Geochronologic evidence for Early Cretaceous volcanic activity on Barton Peninsula, King George Island, Antarctica Hyeoncheol Kim, Jong Ik Lee, Moon Young Choe, Moonsup Cho, Xiangshen Zheng, Haiqing Sang & Ji Qiu Ages of six volcanic and plutonic rocks on Barton Peninsula, King George Island, were determined using “Ar/39Ar and K-Ar isotopic systems. The “Ar/”Ar and K-Ar ages of basaltic andesite and diorite range from 48 My to 74 My and systematically decrease toward the upper stratigraphic section. Two specimens of basaltic andesite which occur in the lowermost sequence of the peninsula, however, apparently define two distinct plateau ages of 52-53 My and 119-120 My. The latter is interpreted to represent the primary cooling age of basaltic andesite, whereas the former is interpreted as the thermally-reset age caused by the intrusion of Tertiary granitic pluton. The isochron ages calculated from the isotope correlation diagram corroborate our interpretation based on the apparent plateau ages. It is therefore likely that volcanism was active during the Early Cretaceous on Barton Peninsula. When the K-Ar ages of previous studies are taken into account with our result, the ages of basaltic andesite i n the northern part of the Barton Peninsula are significantly older than those in the southern part. Across the north-west- south-east trending Barton fault bounding the two parts, there are significant differences in geochronologic and geologic aspects. H . Kim. Isotope Research Team, Koreo Ba.tic Science Institute. Y e o - E m Dorig 52, Rtsung-Ku. Tuejon 305-333, South Korea; J . I . Lee & M . Y . Ciioe. Polar Science Lcihornfor?,, Korea Oceari Research & De~~elopitieni Institute, Str-dorrg 12 70, Anxiti 425- 170, South Korea; M . Clio, Dept. of Geological Scrmces, Seoul Nutionnl Uiiiwrsitv. Seoiil 151-742, South Koreu; X . ZlienR, H. Sang & J . Qiu. Institute of Geology. Chinese Acndernq of Sciences, Beijirig 100029. Cliiiiu. The South Shetland Islands. situated along the Pacific margin of the Antarctic Peninsula (Fig. I ), represent a magmatic arc of Jurassic to Quatern- ary age. Volcanic sequences and plutonic intru- sions in these islands built upon a pre-volcanic. sialic basement of pre-Jurassic schists and deformed sedimentary rocks. The latter is pri- manly composed of late Paiaeozoic-early Meso- zoic turbiditic submarine fan deposits (Smellie. Liesa et al. 1995). Marine sedimentary rocks containing Late Jurassic-Early Cretaceous fossil faunas are succeeded by terrestrial volcanic sandstones and conglomerates, then by basalt to dacite b a s interbedded with andesitic and rhyolitic ignimbrites (Smellie. Pankhurst et al. 1984; Arche et al. 1992; Willan et al. 1994; Smellie, Liesa et al. 1995; Hathway & Lomas 1998). King George Island is the largest island in the South Shetland Islands and mainly consists of volcanic and plutonic rocks of calc-alkaline affinity. Petrography and geochemistry of King George Island have been well documented by previous workers (Barton 1965; Birkenmajer Kim et al. 2000: Polur Research 19f21, 251-260 25 1 F i g . I . Structural elements of King George and Nelson islands (after Birkenmajer 1983) 1983: Smellie, Pankhurst et al. 1984: Tokarski 1988; Kang & Jin 1989; Lee et al. 1996). Ferguson (192 I ) has classified the stratiform volcanic complexes of King George Island into an older and a younger series, based