Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica)
Main objectives of the study were the identification of geological position of the TRGI and the clarification of its geological age. In addition to the field methods of the geological research the samples collected during this and the previous years were studied using optical and electron microscopy...
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| Цитувати: | Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica) / O.V. Mytrokhyn, V.G. Bakhmutov, A.G. Aleksieienko, L.I. Gavryliv, T.V. Mytrokhina // Український антарктичний журнал. — 2017. — № 16. — С. 21-28. — Бібліогр.: 7 назв. — англ. |
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Mytrokhyn, O.V. Bakhmutov, V.G. Aleksieienko, A.G. Gavryliv, L.I. Mytrokhina, T.V. 2019-01-31T17:26:25Z 2019-01-31T17:26:25Z 2017 Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica) / O.V. Mytrokhyn, V.G. Bakhmutov, A.G. Aleksieienko, L.I. Gavryliv, T.V. Mytrokhina // Український антарктичний журнал. — 2017. — № 16. — С. 21-28. — Бібліогр.: 7 назв. — англ. 1727-7485 https://nasplib.isofts.kiev.ua/handle/123456789/145833 55(1)+551.1+552.3+550.93 Main objectives of the study were the identification of geological position of the TRGI and the clarification of its geological age. In addition to the field methods of the geological research the samples collected during this and the previous years were studied using optical and electron microscopy techniques in order to identify petrographic and mineralogical features of the gabbroids. Головною метою досліджень було з’ясування умов залягання та особливостей будови ГІТР, а також уточнення її геологічного віку. Крім польових геологічних методів досліджень, зразки гірських порід, які були зібрані у цей та попередні роки, вивчалися з використанням методів оптичної та електронної мікроскопії для з’ясування мінералого-петрографічних особливостей габроїдів. Главной целью исследований было выяснение условий залегания и особенностей строение ГИТР, а также уточнение ее геологического возраста. Кроме полевых геологических методов исследования, образцы горных пород, собранные в этот и предыдущие годы, изучались с использованием методов оптической и электронной микроскопии для определения минералого-петрографических особенностей габброидов. Large impact on organization and execution of field works conducted belongs to the winterers of XXI Ukrainian Antarctic Expedition namely M.P. Starinec, V.V. Khrapach et al., whose work is highly acknowledged by the authors. Some rock samples for research were courteously provided by P.A.Burtnuy which the authors sincerely appreciate. en Національний антарктичний науковий центр МОН України Український антарктичний журнал Геолого-геофізичні дослідження Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica) Геологічна будова та вік розшарованої габроїдної інтрузії Туксен-Расмусен (Західна Антарктика) Геологическое строение и возраст расслоенной габброидной интрузии Туксен-Расмуссен (Западная Антарктика) Article published earlier |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica) |
| spellingShingle |
Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica) Mytrokhyn, O.V. Bakhmutov, V.G. Aleksieienko, A.G. Gavryliv, L.I. Mytrokhina, T.V. Геолого-геофізичні дослідження |
| title_short |
Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica) |
| title_full |
Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica) |
| title_fullStr |
Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica) |
| title_full_unstemmed |
Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica) |
| title_sort |
geological position and age of tuxen-rasmussen layered gabbroid intrusion (west antarctica) |
| author |
Mytrokhyn, O.V. Bakhmutov, V.G. Aleksieienko, A.G. Gavryliv, L.I. Mytrokhina, T.V. |
| author_facet |
Mytrokhyn, O.V. Bakhmutov, V.G. Aleksieienko, A.G. Gavryliv, L.I. Mytrokhina, T.V. |
| topic |
Геолого-геофізичні дослідження |
| topic_facet |
Геолого-геофізичні дослідження |
| publishDate |
2017 |
| language |
English |
| container_title |
Український антарктичний журнал |
| publisher |
Національний антарктичний науковий центр МОН України |
| format |
Article |
| title_alt |
Геологічна будова та вік розшарованої габроїдної інтрузії Туксен-Расмусен (Західна Антарктика) Геологическое строение и возраст расслоенной габброидной интрузии Туксен-Расмуссен (Западная Антарктика) |
| description |
Main objectives of the study were the identification of geological position of the TRGI and the clarification of its geological age. In addition to the field methods of the geological research the samples collected during this and the previous years were studied using optical and electron microscopy techniques in order to identify petrographic and mineralogical features of the gabbroids.
