Shetland Islands (excluding the non-volcanic islands of the Elephant and Clarence islands groups), followed up by a variety of laboratory studies. These represent a significant advance over previous work based on data from limited geographical areas or thinly spread sampling. Some of the geochemical data resulting from the new programme have already been published [4,5,9] and a review of the major conclusions of stratigraphical, palaeon-
215 _NAZ ~ )
_r"
~
~o's-
Or.*e~ 3 ~ , . . - - ~ - - ~ - ~ J L / (Aluk) ~ S h e t l o n d ANT ~J~C-~-SOUTH SHETLAND /J¢ ISLA ND5
,8o'w#/',
,,~o'w ,
'
.o's-
'
8 ~ , , o ,,)~ ~o ,~,2 ? II~
.... * "'
Livingston
~ j p _ v ~ ~.~
~
,o"
~
. ~
Islond
,J
e, ~.~JRobert I.
3
Greenwich I.
I t ~ J Snow I.
~\
\(¢\"
Deception I~
~ ~" 0 I
100
I
I
km OW I.
2
161
160
159
I 58"W
Fig. 1. Sketch map of the South Shetland Islands showing general sample locations for K-Ar radiometric dating. Inset shows the location of the islands and their relation to present-day plate boundaries, after Barker [6]. Note that the Drake (or Aluk) plate is the last surviving remnant of a much larger Phoenix plate, most of which was subducted in Mesozoic and Tertiary times.
tological and geochemical studies is now complete [10]. Here we are solely concerned with conclusions based on some seventy conventional K-Ar analyses--the first systematic attempt at geochronology in the South Shetland Islands to date--analytical details of which are summarized in Table 1 and more fully documented by Smellie et al. [10]. Fresh rocks were crushed to 60-85 mesh, washed and aliquotted for K determination (by flame photometry with Li as internal standard, usually at least in triplicate) and Ar isotope analysis (mostly in duplicate using both an A.E.I. MS 10 system and a newer VG Isotopes 1200 mass spectrometer). Errors are generally ca. 2-4% (20). All previously published ages quoted in the text have been recalculated using the constant recommended by Steiger and J/ager [11]. The K-Ar age results are summarized graphically in Fig. 2 where they are plotted according to the sample localities marked in Fig. 1. Because of extensive snow and ice cover, and excessive altera-
tion of the volcanic rocks at a large number of localities, geographical coverage is not continuous and the data are best considered in three separate groupings. area. Byers Peninsula, western Livingston Island (localities 2 and 3 in Figs. 1 and 2), is the largest area of summer rock exposure in the South Shetland Islands. Eighteen K-Ar ages from here were reported by Pankhurst et al. [12] as the first stage of the current programme. The oldest ages (122 Ma and 128 Ma) came from the lowest part of the volcanic succession (locality 2) and are compatible with evidence from interbedded fossiliferous marine sediments of latest Jurassic/early Cretaceous (Tithonian-Valanginian) age [13,14]. Samples from the upper, non-marine, sequence (locality 3) gave younger ages (113 to 74 Ma), the spread in which may be due either to Ar loss or to failure to distinguish lavas from later hypabyssal sills. An interval of about 15 Ma may Southwestern
216 TABLE 1 K-Ar data for South Shetland Island samples Sample No.
Rock type
K (%)
Atmos. Ar (%)
Age (Ma)
P.407.4 P.215.1
granodiorite microdamellite
1.384 0.266
23 78
121 120
+3 +5
P.417.2 P.862.3 P.862.4 P.848.1 P.848.5 P.848.7 P.848.12 P.845.1b P.845.2c P.845.3a P.845.8 P.845.9
dacite andesite dyke basalt sill andesite plug andesite plug andesite plug (hornblende) rhyolite basalt basaltic andesite basaltic andesite dolerite plug dolerite plug
0.764 1.531 0.417 0.933 0.808 0.290 4.96 0.529 1.004 0.488 0.257 0.271
79 14 44 42 25 22 9 55 38 48 40 53
46 128 123 110 113 132 109 106 108 86 108 109
___2 _+4 +5 _+4 +4 _+5 +4 +4 -+4 +3 _+4 +4
P.850.8 P.850.11 P.850.5 P.864.1a P.864.1c P.864.2 P.864.4
basalt basalt basalt sill basalt sill basalt sill dolerite plug dolerite plug
0.195 0.295 0.221 0.500 0.532 0.191 0.194
56 58 71 47 51 72 76
94 79 76 77 74 89 95
+3 _+3 +3 _+3 +3 +4 +5
P.225.1a P.225.1b P.428.3 P.1259.2 P.1259.2 P.51.1
basalt sill basalt sill clast in vent tonalite (biotite) tonalite (hornblende) basalt
0.428 0.402 0.441 7.40 0.669 0.433
34 29 45 7 53 99.8
74 + 2 79 + 2 81 ± 2 40 +1 46 +__2 0.1 + 0.4
P.926.1 P.485.1 P.54.1 P.55.1
microgabbro basalt sill basalt basalt
0.248 0.429 0.305 0.300
51 59 99.6 99.5
84 + 2 80 + 2 0.2 + 0.3 0.2 -+0.4
P.840.4 P.840.5 P.840.6 P.842.6 P.842.9 P.1613.1 P.477.1
basalt basalt basalt basalt basalt microgabbro andesite
0.453 0.453 0.428 0.533 0.313 0.573 0.267
34 52 44 30 58 28 55
82 83 80 84 82 60 53
Locality 1 C. Wallace, Low I. C. Hooker, Low I.
Locality 2 President Head, Snow I. Ray Promontory, Byers P. Chester Cove, Byers P.
Vietor Rock, Byers P.
Sealer Hill, Byers P.
Locality 3 Eastern Byers P.
Negro Hill, E. Byers P.
Locality 4 Sayer Nunatak, Livingston I.
Barnard Point, Livingston I. Gleaner Heights, Livingston I.
Locality 5 Express I. Greenwich I. Mt. Plymouth, Greenwich I.
Locality 6 Coppermine P., Robert I.
Kitchen Point, Robert I.
+2 +3 +2 +2 +3 +1 +1
217 T A B L E 1 (continued) Sample No.
Rock type
K (%)
Atmos. Ar (%)
Age (Ma)
P.615.1 P.604.1 P.608.5a P.609.3 P.627.1 P.629.1 P.619.1
andesite andesite andesite andesite altered lava altered lava andesite plug
0.474 0.554 0.408 0.604 0.551 0.203 0.348
54 74 49 67 60 89 50
51 59 58 58 58 31 51
±1 ±2 ±1 ±2 ±1 ±3 ±1
P.1149.1 P.1166.7 P.1147.3 P.1147.4 P.1162.5 P.1125.1 P.1182.1/2 P.1183.2/7 P.611.1
andesite basalt basalt basalt basalt basaltic andesite dacite dacite andesite plug
0.409 0.452 0.242 0.346 0.244 0.620 1.720 1.755 0.945
91 62 76 74 80 66 25 24 69
58 52 48 48 57 43 42 46 44
±4 +1 ±1 ±1 ±3 +1 ±1 ±1 _+1
P.1473.5 P.232.1 P.696.1 P.750.1 P.758.1 P.757.2 P.685.4 P.533.1 P.535.2/3
basaltic andesite basalt basaltic andesite basaltic andesite basaltic andesite andesite andesite plug granodiorite plug granodiorite (biotite)
0.456 0.435 0.286 0.240 0.519 1.040 1.350 2.010 6.35
54 49 70 69 45 39 22 28 33
46 44 45 42 47 48 47 48 46
±1 ±1 ±1 ±1 ±1 +1 ±1 +1 ±1
P.831.2 P.831.3 P.831.4 P.831.5 P.831.8
andesite andesite andesite andesite andesite
1.770 0.623 0.668 1.015 0.878
24 86 82 27 25
45 ± 1 27 ± 1 32 ± 1 46 5:1 47 ± 1
P.560.1 P.1452.2 P.1454.1
andesite dyke andesite dyke andesite dyke
1.213 1.347 0.867
30 44 34
41 44 42
+1 +1 +1
P.438.1
andesite
1.265
16
42
+1
G.28.1
andesite
1.341
22
32
+1
Locality 7 Southern Fildes P., King George I.