on the observa- tion that the Andean intrusive suite intrudes the older series but never the younger series. The older series was tentatively correlated with Jurassic rocks of Graham Land. whereas the younger series with Cenozoic rocks (Tyrrell 1921). The temporal gap i n these volcanic complexes was confirmed by Birkenmajer (1980a-d): Cardozo Cove and Martel Inlet Groups at Admiralty Bay (Fig. Ic), pre- dating the Andean intrusions, are correlated with the older series of Ferguson (19211, whereas the King George Island Supergroup correlates with the younger series. Furthermore, based on lithology and stratigraphy, Birkenmajer (1983) divided King George Island into three tectonic blocks bounded by strike-slip faults (Fig. Ic). The Andean intru- sives are restricted to the central block, or the so-called “backbone”, corresponding to the up- thrown Barton Horst to which Barton Peninsula belongs (Birkenmajer, Francalanci et al. 1991). Birkenmajer (1983) and Kang & Jin (1989) reported Palaeocene K-Ar whole rock and Rb-Sr biotite ages (60-63 My) for mafic plutonic rocks. Recent geochronologic data from Barton Peninsu- la, however, suggest Eocene volcanic and plutonic activity (Park 1989; Lee et al. 1996). K-Ar whole- rock ages of volcanic rocks range from 35.5 to 48.5 My, whereas those of plutonic rocks mostly from 42.1 to 45.2 My (Park 1989). K-Ar biotite ages of plutonic rocks are in the range of 41.2- 41.9My (Lee et al. 1996). Because of intrinsic problems, such as the Ar loss in K-Ar isotopic ages, further discussion on the apparent discre- pancy is not warranted. In this paper, we present “Ar/39Ar as well as K- Ar ages of volcanic and intrusive rocks on Barton Peninsula, King George Island. “Ar/”Ar ages provided us with new geochronologic constraints not only for Cretaceous volcanic activity on King George Island but also the temporal relationship of volcanism in the South Shetland Islands. Geological setting and sample description Barton Peninsula is composed of volcanic and 252 Geochronologic evidence for Early Cretaceous volcanic activity on Barton Peninsula Fig. 2 . Geologic map of Barton and Weaver peninsulas, King George Island, Antarctica. U 4 R Marian Cove a Dhnite Agglomrate I Tuffaceous sandstone and s111Stom LOWW basalt - Fault - plutonic rocks together with minor thin-bedded sandstone and siltstone (Fig. 2 ) . Volcanic rocks in the peninsula mainly consist of basalt, basaltic andesite, agglomerate and lapilli tuff, whereas plutonic rocks include granodiorite and diorite. Acidic dykes are common in diorite. Fine-grained diorite, corresponding to the lower part of granitic pluton, consists of plagioclase, clinopyroxene, hornblende, biotite and Fe-Ti oxides with minor quartz. Medium-grained granodiorite, representing a main body of the pluton, is composed of plagioclase. quartz, alkali feldspar, hornblende, biotite and Fe-Ti oxides. The ubiquitous occur- rence of miarolitic cavities indicates that this pluton was emplaced at a shallow level i n the upper crust. Petrological and geochemical studies by Lee et al. (1996) further suggest that the plutonic rocks are vertically zoned. The pluton is bounded to the south-west by the north-west- south-east trending Barton fault, which apparently penetrates the whole peninsula. Stratification of volcanic rocks is well-pre- served to the north of the pluton along the sea