Geology of Gjelsvikfjella and western Muhlig- Hofmannfjella, Dronning Maud Land, east Antarctica Y . O H T A , B . 0. TBRUDBAKKEN A N D K . SHIRAISHI Ohta, Y . , Terudbakken, B . 0. & Shiraishi, K. 1990: Geology of Gjelsvikfjella and western Miihlig- Hofmannfjella, Dronning Maud Land, east Antarctica. Polar Research 8, 99-126. As a part of the Norwegian Antarctic Research Expedition 1984185, geological mapping was performed in Gjelsvikfjella and western Miihlig-Hofmannfjella, Dronning Maud Land. The northern part of Gjelsvikfjella is dominated by the Jutulsessen metasupracriistals which have been intruded by a major gabbroic body and several generations of dykcs. To the south the metasupracrustals gradually transform into the Risemedet migmatites. In western Miihlig-Hofmannfjella the bedrock is dominated by the large Svarthamaren Charnockite batholith. The batholith is bordered by the Sn0toa metamorphic complex outcropping to the south and west in Miihlig-Hofmannfjella and it is characterized by a high content of partly assimilated country rock inclusions. Mineral paragenesis and geothermometry/geobarometry suggest a two-stagc tectonothermal-igneous history with an initial intermediate pressure, upper amphibolite to granulite facics metamorphism followed by high temperature transformations related to the charnockite intrusion. Thc age of the initial tectonothcrmal event is probably about 1,100 Ma. Geochronological work in the present study (Rb/Sr whole rock) gave an age of 500 2 24 Ma for the Svarthamaren Charnockite, interpreted to record the age of crystallization. Late brittle faulting and undeformed dolerite dykes outcropping in Jutulscssen are believed to be related to Mesozoic crustal stretching in the Jutulstraumen- Pencksokket Rift Zone to the west. Y . O h t a , Norsk Polarinstitutt, P . 0. Box 158, N-1330 Oslo Lufthaon, N o r w a y ; B.O. T@rudhakken, Saga Petroleum AIS, Kj@rboueien 16, N-1301 Sanduika, N o r w a y ; K . Shiraishi, Nationul lnsritute of Polar Research, T o k y o , Japan; A p r i l 1990 (rewised August 1 9 9 0 ) . The studied area is located to 2"-5"35'E and 71"46'-72"15'S, on the eastern side of the Jutulstraumen-Pencksokket Rift Zone (J-P RZ) (Neethling 1972) which separates a Proterozoic platform to the west and high-grade metamorphic areas to the east (Fig. 1). The J-P R Z is a key tectonic structure in correlating this part of Antarctica with the southeastern part of Africa (Grantham et al. 1988). Metamorphic rocks in the study area were first described by Roots (1953) and widespread reconnaissance mapping was undertaken by Soviet geologists (Ravich & Soloviev 1966), who also carried out radiometric dating on the rocks (Ravich & Krylov 1964). A 1 : 1.5 million scale geological map has been edited by Roots (1969) in the Antarctic Map Folio Series. The H.U. Sverdrupfjella area to the southwest has been described by Hjelle (1972). He recognized three lithological units, of which the Sveabreen Formation (granitized gneisses and migmatites of upper amphibolite facies) and the Rootshorga Formation ( psammo-pelitic, metamorphosed supracrustals) extend into the present area. Grantham et al. (1988) have described the geology and petrology of northern H.U. Sverdrupfjella in detail. They separated various igneous rocks from Hjelle's lithostratigraphic formations and carried out some age deter- minations. The oldest obtained age is about 900 Ma (Rb-Sr whole rock) from conformable granitoids in the gneisses. Late tectonic granites are about 470 Ma (Rb-Sr whole rock). The highest grade metamorphism, possibly a late magmatic stage, is determined to about 850°C and 9-11 kb. Most rocks were metamorphosedunder conditions of about 560"-690"C and 5-6 kb, followed by a retrograde overprinting. An alkaline complex near the J-P R Z has been dated to about 182 Ma (40Ar-39Ar) and 170 Ma (Rb-Sr, Allen 1990), indicating young extensional activity along the rift zone. According to Ravich & Krylov (1964), western Muhlig-Hofmannfjella consist of high-grade metasupracrustals of pre-Riphean ages and a 100 Y . Ohta, B. 0. Torudbakken & K . Shiraishi Fig. 1. Distribution of exposures in western Dronning Maud Land. T h e study area is in the eastern part of the map. Numhcrs: radiometric ages in million years ( M a ) obtained hy various methods (Kavich & Krylov 1Y64; Krylov 1972: Wolmarans & K c n t 1982; Moycs 1989). Analytical methods: P: Pb/Pb, U: U/Ph. R : Kh/Sr, S : Sm/Nd, A : 4UAr/3YAr. K: K/Ar. Analyscd samples: w: whole rock, h: biotite. m: muscovite, p: phengite, c: chlorite, z: zircon, f feldspar, wm: whole rock-minerdl. granite-granosyenite complex. Their K/Ar and Rb/Sr whole rock ages range from 400 to 480 Ma, with a 510 Ma age from Gjelsvikfjella. The Norwegian Antarctic Research Expedition 1984-85 mapped the main part of Gjelsvikfjella and western Miihlig-Hofmannfjella. Description of regional geology Gjetsvikfjella In Gjelsvikfjella the geological mapping covered Jutulsessen, Risemedet and Terningskarvet. The southwestern parts, including southwest Jutulsessen, Nupskammen and Von Essenskarvet were not visited, but distant observations were made from Terningskarvet towards these areas (Fig. 2). Three major lithologic units have been recognized: -Metagabbros in northeastern Jutulsessen -Jutulsessen metasupracrustals -Risemedet migmatites recognized: --4plite-pegmatite dykes -Metabask dykes -Mesozoic dolerite Three subordinate dyke types were also Metagabbros The metagabbro in Jutulsessen occurs as an intrusive body about 7 km long and 1 km wide. Geology of Gjelsvikfjella and western Muhlig-Hofmannfjella 101 Fig. 2. Geological map of Gjelsvikfjella, including distant observations by binoculara on the northern sidc of the Fjellimcllom- Bakhallct glaciers, Nupskammcn, Gluvrekletten and Von Esscnskarvet. X and Y i n circle: localities of the dated samples (ref. Tablc 8), BT-, YO-: localities of samples i n Tables 2 , 3 and 4. The rocks are medium-grained, melanocrafc, to white, aplite-pegmatite dykes, generally less hornblende-biotite gabbro, locally coarse- than 0.5 m thick. Most dykes show gentle dips to grained, with no preferred orientation of the the southeast, although some have steep dips. minerals. These rocks include fine-grained Similar dykes are closely associated with the cogenetic, rounded xenoliths with randomly gneisses and migmatites. oriented biotite, and are cut by numerous pink The main constituent minerals of the meta- 102 Y . Ohta, B. 0. Terudbakken & K . Shiraishi gabbro are green hornblende, often dusty with opaques, and commonly including granular clinopyroxene grains. Fresh dark brown biotite flakes are sometimes bent. Plagioclase has normal zoning with andesine-oligoclase composition, and shows blocky and for wavy extinction. The coarse- grained gabbroic facies contains a large proportion of clinopyroxene and locally carries olivine. Small grains of apatite, sphene and opaques are the accessories. The contact to the Jutulsessen meta- supracrustals is intrusive and a wedge of the metagabbro, about 100m wide, cuts the latter north of Stabben. Fine-grained xenoliths, consisting of primary prismatic plagioclase and granular clinopyroxene, may be a doleritic early facies of the metagabbro. Pale green hornblende surrounds the clino- pyroxene grains. Randomly oriented, over- growing dark brown biotite may be due to secondary changes, when the rock was captured as xenoliths. An ore-rich phase which is developed in connection with the later aplite-pegmatite dykes consists of more than 80% magnetite and coarse- grained microcline, plagioclase and sub- idiomorphic quartz. Small amounts of biotite and muscovite surround the ore grains. Jutulsessen metasupracrustals Micaceous and felsic gneisses are the main constituents of this lithological unit (Table 1). The gneisses are found in the northern half of Jutulsessen, and extend to the east, across the Slithallet glacier to Medmulen. The metagabbro discordantly cuts these rocks in the north, whilst on the southern side a gradational transition is observed to the Risemedet migmatites. A steep E-W trending fault, the Armlenet Fault, offsets the rocks in northern Armlenet. a) Lower succession An approximately 4.5 km thick succession of gneissose and schistose rocks is exposed in the cliffs of Grjotlia in western Jutulsessen. The lower succession, 1.3 km in thickness, which consists of alternating felsic and micaceous gneisses, occurs below a layer-parallel shear fault in this area. The micaceous gneisses are mainly biotite- bearing rocks with or without garnet. The other main minerals are microcline, ~ quartz and Table 1 . Lithostratigraphy of the Jutulsessen metasupracrustals. Mesozoic dolerites Aplite - pegmatites Metagabhro Basic dykes and sheets Jutulsessen metasupracrus- tals (c. 4,500m) (lateral change into the Risemedct migmatites) Upper succ. (700 m) Middlc succ. 2,600 m) (2,200- Lower succ. (1,300111) -felsic, migmatitic gneisses with biotite rich and basic .paleosomcs felsic gncisscs with micaceous gneibsc\ and agmatites (500 m) pink felsic gneiss and agmatites (1,800 m) felsic gneiss (300 m ) three sequences of micaceous gneiss with thin biotite rich layers (400 m) micaceous handcd gneisses (550 m) felsic gneisses with basic paleosomes (1SOm) dark biotitc gneisscs (150m) micaceous gneisses (80 m) dark biotite gneisses felsic gneisses (150 m) plagioclase (An1523 with normal zoning) (Tables 2 and 3). The most biotite rich gneisses form thin layers and contain strongly pinitized cordierite and hercynite, both occurring