Horatio Stump, King George I.
Locality 8 Northern Fildes P.
Suffield Point, N. Fildes P.
Locality 9 Marian Cove, King George I. Potter P., King George I.
Three Brothers Hill, King George I. Noel Hill, King George I.
Locality 10 Point Hennequin, King George I. Point Hennequin, King George I.
Locality 11 Keller P., King George I.
Locality 12 Lion's Rump, King George I.
Locality 13 Esther Nunatak, King George I.
Data are summarized from Smellie et al. [10] and Pankhurst et al. [12]. Samples are all extrusive rocks unless otherwise stated. Ages are mostly the means of duplicate radiogenic Ar determinations using A.E.I. MS 10 and Micromass 1200 mass-spectrometers.
218 AREA 1
2
O
~. •
5 •g
6
7
$
9
10
11
12
~3
20
~0
.'...i,
,? ee
•
60
•
°8°
,
I
AGE (Ha) 80
100
120
~
•
V o l c a n i c & hypQbyssol rocks
0
P l u l o n i c rocks
140
Fig. 2. Plot of K-Ar ages of South Shetland Islands' volcanic and plutonic rocks determined at IGS, grouped according to the sample locality identified in Fig. 1 and Table 1.
have elapsed between the deposition of the youngest fossiliferous marine rocks and that of the non-marine sequence. The most reliable ages are indicated by a Rb-Sr whole-rock isochron age of 111 + 4 Ma for acid volcanics from Chester Cone [10] and K-Ar ages of 89 Ma and 95 Ma for a cross-cutting dolerite plug near the top of the sequence [12]. Later Jurassic marine beds on Low Island [15] (locality 1) are cut by granodiorite and micro-ademellite plutons at Cape Wallace and near Cape Hooker [16], which have yielded new ages of 121 + 3 Ma and 1 2 0 _ 5Ma, respectively (early Cretaceous). Elliot et al. [17], using t h e 4°Ar/39Ar method, obtained an age of 127 Ma for a dacitic intrusion at Cape Wallace, but like us report younger ages (down to ca. 50 Ma) for samples from President Head, eastern Snow Island, in a sequence containing Jurassic to Early Cretaceous macro- and micro-floras [18,19] (locality 2).
C e n t r a l area. There is no published palaeontological control on the stratigraphy of volcanic rocks cropping out in eastern Livingston Island, Greenwich Island and Robert Island. A minimum mid-Cretaceous age for at least part of the sequence is indicated by a published K-Ar age of 102 Ma for a quartz diorite intrusion on Half Moon Island in Macfarlane Strait [20] between Livingston and Greenwich islands. In addition, a tonalite pluton at Barnard Point, southeastern Livingston Island, has yielded an Eocene age (40 Ma) [21], confirmed by Rb-Sr dating [10,22]. We have also analysed nine samples of volcanic and hypabyssal rocks from several localities between Williams Point, Livingston Island, and Coppermine Peninsula, Robert Island (localities 4-6). These mostly fall within a relatively limited K-Ar age range of 78-85 Ma. Elliot et al. [17] have obtained comparable ages of 80-82 Ma for three samples from the northwestern end of Greenwich Island (locality 5). Thus there seems little doubt that the major period of volcanic activity in this area was during the late Cretaceous. One age of 60 + 1 Ma from Coppermine Peninsula may represent Ar loss, but an even younger age of 53 + 1 Ma from Kitchen Point, easternmost Robert Island, taken in conjunction with the age of the Barnard Point intrusion, probably indicates an overlap with Tertiary activity within the next area considered here.
The geology of King George Island was first described by Ferguson [23] who recognized an older and a younger suite of volcanic rocks of which the latter has been subdivided into several formations by subsequent workers [24-28]. The older suite, which was recognized by a much greater degree of low temperature alteration, was thought to be of possible Jurassic age by analogy with the then current interpretation of Antarctic Peninsula volcanism. Palaeobotanical studies at first suggested a Miocene age [29] for the younger suite but an earlier Tertiary age has since been suggested by studies of bird tracks [30], palaeomagnetism [31] and previous geochronology [20,31]. An early Eocene age for the Fildes Formation [10], southwestern King George Island, is confirmed by the new results. One age of 51 + 1 N o r t h e a s t e r n area.
219 Ma and three of 58-59 Ma were obtained from southern Fildes Peninsula (locality 7) and eight in the range 42-58 Ma from northern Fildes Peninsula (locality 8). A number of cross-cutting intrusive rocks have also been dated: 44 + 1 Ma (Suffield Point), 47 + 1 Ma (Noel Hill) and 51 + 1 Ma (Horatio Stump). Watts [31] obtained ages of 47-52 and 56 + 1 Ma for the last two intrusions. Slight age discordance relative to stratigraphical height was found in the Fildes Peninsula succession, and we suggest a certain amount of Ar loss to be responsible. Previous workers have reported irreproducible and erratic ages (27-109 Ma) [31,32] for the supposed older suite of altered volcanics. We have obtained two further K-Ar dates of 31 + 3 Ma and 58 + 1 Ma, the latter quite reproducible. At this stage we do not consider that any of the available evidence demonstrates a preTertiary age for these rocks, and propose that their Ar contents were severely disturbed during alteration, possibly during the late intrusive phase. However, apart from these hydrothermally altered lavas, Polish workers have apparently obtained K - A r ages of 67-77 Ma for the lowest exposed part of the succession southwest of Admiralty Bay, suggesting that Late Cretaceous volcanics underlie the Tertiary volcanics (K. Birkenmajer and W. Narebski, personal communication). Five samples of the Fildes Formation [10] from Potter Peninsula (locality 9) give ages of 42-48 Ma, rather younger than those of 51, 52 and 59 Ma reported by Watts [31] and again caused by Ar loss; a minimum age is provided by the cross-cutting andesite plug at Three Brothers Hill, dated by us 47 + 1 Ma, by Watts [31] at 52 + 1 Ma and by Elliot et al. [17] at 46 Ma. Our data from the Hennequin Formation [10] (locality 10) which is in general more evolved than and interdigitates with the Fildes Formation, have a bimodal distribution. Three andesite lavas gave ages of 45-47 Ma, but two others have apparent ages of 27 Ma and 32 Ma. Elliot et al. [17] report 4°Ar/39Ar ages of 48, 51 and 52 Ma for samples from the Point Thomas area which tends to support an early Eocene stratigraphy. Consistently younger ages of 41-44 Ma for relatively unaltered dykes cross cutting altered lavas on Keller peninsula (locality 11; previously ascribed to the
"Jurassic" group [25,33]) and for a lava beneath the Plio-Pleistocene conglomerate [25,34] at Lion's Rump (locality 12) may be real--Elliot et al. [17] obtained an age of 42 Ma for an intrusive andesite at Jerzak Hills, Point Thomas. For the most northeasterly sample dated from King George Island, and andesite plug at Esther Nunatak (locality 13), we obtained a still younger age of 32 + 1 Ma. Finally a granodiorite on Cornwallis Island, 200 km northeast in the Elephant and Clarence islands group, has given an age of 9.8 Ma [35]. Thus, although the interpretation is slightly clouded by the uncertainty associated with loss of radiogenic Ar from whole-rock samples, our data indicate a clear northeasterly decrease in the age of activity on King George Island, from early Eocene to late Oligocene. This appears to be a continuation of the younging trend observed throughout the archipelago when the two previous areas and possibly Cornwallis Island are considered (Fig. 2). The occurrence of older rocks towards the southwest can scarcely be due to differential erosion of a previously omnipresent Tertiary sequence since (a) the early Cretaceous and Tertiary volcanic sequences show closely comparable alteration ascribed to diagenesis and very low-grade burial metamorphism [10], and (b) there is a general lack of Tertiary dykes or other intrusions within the Cretaceous outcrops, which could have acted as feeders for an overlying succession. However, in consideration of the prolonged history of subduction at the South Shetland trench, it is possible that pre-Tertiary volcanic rocks are present at depth beneath the northeastern islands. The observed easterly younging of volcanism must be related to the subduction history of the southeastern Pacific Ocean floor, but cannot at this stage be explained in detail. Whereas spatial migration of volcanism in an intraplate situation is relatively common and maybe ascribed to hot-spot or fracture propagation, migration of subductionrelated igneous activity is more problematical, due partly to the disappearance of much of the evidence embodied in the ocean floor. According to reconstructions by Barker [6], the Mesozoic plate configuration of the Pacific margin of Antarctica and South America was relatively simple--a single oceanic plate (Phoenix) being produced at spread-