cliff. Bedding planes of the volcanic sequences generally strike north-east or north-west and consistently dip south at 1040’ N E o r NW (Kang & Jin 1989). The lapilli tuff mainly occurs to the north of the Barton fault and locally to the south. The agglomerate occurs as a ca. 10 m thick layer along the southern coast of the peninsula. Both agglomerate and lapilli tuff contain a variety of volcanic glasses and breccias, but lithic fragments of crystalline rocks are absent. Basaltic andesites overlain by the lapilli tuff are widespread in the peninsula and generally por- phyritic in texture. Phenocrysts consist primarily of plagioclase together with minor clinopyroxene and orthopyroxene. Holocrystalline groundmass of basaltic andesite commonly shows flow textures defined by the preferred orientation of plagioclase microlites. Volcanic rocks on Barton Peninsula are affected by hydrothermal alteration ( S o et al. 1995) and low-temperature thermal metamorphism (Kim et al. 1995). Kim et al. (1995) defined two meta- morphic zones, changing from a calcite-chlorite zone to a amphibole-chlorite zone toward the pluton on the peninsula. Mineral assemblages of metavolcanics are represented by epidote + chlor- ite f hornblende f actinolite f calcite (+plagio- clase, quartz, opaque minerals), suggesting the greenschist to lower-amphibolite facies meta- morphism. Further details on thermal metamorph- ism will be reported elsewhere (Kim et al. 2000). We have analysed three basaltic andesites and one diorite for the “Ar/”Ar age determination and Kim et al. 2000: Polar Research 19(1), 251-260 2 5 3 two basaltic andesites for the K-Ar age determina- tion. All samples were collected from the northern part of the Barton fault. Fresh samples were chosen to minimize the effect of argon loss or contamination, but most of these samples con- tained small amounts of secondary minerals such as chlorite and white mica. In addition, all of amphibole crystals of diorite are altered comple- tely to actinolite. Analytical procedure "hrt"'Ar method The analysis for "'Ar/"Ar age determination was carried out at the Institute of Geology, Chinese Academy of Sciences, following the procedures described by Hu, Wang et al. (1985). The samples were ground into sizes of 60-80 mesh and packaged in aluminum foil together with the flux monitors. The flux monitors include ZBJ hornblende (132.8 f 3.1 Mya), ZBH-25 biotite (132.7 f 2.8 Mya) and international standard sam- ple, BSP-I hornblende (2060 f 18 Mya). Both samples and flux monitors were evacuated and sealed hermetically in quartz ampoules. K2S04 and CaF2 were added into each package to monitor the interfering reactions "K(n,p)"Ar, 11 K(n.a)40Ar. "OCa(t~,a)~~Ar, 4"Ca(n,na)36Ar and "Ca(n,a)"Ar. Then, two basaltic andesite (BPO5 and BP13) samples and one diorite (BP03) sample were put in the B4 channel of 49-2 Reactor for 4320 minutes and irradiated with fast neutron flux (0.56 x n/cm2sec). The total amount of the integrated fast neutron flux is 1.45 x 10l8 dcm'. On the other hand, one basaltic andesite sample (BP04) was placed in the B8 channel of 49-2 Reactor for 3174 minutes and irradiated with instantaneous fast neutron flux (4.22 x 10" n/ cm'sec) and the total amount of the integrated fast neutron flux is 8.04 x 10" n/cm2. A 0.5 mm thick cadmium foil was used as a shield to prevent the interference of slow neutrons. During