Головною метою досліджень було з’ясування умов залягання та особливостей будови ГІТР, а також уточнення її геологічного віку. Крім польових геологічних методів досліджень, зразки гірських порід, які були зібрані у цей та попередні роки, вивчалися з використанням методів оптичної та електронної мікроскопії для з’ясування мінералого-петрографічних особливостей габроїдів.
Главной целью исследований было выяснение условий залегания и особенностей строение ГИТР, а также уточнение ее геологического возраста. Кроме полевых геологических методов исследования, образцы горных пород, собранные в этот и предыдущие годы, изучались с использованием методов оптической и электронной микроскопии для определения минералого-петрографических особенностей габброидов.
|
| issn |
1727-7485 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/145833 |
| citation_txt |
Geological position and age of Tuxen-Rasmussen layered gabbroid intrusion (West Antarctica) / O.V. Mytrokhyn, V.G. Bakhmutov, A.G. Aleksieienko, L.I. Gavryliv, T.V. Mytrokhina // Український антарктичний журнал. — 2017. — № 16. — С. 21-28. — Бібліогр.: 7 назв. — англ. |
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21
УКРАЇНСЬКИЙ АНТАРКТИЧНИЙ ЖУРНАЛ
УАЖ, №16, 21—28 (2017)
UDC 55(1)+551.1+552.3+550.93
GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID
INTRUSION (WEST ANTARCTICA)
O. V. Mytrokhyn1, V. G. Bakhmutov2, A. G. Aleksieienko1, L. I. Gavryliv1, T. V. Mytrokhina1
1Taras Shevchenko National University of Kyiv, 90 Vasylkivska Str. Kyiv, 03022, Ukraine;
mitrokhin.a.v@ukr.net; State Institution National Antarctic Scientific Center, Ministry of Education and Science of
Ukraine, 16 Taras Shevchenko Blvd. Kyiv, 01601, Ukraine
2 S.I. Subbotin Institute of Geophysics, National Academy of Sciences of Ukraine, 32 Akad. Palladin Ave, Kyiv,
03142, Ukraine; State Institution National Antarctic Scientific Center, Ministry of Education and Science of
Ukraine, 16 Taras Shevchenko Blvd. Kyiv, 01601, Ukraine
Abstract. The latest geological research carried out during seasonal work in 2017 revealed that metamorphosed layered gabbroids,
which partially outcrop along the Antarctic Peninsula Coast on Cape Tuxen and Rasmussen Island, compose sheet-like intrusive
body of total dimensions more than 3 km2. Significant part of the Tuxen-Rasmussen Gabbroid Intrusion (TRGI) is submerged under
Waddington Bay. Main objectives of the study were the identification of geological position of the TRGI and the clarification of
its geological age. In addition to the field methods of the geological research the samples collected during this and the previous
years were studied using optical and electron microscopy techniques in order to identify petrographic and mineralogical features of
the gabbroids. Results of the studies confirmed previous observation that TRGI was embedded in Upper Jurassic Volcanic Group
(UJVG) of Antarctic Peninsula and it was responsible for contact metamorphism of the UJVG and suffered contamination by vol-
canic material. Gabbroids and volcanic rocks were later intruded by granites of Late Cretaceous age, which belong to the Andean
Intrusive Suite (AIS) of Graham Land. Authors define geological age of TRGI as Early Cretaceous. Previous U-Pb isotope datings
of the gabbroids are believed to be “rejuvenated”. It was discovered that TRGI strikes in northeastern direction and dips steep in
northwestern direction. It is assumed that intrusive body continues underwater and can outcrop at the northern shore of the Wad-
dington Bay and at Barros Rocks direction to the southwest. Petrographical researches showed that gabbroids underwent metamor-
phic alteration in conditions of epidote-amphibolite facies. They bear, nevertheless, relict structural-textural features and mineral
associations of mafic igneous rocks. Authors drew a conclusion that geological position and petrographical peculiarities of TRGI
correspond to hypabyssal level of crystallization. Primary igneous origin is proved for the microrhythmic layering of the gabbroids.