as inclusions i n later andalusite. Reddish brown biotite, with T i 0 2 contents up to 4.4wt%, is stable in most rocks; locally it displays symplectite texture with fibrous sillimanite (Tables 2 and 3). Garnet- biotite clusters of cm size in a plagioclase matrix are often observed in the biotite rich gneisses. The cores of the clusters are composed of flaky, brown biotite (TiOz approximately 1.5 wt%) and granular garnet. Most garnet grains are strongly fragmented and are usually free from inclusions, but a few contain biotite and quartz inclusions. The garnet has a Mg content which decreases, while the Fe and Mn contents increase from centre t o rim, suggesting a retrograde readjustment of Geology of Gjelsvikfjella and western Miihlig-Hofrnannfjella 103 Table 2 . Metamorphic mineral assemblages of the micaceous gneisses from Gjelsvikfjella, Jutulsessen metasupracrustals. sil ky and cord sp ga hb mus bi pl kf q z il rut BT5 x x x X X BT13 X x x x X X YO14 X x x x X YO58 X X x x x X Y O E G X X X X X Y 0 1 4 B X X X x x X X BT4 I X X X YO54 X X X YO133 x YOS8B x X X X X x x x X X Accessories: apatite, zircon, allanite, monazite and secondary sphene, cpidote, chlorite and sericite. Muscovites (mus) listcd are large lepidohlastic flakes (not alteration prciducts). Magnetite may exist and has not been discriminated from allanite. x x x x x x x x x X X X X X X X x x the rim composition. Porphyroblasts of plagioclase and microcline augen are occasionally developed in the micaceous gneisses. Myrmekitic plagioclase also occurs occasionally. Sphene, apatite, zircon, graphite and monazite are the accessories. The most biotite rich rock, containing domains T d d r 3. Keprcscntativc microprobe analyses of metamorphic minerals from t h c rocks of the Jutulsesscn metasupracrustals. Lociilitics of the samples i n Fig. 2. ... Y058B: garnct biotite gneiss, Grjotlia garnet biotite SP core rim core flaky pscudm. i n ga SIO: TIO? A1201 C r ? 0 3 FcO* MnO ( ' a 0 Na10 K?O ZnO Total MgO 38 32 36 89 0 0 U. 14 0.06 32.29 35.26 1.20 2.51 6.12 3.60 1.20 0.92 0 0 0 0 21.56 21.06 35.62 3.29 18.75 0.06 18.68 0 . 0 5 11.41 0 0.28 9.04 97.18 - 35.41 3.20 18.75 0.12 18.40 0.04 1 I .26 0 0.21 8.83 96.22 - 34.62 2.71 19.56 0.03 17.33 0 11.14 0 9.05 94.69 0.25 - Kation ratio 0 12 12 22 22 22 Si 2.999 2.970 5.2Y1 5.301 5.251 TI 0 0 0.367 0.360 0.309 A l 1.988 1.998 2.709" 2.6')9* 2.749* Cr 0.009 0.004 0.007 0.014 0.004 Fc 2.113 2.374 2.320 2.303 2.198 Mn 0.080 0.171 o.no6 0.00s 0 Mg 0.714 0.432 2.526 2.513 2.519 Ca 0.101 0.079 ( I 0 0 N a 0 0 0.081 0.061 0.074 K 0 0 1.713 1.686 1.751 Zn 0.573 * * 0.609" * 0.748** ~ - - - - X M g 0.253 0.154 0.521 0.522 0.534 FeO* = total Fe as FeO, Al* = All", A l * * = AI"', XMg = Mg/Mg + Fe in all analyses. Feldspars single separated grains in the matrix, unless specified. pl:An=27.9,Ab=71.3,Or=0.9andAn=26.1,Ab=73.5,Or=0.4 0.02 0.09 59.70 0.19 34.31 0.18 4.16 - 1.37 100.02 4 0.001 0.002 i.on2 0.004 0.808 0.004 0.004 0.175 - 0.028 0.178 104 Y . Ohta, B . 0. T@rudbakken & K . Shiraishi Table 3 . (continued) garnet core rim Y014: garnet biotite gneiss, SW of Stabben matrix biotite flaky' in ga S i 0 2 T i 0 2 A1203 C r 2 0 3 FeO' MnO CaO N a 2 0 K20 Total MgO 37.40 0.01 20.82 0 32.56 2.53 3.81 2.29 0 0 99.42 36.36 0 20.56 0 33.38 6.34 2.22 1.30 0 0 100.16 Kation ratio 0 12 12 Si 3.009 2.967 Ti 0.001 0 A1 1.975 1.977 Cr Fe Mn Ca Na K Mg XME 0 2.191 0.172 0.457 0.197 0 0 0.173 0 2.278 0.438 0.270 0.114 0 0 0.106 34.39 3.10 16.61 0.07 21.57 0.27 9.12 0.02 0.06 9.34 94.55 22 5.374 0.364 2.626* 0.433** 0.009 2.819 0.036 2.125 0.003 0.018 1.862 0.430 35.29 1.78 17.09 0.05 21.41 0.12 9.68 0.01 0.05 9.45 94.93 22 5.465 0.207 2.535* 0.5X5"* 0,006 2.773 0.016 2.235 0.002 0.015 1.867 0.446 33.72 0.61 21.80 0.17 19.71 0.25 8.25 0.01 0.05 8.31 92.88 22 5.244 0.01 1 2.756* 1.240** 0.021 2.564 0.033 1.913 0.002 0.015 1.649 0.427 Fcldspars pl: An = 23.3, Ah = 75.1, Or = 1.6 and An = 15.8, Ab = 82.9, Or = 1.4 flaky': very thin biotite flake near garnet YOEG: garnet biotite gneiss, Medmulen garnet biotite core rim* rim** coarse small' S i 0 2 38.48 Ti02 0.04 A1203 21.43 ( 3 2 0 3 0.01 FeO* 25.92 MnO 0.57 MgO 6.13 CaO 7.11 N a 2 0 0 K 2 0 0 Total 99.69 Kation ratio 0 12 Si 3.002 Ti 0.002 A1 1.970 Cr 0.001 Fe 1.691 M n 0.038 37.35 0 20.30 0.04 31.48 1.90 2.23 5.37 0 0 98.67 12 3.036 0 1.944 0.003 2.140 0.131 38.51 0 21.72 0.05 26.79 0.43 7.06 4.66 0 0 99.22 12 3.005 0 1.998 0,003 1.748 0.028 34.59 4.20 15.42 0 24.15 0.07 8.42 0.02 0.01 8.99 95.87 22 5.384 0.492 2.616* 0.213'* 0 3.144 0.009 35.54 2.86 15.72 0.23 22.59 0.07 9.09 0.11 0.04 8.73 94.98 22 5.519 0.334 2 481' 0.396** 0.028 2.934 0.009 Geology of Gjelsuikfjella and western Muhlig-Hofmannfjella 105 Table 3 . (continued) Ca 0.594 0 468 Na 0 0 K 0 0 Mg n.713 0.271) XMg 0.297 0.112 0.821 1.954 2 104 0 0.003 0.012 0 1.785 1.729 0.320 0.383 0.418 0.390 0.003 0.018 rim' = adjaccnt to plagioclasc rim" = adjaccnt to sillimanitc small': thin biotite flake around garnet Feldspars PI PI Pl PI kf An 44.3 40.4 46.3 50.8 0.3 Ah 54.8 59 .0 53.2 48.6 0.5 Or 0.9 0.6 0.5 0.6 99.2 A kyanite grain occurs as inclusion in thc garnct OPX biotite BT16: granulitic gneiss, N E Stabbcn h o r n b cumming. SiOL 50.86 TiOL 0.05 A120, 0.54 Cr203 0 FeO* 27.62 MnO 0.65 MgO 19.60 CaO 0.35 Na,O 0.02 K 2 0 0 Total 99.6') Kation ratio 0 6 Si 1.956 TI 0.001 Al 0.1)24* 0 Cr I) Fc 0.888 Mn 0.02 1 Mg 1.124 Ca 0.014 N a 0.001 K 0 XME 0.559 Feldspars 36.81 4.49 14.35 0.02 17.56 0.04 P3.46 0.04 0.21 X .90 95.88 22 5.529 0.507 2.471* O.O69** 0.002 2.206 0.005 3.014 0.006 O.061 1.705 0.577 pl: An = 48.3, Ah = 51.2. O r = 0 . 5 and An = 41.5, Ah = 58.1, O r = 0.4 48.73 1.00 6.63 0.04 13.77 0.20 14.88 10.79 0.99 0.34 97.37 23 7.123 0.110 0.877" O.2hb* * 0.005 1.683 0.025 3.243 1 .6YO 0.281 0.063 0.658 54.28 0.03 0.39 n 22.90 0.53 18.44 0.70 0.07 0 97.34 23 7.942 0.003 0.058* 0.009 * .* 0 2.802 0.066 4.022 0 . l l O 0.020 0 0.589 garnct corc rim BT24: amphibole gneirs, E Jutulratcr O P X CPX biot , hornbl. cumming S O L 39.79 36.61 48.75 49.58 36.29 43.09 51.37 T i 0 2 0.13 0.02 0.04 0.12 3.66 1.61 0.02 A1203 21.25 20.76 0.44 0.74 14.78 9.42 0.33 MnO 1.12 2.32 1 . O Y 0.39 0 0.27 0.83 C r K h 0.01 0 0.01 0.02 0 0 0.03 FeO* 25.73 30.14 37.32 15.66 19.21 21.13 30.58 106 Y . Ohta, B . 0. Torudbakken d K . Shiraishi Table 3 . (continued) MgO 4.10 C d 0 9.20 NazO - K2O Total 99.33 Kation ratio 0 12 Si 2.988 Ti 0.008 Al 1.Y80 - Cr 0.001 FC 1.701 M n 0.075 Mg 0.483 Ca 0.779 Na - K - XMg 0.221 Feldspars PI' 2.23 6.57 - - 98.65 12 2.979 0.001 1.991 0 2.051 0.160 0.270 0.573 - - 0.1 I 7 PI2 10.84 0.84 0 0 99.33 6 1.982 0.001 0.018* 0.004** 0 t 269 0.038 0.657 0.037 0 0 0.341 PI3 8.90 21.12 0.20 0 96.73 6 1.981 0.004 0.019* 0.016** 0.001 0.523 0.013 0.530 0.904 0.015 0 0.503 11.05 0.07 0.01 9.24 94.31 22 5.594 0.424 2.406' 0.279** 0 2.476 0 2.539 0.012 0.003 1.817 0.506 7.81 11.49 10.65 0.82 1.51 0.02 0.67 0 96.16 95.49 23 23 6.692 8.000 0.188 0.002 1.308* 0* 0.416** 0.060** 0 0.004 2,744 3.983 0.036 0 109 1.808 2.668 1.772 0.137 0.455 0.006 0.133 0 0.397 0.401 An 48.1 44.7 52.4 Ah 51.0 54.4 46.9 Or 1.0 0.9 0.7 pl': core, pI2: rim, PI': adjacent to garnet. Ga-pl and hb-pl symplectites occur locally. with and without quartz, occurs as restite in a migmatitic gneiss (Tables 2 and 3). In the quartz bearing domains garnet porphyroblasts are converted into a biotite, plagioclase assemblage. Biotite, rutile, ilmenite and hercynite (Mg/ Fe + Mg = 0.22, ZnO = 0.6 wt%) also occur as inclusions i n garnet. The Mg/Fe + Mg ratios of the central parts of the garnet are as high as 0.28, whereas that of the biotite, which is constant and independent of the texture and assemblages, is 0.50-0.53. Biotite inclusions in the garnet have slightly higher ratios of about 0.55. Plagioclase compositions in the matrix are An14 to An17. In the quartz-free domains aggregates of sillimanite, hercynite (Mg/Fe + Mg = 0.15-0.19, ZnO = 1.4 wt %), plagioclase (An23-27), biotite and small amounts of muscovite, chlorite, rutile, ilmenite and secondary kaolinite are found. Her- cynite is also found as scattered inclusions in large biotite flakes. Andalusite occurs as porphyro- blasts, surrounded by hercynite-plagioclase aggregates. These are overgrown by biotite which is chloritized along the margins. Sillimanite is partly converted to muscovite. The described tex- tural relationships of andalusite and sillimanite may indicate a late temperature increase under relatively low pressure conditions. The felsic gneisses contain less than 5% dark brown lepidoblastic biotite in a granoblastic matrix consisting of microcline, plagioclase and quartz. Plagioclase grains sometimes have albite rims and myrmekitic texture. Small amounts of pink garnet occur in some aplitic gneisses. An isolated small grain of orthopyroxene has been found in a felsic gneiss, coexisting with anti- perthitic plagioclase. The basic rocks always occur as blocky paleo- somes in the felsic and banded biotite gneisses and such agmatitic gneisses form distinct layers of some tens of metres in thickness. The basic rocks show varying degrees of disruption, ranging from conformable lenses to torn apart dykes or sheets, indicating their emplacements at different stages of the tectonothermal history. Their tex- tures also vary, ranging from completely recrys- tallized granoblastic amphibole gneiss to weakly recrystallized igneous fabrics. The granoblastic amphibolitic gneisses consist of alternating bands of hornblende-biotite-plagioclase and clinopy- roxene-plagioclase-quartz (Tables 