the irradiation, the sample box was cooled by water, 42°C at the outlet. and was rotated 2 to 8 times per minute to eliminate a transverse gradient of neutron flux. After cooling to a safe dosage, the irradiated samples were put i n an Ar-extraction system to carry out step heating analysis. A high frequency oven was used to heat the samples at each step for 2 0 minutes. The extracted argon was purified by CuO and sponge titanium and then introduced directly to RAG-10 mass spectrometer for argon isotope analysis. Apparent ages were corrected for mass discrimination, interference of K and Ca to Ar isotope, and "Ar radioactive decay. The measured correction factors for the interference of K and C a to Ar isotope are ("Ar/39Ar)K = 3.05 x lo-', ("Arl'7Ar)Ca = 2.64 x lop4 and ( " A I - / ~ ' A ~ ) ~ ~ = 6.87 x lo-' (Hu, Wang et al. 1985). A half life of 35.1 days was adopted to correct the radioactive decay of "Ar. Uncertainty is quoted as one sigma and does not include that of the calculated J-factor. Isochrons were regressed following the method of York (1969). K - A r method Quantitative analysis of argon was performed by isotopic dilution method using a "Ar spike at the Institute of Geology, Chinese Academy of Sciences. Samples were fused at 1450 C-15OO'C for 4 0 minutes. CuO and sponge titanium were used as purifiers and 38Ar spike was added during the purification process. The purified argon was directly introduced into RAG- 10 mass spectro- meter for isotopic analysis. Potassium concentra- tion was measured by a flame photometer. The decay constants and isotopic abundance ratio of K used in the age calculation are as follows: A = 5.543 x 10-'oy-', I.,{= 4.962 x 1 0 - ' o y - l , A, = 0.581 x IO-'"y-'. 'OK/K = 0.01 167 (atomic %) (Steiger & Jager 1977). Results The K-Ar whole-rock ages of two basaltic andesites are listed in Table 1. The K-Ar age of BP09 is older than that of BP12, which was collected from a stratigraphically upper part near the Barton fault (Fig. 2). The '"Ar/"Ar incre- mental heating data and ages of basaltic andesite and diorite are listed i n Table 2. The results of 40Ar/39Ar whole-rock ages are shown in Fig. 3. Isotope correlation ages of basaltic andesite (BP04, BP05 and BP13) range apparently from 53 & 2 My to 1 18 f 4 My, whereas that of diorite (BP03) is 48.8 -+ 1.5 My. Specimen BP04 records an internally discordant 40Ar/39Ar age spectrum that apparently defines two "plateau" ages: 52 ? I My from low-tempera- ture fractions (steps 2 to 4 ) and 119 +. 1 My from high-temperature fractions (steps 6 to 9 ) (Fig. 3a). 254 Geochronologic evidence for Early Cretaceous volcanic activity on Barton Peninsula Tnhlr I . K-Ar whole-rock age\ of basaltic andesite BPO9 0. I761 0.75 o.9xiox 68.1 0.004383 73.9 It 4.4 BPI1 0.2780 1.04 0.83859 79.8 0.002702 -15.9 * 3.8 These plateau data define, respectively. two isochron ages in 36Ar/40Ar versus 39ArT"'Ar isotope correlation diagram (Fig. 3b). Four high- temperature steps, recording similar apparent ages, yield a plateau isotope correlation age of 118 f 4 My with a 40Ar/"6Ar intercept value of 290 f 3. These values are indistinguishable from the plateau age and the atmospheric ratio of 295.5, respectively. On the other hand, three or four steps at temperatures of 460Y-9OO"C yield an isochron age of 53 f 2 My which is identical to the apparent "plateau" age of 52 f I My within error range. The 40Ar/36Ar intercept value of 288 rf: 4 is larger than the 4oAr/3yAr ratio in the present-day atmos- phere, indicating the presence of minor excess Ar in low-temperature fractions. Specimen BP05 yield a '"Ar/"'Ar apparent age spectrum with characteristics similar to those of BP04, defining two apparent "plateau" ages of 53.1 f 1.5 Myand 1 2 0 . 