220 ing centres to the west (Pacific-Phoenix and FaralIon-Phoenix) and being subducted as the triple junction migrated towards the continents. Rough parallelism between the peninsula and the former spreading centre would account for calc-alkaline igneous activity throughout the area in Jurassic and Cretaceous times, presumably including its expression in the forward-sited island arc of the South Shetland Islands. At the start of the Tertiary period that part of the Phoenix plate southwest of the Antarctic Peninsula became coupled to the Antarctic plate and northeast of this a new spreading centre (Aluk) appeared close to the continent. The southern end of this ridge was itself progressively consumed beneath the peninsula northwards during Tertiary times until about 4 Ma ago, when spreading essentially ceased with the configuration of Fig. 1. Barker [6] related an apparent northward migration in the cessation of calc-alkahne magmatism on the Antarctic Peninsula with consumption of the Aluk Ridge, although it seems as though cessation took place as the ridge-crest approached, up to 50 Ma prior to collision, i.e. when young oceanic crust was being subducted. This effect was ascribed to dehydration of younger, more buoyant, oceanic crust at shallow depths where melting could not occur [36]. Thus the migration effect could result from variation along the arc in the age of adjacent oceanic crust, either due to non-parallelism to the ridge, or due to segmentation of the slab resulting in adjacent sections on either side of fracture zones having different ages and thermal structures [37-39]. Sinistral offsetting of the segments, as indicated by the present-day magnetic anomaly pattern, would cause younger crust to be subducted first in the southwest areas and progressively northeastwards. It is not entirely clear whether this explanation can be applied to the South Shetland Islands themselves, as well as the Antarctic Peninsula as a whole, since the apparent migration is more rapid (i.e. compressed) in the former situation. It may be that we are observing the overlap of two effects: a northeasterly migration of cessation as described above during the Jurassic and Cretaceous periods and a sudden change in the segmentation/thermal structure of the subducted ocean floor associated with initiation of the Aluk spreading centre at the start of Tertiary times.
A final point of interest is the unexpected discovery of Recent volcanism along the central axis of the South Shetland Islands--less than 0.5 Ma from localities in Livingston and Greenwich islands. Elliot et al. [17] report an age of 2.4 + 0.4 Ma for a plug at Edinburgh Hill, easternmost Livingston Island. The rocks concerned are typically very fresh olivine- and clinopyroxene-phyric basalts (lavas, plug and the only know palagonitized hyaloclastites). They are mildly alkaline (with up to 6.3% ne in the norm) and plot close to or in the field of alkaline rocks in the alkali-silica diagram (due to high N a 2 0 rather than high K 2 0 ). There is little obvious enrichment in incompatible trace elements compared to other South Shetland Islands' volcanic rocks, but since they clearly post-date the cessation of subduction 4 Ma ago they must be presumed to be related to the more conspicuously alkaline rocks of James Ross Island [40] and Alexander Island [41] (Antarctic Peninsula) or to the Recent volcanoes in Bransfield Strait-Deception, Bridgeman and Penguin islands [42]. All of these are associated with a change to extensional tectonics, although Bransfield Strait, an incipient back-arc spreading centre, may be due to continued gravitational sinking of oceanic lithosphere. In terms of geochemistry, the young samples analysed here most resemble the Penguin Island basalts rather than the axial Bransfield Strait volcanoes.
References 1 I.W.D. Dalziel, Large-scale folding in the Scotia arc, in: Antarctic Geology and Geophysics, R.J. Adie, ed., pp. 47-55, Universitetsforlaget, Oslo, 1972. 2 I.W.D. Dalziel, The early (pre-Middle Jurassic) history of the Scotia arc region: a review and progress report, in: Antarctic Geoscience, C. Craddock, ed., pp. 111-126, University of Wisconsin Press, Madison, Wisc., 1982. 3 J.L. Smellie, A complete arc-trench system recognized in Condwana sequences of the Antarctic Peninsula region, Geol. Mag. 118, 139-159, 1981. 4 S.D. Weaver, A.D. Saunders and J. Tarney, MesozoicCenozoic volcanism in the South Shetland Islands and the Antarctic Peninsula: geochemicalnature and plate tectonic significance, in: Antarctic Geoscience, C. Craddock, ed., pp. 263-274, Universityof WisconsinPress, Madison, Wisc., 1982. 5 A.D. Saunders, S.D. Weaver and J. Tarney, The pattern of Antarctic Peninsula plutonism, in: Antarctic Geoscience, C.
221
6
7
8
9
10 11
12
13
14
15
16
17 18
19
20
Craddock, ed., pp. 305-346, University of Wisconsin Press, Madison, Wisc., 1982). P.F. Barker, The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: ridge crust-trench interactions, J. Geol. Soc. London 139, 787-801, 1982. P.F. Barker, in: Symposium on Andean and Antarctic Volcanology Problems, O. Gonzalez-Ferran, ed., pp. 330-346, Proc., IAVCEI, Santiago, 1976. W.A. Ashcroft, Crustal structure of the South Shetland Islands and Bransfield Strait, Sci. Rep. Br. Antarct. Surv. 66, 43 pp., 1972. J.L. Smellie, A geochemical overview of subduction-related igneous activity in the South Shetland Islands, Western Antarctica, in: Antarctic Earth Science, R.L. Oliver, J.B. Jago and P.R. James, eds., Australian Academy of Science, Canberra, A.C.T., 1983. J.L. Smellie, R.J. Pankhurst, M.R.A. Thomson and R.E.S. Davies, Sci. Rep. Br. Antarct. Surv. (in press). R.H. Steiger and E. J~iger, Subcommission on Geochronology: convention on the use of decay constants in geo- and cosmo-chronology, Earth Planet. Sci. Lett. 36, 359-362, 1977. R.J. Pankhurst, S.D. Weaver, M. Brook and A.D. Saunders, K-Ar chronology of Byers Peninsula, Livingston Island, South Shetland Islands, Bull. Br. Antarct. Surv., 49, 277-282, 1980. E. Valenzuela and F. Herv~, Geology of Byers Peninsula, Livingston Island, South Shetland Islands, in: Antarctic Geology and Geophysics, R.J. Adie. ed., pp. 83-89, Universitetsforlaget, Oslo, 1972. J.L. Smellie, R.E.S. Davies and M.R.A. Thomson, Geology of a Mesozoic intra-arc sequence on Byers Peninsula, Livingston Island, South Shetland Island, Bull. Br. Antarct. Surv. 50, 55-76, 1980. M.R.A. Thomson, Late Jurassic fossils from Low Island, South Shetland Islands and Austin Rocks, Bull. Br. Antarct. Surv. 56, 25-36, 1982. J.L. Smellie, The geology of Low Island, South Shetland Islands and Austin Rocks, Bull. Br. Antarct. Surv. 49, 235-257, 1980~ D.H. Elliot, D.D. Duprb and T.M. Gracanin, personal communication to R.J.P. H. Fuenzafida, R. Araya and F. Herv~, Middle Jurassic flora from north-eastern Snow Island, South Shetland Islands, in: Antarctic Geology and Geophysics, R.J. Adie, ed., pp. 93-97, Universitetsforlaget, Oslo, 1972. R.A. Askin, Tithonian-Barremian spores, pollen and microplankton from the South Shetland Islands, Antarctica, in: Antarctic Earth Science, R.L. Oliver, J.B. Jago and P.R. James, eds., Australian Academy of Science, Canberra, A.C.T., 1983. G.E. Grikurov, A.Ya. Krylov, M.M. Polyakov and Ya.N. Tsovbun, Vorast porod v severnoi chasti Antarkticheskogo poluostrova i na Yudznykh Shetlandskikh ostrovakh (po dannym Kaliyargonovogo metoda). [Age of rocks from the northern part of the Antarctic Peninsula and the South Shetland Island (from data of the potassium-argon method)], Inf. Byull, Sov. Antarkt. Eksped. 80, 30-34, 1970.