3 and 4). Some bands have abundant garnet porphyroblasts in polygonal plagioclase domains. The compo- sitional profile of the garnet shows that the Mg/ Fe + Mg ratio decreases towards the margins Geology of Gjelsvikfjella and western Miihlig-Hofmannfjella 107 (Fig. 3a). Brown hornblende is replaced by green amphibole around its margins and orthopyroxene is converted to cummingtonite, which coexists with the green hornblende (Tables 3 and 4). Sub- angular basic paleosomes commonly consist dom- inantly of prismatic hornblende which often includes clinopyroxene grains. Most hornblende, however, displays a coarse-grained lepidoblastic texture which is associated with plagioclase of andesine-oligoclase composition, and is over- grown by biotite. One basic paleosome displays garnet-plagioclase clusters, whilst some garnet porphyroblasts are partly overgrown by a hornblende-clinopyroxene-plagioclase mosaic. Brownish green hornblende of this rock has both ortho- and clinopyroxene inclusions. b) Middle succession Above the layer parallel shear fault about 2,600 m of the middle succession are exposed. The lower 700 rn dominantly consist of felsic gneisses, but include three layers, 50-150111 thick, of fine- grained, garnet-bearing, micaceous gneisses. Some mica rich layers are fine-grained and schistose, whilst others have feldspar por- phyroblasts. The middle 1,800 m are dominated by pink felsic gneisses, often granitic i n composition. Sev- eral layers of agmatite, including basic and biotite rich paleosomes, occur concordantly in the , I Table 4. Metamorphic mineral assemblages of the amphibolites from the Jutulsessen metasupracrustals. YO18 YO9 BT17 Y 060 BTlO B'I2.4 YO60 YO92 BT24 includes garnet and cummingtonite. Accessories: apatite, zircon, allanite and secondary epidote, zoisite, muscovite, chlorite, sphcne. BT16 (Table 3) has a similar assemblage to YO42 and secondary cummingtonite surrounding hornblende. ( ): almost totally converted into hornblende and possibly plagioclase. opx cpx hb hi pl kf qz il x x x x x x x x x x x x X x x x x x x x x x x x x x (4 x x X X x (x) x x x X X x x x x x x x gneisses. The basic paleosomes are aligned in some places, and indicate that the original trend of the dykes was oblique to the gneissosity, while others are concordant lenses. The upper 500 m of the middle succession con- tain two layers of banded biotite gneisses and several thin agmatitic layers in felsic gneisses. These rocks form the western ridge of Grjotlia, and extend along the southern ridge of Jutuls- essen. The felsic gneisses in the upper part of the middle succession have a more granitic com- position than those described in the lower part. The matrix is microcline rich and plagioclase Mg/Mg+Fe 0.16 1 0.1 L 0.12 , 1mm , 0.8 1 1 0.L l m m I I 0.3 Fig. 3 . Compositional profiles of garnets: a) from a garnet amphiholite (BT24. ref. Tables 3 and 4 ) , eastern part of Jutulsessen; b) from a garnet biotitc gneiss (ref. TdbkS 2 and 3 ) . northern Medmulcn, castcrn Gjclsvikfjclla. 108 grains, mainly oligoclase, are commonly myr- mekitic. Biotite, which is the only major mafic constituent, shows signs of chloritization around the margins. Disruption and rotation of basic and biotite rich paleosomes and modal increase of microcline may indicate that parts of the felsic gneisses were mobilized in this area. Y . Ohta, B . 0. TGrudbakken & K . Shiraishi c) Upper succession The upper succession, which is more than 700 m in thickness, is dominated by felsic-granitic gneisses. The succession extends from the western ridge of Grjotlia to Jutulhogget to the east, where it forms a cliff approximately 500 m high. The micaceous and agmatitic layers are rare, and become more felsic, shadow-like discontinuous lenses. The fel- sic gneisses laterally change into granitic mig- matites of various textures, and increase in thickness towards the north. Some basic paleo- somes are enriched in biotite, and display ptyg- matic folds. The upper gneisses extend to Brugda where the thickness is estimated to be more than 500 m. Various migmatites are present in Brugda and in the eastern side of Jutulsessen, and these rocks are similar to the Risemedet migmatites. Garnet-biotite gneisses and agmatites with basic paleosomes make up the middle part of the eastern slope of Armlenet. These rocks are interlayered with granitic lenses, and have a thick- ness of about 700m. Stabben, a distinct tower in northeastern Jutulsessen, is composed of a relatively homogeneous, coarse-grained granite with sharp contacts to the gneisses and migma- tites. Aplites in this area often contain small grains of red garnet. Some garnet-biotite gneisses have small prismatic sillimanite. Felsic gneisses occur in western Armlenet and northern Stabben, whilst the area south of Stab- ben is dominated by various migmatites. In west- ern Armlenet the succession consists of an approximately 500 m thick unit of felsic gneiss, interbedded by numerous agmatite layers and overlain by a garnet-biotite gneiss of 100 m thick- ness and a 200 m thick felsic gneiss. The basic paleosomes are enriched by hornblende and con- verted into small hornblendite bodies some tens of cm in size. An alternation of plagioclase por- phyroblastic biotite gneiss and granitic gneiss occurs between the two peaks of Stabben. These rocks laterally grade into agmatitic and shadow- like migmatites northeast of Stabben. A biotite rich gneiss from northeastern Stabben has granoblastic ortho- and clinopyroxene with reddish biotite. Antiperthitic plagioclase and partly mesoperthitic orthoclase are common in this rock. A basic paleosome in the same area has granular clinopyroxene and ortho-pyroxene- biotite aggregates in a complex symplectite texture. Plagioclase shows irregular com- positional domains in each grain. Pale green, poi- kiloblastic hornblende includes clinopyroxene grains, and is partly converted into cummingtonite during a retrograde process. The Rabben nunataks 10km north of Jut- ulsessen are composed of coarse-grained, biotite and hornblende gneisses, each 20-30 m thick. Dark, biotite rich boudins represent an older basic rock, whereas fine-grained schistose amphibolite is younger. A 7 m wide metagabbro dyke cuts the gneisses. A 100 m thick, regularly banded biotite gneiss occurs in the southern part of Rabben. A separated small nunatak to the south consists of granitic gneiss. Granitic gneiss with thin layers of garnet-biotite gneiss and agmatite with basic paleosomes are present to the east in Medmulen, Medhovden and Gygra-Risen at the northeastern edge of Gjelsvikfjella. Some biotite rich paleosomes in this area have large poikiloblastic garnets with inclusions of kyanite, ilmenite, rutile, potash feld- spar and plagioclase (An51). A compositional profile of the garnet shows a convex shape for Mg with a decrease near the margins (Fig. 3b). Sillimanite rarely occurs in contact with garnet and plagioclase in the matrix. d) Original rocks of the Jutulsessen metasupracrustals Complex deformation and metamorphic recrys- tallization make estimation of protolith rocks dif- ficult and uncertain. However, the occurrence of Al-silicates, cordierite and garnet in dark micaceous layers may suggest originally argil- laceous to sandy sediments. A possibility of acid volcanic rock origin for the felsic gneisses cannot be ruled out. Basic rocks probably originated from both syn- and late-tectonic basic dykes and sheets. e) Metamorphic conditions The metamorphic mineral assemblage in the pel- itic gneisses commonly includes biotite and Geology of Gjelsuikfjella and western Muhlig-Hofmannjjella 109 garnet. Sillimanite has been found to b e a major constituent in some rocks (Tables 2, 3 and 4). Occurrence of kyanite as inclusions in garnet (Tables 2 and 3, sample Y O E G ) indicates an intermediate pressure facies series for the pro- grade process. Orthopyroxene occurs in some silica undersaturated rocks, suggesting the maxi- mum metamorphic grade is transitional from upper amphibolite facies t o granulite facies. Cordierite and hercynite in some rocks may reveal a pressure fall after the metamorphic climax. Andalusite porphyroblasts found in a silica undersaturated gneiss a r e replacing an older gar- net-biotite assemblage, and are overgrown by sillimanite and biotite. T h e sillimanite is partly converted into muscovite. These reactions reveal a high temperature facies metamorphism fol- lowing a cooling period after the older meta- morphism, which is probably related to a thermal event during the emplacement of large masses of charnockite, which will be described later. For the calculation of the maximum meta- morphic conditions, geothermometry and geo- barometry were applied. Normally, garnet has a high content of C a and Mn and biotite has dis- tinctive amounts of Ti. Therefore, garnet-biotite geothermometry is not applicable for most rocks. However, a central part of the garnet and its inclusion biotite from a gneiss of Grjotlia gave a temperature range of 650-750°C by the cali- bration given by Thompson (1976). The garnet- rutile-sillimanite-ilmenite ( G R A I L ) geobarome- t e r (Bohlen e t al. 1983) for the same rock gave 8 * 1 kb at -760°C. Rocks from Sverdrupfjella to the west of Gjelsvikfjella were examined for comparison, using the samples collected by Iijelle in 197CL-71. In this area orthopyroxene has not been found in intermediate and basic rocks, suggesting that the metamorphic grade did not reach granulite facies. A garnet porphyroblast has sillimanite and quartz