4 f 1.6My(Fig.3c).These ages are consistent with the plateau ages of BP0-t. The old plateau age is based on three high- temperature steps comprising >70% of the 3"ArlC released. Furthermore, the C d K ratio of these steps (Table 2 ) are consistently high, suggesting the outgassing from actinolite-hornblende phenocrysts. In the isotope correlation diagram (Fig. 3d), data points of low-temperature fractions are too concen- trated in a small area to define an isochron age, high- temperature steps yield a plateau isotope correlation age of 105 k 5.5 My. Because the isotope correle- tion ages do not assume any "Ar/"Arratio, they are considered to be more reliable than the apparent plateau ages. Hence, the 105 rf: 5.5 My age is geologically significant and is interpreted to be close to the formation age of basaltic andesite BPOS. Specimens BP13 and BP03 record concordant whole-rock '"Ar/'"Ar apparent age spectra (Figs. 3e, g) and define the isochron ages of 60.6 f 1.4 My and 48.8 f 1.5 My, respectively. which are identical to plateau age within error range (Figs. 3f. g). When we consider the stratigraphic relationship of each sample, the K-Ar and 40Ar/"'Ar whole- 40 rock ages of basaltic andesite decrease progres- sively from the stratigraphically lower part (BP04 and BP05) to the upper part (BP13) (Fig. 4). The apparent ages from low-temperature fractions of BP04 and BP05 indicate that they were partly reset by the emplacement of plutonic rocks about 49 Mya (BP03). The "Ar/"Ar ages from high- temperature fractions. however, indicate the pre- sence of Cretaceous volcanic activity on Barton Peninsula. On the other hand, K-Ar and 4"Ar/"Ar ages of B P I 2 and BP03, respectively, are con- sistent with the previously reported ages (Fig. 4), suggesting an early Tertiary magmatism. Discussion and geologic implications Birkenmajer ( 1989) suggested considerable differ- ences in stratigraphic succession, ages and charac- teristics of three tectonic blocks: i.e. Fildes Block, Barton Horst and Warszawa Block. Barton Horst, upthrown relative to Fildes and Warszawa blocks and more deeply eroded than the other two blocks, exposes the plutonic rocks intruding into the old volcanic strata. Although Mesozoic volcanic activities are suggested in the Fildes Peninsula and Admiralty Bay of King George Island (Ferguson 1921: Davies 1982: Birkenmajer 1983; Birkenmajer. Narebski et al. 1983a), no Mesozoic isotope age of volcanic rocks are reported from Barton Peninsula. K-Ar and Rb-Sr ages of volcanic rocks i n the Barton Horst apparently range from Paleocene to Eocene (Birkenmajer, Narebski et al. 1983a, b; Kawashita & Soliani 1988; Park 1989). The K-Ar ages of volcanic specimens collected from the southern part of the Barton fault (Park 1989) (Fig. 4) range from 35.5 to 48.5 My. These ages are consistent with the K-Ar age of basaltic andesite, B P I 2 (45.9 f 3.8 My). As mentioned above. all of the analysed speci- mens were slightly to moderately altered and/or experiencsd low-grade metamorphism. Nonethe- less, the 