21 R. del Valle, J. MoreUi and C. Rinaldi, Contrnes. Inst. Antarct. Argent. 175, 1974. 22 I.W.D. Dalziel, R. Kligfield, W. Lowrie and N.D. Opdyke, Palaeomagnetic data from the southernmost Andes and the Antarctandes, in: Implications of Continental Draft to the Earth Sciences, D.H. Tarling and S.K. Runcorn, eds., pp. 37-101, Academic Press, New York, N.Y., 1973. 23 D. Ferguson, Geological observations in the South Shetland Islands, the Palmer archipelago, and Graham Land, Antarctica, Trans. R. Soc. Edinburgh 53, 29-55, 1921. 24 D.D. Hawkes, The geology of the South Shetland Island, I. The petrology of King George Island, Sci. Rep. Falkld. Inst. Depend. Surv. 26, 28 pp., 1961. 25 C.M. Barton, The geology of the South Shetland Islands, III. The stratigraphy of King George Island, Sci. Rep. Br. Antarct. Surv. 44, 33 pp., 1965. 26 K. Birkenmajer, A revised lithostratigraphic standard for the Tertiary of King George Island, South Shetland Islands (West Antarctica), Bull. Acad. Pol. Sci., Ser. Sci. Terre 27, 49-57, 1980. 27 K. Birkenmajer, Tertiary volcanic-sedimentary succession at Admiralty Bay, King George Island (South Shetland Islands, Antarctica), Stud. Geol. Pol. 64, 7-65, 1980. 28 K. Birkenmajer, Lithostratigraphy of the Point Hennequin Group (Miocene volcanics and sediments) at King George Island (South Shetland Islands, Antarctica), Stud. Geol. Pol. 72, 59-73, 1981. 29 H.A. Orlando, The fossil flora of the surroundings of Ardley Peninsula (Ardley Island), 25 de Mayo Island (King George Island), South Shetland Islands, in: Antarctic Geology, R.J. Adie, ed., pp. 629-636, North-Holland, Amsterdam, 1964. 30 V. Covacevich and P.V. Rich, New bird ichnites from Fildes Peninsula, King George Island, West Antarctica, in: Antarctic Geoscience, C. Craddock, ed., pp. 245-253, University of Wisconsin Press, Madison, Wisc., 1982. 31 D.R. Watts, Potassium-argon ages and palaeomagnetic resuits from King George Island, South Shetland Islands, in: Antarctic Geoscience, C. Craddock, ed., pp. 255-262, University of Wisconsin Press, Madison, Wisc. 1982. 32 D.A. Valencio, J.E. Mendia and J.F. Vilas, Palaeomagnetism and K-Ar age of Mesozoic and Cenozoic igneous rocks from Antarctica, Earth Planet. Sci. Lett. 45, 61-68, 1979. 33 K. Birkenmajer, Mesozoic stratiform volcanic-sedimentary succession and Andean intrusions at Admiralty Bay, King George Island (South Shetland Islands, Antarctica), Stud. Geol. Pol. 74, 105-154, 1982. 34 K. Birkenmajer, Pliocene tillite-bearing succession of King George Island (South Shetland Islands, Antarctica), Stud. Geol. Pol. 72, 7-72, 1982. 35 D.C. Rex and P.E. Baker, Age and petrology of Cornwallis Island granodiorite, Bull. Br. Antarct. Surv. 32, 55-61, 1973. 36 S.E. DeLong, W.M. Schwarz and R.N. Anderson, Thermal effects of ridge subduction, Earth Planet. Sci. Lett. 44, 239-246, 1979. 37 P. Molnar and T. Atwater, Interarc spreading and cordilleran tectonics as alternates related to the age of subducted
222 oceanic lithosphere, Earth Planet. Sci. Lett. 41, 330-340, 1978. 38 P. England and R. Wortel, Some consequences of the subduction of young slabs, Earth Planet. Sci. Lett. 47, 403-415, 1980. 39 K.P. Furlong, D.S. Chapman and P.W. Alfield, Thermal modelling of the geometry of subduction with implications for the tectonics of the overriding plate, J. Geophys. Res. 87, 1786-1802, 1982. 40 P.E. Baker, F. Buckley and D.C. Rex, Cenozoic volcanism
in the Antarctic, Philos. Trans. R. Soc. London, Ser. B 279, 131-142, 1977. 41 C.M. Bell, The geology of Beethoven Peninsula, south-western Alexander Island, Bull. Br. Antarct. Surv. 32, 75-83, 1973. 42 S.D. Weaver, A.D. Saunders, R.J. Pankhurst and J. Tarney, A geochemical study of magmatism associated with the initial stages of back-arc spreading: Quaternary volcanics of Bransfield Strait, from South Shetland Islands, Contrib. Mineral. Petrol. 68, 151-169, 1979.
Earth and Planetary Science Letters, 66 (1983) 214-222 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
[21
K-Ar geochronology of the South Shetland Islands, Lesser Antarctica: apparent lateral migration of Jurassic to Quaternary island arc volcanism R.J. Pankhurst 1 and J.L. Smellie 2 I British Antarctic Survey, Natural Environment Research Council, c / o Institute of Geological Sciences, 64- 78 Gray's Inn Road, London W C 1 X 8NG (U.K.) 2 Institute of Geological Sciences, Natural Environment Research Council, Murchison House, West Mains Road, Edinburgh EH9 3LA (U.K.)
Received June 5, 1983 Revised version accepted August 24, 1983
Approximately 70 K-Ar whole-rock ages for low-K tholeiitic and andesitic volcanic and intrusive rocks from the South Shetland Islands, Antarctica, including about 50 not previously published, are reviewed. Activity mainly spanned the range 130 to 30 Ma (Jurassic/Cretaceous to Oligocene/Miocene) with a very recent ( - 2 Ma), more alkaline, renewal. Throughout the main period of activity magmatism (or, perhaps, its cessation) migrated continuously northeastwards along the length of the island chain.