inclusions, and indicates upper amphibolite facies. Garnet-biotite geothermometry (Thomp- son 1976) gave 750°C for t h e central part of the garnet, t h e geobaronietry of Newton & Haselton (1981) yielded about 7 k b at -750°C and G R A I L gave 8 k b at -750°C for a garnet and its inclusion biotite. These calculations and the lack of ortho- pyroxene show a transition from upper amphibo- lite facies t o granulite facies, which is similar to that of t h e rocks of Gjelsvikfjella. These results a r e somewhat higher than those obtained by Grantham e t al. (1988): 560-690°C and 5-6 kb for the main metamorphism in northern H . U . Sverdrupfjella. An ultramafic rock sampled in this area shows late magmatic conditions of 850°C and 9-10 kb. T h e metamorphic conditions cal- culated for the rocks in the areas from Kir- wanveggen t o middle Miihlig-Hofmannfjella are summarized in Fig. 4. Risemedet migmatites This migmatite complex is defined as a lithologic unit. T h e Jutulsessen metasupracrustals show gra- dational transition t o t h e Risemedet migmatites in the south. Banding in biotite gneisses gradually disappears and biotite is concentrated in thin seams in granitic-aplitic rocks. T h e latter contain microcline and plagioclase. Plagioclase has albite rims and well developed myrmekite texture. T h e thick felsic gneisses around Jutulhogget show a very faint gneissosity of irregular orientation and basic dykes show strong ptygmatic folds and rotated fragments, indicating mobilization of the felsic gneisses. T h e gneisses in the middle part o f the eastern slope of Armlenet have agmatitic and shadow-like paleosomes in a medium-grained granitic gneiss. Gneissosity is locally difficult to detect. Similar migmatites occur in the eastern side of Stabben. T h e basic paleosomes in t h e gneisses change into more biotite enriched conformable layers, having distinct amounts of microcline and quartz. Some basic rocks in Armlenet cut the gneissosity of both gneiss paleosomes and granitic metatect and include angular blocks of granite. The basic rocks themselves a r e cut by aplite and pegmatite dykes. Migmatites with streaky, shadow-like and biotite rich paleosomes occupy the Risemedet and Horten areas. Similar rocks are probably also present in Medhovden based on distant observation. T h e modal content of hornblende is exceeded by biotite in the basic paleosomes. Orthoclase perthites are often present in both gneissic and basic paleosomes. T h e gneissic paleosomes lose their clear sharp boundaries t o the granitic metatect, and gradually transform into homogeneous granitic rock. Migmatite varieties also make up most of Terningskarvet. Biotite gneiss-rich layers, about 100 m thick, and granitic migmatite a r e mappable units in t h e area. T h e biotite gneiss is often feldspar porphyroblastic and strongly folded, and contains many basic paleosomes. A massive 110 Y . Ohta, B . 0. T#rudbakken & K . Shiraishi kb 10 8 6 4 1 MH: Muhlig Hofmannfj. G : Gjelsvikfj. Sn: north Sverdrupfj. Ss: south Sw: west K: Kirwanryggen #:: Ahimannryggen- krgmaSSivet I AMPH.FJ I I a ’ h I I I . 600 700 8 00 9oooc Fig. 4. Pressure-temperature conditions of the metamorphic rocks from western Dronning Maud Land. Ahlmannryggen- Borgmassivet: Wolmarans & Kent (1982); Kirwanveggen and H. U . Sverdrupfjella: Wolmarans & Kent (1982), Grantham et al. (1988), Allen (1990); Gjelsvikfjella and Miihlig-Hofmannfjella: present study. Reference curves: 1. Holdaway (1971), 2 . Ringwood & Green (1966). 3 . Richardson (1968), 4. Percival (1983), 5. Holdaway & Lee (1977). Broken curves: estimated retrograde conditions. diorite, about 200 m wide, occurs concordantly in the granitic migmatites in the southwestern part of Terningskarvet. A plagioclase porphyroblastic biotite schist occurs in the same area, alternating with garnet- bearing aplite layers and schistose amphibolites. These rocks form a km wide, large paleosome in the migmatites. The granitic metatects are rich in pink coloured microcline, and contain as much as 5% biotite. The basic paleosomes have clino- pyroxene inclusions in hornblende grains and biotite overgrows on the hornblende. Medium- grained oligoclase is the main constituent and no hydration of mafic minerals has been observed. Gneisses and granitic layers, some tens to a few hundred metres thick, were traced by binocular observation on Nupskammen, Von Essenskarvet and the southwestern side of Jutulsessen, and the observed outlines are shown in Fig. 2. Geological structures of Gjelsvikfjella a) Mesoscopic structures All rocks of the Jutulsessen metasupracrustals show distinct gneissosity represented by dif- ferentiated layering and preferred orientation of mafic minerals. The gneissosity is considered to be the result of the main period of deformation and metamorphism. The observed orientation of the foliation is summarized on the equal-area lower hemisphere stereographic projection for six subareas in Gjelsvikfjella (Fig. 5). Some layers, cm to dm thick, include isolated isoclinal folds, indicating that the gneissosity is a transpositional foliation. Compositionally distinct units, some hundreds of metres thick and with 5- 10 km lateral persistency, can be considered to reflect differences in primary lithologies to some extent. Some granoblastic basic rocks are com- Geology of Gjelsvikfjella and western Miihlig-Hofmannfjella 111 Fig. 5 . Stereographic projections, lower hemisphere, of foliations and lincations from Gjclsvikfjclla. Division of the wharca\ and general structures are shown i n the inserted map. ( ): number of measured foliation poles; 1-5-10: pcrccntagc of thc contours for the foliation plots; solid dot: mineral lineation; opcn circle: fold axis. pletely conformable with the gneissosity of sur- rounding rocks. These are believed to represent pre-metamorphic basic sheets or dykes. Basic rocks with clinopyroxene in hornblende show oblique alignment t o the gneissosity, and are interpreted as syn-metamorphic intrusions. The isoclinal folds are refolded by later tight folds which represent the main phase of deform- ation (D2). The isoclinal folds therefore indicate a strong deformation, D,, prior to the main recrystallisation phase DZ. The most common mesoscopic fold axes coincide with the mineral lineations defined by preferred orientation of mafic constituents. These are formed during the main deformation-meta- morphism, D2, together with the formation of gneissosity and differentiated layering. These mesoscopic linear structures have by later deform- ation (D3) been rotated into a wide large girdle along the maximum gneissosity girdle in the type locality of the Jutulsessen metasupracrustals in Grjotlia (subarea I1 in Fig. 5). Similar rotations of the lineations are seen in the diagrams for subareas 111 and VI, but the axes of rotation are oblique to the axes defined by the gneissosity girdles. The D3 deformation resulted in a kind of dome-basin structure, as shown by the general trends of gneissosity in the inserted map of Fig. 5. The wide girdle deviations of lineation in sub- areas I, 11, I11 and IV can be explained by the rotation during the D3 migmatization. b) Faults A distinct fault, the Armlenet Fault, occurs in northern Armlenet (Fig. 2 ) . It has an E-W strike, subvertical dip and a downthrow to the south. The fault plane itself is not directly exposed, but appears to be manifested by a narrow discrete break buried beneath a 10 m wide zone of scree cover. The northern side of the fault forms a 150 m high vertical cliff, with two types of basic dykes present. One type is a disrupted and folded biotite amphibolite which was possibly emplaced during the migmatization. The second type con- sists of a network of unmetamorphosed dolerite of possible Mesozoic age. From the dip change of the gneissosity across the fault it is considered that the fault intersects the crest of a regional antiform. The eastern extension of the fault prob- ably passes between Medmulen-Medhovden and Gygra-Risen and the gneissosity in these two 112 Y . Ohta, B . 0. Torudbakken & K . Shiraishi areas also shows opposing dips. T h e occurrence of many basic dykes along t h e fault and its structural relationship t o the major regional structures sug- gest that this fault may have been initiated during the late stages of the D2 deformation episode, and was subsequently reactivated during the Mesozoic. A NE-SW striking fault, dipping 60" eastwards, occurs about 2 km south of the Armlenet fault (Fig. 2), and cuts strongly migmatized gneisses. This fault separates an easterly dipping western block from a westerly dipping eastern one, but its strike is oblique t o that of the gneissosity. This fault may b e a splay fault of the Armlenet fault. A 3-5 m wide shear zone was observed in the middle part of t h e eastern ridge of Grjotlia (Fig. 2). Cataclastic structures a r e developed, but no associated retrograde metamorphism has been identified. This fault is nearly parallel to the com- positional layering in the biotite rich gneisses and the sense of displacement is unknown. c) Fold structures There is n o structural discordance between the Jutulsessen metasupracrustals and the Risemedet migmatites, and their gneissosities define an oval, incomplete d o m e structure, the Jutulsessen dome, with its axis elongated in a NW-SE direc- tion (subareas 11, I11 and IV). T h e Armlenet Fault cuts t h e northern part of the dome. North of the fault t h e gneiss-migmatites form a steep northeast trending fold (subarea I ) . T h e gneissosities in the