4"Ar/3yAr ages obtained in this study are Kim et al. 2000: Polnr- Reseorch 1 9 ( 2 ) , 251-260 255 N V I m T u bk 2 . In cr em en ta l he at in g da ta f or '" A r/" A r ag e de te rm in at io n o f ba sa lt ic a nd es it e an d di or it e. T em p. '"A rc : '0 A r* /'9 A rK 3'1 A rc : A pp ar en t ag e S te p (' C ) (J "A r/ "A r) M (Z hA r/ 39 A r) k, (7 7A r/ 3" A r) M ('x A r/ 74 A r) M (1 0 - l2 m ol ) (+ I u) (% ) (* I c , M y) rn 4 G < 0 2. e, 0 W 6 B PO 4 1 (b as al ti c an de si te ) 2 J = 0 .0 13 39 3 w = 0 .3 5 g 3 5 6 7 8 9 46 0 65 0 80 0 90 0 I0 0 0 11 00 I2 00 I3 00 14 50 3 I S 38 0 8. 52 46 4. 48 28 4. 86 91 16 .4 52 0 9. 24 1 3 7. 42 05 11 .2 76 0 16 .8 75 0 0. 10 26 0. 02 05 0. 00 79 0. 00 89 0. 04 35 0. 01 45 0. 00 8 1 0. 02 23 0. 03 96 I . 97 4 1 0. 55 91 0. 91 36 0. 73 87 1. 87 61 2. 27 29 1. 32 34 4. 81 23 1. 03 22 0. 11 54 0. 07 65 0. 06 14 0. 03 82 0. 15 81 0. 04 97 0. 02 6 I 0. 06 59 0. I1 67 0. 18 0. 85 1. 36 3. 33 0. 72 1. 64 3. 68 1. 19 0. 57 1. 37 f 0 .1 0 I . 3 35 .0 f 2. 0 2. 11 f O .0 3 6. 2 50 .0 f 3. 0 2. 19 f 0 .0 2 10 .0 52 .0 f 1. 0 2. 28 f 0 .0 2 25 .2 54 .0 t 2. 0 3. 76 t 0 .0 5 5. 3 89 .0 f 5. 0 12 0. 0 f 3. 0 5. 13 f 0 .0 3 12 .1 5. 1 I f 0 .0 3 27 .0 11 9. 0 i 1. 0 11 8. 0 f 4. 0 5. 05 t 0 .0 4 8. 7 5. 28 f 0 .0 5 4. 2 12 3. 0 f 6. 0 B P0 5 1 (b as al ti c an de si te ) 2 J = 0 .0 1 0 68 3 W = 0 .2 5 g 4 5 6 7 8 52 0 65 0 78 0 90 0 I0 00 11 00 12 50 14 50 64 3. 67 00 2 13 .2 30 0 92 .7 10 0 10 3. 25 00 76 .2 40 0 2 1. 2 1 00 33 . I 80 0 14 8. 75 00 2. 16 32 0. 7 16 5 0. 30 58 0. 34 28 0. 23 09 0. 05 34 0. 09 88 0. 47 22 2. 74 77 1. 76 86 2. 16 33 0. 05 75 13 .2 30 0 13 .3 34 0 13 .8 13 0 5. 7 13 -3 0. 46 94 0. 16 54 0. 08 25 0. 08 94 0. 10 23 0. 10 43 0. 13 02 0. 18 75 0. 1 I 0. 27 0. 63 0. 79 0. 65 2. 49 1. 85 0. 3 1 13 7. 9 f 3 6. 1 7. 44 f 2 .1 0 1. 6 2. 53 f 0 .6 0 3. 8 48 .2 c 1 2. 6 2. 88 f 0 .3 0 8. 9 54 .6 t 5. 4 52 .5 f 5. 3 2. 77 f 0 .3 0 11 .1 6. 41 t 0 .2 4 9. 2 11 9. 5 f 4. 5 6. 56 f 0 .0 7 35 .0 12 2. 1 f 1. 8 6 .2 9 f0 .1 1 26 .1 11 7. 3 t 2. 5 18 7. 8 f 1 2. 2 10 .3 0 f 0 .5 0 4. 4 B P I3 I (b as al ti c an de si te ) 2 J = 0 .0 10 68 3 w = 0 .2 5 g 3 5 6 7 8 45 0 64 0 78 0 90 0 I0 50 12 00 I3 50 I5 00 38 .4 0 1 0 11 .9 18 0 I 1 .4 43 0 9. 01 52 11 .1 32 0 6. 32 86 5. 75 37 45 .7 14 0 0. 11 60 0. 03 33 0. 02 78 0. 01 97 0. 02 74 0. 01 05 0. 00 89 0. 13 43 2. 35 5 I 1. 07 63 1. 53 10 1. 04 47 1. 67 43 0. 56 39 0. 52 80 3. 95 59 0. 07 60 0. 02 47 0. 02 33 0. 0 I8 9 0. 02 55 0. 01 71 0. 02 15 0. 07 71 0. 53 1. 57 1. 93 2. 84 2. 28 9. 20 13 .7 0 0. 75 3. 43 t 0 .1 2 1. 6 1. 89 f 0 .0 4 4. 7 3. 36 f 0 .0 4 5. 9 3. 27 f 0 .0 3 8. 6 3. 19 i : 0. 03 6. 9 3. 36 f 0 .0 2 28 .1 3. 13 f 0 .0 2 31 .8 6. 50 f 0 .1 3 2. 3 83 .4 f 2. 4 36 .2 t 1. 3 63 .7 f 1. 3 6 1 .9 f 1. 1 60 .4 f 0. 9 63 .6 t 0. 8 59 .3 f 0. 7 12 1. 1 f 3. 9 B P0 3 (d io ri te ) J = 0 .0 10 68 w = 0 .2 0 g 46 0 64 0 75 0 85 0 95 0 10 50 12 00 13 50 15 00 14 0. 91 00 86 .6 67 0 38 .4 35 0 5. 02 59 5. 52 35 3. 51 72 4. 28 57 9. 48 72 11 4. 10 30 0. 45 45 0. 28 05 0. 12 41 0. 00 9 1 0. 01 05 0. 00 66 0. 00 59 0. 02 35 0. 33 89 13 .2 1 20 3. 13 29 1. 78 25 3. 06 52 0. 65 37 0. 30 2. 5 0. 39 85 0. 97 65 15 .7 87 0 0. 12 12 0. 08 21 0. 04 42 0. 03 55 0. 01 68 0. 0 15 9 0. 0 14 9 0. 02 0 I 0. 20 51 0. 14 0. 42 0. 63 16 .6 0 9. 22 18 .7 0 25 .6 0 5. 03 0. 17 8. 25 t 0 .4 5 0. 2 3. 91 i : 0. 27 0. 5 2 .0 2 fO .1 2 0. 