T h e South Shetland Islands, a northeasterly trending archipelago some 450 km in length, are situated about 100 km north of the Antarctic Peninsula (Fig. 1). The majority of the islands are formed of volcanic and hypabyssal rocks, with interbedded sediments, erupted subaerially in part on a continental basement of which only a few metasedimentary representatives are exposed [1-3]. Stratigraphical, petrological and geochemical studies suggest that the volcanism was related to the Mesozoic/Cenozoic igneous evolution of the Antarctic Peninsula, controlled by subduction of the Pacific Ocean floor beneath Lesser Antarctica [4-6]. Compositions are fairly restricted (mainly basalt andesite) and show low contents of lithophile elements in comparison with the calc-alkaline rocks of the peninsula, in keeping with closer proximity to the South Shetland Islands trench. Subduction virtually ceased about 4 Ma ago [7], since when the South Shetland Islands have separated from the mainland due to spreading in Bransfield Strait, a very young marginal 0012-821X/83/$03.00
© 1983 Elsevier Science Publishers B.V.
basin [6,8]. Here we report the results of the first comprehensive radiometric chronology of South Shetland Islands' volcanic rocks, which shows that the recorded age of magmatism migrated progressively northeastwards along the length of the archipelago from ca. 130 Ma to 30 Ma (with a slight renewal of magmatism in Pliocene-to-Recent times) although the mechanism responsible for this migration has not been identified. During two summer field seasons between 1974 and 1976 scientists of the British Antarctic Survey and the University of Birmingham carried out a comprehensive geological survey of the South Shetland Islands (excluding the non-volcanic islands of the Elephant and Clarence islands groups), followed up by a variety of laboratory studies. These represent a significant advance over previous work based on data from limited geographical areas or thinly spread sampling. Some of the geochemical data resulting from the new programme have already been published [4,5,9] and a review of the major conclusions of stratigraphical, palaeon-
215 _NAZ ~ )
_r"
~
~o's-
Or.*e~ 3 ~ , . . - - ~ - - ~ - ~ J L / (Aluk) ~ S h e t l o n d ANT ~J~C-~-SOUTH SHETLAND /J¢ ISLA ND5
,8o'w#/',
,,~o'w ,
'
.o's-
'
8 ~ , , o ,,)~ ~o ,~,2 ? II~
.... * "'
Livingston
~ j p _ v ~ ~.~
~
,o"
~
. ~
Islond
,J
e, ~.~JRobert I.
3
Greenwich I.
I t ~ J Snow I.
~\
\(¢\"
Deception I~
~ ~" 0 I
100
I
I
km OW I.
2
161
160
159
I 58"W
Fig. 1. Sketch map of the South Shetland Islands showing general sample locations for K-Ar radiometric dating. Inset shows the location of the islands and their relation to present-day plate boundaries, after Barker [6]. Note that the Drake (or Aluk) plate is the last surviving remnant of a much larger Phoenix plate, most of which was subducted in Mesozoic and Tertiary times.
tological and geochemical studies is now complete [10]. Here we are solely concerned with conclusions based on some seventy conventional K-Ar analyses--the first systematic attempt at geochronology in the South Shetland Islands to date--analytical details of which are summarized in Table 1 and more fully documented by Smellie et al. [10]. Fresh rocks were crushed to 60-85 mesh, washed and aliquotted for K determination (by flame photometry with Li as internal standard, usually at least in triplicate) and Ar isotope analysis (mostly in duplicate using both an A.E.I. MS 10 system and a newer VG Isotopes 1200 mass spectrometer). Errors are generally ca. 2-4% (20). All previously published ages quoted in the text have been recalculated using the constant recommended by Steiger and J/ager [11]. The K-Ar age results are summarized graphically in Fig. 2 where they are plotted according to the sample localities marked in Fig. 1. Because of extensive snow and ice cover, and excessive altera-
tion of the volcanic rocks at a large number of localities, geographical coverage is not continuous and the data are best considered in three separate groupings. area. Byers Peninsula, western Livingston Island (localities 2 and 3 in Figs. 1 and 2), is the largest area of summer rock exposure in the South Shetland Islands. Eighteen K-Ar ages from here were reported by Pankhurst et al. [12] as the first stage of the current programme. The oldest ages (122 Ma and 128 Ma) came from the lowest part of the volcanic succession (locality 2) and are compatible with evidence from interbedded fossiliferous marine sediments of latest Jurassic/early Cretaceous (Tithonian-Valanginian) age [13,14]. Samples from the upper, non-marine, sequence (locality 3) gave younger ages (113 to 74 Ma), the spread in which may be due either to Ar loss or to failure to distinguish lavas from later hypabyssal sills. An interval of about 15 Ma may Southwestern
216 TABLE 1 K-Ar data for South Shetland Island samples Sample No.
Rock type
K (%)
Atmos. Ar (%)
Age (Ma)
P.407.4 P.215.1
granodiorite microdamellite
1.384 0.266
23 78
121 120
+3 +5
P.417.2 P.862.3 P.862.4 P.848.1 P.848.5 P.848.7 P.848.12 P.845.1b P.845.2c P.845.3a P.845.8 P.845.9
dacite andesite dyke basalt sill andesite plug andesite plug andesite plug (hornblende) rhyolite basalt basaltic andesite basaltic andesite dolerite plug dolerite plug
0.764 1.531 0.417 0.933 0.808 0.290 4.96 0.529 1.004 0.488 0.257 0.271
79 14 44 42 25 22 9 55 38 48 40 53
46 128 123 110 113 132 109 106 108 86 108 109
___2 _+4 +5 _+4 +4 _+5 +4 +4 -+4 +3 _+4 +4
P.850.8 P.850.11 P.850.5 P.864.1a P.864.1c P.864.2 P.864.4
basalt basalt basalt sill basalt sill basalt sill dolerite plug dolerite plug
0.195 0.295 0.221 0.500 0.532 0.191 0.194
56 58 71 47 51 72 76
94 79 76 77 74 89 95
+3 _+3 +3 _+3 +3 +4 +5
P.225.1a P.225.1b P.428.3 P.1259.2 P.1259.2 P.51.1
basalt sill basalt sill clast in vent tonalite (biotite) tonalite (hornblende) basalt
0.428 0.402 0.441 7.40 0.669 0.433
34 29 45 7 53 99.8
74 + 2 79 + 2 81 ± 2 40 +1 46 +__2 0.1 + 0.4
P.926.1 P.485.1 P.54.1 P.55.1
microgabbro basalt sill basalt basalt
0.248 0.429 0.305 0.300
51 59 99.6 99.5
84 + 2 80 + 2 0.2 + 0.3 0.2 -+0.4
P.840.4 P.840.5 P.840.6 P.842.6 P.842.9 P.1613.1 P.477.1
basalt basalt basalt basalt basalt microgabbro andesite
0.453 0.453 0.428 0.533 0.313 0.573 0.267
34 52 44 30 58 28 55
82 83 80 84 82 60 53
Locality 1 C. Wallace, Low I. C. Hooker, Low I.
Locality 2 President Head, Snow I. Ray Promontory, Byers P. Chester Cove, Byers P.
Vietor Rock, Byers P.
Sealer Hill, Byers P.
Locality 3 Eastern Byers P.
Negro Hill, E. Byers P.
Locality 4 Sayer Nunatak, Livingston I.
Barnard Point, Livingston I. Gleaner Heights, Livingston I.
Locality 5 Express I. Greenwich I. Mt. Plymouth, Greenwich I.
Locality 6 Coppermine P., Robert I.
Kitchen Point, Robert I.