Medmulen-Medhovden area have roughly E-W strikes and moderate southern dips, while those of t h e Gygra-Risen area have northern dips. This may show an open anticline with an E - W trend between the two areas. A n asymmetric, NW-SE trending antiform occurs around t h e Horten nunatak with 50"-60" SW dips in t h e western limb and 60"-75" N E dips in the eastern limb. T h e eastern limb of this fold can be followed into a synform t o t h e east of Horten, where t h e opposite flank dips about 10"- 15" to t h e west. These structures have not been observed on t h e northern cliffs of Medmulen- Medhovden which lie in t h e strike direction of the folds. T h e antiform is therefore probably terminated in Risemedet as an elongate dome. A small open synform with a NW-SE trend and a southeasterly plunging axis occurs in the southeastern part of Brugda. This synform is a subordinate structure on the southern slope of the Jutulsessen dome. A major S E plunging synform occurring in Terningskarvet is divided in two by a small anti- form around t h e highest part of the nunataks. T h e axes of these folds plunge t o the SE, subparallel to those in Brugda and Horten. Compositional layers include many isoclinal folds indicating that they a r e of D1 structure. T h e folds described above are from t h e D2 deformation, with some later rotation by the D3 deformation. Geological event history in Gjelsvikfjellu T h e following successive events can be recognized from t h e petrographic and structural observa- tions: 1) Argillaceous and sandy sedimentary rocks may b e protoliths for the dark micaceous gneisses occurring in t h e Jutulsessen metasupracrustals. Felsic gneisses may alternatively have been derived from acidic volcanic rocks. W e cannot exclude that it is possible t o discriminate between some plutonic rocks in the metasupracrustals. 2) Strong deformation resulted in the for- mation of isoclinal and tight folds and trans- positional foliation (D1 and Dz phases of deformation). T h e deformation phases were associated with major metamorphism under upper amphibolite to granulite facies conditions. Increasing temperature or PHZ0 led to regional migmatite formation especially in the southern areas. 3) Regional open fold structures (D,) refolded the earlier folds into dome-basin-like structures. Metagabbro intrusions a r e associated with the D3 deformation phase and the emplacement of basic, aplitic and pegmatitic dykes took place in the later stages of this phase. 4) Brittle activation of the Armlenet Fault. 5 ) Emplacement of the Mesozoic dolerite dykes associated with brittle deformation - reac- tivation of the Armlenet Fault. Western Muhlig-Hofrnannfjellu O u r mapping covered t h e area from Skigarden (4"30'E) t o Ahlstadhottane (S"30'E) along the northern escarpments of western Miihlig-Hof- mannfjella. Reconnaissance observations were made t o the south (Fig. 6). T h e Svarthamaren charnockites and the Snotoa metamorphic complex dominate in the area. The former intrude into t h e latter. T h e term char- Geology of Gjelsvikfjella and western Muhlig-Hofmannfjella 113 114 Y . Ohta, B . 0. TGrudbakken & K . Shiraishi nockite is used for rocks with orthopyroxene and mesoperthite as essential constituents, which have a dark brownish-green colour due to the felsic constituents. Some varieties have biotite and hornblende as main minerals. The charnockite intrusion will be described from west to east. Skigarden-SnBtoa area and the area to the west The Skigarden and B j ~ r n s a k s a nunataks are mainly composed of coarse-grained, dark char- nockite (Fig. 6). The rocks have large idiomorphic crystals of orthoclase mesoperthite, a large modal content of biotite and both ortho- and clino- pyroxene are present. Plagioclase grains are often antiperthitic. Isolated pale coloured blocks, up to 50m across, mainly of heterogeneous granitic com- position, often with gneissic and basic paleo- somes, are included in the charnockite mass. Size and frequency of the granitic xenoliths increase to the south. A one km wide, vertically dipping xenolith rich zone of charnockite occurs in southern Skigarden. The zone has a roughly E- W strike, and forms the boundary to the shadow- like migmatites of Snatoa (Fig. 7). Small gneiss and granite blocks have diffuse margins and the adjacent charnockite shows a red coloured contact zone, cm t o dm wide, probably formed by assimilation. The migrnatites of Sncbtoa. commonly with shadow-like and streaky paleosomes and some- times agmatitic, have an E-W strike, dip mod- erately to the south and display upper amphibolite facies mineral assemblages (Table 5 ) . Parts of the Sncbtoa migmatites form a sheet about 500 m thick in the charnockite. A similar sheet-shaped occurrence of char- nockite and migmatites was observed in the north- ern cliff of Tjuvholene, northern Gryt~yrfjellet. In these mountains dark granulitic rocks contain sheets of granitic rocks with relatively sharp con- tacts. Grinda nunataks to the north of Tjuvholene consist entirely of charnockite. A zone from southern Skigarden to western Grytcbyrfjellet can be regarded as an intrusive transition zone between a large charnockite mass t o the north and metamorphic rocks to the south. Fig. 7. Contrasting mountain shapes between the charnockite (pinnacles) and the Snotoa metamorphic complex (massive mountains); Skigarden i n front and Sncdtoa in the background. Geology of Gjelsvikfjella and western Muhlig-Hofmannfjella 115 Table 5. Metamorphic mineral assemblages of the gneisses and migmatites of the Sndtoa metamorphic complex. sil ga sp co opx cpx hb bi pl kf qz il BT80 BT79 BT58 BT97 BT95 BT108 BT120 BT104 BT94 BT85 X x x x x x x x x x x x x x x x x x X x x x x x x x x x x x x x x x x x x x X X x x x x x X X x x x x x x x x x x x x x x x X Accessories: apatite, zircon, allanite, monazite and secondary epidote, sphene, serpentine, chlorite, vermiculite, hematite. Sample YO169 (Table 6) has the same assemblage as sample BT95. Sample YO167 (Table 6) has the same assemblage as BT97, but the content of opx is very small. West of Flogeken A dark charnockite, more than one km wide, occurs t o the south of Hoggestabben (Fig. 6), and has E-W strikes and steep dips. Hoggestabben consists of a granitic migmatite with banded gneiss relics. Weakly banded granulitic rocks dominate in Hochlinfjellet. Distinct banded gneisses, a few km wide, occur in the northern part of Vedskllen. A 15 m thick skarn layer has been observed in its eastern part. Towards the western part of Vedskdlen the banded gneiss is replaced laterally by a granulitic granite . Festninga further to the west (Fig. 6) is dom- inated by granitic migmatites with some distinct layers of banded gneisses and agmatites. These rocks have a roughly E-W strike and gentle to moderate southward dips. The banded gneiss- migmatites in Festninga and Bvrevollen are prob- ably the continuation of the Jutulsessen meta- supracrustals and the Risemedet migmatites of eastern Gjelsvikfjella. Petrellfjellet and Harnarskoruene areas Petrellfjellet to the south of Skigarden (Fig. 6) has outcrops of granulitic metamorphic rocks along the northwestern foothills, on the northern ridge (with the 2,285m peak) and the eastern part. Dark charnockite which makes up the north- ern tip of the eastern ridge is a roughly concordant layer to the migmatites, and has a nearly E-W strike and 20°-40' southward dips. Another char- nockite in southern Petrellfjellet overlies the metamorphic rocks. Hamar@ya and a nunatak to the south are composed of massive charnockite. Granulitic migmatites with some banded gneiss and agmatite layers with E-W to ENE-WSW strikes and 10'40" southward dips occupy Slok- stallen and Kvithamaren (Fig. 6). These rocks were also observed in Hamarskorvene to the southeast, where the banded gneisses consisted of pink feldspar porphyroblastic biotite-garnet gneiss, felsic gneiss, agmatitic migmatite with basic paleosomes and pink aplitic gneiss layers. Some of the pink gneisses are augen gneiss and mylonite gneiss. Ptygmatic folds are frequently observed in the gneisses and migmatites. An approximately 5 m thick, coarse-grained marble layer, containing diopside-wollastonite-grossular aggregates, occurs in the gneisses. Coarse-grained mosaic texture of subangular hornblende and plagioclase has been preserved in an agmatitic basic paleosome found in the agmatitic basic rocks. The rock consists of plagio- clase, An 38-26, normally zoned, biotite, ortho- and clinopyroxene and quartz with accessory ilmenite and apatite. Randomly orientated, yel- lowish brown biotite is dominant, and replaces orthopyroxene in places. Irregularly shaped orthopyroxene with fine lamellae of clinopyroxene is decomposed into biotite-quartz aggre- gates. This rock is considered to be a basic intrusion which was emplaced after the formation of the gneisses, but prior to the intrusion of charnockite. These banded granulitic rocks and upper amphibolite facies rocks seem to dominate in the southern part of western Muhlig-Hofmannfjella and are continuous with those of Gjelsvikfjella. The charnockite extends eastwards from Vestre Skorvebreen. An isolated nunatak to the north, Larsgaddane, is composed of homogeneous, coarse-grained pink granite, with large idiomorphic potash feld- spar phenocrysts. This may be a large xenolith in the charnockite, granitized charnockite or poss- ibly a younger intrusive. Hamarskaftet The northern nunatak of Hamarskaftet, 1,661 m high (Fig. 6), consists of pink granite with small feldspathized gneissic paleosomes. The southern nunatak, 1,745 m high, is composed of dark char- 116 nockite, while a small nunatak about 1.5 km t o the north consists of migmatite with shadow-like paleosomes including biotite gneiss blocks. T h e migmatites may b e part of a large xenolith in the charnockite. Y . Ohta, B . 