8 2. 56 f 0 .0 2 21 .6 2. 46 f 0 .0 2 12 .1 2. 57 f 0 .0 2 24 .4 2. 56 f 0 .0 2 33 .5 2. 62 f 0 .0 3 6. 5 9. 79 f 0 .3 6 0 3 15 2. 4 f 73 .8 f 38 .5 f 48 .7 f 46 .8 f 48 .8 f 18 .6 t 49 .8 f 17 9. 5 t 8. 6 5. 2 2. 5 1. 5 I . 4 I . 5 I .5 1. 7 9. 5 .A r* : r ad io ge ni c A r: J: c al cu la te d J- fa ct or ; W : sa m pl e w ei gh t: M : m ea su re d va lu e: " A rt i: ca lc ul at ed ' "A r or ig in at ed f ro m p ot as si um 150 100 - Y r” 0) ul m c m a a a c E - : (a) B P O ~ +-1,2=119*1 My + I 2 - - - - 0.004 I 50 , 1 , , = 5 2 + 1 M y 0 1 1 1 1 1 1 1 1 1 , 0.003 0.002 0.001 0.000 200 X 150 W CI) z m 100 - c E a a 50 0 l , , = 53 i 2 Mv I 0.0 0.1 0.2 0.3 0.4 0 . 5 3PAr/ ‘OAr 0 10 2 0 30 40 5 0 60 70 80 90 100 Fraction of 39ArK released 0.003 L 4 0.002 . L a 0 0.001 0.000 0.0 0.1 39Arl ‘OAr 0.2 100 50 0.004 0.003 L a ; 0.002 4: 0 0.001 0.000 0 10 20 30 40 50 60 7 0 80 90 100 Fraction of 39ArK released 1 , = 60.6 f 1.4 My 4QArl’’Ar=294.7 i 2 . 3 0.0 0.1 0 . 2 0.3 0.4 3gArl ‘OAr 200 ( S ) BP03 W m m c m CL 4 I 100 E a 50 0 L t f--- 1 = 48.4 i 0 . 5 My -b l--===- 0 10 20 30 40 50 60 7 0 80 90 100 Fraction of 39Ar, released 0.003 0.002 0.001 0.000 1 , = 4 8 . 8 * 1 . 5 My ‘OArI 1sAr=295.i f 2.1 1 , = 4 8 . 8 * 1 . 5 My ‘OArI 1sAr=295.i f 2.1 0 0 0.1 0.2 0.3 0.4 39Ar/ ‘OAr Fig. 3. ‘”Ar/”Ar apparent ages and isotope correlation diagrams of hlrsdtic andesite and diorite from Barton Peninsula, King George Island. Antarctica. Abbreviations: P = apparent plateau age; 1 = isochron age: 1 = low-temperature fraction,; 2 = high-temperature fractions. (a-f) are for basaltic andesite. and (g-h) for dioritr. Kim et al. 2000: Polar Reseurch 19(2), 251-260 257 580 47' w 58" 44' w U ' I 0 c- F i g . 4. Map showing the 4"Ar/3"Ar and K-Ar ages compiled from this and previous studies of Barton Peninsula. King George Island. Filled circles represent the results of this study. Filled and open squares deiiote the results of Park (1989) and Lee et al. (1996), respectively. Filled stars refer to the locations of plant fossils reported by Chun et al. (1994). Abbreviations: tg, = biotite age; tw = whole- rock age: t l = isochron age; t , , = low-temperature fraction isochron age: t12 = high- temperature fraction isochron age. considered to be reliable because the initial 4"Ar/'6Ar ratios of analysed specimens are con- sistent with that of present atmospheric argon (295.5). Furthermore, the apparent plateau ages are compatible with isochron ages, which support this interpretation. The old isochron ages deter- mined in this study thus suggest an Early Cretaceous volcanic activity on Barton Peninsula, whereas the young ages are compatible with the Eocene ages previously determined from the plutonic rocks. Thus, the latter are interpreted to be the ages reset by the thermal effect during the emplacement of plutons. Plant fossils of late Palaeocene to early Oligocene ages occur in reddish to black shales intercalated with agglomerates in the southern part of Barton Peninsula (Fig. 4) (Del Valle et al. 1984; Chun et al. 1994). These results are incompatible with the K-Ar and "Ar/39Ar ages determined by this study, as well as with the previous suggestion of a Mesozoic age of volcanic rocks on the Barton Peninsula (Davies 1982; Birkenmajer, Narebski et al. 1983b). On the other hand. Tokarski et al. ( 1987) suggested a possible presence of Mesozoic volcanic rocks on Barton