+2 +3 +2 +2 +3 +1 +1
217 T A B L E 1 (continued) Sample No.
Rock type
K (%)
Atmos. Ar (%)
Age (Ma)
P.615.1 P.604.1 P.608.5a P.609.3 P.627.1 P.629.1 P.619.1
andesite andesite andesite andesite altered lava altered lava andesite plug
0.474 0.554 0.408 0.604 0.551 0.203 0.348
54 74 49 67 60 89 50
51 59 58 58 58 31 51
±1 ±2 ±1 ±2 ±1 ±3 ±1
P.1149.1 P.1166.7 P.1147.3 P.1147.4 P.1162.5 P.1125.1 P.1182.1/2 P.1183.2/7 P.611.1
andesite basalt basalt basalt basalt basaltic andesite dacite dacite andesite plug
0.409 0.452 0.242 0.346 0.244 0.620 1.720 1.755 0.945
91 62 76 74 80 66 25 24 69
58 52 48 48 57 43 42 46 44
±4 +1 ±1 ±1 ±3 +1 ±1 ±1 _+1
P.1473.5 P.232.1 P.696.1 P.750.1 P.758.1 P.757.2 P.685.4 P.533.1 P.535.2/3
basaltic andesite basalt basaltic andesite basaltic andesite basaltic andesite andesite andesite plug granodiorite plug granodiorite (biotite)
0.456 0.435 0.286 0.240 0.519 1.040 1.350 2.010 6.35
54 49 70 69 45 39 22 28 33
46 44 45 42 47 48 47 48 46
±1 ±1 ±1 ±1 ±1 +1 ±1 +1 ±1
P.831.2 P.831.3 P.831.4 P.831.5 P.831.8
andesite andesite andesite andesite andesite
1.770 0.623 0.668 1.015 0.878
24 86 82 27 25
45 ± 1 27 ± 1 32 ± 1 46 5:1 47 ± 1
P.560.1 P.1452.2 P.1454.1
andesite dyke andesite dyke andesite dyke
1.213 1.347 0.867
30 44 34
41 44 42
+1 +1 +1
P.438.1
andesite
1.265
16
42
+1
G.28.1
andesite
1.341
22
32
+1
Locality 7 Southern Fildes P., King George I.
Horatio Stump, King George I.
Locality 8 Northern Fildes P.
Suffield Point, N. Fildes P.
Locality 9 Marian Cove, King George I. Potter P., King George I.
Three Brothers Hill, King George I. Noel Hill, King George I.
Locality 10 Point Hennequin, King George I. Point Hennequin, King George I.
Locality 11 Keller P., King George I.
Locality 12 Lion's Rump, King George I.
Locality 13 Esther Nunatak, King George I.
Data are summarized from Smellie et al. [10] and Pankhurst et al. [12]. Samples are all extrusive rocks unless otherwise stated. Ages are mostly the means of duplicate radiogenic Ar determinations using A.E.I. MS 10 and Micromass 1200 mass-spectrometers.
218 AREA 1
2
O
~. •
5 •g
6
7
$
9
10
11
12
~3
20
~0
.'...i,
,? ee
•
60
•
°8°
,
I
AGE (Ha) 80
100
120
~
•
V o l c a n i c & hypQbyssol rocks
0
P l u l o n i c rocks
140
Fig. 2. Plot of K-Ar ages of South Shetland Islands' volcanic and plutonic rocks determined at IGS, grouped according to the sample locality identified in Fig. 1 and Table 1.
have elapsed between the deposition of the youngest fossiliferous marine rocks and that of the non-marine sequence. The most reliable ages are indicated by a Rb-Sr whole-rock isochron age of 111 + 4 Ma for acid volcanics from Chester Cone [10] and K-Ar ages of 89 Ma and 95 Ma for a cross-cutting dolerite plug near the top of the sequence [12]. Later Jurassic marine beds on Low Island [15] (locality 1) are cut by granodiorite and micro-ademellite plutons at Cape Wallace and near Cape Hooker [16], which have yielded new ages of 121 + 3 Ma and 1 2 0 _ 5Ma, respectively (early Cretaceous). Elliot et al. [17], using t h e 4°Ar/39Ar method, obtained an age of 127 Ma for a dacitic intrusion at Cape Wallace, but like us report younger ages (down to ca. 50 Ma) for samples from President Head, eastern Snow Island, in a sequence containing Jurassic to Early Cretaceous macro- and micro-floras [18,19] (locality 2).
C e n t r a l area. There is no published palaeontological control on the stratigraphy of volcanic rocks cropping out in eastern Livingston Island, Greenwich Island and Robert Island. A minimum mid-Cretaceous age for at least part of the sequence is indicated by a published K-Ar age of 102 Ma for a quartz diorite intrusion on Half Moon Island in Macfarlane Strait [20] between Livingston and Greenwich islands. In addition, a tonalite pluton at Barnard Point, southeastern Livingston Island, has yielded an Eocene age (40 Ma) [21], confirmed by Rb-Sr dating [10,22]. We have also analysed nine samples of volcanic and hypabyssal rocks from several localities between Williams Point, Livingston Island, and Coppermine Peninsula, Robert Island (localities 4-6). These mostly fall within a relatively limited K-Ar age range of 78-85 Ma. Elliot et al. [17] have obtained comparable ages of 80-82 Ma for three samples from the northwestern end of Greenwich Island (locality 5). Thus there seems little doubt that the major period of volcanic activity in this area was during the late Cretaceous. One age of 60 + 1 Ma from Coppermine Peninsula may represent Ar loss, but an even younger age of 53 + 1 Ma from Kitchen Point, easternmost Robert Island, taken in conjunction with the age of the Barnard Point intrusion, probably indicates an overlap with Tertiary activity within the next area considered here.
The geology of King George Island was first described by Ferguson [23] who recognized an older and a younger suite of volcanic rocks of which the latter has been subdivided into several formations by subsequent workers [24-28]. The older suite, which was recognized by a much greater degree of low temperature alteration, was thought to be of possible Jurassic age by analogy with the then current interpretation of Antarctic Peninsula volcanism. Palaeobotanical studies at first suggested a Miocene age [29] for the younger suite but an earlier Tertiary age has since been suggested by studies of bird tracks [30], palaeomagnetism [31] and previous geochronology [20,31]. An early Eocene age for the Fildes Formation [10], southwestern King George Island, is confirmed by the new results. One age of 51 + 1 N o r t h e a s t e r n area.