0. T#rudbakken h K . Shiraishi Plogska ftet Occurrences of gneiss-migmatite-granite xeno- liths in charnockite a r e well demonstrated in these nunataks (Figs. 6 and 8). T h e northernmost nun- atak, 1,595 m high, is composed o f a streaky migmatite including tightly folded, biotite-horn- blende gneiss. Charnockite intrudes along joints in the gneiss, crosscutting the gneissosity. This gneiss mass may b e a large block enclosed in the charnockite. Another type of xenolith which occurs in this nunatak is medium-grained granoblastic gneiss containing Fe-rich minerals (Tables 5 and 6). T h e rock consists of orthopyroxene, biotite, plagio- clase, potash feldspar quartz and ilmenite. T h e orthopyroxene is a ferrosilite relatively rich in M n ( E n = 18.5, Fs = 77.2, R h = 2.0, Wo = 1.9) and the biotite is rich in FeO and Ti02 (28.65 and 4.85 wt%, respectively). Plagioclase shows weak zonation from An32 t o An28. Another granulitic rock sample of this xenolith type contains ortho- and clinopyroxene, biotite, antiperthitic plagio- clase, ilmenite and monazite. T h e orthopyroxene is broken down t o biotite and quartz around its margins. Two-pyroxene geothermometry (Wood & Banno 1973) yields 780°C for this rock. T h e high-temperature mineral assemblages possibly resulted from re-equilibration d u e t o the thermal influence of t h e charnockite o n the original granu- lite facies assemblages. T h e 1,623 m high nunatak reveals typical occur- rences of charnockite dykes cutting migmatites of shadow-like, agmatitic and ptygmatic types. Biotite rich and basic gneisses occur as numerous paleosomes in the migmatites, showing various degrees of granitization. Homogeneous white granite is also found as xenoliths in the charnock- ite. O n e biotite rich gneiss paleosome has a gar- net-sillimanite assemblage. Many aplite and pegmatite dykes cut the migmatites and numerous joints a r e filled by leucocratic veins. T h e char- nockite cuts all rocks described above with sharp contacts. Only a few, narrow white veins a r e injected along joints in t h e charnockite and these a r e probably t h e e n d products of the charnockite consolidation. A weak foliation is revealed by Fig. 8. Occurrencc of xenoliths of gneisscs and migmatitcs in the charnockitc. Plogskaftct. the alignment of subidiomorphic potash feldspar crystals in the charnockite. T h e three middle nunataks, 1,731 m , 1,625 m and 1,755 m high, also show good examples of xenolithic occurrences of gneisses, migmatites and granite in the charnockite (Fig. 8). Many angular blocks, u p to 50 m across, are aligned in the charnockite along its gentle, west dipping weak foliation. Even small xenoliths of some dm size have sharp angular outlines. Charnockite dykes, cutting agmatitic migma- tites, a r e exposed on the northern face of the 1,755 m high nunatak. Both granitic metatect and gneissic paleosomes a r e sharply transected by white pegmatites and all these rocks are again cut by dark charnockitic dykes. Along the margins of the charnockite fine-grained chilled facies, about 1 m in width, a r e locally developed. T h e southernmost nunatak consists of dark charnockite in its western half, whilst a homo- geneous, white potash feldspar granite occurs in t h e eastern part. Several dykes of charnockite cut Geology of Gjelsuikfjella and western Muhlig-Hofnzannfjella 117 Table 6. Representative microprobe analyses of thin-sections from the SnQtoa mctarnorphic complex and the Svarthamaren charnockite batholith (ref. Tables 5 and 7). Sample localities are shown in Fig. 6. SiO: TiOl AI1O, C r 2 0 i FCO^ M n O MgO c a 0 Na:O K 1 O Total Kation ratio 0 Si Ti Al Cr FL. M n ('a Nil K X M g Mg BT104, charnockitc, Hegsenga 01 CPX hornblende 19 .69 0 0 0.04 hX.hl 1 . X I 0.32 0 0 100.47 4 I .no0 1) 0 o.no1 1.932 0.052 0.016 0 0 0. ()OX ~ - 4X. 22 0.03 0.97- 0 2x. I X 0.02 0.93 20.47 0.25 99. (1 1 0 I .9xs 0.001 0 . 0 IS* o.030'r 0 0.970 0.022 0.057 0.903 0.020 0.056 ~ - 37.9s 0.97 11.64 0.03 32.25 0.26 0.56 10.46 I .68 1.41 97.22 23 6.246 0.121 I .754* 0.504* O.OO4 4.439 0.036 0 . 137 1 .x44 0.536 0.296 0 . 0 3 0 FeO* =total Fe as FcO, Al" = All". Al** = AI"', XMg = Mg/Fe + Mg i n a l l analyses. Feldspars pl: core: An = 33.6, Ah = 65.3, Or = 1.1 rim: An = 29.9. Ab = 69.3. Or = 0.8 BT95: gneiss xenolith in charnockite, H ~ g s e n g a opx C P X hornbiendc grain lamell. poiki to opx biot $10, T i 0 , AlzO, C r 2 0 3 FeO* MnO CdO N d @ K z O Totdl MgO Kation ratio 0 Si TI Al Cr Fe Mn Ca Mg ~ 4x 59 0 09 0 23 0 02 40 23 1 5 1 x 53 0 82 0 0 100 02 6 1.993 0.003 0.007* 0.004** 0 001 1.380 0.052 0.522 0.036 50.00 0.06 0.58 19.57 0.63 7.46 20.40 0.23 0 99.03 0.10 6 1.9x2 0.002 0.018* 0.003 0.649 0.021 0.441 0.X67 n , on9 * * 50.01 0. I3 0.77 0.04 19.77 0.63 7.44 20.49 0.34 0 99.62 6 1.973 0.004 0.027% o.n09* * 0.001 0.652 0.021 0.438 0.866 42.91 8.34 0.03 21 45 0.32 8.76 10.97 1 .62 1.41 96.18 0.37 23 6.730 1.270* 0.272** 2.813 0.043 2.048 1.843 0.044 0.004 39.94 1.21 10.22 0.02 27.80 0.13 3.85 10.71 1 .66 1.63 97.17 23 6.423 0.146 1.577* 0.360** 0.003 3.739 0.923 1.845 0 . m 37.65 3.20 12.42 0.OX 20.34 0 1 l J ILS7 0.02 9.18 94.72 0.07 22 5.81 1 0.371 2.189* 0.071 * * 0.010 2.626 0.025 2.662 0.003 118 Y . Ohta, B . 0. Tfirudbakken & K . Shiraishi Table 6. (continued) Na 0 0.018 0.026 K 0 0 0 XMg 0.274 0.405 0.402 Feldspars pl: core: An = 32.6, Ah = 65.2, Or = 2.2 rim: An = 32.4, Ah = 65.2, Or = 2.4 poiki: poikiloblastic to opx: contact with orthopyroxene 0.493 0.282 0.421 0.518 0.334 0.198 0.021 1.808 0.503 Y0167: gneissic xenolith i n charnockite. N Plogskaftet OPX biotite SiOz T i 0 2 CrzOi FeO' MnO CaO N a 2 0 K 2 0 Total , 4 1 2 0 3 MgO 47.22 0.03 0.32 0.07 43.69 1.12 6.00 0.82 0.05 0 99.32 34.YO 4.85 13 12 0 28 65 0.14 5.08 0.01 0 01 8.75 95 51 Kation ratio 0 6 22 Si 1.988 5.582 Ti 0.001 0.583 A1 0.0121 2.418' Cr 0.002 0 Fe 1.539 3.832 Mn 0.040 0.019 Mg 0.377 1.211 Ca 0.037 0.002 Na 0.004 0.003 K 0 1.786 XMg 0.197 0.240 Fcldspars pl: core: An = 30.4, Ah = 68.3. Or = 1.3 rim: An = 29.0, Ah = 70.2, O r = 0.8 opx: En = 18.9, Fs = 77.2, Rh = 2.0, Wo = 1.9 0.004** 0.056** Y0169: gneissic xenolith in charnockite. N Plogskaftet OPX CPX biotite S i 0 2 TiOz A1209 ( 3 2 0 3 FeO* M n O CaO NazO K2O Total Kation ratio 0 Si Ti MgO 49.33 0.11 0.34 0 33.53 1.20 13.85 0.81 0 0 99.17 6 1.973 0.003 51.07 0.11 0.64 0 14.44 0.5 I 10.66 21.09 0.29 0 98.81 6 1.981 0.003 36.66 5.05 13.17 0.08 19.62 0.16 10.84 0.06 0.05 9.20 94.89 22 5.641 0.584 Geology of Gjelsuikfjha and western Muhlig-Hofmunnfjella 119 Table 6. (continued) Al 0.016* 0.019* 0 0.010 Cr 0 0 Fe 1.122 0.468 Mn 0.041 0.017 Ca 0.035 0.876 N a 0 0.022 K 0 0 XMg 0.424 0.568 Feldspars pl: An = 27.8. Ab = 70.9, Or = 1.2 and An = 26.9, Ah = 71.9, Or = 1.2 Mg 0.826 0.616 2.359* 0.029 0.010 2.525 0.021 2.487 0.010 0.015 1.806 0.496 the granite and t h e contact is partly sheared with a 5-10 cm wide mylonite zone. Fe-rich, medium- grained charnockite from this area consists of fayalite (Fa = 99), hedenbergite ( E n = 2.5, Fs = 50.5, W o = 47), plagioclase (An35-26from centre t o rim), quartz, ilmenite and magne- tite (Tables 6 and 7). T h e quartz-fayalite-mag- netite assemblage from this rock gave fo2 = 10-(’@15) bars and 750°C (Wones & Gilbert 1969) as the formation conditions for t h e charnockite. Svarthamaren to Ahlstadhottane T h e high mountains in this area are exclusively composed of charnockite (Fig. 6). Weak foliations a r e locally present and they generally show NW- SE strikes and moderate westward dips to the west of Svarthamaren and eastward dips t o the east of BBsbolken. T h e charnockite can be divided into two facies: a ) a medium-grained, greenish grey rock with less than 10% mafic constituents (ortho- and clino- Table 7. Mincral assemblages of the Svarthamaren charnockite, Muhlig-Hofmannfjclla. 01 opx cpx hh hi pl k f qz i l BT46 BT48 BT98 BT106 BT104 BTll5 BT69 BT144 x x x x x x x x x x x x x x x x x x x x X x x x x x x X x x X x x x x x x x x x x x x x x x x x x x x x x x x x x Accessoiies: apatite, zircon, allanitc. monazite and secondary cpidote, sphene, serpcntinc. pyroxene and locally biotite), b) a dark brown, coarse-grained rock, characterized by large idiomorphic potash feldspar crystals and smoky quartz. These two facies show sharp contacts. T h e medium-grained variety is restricted in its occur- rence t o northern Svarthamaren, southern Cumu- lusfjella and t h e middle part of Hogsenga. Large amounts of deep green hornblende is t h e major mafic constituent. Ortho- and clinopyroxene a r e always present and often included in the horn- blende. Pale coloured, irregular flame-like patches occur in the medium-grained charnockite with high angles t o the weak foliation (Fig. 9). T h e flame-like patches d o not display clear lithological differences to t h e dark charnockite, except for t h e colour of the felsic constituents. T h e centres of some flame domains are granitic in composition. These flames may have formed by late hydration of the charnockite or by assimilation of granitic xenoliths in t h e charnockite. Small gneissic, migmatitic and granitic xeno- liths occurring elsewhere in the charnockite show varying degrees of assimilation, revealed by vari- ous widths of t h e transition zone of the block margins. Isolated large granitic xenoliths, tens t o hundreds of metres in size, are observed in the southeastern cliff of Svarthamaren, t h e north- western tip of B h b o l k e n , the middle part of the western cliff and northeastern slope of Hogsenga, the southern and southeastern parts of Cumu- lusfjellet and t h e middle and eastern parts of Ahlstadhottane. T h e 2,695 m peak of south- western Breplogen consists of granitic migmatites containing basic paleosomes and together they form a large xenolithic block in the charnockite. Some xenoliths show diffuse margins of white tint, but many others have very sharp angular 120 Y . Ohta, B . 