Peninsula. Furthermore, plant fossils documented from the Jurassic strata of Hope Bay, Antarctic Peninsula, are also reported from the Cardozo Cove Group of the Barton Horst (Zastawniak 1981; Birkenmajer & Zastawniak 1986). The Cardozo Cove Group of the Admiralty Bay (Fig. Ic) is correlated with the volcanic sequence of Barton Peninsula (Birken- majer 1982). Thus, we cannot preclude the occurrence of Mesozoic volcanic and volcaniclas- tic rocks on Barton Peninsula. Based on the similarity of K-Ar ages, Jwa et al. (1 992) suggested that the displacement ofthe Barton Horst with respect to the Fildes Block is negligible. K-Ar and 40Ar/39Ar ages from the eastern part of pluton, however, indicate the presence of Early to Late Cretaceous volcanism on Barton Peninsula and do not coincide with an age of 40-60 My for the Fildes Block. The agglomerate, the lowermost volcanic unit of southern Barton Peninsula, contains Eocene flora fossils but is absent in the north- western peninsula across the Barton fault. On the other hand, lapilli tuff abruptly diminishes in its occurrence to the south of the Barton fault. 25 8 Geochronologic evidence for Early Cretaceous volcanic activity on Barton Peninsula The apparent discrepancies in geologic and geochronologic data across the Barton fault can be accounted for by the presence of significant displacement across the fault. This fault is thought to be a transform fault which is related to the opening of the Bransfield Strait. Previously determined K-Ar ages from the volcanic rocks of the southern part of Barton Peninsula (Park 1989) are well correlated with Eocene volcanism having widely occurred in the Fildes and the Warszawa blocks of King George Island. Hu, Zheng et al. (1996) reported Late Cretac- eous to late Eocene (92-40 Mya) ‘“Ad3‘Ar and K- Ar ages in the northern Fildes Block of King George Island. These ages successively decrease toward the north-east, and Hu. Zheng et al. (1996) have attributed this systematic change to the gradual migration of the volcanic centre (Pank- hurst & Smellie 1983: Smellie, Pankhurst et al. 1983). Our result, however, indicates a large temporal gap along a vertical set of volcanic sequences in a narrow area. Furthermore. recent geochronologic studies on Livingston Island indicate an Early to Late Cretaceous (137- 73 Mya) volcanic activity (Smellie, Pallhs et al. 1996: Zheng. Hu et al. 1997; Zheng, Sang et al. 1998) corroborating our result. I t is thus likely that at least parts of King George and Livingston islands are composed of Early Cretaceous volcanic sequences. Conclusions 1 ) “Ar/3’Ar step-heating whole-rock ages deter- mination of basaltic andesite from Barton Penin- sula in King George Island suggest two separate magmatic activities: Early Cretaceous and middle Eocene. 2) “oAr/3yAr and K-Ar ages of volcanic rocks i n the northern pluton part are compatible with the stratigraphic relationship among sedimentary and volcaniclastic sequences. In spite of thermal resetting caused by the intrusion of pluton. 4oAr/39Ar step-heating ages of basaltic andehite preserve the evidence for Early Cretaceous volcanic activity in the northern Barton Peninsula. 3) Geologic and geochronologic differences across the north-west-south-east trending fault indicate the presence of tectonic discontinuity on Barton Peninsula. 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