219 Ma and three of 58-59 Ma were obtained from southern Fildes Peninsula (locality 7) and eight in the range 42-58 Ma from northern Fildes Peninsula (locality 8). A number of cross-cutting intrusive rocks have also been dated: 44 + 1 Ma (Suffield Point), 47 + 1 Ma (Noel Hill) and 51 + 1 Ma (Horatio Stump). Watts [31] obtained ages of 47-52 and 56 + 1 Ma for the last two intrusions. Slight age discordance relative to stratigraphical height was found in the Fildes Peninsula succession, and we suggest a certain amount of Ar loss to be responsible. Previous workers have reported irreproducible and erratic ages (27-109 Ma) [31,32] for the supposed older suite of altered volcanics. We have obtained two further K-Ar dates of 31 + 3 Ma and 58 + 1 Ma, the latter quite reproducible. At this stage we do not consider that any of the available evidence demonstrates a preTertiary age for these rocks, and propose that their Ar contents were severely disturbed during alteration, possibly during the late intrusive phase. However, apart from these hydrothermally altered lavas, Polish workers have apparently obtained K - A r ages of 67-77 Ma for the lowest exposed part of the succession southwest of Admiralty Bay, suggesting that Late Cretaceous volcanics underlie the Tertiary volcanics (K. Birkenmajer and W. Narebski, personal communication). Five samples of the Fildes Formation [10] from Potter Peninsula (locality 9) give ages of 42-48 Ma, rather younger than those of 51, 52 and 59 Ma reported by Watts [31] and again caused by Ar loss; a minimum age is provided by the cross-cutting andesite plug at Three Brothers Hill, dated by us 47 + 1 Ma, by Watts [31] at 52 + 1 Ma and by Elliot et al. [17] at 46 Ma. Our data from the Hennequin Formation [10] (locality 10) which is in general more evolved than and interdigitates with the Fildes Formation, have a bimodal distribution. Three andesite lavas gave ages of 45-47 Ma, but two others have apparent ages of 27 Ma and 32 Ma. Elliot et al. [17] report 4°Ar/39Ar ages of 48, 51 and 52 Ma for samples from the Point Thomas area which tends to support an early Eocene stratigraphy. Consistently younger ages of 41-44 Ma for relatively unaltered dykes cross cutting altered lavas on Keller peninsula (locality 11; previously ascribed to the
"Jurassic" group [25,33]) and for a lava beneath the Plio-Pleistocene conglomerate [25,34] at Lion's Rump (locality 12) may be real--Elliot et al. [17] obtained an age of 42 Ma for an intrusive andesite at Jerzak Hills, Point Thomas. For the most northeasterly sample dated from King George Island, and andesite plug at Esther Nunatak (locality 13), we obtained a still younger age of 32 + 1 Ma. Finally a granodiorite on Cornwallis Island, 200 km northeast in the Elephant and Clarence islands group, has given an age of 9.8 Ma [35]. Thus, although the interpretation is slightly clouded by the uncertainty associated with loss of radiogenic Ar from whole-rock samples, our data indicate a clear northeasterly decrease in the age of activity on King George Island, from early Eocene to late Oligocene. This appears to be a continuation of the younging trend observed throughout the archipelago when the two previous areas and possibly Cornwallis Island are considered (Fig. 2). The occurrence of older rocks towards the southwest can scarcely be due to differential erosion of a previously omnipresent Tertiary sequence since (a) the early Cretaceous and Tertiary volcanic sequences show closely comparable alteration ascribed to diagenesis and very low-grade burial metamorphism [10], and (b) there is a general lack of Tertiary dykes or other intrusions within the Cretaceous outcrops, which could have acted as feeders for an overlying succession. However, in consideration of the prolonged history of subduction at the South Shetland trench, it is possible that pre-Tertiary volcanic rocks are present at depth beneath the northeastern islands. The observed easterly younging of volcanism must be related to the subduction history of the southeastern Pacific Ocean floor, but cannot at this stage be explained in detail. Whereas spatial migration of volcanism in an intraplate situation is relatively common and maybe ascribed to hot-spot or fracture propagation, migration of subductionrelated igneous activity is more problematical, due partly to the disappearance of much of the evidence embodied in the ocean floor. According to reconstructions by Barker [6], the Mesozoic plate configuration of the Pacific margin of Antarctica and South America was relatively simple--a single oceanic plate (Phoenix) being produced at spread-
220 ing centres to the west (Pacific-Phoenix and FaralIon-Phoenix) and being subducted as the triple junction migrated towards the continents. Rough parallelism between the peninsula and the former spreading centre would account for calc-alkaline igneous activity throughout the area in Jurassic and Cretaceous times, presumably including its expression in the forward-sited island arc of the South Shetland Islands. At the start of the Tertiary period that part of the Phoenix plate southwest of the Antarctic Peninsula became coupled to the Antarctic plate and northeast of this a new spreading centre (Aluk) appeared close to the continent. The southern end of this ridge was itself progressively consumed beneath the peninsula northwards during Tertiary times until about 4 Ma ago, when spreading essentially ceased with the configuration of Fig. 1. Barker [6] related an apparent northward migration in the cessation of calc-alkahne magmatism on the Antarctic Peninsula with consumption of the Aluk Ridge, although it seems as though cessation took place as the ridge-crest approached, up to 50 Ma prior to collision, i.e. when young oceanic crust was being subducted. This effect was ascribed to dehydration of younger, more buoyant, oceanic crust at shallow depths where melting could not occur [36]. Thus the migration effect could result from variation along the arc in the age of adjacent oceanic crust, either due to non-parallelism to the ridge, or due to segmentation of the slab resulting in adjacent sections on either side of fracture zones having different ages and thermal structures [37-39]. Sinistral offsetting of the segments, as indicated by the present-day magnetic anomaly pattern, would cause younger crust to be subducted first in the southwest areas and progressively northeastwards. It is not entirely clear whether this explanation can be applied to the South Shetland Islands themselves, as well as the Antarctic Peninsula as a whole, since the apparent migration is more rapid (i.e. compressed) in the former situation. It may be that we are observing the overlap of two effects: a northeasterly migration of cessation as described above during the Jurassic and Cretaceous periods and a sudden change in the segmentation/thermal structure of the subducted ocean floor associated with initiation of the Aluk spreading centre at the start of Tertiary times.
A final point of interest is the unexpected discovery of Recent volcanism along the central axis of the South Shetland Islands--less than 0.5 Ma from localities in Livingston and Greenwich islands. Elliot et al. [17] report an age of 2.4 + 0.4 Ma for a plug at Edinburgh Hill, easternmost Livingston Island. The rocks concerned are typically very fresh olivine- and clinopyroxene-phyric basalts (lavas, plug and the only know palagonitized hyaloclastites). They are mildly alkaline (with up to 6.3% ne in the norm) and plot close to or in the field of alkaline rocks in the alkali-silica diagram (due to high N a 2 0 rather than high K 2 0 ). There is little obvious enrichment in incompatible trace elements compared to other South Shetland Islands' volcanic rocks, but since they clearly post-date the cessation of subduction 4 Ma ago they must be presumed to be related to the more conspicuously alkaline rocks of James Ross Island [40] and Alexander Island [41] (Antarctic Peninsula) or to the Recent volcanoes in Bransfield Strait-Deception, Bridgeman and Penguin islands [42]. All of these are associated with a change to extensional tectonics, although Bransfield Strait, an incipient back-arc spreading centre, may be due to continued gravitational sinking of oceanic lithosphere. In terms of geochemistry, the young samples analysed here most resemble the Penguin Island basalts rather than the axial Bransfield Strait volcanoes.
References 1 I.W.D. Dalziel, Large-scale folding in the Scotia arc, in: Antarctic Geology and Geophysics, R.J. Adie, ed., pp. 47-55, Universitetsforlaget, Oslo, 1972. 2 I.W.D. Dalziel, The early (pre-Middle Jurassic) history of the Scotia arc region: a review and progress report, in: Antarctic Geoscience, C. Craddock, ed., pp. 111-126, University of Wisconsin Press, Madison, Wisc., 1982. 3 J.L. Smellie, A complete arc-trench system recognized in Condwana sequences of the Antarctic Peninsula region, Geol. Mag. 118, 139-159, 1981. 4 S.D. Weaver, A.D. Saunders and J. Tarney, MesozoicCenozoic volcanism in the South Shetland Islands and the Antarctic Peninsula: geochemicalnature and plate tectonic significance, in: Antarctic Geoscience, C. Craddock, ed., pp. 263-274, Universityof WisconsinPress, Madison, Wisc., 1982. 5 A.D. Saunders, S.D. Weaver and J. Tarney, The pattern of Antarctic Peninsula plutonism, in: Antarctic Geoscience, C.