0. Torudbakken & K . Shiraishi Fig. 9. Flamc-likc. pale coloured facies in thc charnockitc, Hsgsenga block boundaries. A gneissic xenolith in the char- nockite of Hogsenga consists of ortho- and clinopyroxene, dark green hornblende, brown biotite, plagioclase, potash feldspar, quartz and accessory ilmenite (Tables 6 and 7). Ortho- pyroxene with exsolved Ca-clinopyroxene ( ferroaugite-ferrosalite) lamellae is surrounded by poikiloblastic hornblende. Clinopyroxene also occurs as individual medium-sized grains in the rock and the composition is similar t o that of the lamellae. T h e hornblende is heterogeneous both in colour and composition within one grain. Two pyroxene geothermometry (Wood & Banno 1973) gave a possible blocking temperature of about 795°C for this rock. Distant observation by binoculars from Ahlstadhottane and Breplogen t o t h e mountains and nunataks t o t h e east of Austre Skorvebreen shows that all visible exposures are dark red coloured charnockite. Dyke rocks Several different types of dykes, which cut the medium-grained charnockite, are well exposed in a 300 rn high cliff in southern Svarthamaren: 1) T h e oldest dykes a r e agmatitic and irregu- larly shaped. They consist of biotite rich basic rock fragments of 1G-20 cm size, and occur in a fine-grained, dark yellow, felsic granulitic matrix. 2 ) Yellowish felsic charnockite forms thin dykes, locally with pink-coloured seams in the centre. 3) A pink, impregnation-like aplitic rock forms irregular dykes. They cut t h e felsic charnockite dykes. 4) Hornblende rich basic dykes with large, white idiomorphic potash feldspar cut t h e pink aplitic dykes. 5 ) D a r k coloured, basic dykes with biotite clus- ters cut the feldspar idiomorphic dykes. These five types of dykes a r e considered to be emplaced during a late consolidation stage of the medium-grained charnockite. There a r e two more types of sharply cross- cutting dykes: 6) Dark metabasite dykes, 3-5 m thick, are the most distinct dykes in the upper part of the cliff. Geology of Gjelsuikfjella and western Muhlig-Hofmannfjella 121 They can b e followed in a distance of more than 3 k m with a gentle dip to the south. 7) Grey aplite dykes cut t h e metabasite dykes, and sometimes form composite dykes with the latter. Dyke types 6) and 7 ) d o not show any mobil- ization, but they a r e moderately recrystallized. All dykes a r e cut by the coarse-grained, dark charnockite in the northern part of Svarthamaren. Svarthamaren is t h e only locality with such a high concentration of basic dykes in the medium- grained charnockite, which indicates that the area may be in a special structural position near the margin of t h e charnockite massif. S u m m a r y o f the geology in western Miihlig- H o f m a n n f j e l h T h e observations in western Miihlig- Hofmannfjella show that a large intrusive char- nockite complex, the Svarthamaren charnockite batholith, occurs in the northeastern part of the area. T h e main rock types a r e coarse-grained and medium-grained charnockite. A westward projection of the coarse-grained charnockite extends from Svarthamaren to Ski- garden. This projection splits into several major subconcordant sheets and dykes and they cut into the Snatoa metamorphic complex along the east- e r n parts of Grytoyrfjellet and northern Hoch- linfjellet. T h e coarse-grained charnockite has sharp cutting contact relationships t o t h e medium- grained charnockite, showing that t h e coarse- grained charnockite is younger than the former. T h e medium-grained charnockite includes a large proportion of xenoliths probably derived from the Snotoa metamorphic complex. These xenoliths a r e assimilated t o a varying degree. However, many xenoliths, even small ones, are almost devoid of assimilation and have retained their angular outlines a t many localities. Strongly assimilated rocks a r e mostly of granitic compo- sition. Partly assimilated gneissic and basic xeno- liths commonly have white coloured margins in which t h e granulitic mineral assemblages are lost. White patches may also b e hydrated charnockite by volatiles derived from t h e xenoliths. T h e Sn0toa metamorphic complex occupies the high mountains in t h e southern part of western Miihlig-Hofmannfjella. O n the western side of Vestre Skorvebreen t h e Snotoa metamorphic complex contains granulite facies gneisses and migmatites which probably were partially recrys- tallized during t h e emplacement of the Svart- hamaren charnockite batholith. Geochronology Previous works Two distinctly different age provinces are separated by t h e J-P RZ in western Dronning Maud Land (Fig. 1). To the west of the rift zone Archean basement, 3,100-2,800 Ma old (Halpern 1970; Elworthy 1982), covered by almost undeformed 1,850-760 Ma old fluvial and volcanic sediments. To the east of the zone isotope data have been interpreted t o record a regional metamorphic phase of 1,00&1,200 M a (Elworthy 1982), coeval with the Bunger Orogeny of east Antarctica (Angino & Turner 1964; Deutsch & Webb 1964) and the Nimrod Orogeny of the Trans Antarctic Mountains (Grindley & McDougall 1969). Elworthy (1982) also inter- preted a thermal event about S O ( H 0 0 M a old, comparable t o the Ross Orogeny (Elliot 1975). Because the two provinces show such a contrasting history, different degrees of deformation and metamorphic grade, the J-P RZ is believed to be a deep crustal discontinuity with probably significant lateral displacement. T h e youngest igneous activity associated with the J-P R Z is a 155-140Ma old nepheline syenite in the northeastern part of H . U . Sverdrupfjella. 170 Ma old Mesozoic dolerite lavas and dykes, similar to the Ferrar dolerite of the Trans Antarctic Mountains, are known from some places to the east of the zone. T h e radiometric ages obtained until 1989 in western Dronning Maud Land are shown in Fig. 1. Both in Kirwanveggen (Wolmarans & Kent 1982) and H . U . Sverdrupfjella (Grantham e t al. 1988) in the southwestern extension of the present area t h e older gneisses and migmatites have an age range from 1,186 to 8 6 1 M a (Rb-Sr, whole rock and U-Pb zircon, Elworthy 1982; Moyes 1989). Elworthy (1982) interpreted some of these ages as t h e record of the main metamorphism, and inferred that the rocks had a relatively short history prior t o the metamorphism, on the basis of low initial 87Sr/86Sr ratios. Granitic, intermediate and mafic intrusions from the same area fall in an age range from 918 t o 519Ma (Moyes 1989). Nearly all Rb-Sr mineral ages and K-Ar mineral and whole rock ages from Kirwanveggen in t h e west t o Miihlig-Hof- 122 Y . Ohta, B . 0. TQrudbakken & K . Shiraishi mannfjella in the east fall in a 55C-330Ma age range (Ravich & Krylov 1964; Krylov 1972; Elworthy 1982; Moyes 1989). Location and description of the samples Five types of rocks were sampled at three localities, x, y and z shown in Figs. 2 and 6. Each sample weighed 3-6 kg and explosives were used to obtain fresh rock samples. Felsic gneiss of the Jutulsessen metasupracrustals. - Ten samples of felsic gneiss were collected from the middle succession of the Jutulsessen metasupracrustals. The bulk composition of the rock is granodioritic and the main minerals are oligoclase, microline, quartz and some biotite. The samples have only very weak gneissic fabric. Metagabbro. - Nine samples of metagabbro were collected northwest of Stabben in the northeastern part of Jutulsessen. The metagabbro has a sharp intrusive contact to the Jutulsessen metasupracrustals. The samples were chosen to cover as many lithological varieties of the gabbro as possible. The mineral assemblage is plagioclase (andesine-oligoclase), pale green clinopyroxene, brown biotite, green hornblende and olivine in some samples. The modal composition varies considerably between the samples. Opaque minerals, sphene and apatite are the accessories. Clinopyroxene is partly converted into green hornblende. The lithological variation is believed to be caused by magmatic differentiation and successive hydrothermal processes, and original igneous textures are well preserved. Charnockite. -This rock was sampled in southern Svarthamaren and care was taken to avoid the light coloured parts which may be contaminated facies with assimilated foreign materials. The nine samples collected are medium-grained, dark yellow-brown charnockite without preferred mineral orientation. These rocks have tabular potash-feldspar often with very small opaque inclusions, quartz, oligoclase, dark brown biotite, brown clinopyroxene and occasional orthopyroxene grains as the main constituents. The bulk composition is granitic to granodioritic. Grey aplitic and dark basic dykes, cutting the medium-grained charnockite. -Two types of dyke cutting the charnockite in Svarthamaren were chosen for the isotope study. The gray aplitic dyke is the youngest dyke, and consists of oligoclase, microcline, brown-green biotite, green hornblende, quartz and opaque grains. The samples are uniform and have granodioritic bulk composition. The dark dyke (metabasite) has a basic bulk composition, and consists of oligoclase, dark brown biotite, clinopyroxene and accessory amounts of opaque grains and apatite. This rock is cut by the grey aplitic dyke and locally they form composite dykes. Analytical techniques Rb-Sr ratios were determinzd by X-ray fluor- escence spectrometry. Measurements of unspiked 87Sr/86Sr were made on a VG Micromass 30 at Mineralogisk-Geologisk Museum, Oslo, using procedures similar to those described by Pankhurst & O'Nions (1973). Variable mass discrimination in 87Sr/86Sr was corrected by normalizing 88Sr/ 86Sr to 8.3754. The 87Rb decay constant used is 1.42 x 10-"y-' and the regression technique was that of York (1969). Age and intercept errors are quoted at the 2 sigma level. Results and discussion Felsic gneiss, Middle Jutulsessen. - High Sr and relatively low Rb contents did not give a spread in the Rb/Sr ratios sufficient enough to allow an age calculation (Table 8). Therefore, only two samples were analysed for Sr isotope composition. The obtained 87Sr ratios, 0.70576 t 0.00016 and 0.70482 ? 