221
6
7
8
9
10 11
12
13
14
15
16
17 18
19
20
Craddock, ed., pp. 305-346, University of Wisconsin Press, Madison, Wisc., 1982). P.F. Barker, The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: ridge crust-trench interactions, J. Geol. Soc. London 139, 787-801, 1982. P.F. Barker, in: Symposium on Andean and Antarctic Volcanology Problems, O. Gonzalez-Ferran, ed., pp. 330-346, Proc., IAVCEI, Santiago, 1976. W.A. Ashcroft, Crustal structure of the South Shetland Islands and Bransfield Strait, Sci. Rep. Br. Antarct. Surv. 66, 43 pp., 1972. J.L. Smellie, A geochemical overview of subduction-related igneous activity in the South Shetland Islands, Western Antarctica, in: Antarctic Earth Science, R.L. Oliver, J.B. Jago and P.R. James, eds., Australian Academy of Science, Canberra, A.C.T., 1983. J.L. Smellie, R.J. Pankhurst, M.R.A. Thomson and R.E.S. Davies, Sci. Rep. Br. Antarct. Surv. (in press). R.H. Steiger and E. J~iger, Subcommission on Geochronology: convention on the use of decay constants in geo- and cosmo-chronology, Earth Planet. Sci. Lett. 36, 359-362, 1977. R.J. Pankhurst, S.D. Weaver, M. Brook and A.D. Saunders, K-Ar chronology of Byers Peninsula, Livingston Island, South Shetland Islands, Bull. Br. Antarct. Surv., 49, 277-282, 1980. E. Valenzuela and F. Herv~, Geology of Byers Peninsula, Livingston Island, South Shetland Islands, in: Antarctic Geology and Geophysics, R.J. Adie. ed., pp. 83-89, Universitetsforlaget, Oslo, 1972. J.L. Smellie, R.E.S. Davies and M.R.A. Thomson, Geology of a Mesozoic intra-arc sequence on Byers Peninsula, Livingston Island, South Shetland Island, Bull. Br. Antarct. Surv. 50, 55-76, 1980. M.R.A. Thomson, Late Jurassic fossils from Low Island, South Shetland Islands and Austin Rocks, Bull. Br. Antarct. Surv. 56, 25-36, 1982. J.L. Smellie, The geology of Low Island, South Shetland Islands and Austin Rocks, Bull. Br. Antarct. Surv. 49, 235-257, 1980~ D.H. Elliot, D.D. Duprb and T.M. Gracanin, personal communication to R.J.P. H. Fuenzafida, R. Araya and F. Herv~, Middle Jurassic flora from north-eastern Snow Island, South Shetland Islands, in: Antarctic Geology and Geophysics, R.J. Adie, ed., pp. 93-97, Universitetsforlaget, Oslo, 1972. R.A. Askin, Tithonian-Barremian spores, pollen and microplankton from the South Shetland Islands, Antarctica, in: Antarctic Earth Science, R.L. Oliver, J.B. Jago and P.R. James, eds., Australian Academy of Science, Canberra, A.C.T., 1983. G.E. Grikurov, A.Ya. Krylov, M.M. Polyakov and Ya.N. Tsovbun, Vorast porod v severnoi chasti Antarkticheskogo poluostrova i na Yudznykh Shetlandskikh ostrovakh (po dannym Kaliyargonovogo metoda). [Age of rocks from the northern part of the Antarctic Peninsula and the South Shetland Island (from data of the potassium-argon method)], Inf. Byull, Sov. Antarkt. Eksped. 80, 30-34, 1970.
21 R. del Valle, J. MoreUi and C. Rinaldi, Contrnes. Inst. Antarct. Argent. 175, 1974. 22 I.W.D. Dalziel, R. Kligfield, W. Lowrie and N.D. Opdyke, Palaeomagnetic data from the southernmost Andes and the Antarctandes, in: Implications of Continental Draft to the Earth Sciences, D.H. Tarling and S.K. Runcorn, eds., pp. 37-101, Academic Press, New York, N.Y., 1973. 23 D. Ferguson, Geological observations in the South Shetland Islands, the Palmer archipelago, and Graham Land, Antarctica, Trans. R. Soc. Edinburgh 53, 29-55, 1921. 24 D.D. Hawkes, The geology of the South Shetland Island, I. The petrology of King George Island, Sci. Rep. Falkld. Inst. Depend. Surv. 26, 28 pp., 1961. 25 C.M. Barton, The geology of the South Shetland Islands, III. The stratigraphy of King George Island, Sci. Rep. Br. Antarct. Surv. 44, 33 pp., 1965. 26 K. Birkenmajer, A revised lithostratigraphic standard for the Tertiary of King George Island, South Shetland Islands (West Antarctica), Bull. Acad. Pol. Sci., Ser. Sci. Terre 27, 49-57, 1980. 27 K. Birkenmajer, Tertiary volcanic-sedimentary succession at Admiralty Bay, King George Island (South Shetland Islands, Antarctica), Stud. Geol. Pol. 64, 7-65, 1980. 28 K. Birkenmajer, Lithostratigraphy of the Point Hennequin Group (Miocene volcanics and sediments) at King George Island (South Shetland Islands, Antarctica), Stud. Geol. Pol. 72, 59-73, 1981. 29 H.A. Orlando, The fossil flora of the surroundings of Ardley Peninsula (Ardley Island), 25 de Mayo Island (King George Island), South Shetland Islands, in: Antarctic Geology, R.J. Adie, ed., pp. 629-636, North-Holland, Amsterdam, 1964. 30 V. Covacevich and P.V. Rich, New bird ichnites from Fildes Peninsula, King George Island, West Antarctica, in: Antarctic Geoscience, C. Craddock, ed., pp. 245-253, University of Wisconsin Press, Madison, Wisc., 1982. 31 D.R. Watts, Potassium-argon ages and palaeomagnetic resuits from King George Island, South Shetland Islands, in: Antarctic Geoscience, C. Craddock, ed., pp. 255-262, University of Wisconsin Press, Madison, Wisc. 1982. 32 D.A. Valencio, J.E. Mendia and J.F. Vilas, Palaeomagnetism and K-Ar age of Mesozoic and Cenozoic igneous rocks from Antarctica, Earth Planet. Sci. Lett. 45, 61-68, 1979. 33 K. Birkenmajer, Mesozoic stratiform volcanic-sedimentary succession and Andean intrusions at Admiralty Bay, King George Island (South Shetland Islands, Antarctica), Stud. Geol. Pol. 74, 105-154, 1982. 34 K. Birkenmajer, Pliocene tillite-bearing succession of King George Island (South Shetland Islands, Antarctica), Stud. Geol. Pol. 72, 7-72, 1982. 35 D.C. Rex and P.E. Baker, Age and petrology of Cornwallis Island granodiorite, Bull. Br. Antarct. Surv. 32, 55-61, 1973. 36 S.E. DeLong, W.M. Schwarz and R.N. Anderson, Thermal effects of ridge subduction, Earth Planet. Sci. Lett. 44, 239-246, 1979. 37 P. Molnar and T. Atwater, Interarc spreading and cordilleran tectonics as alternates related to the age of subducted
222 oceanic lithosphere, Earth Planet. Sci. Lett. 41, 330-340, 1978. 38 P. England and R. Wortel, Some consequences of the subduction of young slabs, Earth Planet. Sci. Lett. 47, 403-415, 1980. 39 K.P. Furlong, D.S. Chapman and P.W. Alfield, Thermal modelling of the geometry of subduction with implications for the tectonics of the overriding plate, J. Geophys. Res. 87, 1786-1802, 1982. 40 P.E. Baker, F. Buckley and D.C. Rex, Cenozoic volcanism
in the Antarctic, Philos. Trans. R. Soc. London, Ser. B 279, 131-142, 1977. 41 C.M. Bell, The geology of Beethoven Peninsula, south-western Alexander Island, Bull. Br. Antarct. Surv. 32, 75-83, 1973. 42 S.D. Weaver, A.D. Saunders, R.J. Pankhurst and J. Tarney, A geochemical study of magmatism associated with the initial stages of back-arc spreading: Quaternary volcanics of Bransfield Strait, from South Shetland Islands, Contrib. Mineral. Petrol. 68, 151-169, 1979.