0.00018, may be regarded as maximum numbers of the initial 87Sr/86Sr ratio. Assuming an age of 1,100 Ma, referring to the results from H.U. Sverdrupfjella and Kirwanveggen, the calculated initial 87Sr/86Sr ratios for the two samples are 0.7029 and 0.7014 (Fig. 10). These are close to the lowest values which are geologically reasonable. Although no definite conclusion is possible, the calculation suggests that the felsic gneiss could fall into the same groupasthegneissesdatedin H.U. Sverdrupfjella, or be younger. Metagabbro, northeastern Jutulsessen. - Very similar Rb/Sr ratios made age determination impossible for this rock (Table 8). The Sr isotopes were analysed in four samples and they show some scatter on the isotope diagram (Fig. 10). This scatter may be due to a slight disturbance Geology of Gjelsuikfjella and western Miihlig-Hofmannfjella 123 JJTULSESSEN I %r/%r 710 + + + Met a g a b b r o + I / 4 ' 'Felsic gneiss I I 705, + 87 R b/%r . 2 ./l .6 .a Fig. 10. Isotope diagram for the felsic gneiss of the Jutulscsscn mctasupracrustals and mctagabbro (rcf. Table 8). of the isotope system. However, all samples show relatively high 87Sr/86Sr ratios from 0.70785 to 0.70874. This may indicate that a relatively high initial ratio can be inferred for the metagabbro. Such high initial ratios have commonly been obtained from the mafic rocks to the west of the J-P R Z , and have been interpreted by Elworthy (1982) and Barton & Copperthwaite (1983) to be due to an undepleted mantle source or crustal contamination. Charnockite, Suarthamaren, Miihlig-Hofmann- fjella. - Acceptable spread in the Rb/Sr ratios enabled whole rock age determination of the medium-grained charnockite (Table 9). The regression calculation gave an isochron age of 500 f 24Ma with M.S.W.D. = 1.64 (Fig. 11). This age is interpreted as the age of crystallization, and is regarded as the most reliable age for the emplacement of the large charnockite massif in Muhlig-Hofmannfjella (e.g. Ravich & Krylov 1964; Krylov 1972). This intrusion event is coeval to the Ross Orogeny of the Trans Antarctic Tuhle 8. R b and Sr analyscs of the fclsic gnciss and metagabbro from Jutulscsscn F c l ~ gneiss Sample n o Rh' Sr* X7Rb/X6Sr 87Sr/86Sr S E ' * ES E6a E l E2 EB E4 Ehb E6C E7 EXa E8b E8c EY El08 ElOb 83 Y5 83 70 82 95 93 85 95 98 111 63 88 91 86 1322 1264 1294 12x1 I278 l2bY 130Y 1294 I170 I253 1290 1297 1596 1132 1170 Metagabbro Sample no. Rb* Sr* 87Rb/86Sr 87Sr/86Sr SE** A 46 790 171 70874 00012 DI 114 2Y01 1 I 3 70785 000 14 D2 116 2697 124 70868 00012 ti 71 381h 052 70804 00010 B 120 3208 c1 20 1408 c 2 23 1308 - F1 75 3464 F7. 71 3846 *ppm **2 sigma - - - - - - - - - - - - - - 124 Y . Ohta, B. 0. Tprudbakken & K . Shiraishi Table 9. Rh and Sr analyses of the charnockite and associated dyke rocks from Svarthaniaten Charnockite Sample n o . Rh* Sr* 87Rhl86Sr 87Sr186Sr SE*' H1 H2 H4 H5a H5b H 6 H 7 H8 H9 26 1 264 27 1 385 290 226 211 205 211 Greyaplitic dyke Sample no. Rb' 1 50 156 153 192 159 159 I76 158 184 Sr* 5.06 4.92 5.15 5.83 5.30 4.13 3.58 3.17 3.33 .74685 .74580 .74753 ,75123 ,74834 .74004 ,73593 ,13796 ,73401 87Rb186Sr .00018 ,0001 4 .000 12 ,00014 .00012 .O(JO02 .00002 .00002 .00002 SE*' ~~ JI 152 454 ,970 ,71527 .00002 J4 234 192 3.54 .73635 .moo2 J7 15 1 456 ,961 ,71520 .00002 JY 153 448 ,987 ,71538 ,00003 53 149 464 J5 150 464 - 56 151 452 - J X 152 451 Dark dyke, Svarthamaren Sample no. Rb* Sr* 87Rb186Sr 87Sr/86Sr SE** K I 88 1694 ,150 ,70996 .00002 - - - - - - - - - - *ppm '*2 sigma Mountains. Evidence of deformation related to this orogeny in western Dronning Maud Land is so far restricted t o the folding of the Lower Paleozoic sediments (the Urfjell Group), out- cropping in southern Kirwanveggen (Aucamp et al. 1972). A relatively high initial 87Sr/86Sr ratio of 0.71057 * 0.00071 for the charnockite is probably related to crustal contamination, since many partly assimilated xenoliths of gneiss and migmatite have been observed along the marginal areas of the charnockite body. Grey aplitic and dark basic dykes, Suarthamaren, Miihlig-Hofmannfjella. - No significant spread in Rb/Sr ratio, except for one sample, was obtained for the grey aplitic dyke and accordingly, only four samples were analysed for Sr isotope composition (Table 9). The results, together with the analysis of the dark basic dyke and a reference line corresponding to the charnockite isochron, are plotted in Fig. 12. From their crosscutting occurrence both types of dykes are younger than the medium-grained charnockite. I n addition, the two kinds of dykes sometimes occur as composite dykes, suggesting their comagmatic derivation. No meaningful age, therefore, can be obtained for the grey aplitic dyke, or for any combination of this rock with the dark basic dyke. The scatter in the isotope diagram possibly reflects originally different initial Rb/Sr ratios, due to magmatic differentiation processes. Conclusions 1. The bedrock of Gjelsvikfjella and western Muhlig-Hofmannfjella is essentially composed of two metamorphic-igneous rock units: firstly the Jutulsessen metasupracrustals and Snotoa metamorphic complex and secondly the Svart- hamaren charnockite batholith. 2 . The age of the regional metamorphism could not be determined in the present study. However, it can be assumed to be about 1,100 Ma old from the structural and lithological continuities with the Geology of Gjelsuikfjellu and western Miihlig-Hofmannfjellu 125 87s r-186~ r -15 SVARTHAMAREN I Charnockite / Age: 5 0 0 2 2 4 Ma I R:0.7 1037 20.00071 MS W D: 1.64 Fig. 11. Isotope diagram for thc 87R bIB6Si 2 3 4 5 6 charnockite from southern Svarthamaren (ref. Table 9). rocks of H.U. Sverdrupfjella and Kirwanveggen. Around 1,100 M a is a possible age for the felsic gneiss of the Jutulsessen metasupracrustals. 3. The Sngitoa metamorphic complex consists of granulite facies banded gneisses and migmatites intruded by medium-grained charnockite. T h e medium-grained charnockite includes a large proportion of gneiss and migmatite xenoliths and intruded into the S n ~ t o a metamorphic complex about 500 M a ago. Coarse-grained charnockite ' 7 ~ r ~ 8 6 ~ r ' / p' SVARTHAMAREN ,// / / 73 / / / / / / , / / 0 o Grey felsic dyke 0 Dark basic dyke 1 2 3 87RW86S~ Fig. 12. Isotope diagram for the dykes from southern Svarthamaren (ref. Tahlc 9). makes u p the major part of the Svarthamaren charnockite batholith, which is extending far to the east of the mapped area. T h e cooling process after this thermal event is probably recorded in the mineral ages u p to about 400 Ma. 4. T h e regional metamorphism was originally an intermediate pressure type, upper amphibolite facies t o granulite facies. This was probably modified into a high temperature type facies by later thermal influence during the emplacement of the Svarthamaren charnockite batholith. 5 . A similar two-stage igneous-metamorphic history is also known from eastern Dronning Maud Land from Sgir Rondane to Prins Olav Coast by t h e studies of Japanese Expeditions (e.g. Shiraishi e t al. 1987). Although the area in middle Dronning Maud Land from 6"E t o 20"E has not been mapped in detail, this two-stage tectonic history seems t o be common in Dronning Maud Land east of t h e J-P RZ. Acknowledgements. - Thc authors would like to thank two anonymous reviewers and Alastair Allen for helpful suggcstions and commcnts on the manuscript. Kurt Buchcr-Nurmincn and HBkon Austrheim rcad and commented on an early draft of the manuscript. We would also like to thank Anne Lund for her exccllcnt handling of thc word processor. References Allen, A . R. 1990: Thc tcctonic and metamorphic evolution of H.U. Sverdrupfjella, wcstcrn Dronning Maud Land, Antarctica. In Thomson ct al. (cds.): Geological evolution of Anfurcticu. Cambridge University Press (in press). Angino, E. E. & Turner, M. D. 1964: Antarctic orogenic belts as delineated by absolute age dates. Pp. 552-556 in Adie, R. J. (ed.): Anrurcric geology. North Holland Puhl. C o . , Amsterdam. 126 Y . Ohta, B . 0. Torudbakken & K . Shiraishi Aucamp, A . P. H . , Wolmarans, L. G . & Neethling, D. C. 1972: The Urfjell Group, a deformed (?) Early Palaeozoic sedimentary sequencc, Kirwanveggen, western Dronning Maud Land. 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