Relict mineral associations allow to identify the rocks as olivine gabbro-norites. Potential Fe-Ti-V ore specialization is emphasized
according to the revealed patterns of crystal fractionation and accumulation of ilmenite and magnetite.
Key words: geology of Antarctica, Andean Intrusive Suite, layered intrusions, gabbroids.
ГЕОЛОГІЧНА БУДОВА ТА ВІК РОЗШАРОВАНОЇ ГАБРОЇДНОЇ ІНТРУЗІЇ
ТУКСЕН-РАСМУСЕН (ЗАХІДНА АНТАРКТИКА)
О. В. Митрохин1, В. Г. Бахмутов2, А. Г. Алексєєнко1, Л. І. Гаврилів1, Т. В. Митрохина1
1 Київський національний університет імені Тараса Шевченка, м. Київ; mitrokhin.a.v@ukr.net; Державна
установа Національний антарктичний науковий центр МОН України, м. Київ
2 Институт геофізики ім. С.І.Суботина НАН України 03680, м. Київ; Державна установа Національний
антарктичний науковий центр МОН України, м. Київ
Реферат. Новітні геологічні дослідження 2017 року показали, що метаморфізовані шаруваті габроїди, які фрагментарно
відслонюються вздовж узбережжя Антарктичного півострова в районі мису Туксен та острова Расмусен, складають
пластоподібне інтрузивне тіло загальною площею не менш ніж 3 км2. Значна частина габроїдної інтрузії Туксен-Расму-
сен (ГІТР) знаходиться під водами затоки Вадінгтон. Головною метою досліджень було з’ясування умов залягання та
особливостей будови ГІТР, а також уточнення її геологічного віку. Крім польових геологічних методів досліджень, зраз-
ки гірських порід, які були зібрані у цей та попередні роки, вивчалися з використанням методів оптичної та електронної
мікроскопії для з’ясування мінералого-петрографічних особливостей габроїдів. Результати виконаних досліджень під-
твердили спостереження попередників, які засвідчили, що ГІТР вкорінена у орогенні утворення верхньо-юрської вул-
канічної групи Антарктичного півострова, спричинила їх контактовий метаморфізм та зазнала контамінації вулканоген-
22
O. V. Mytrokhyn, V. G. Bakhmutov, A. G. Aleksieienko, L. I. Gavryliv, T. V. Mytrokhina
GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID INTRUSION (WEST ANTARCTICA)
ним матеріалом. Габроїди і вулканіти, у свою чергу, інтрудуються гранітоїдами пізньо-крейдяного віку, що належать до
Андійської інтрузивної серії Землі Греяма. Геологічний вік ГІТР визначається авторами як ранньо-крейдяний, попередні
U-Pb ізотопні датуванні цирконів з габроїдів вважаються «омолодженими». Виявлено, що ГІТР має північно-східне про-
стягання та стрімке північно-західне падіння. Виходячи з умов залягання, припускається продовження інтрузивного тіла
на північному узбережжі затоки Вадінгтон, а також у напрямку на південний захід у бік островів Барос Рокс. Петро-
графічні дослідження показали, що габроїди ГІТР підлягають метаморфічнічним перетворенням в умовах епідот-амфі-
болітової фації. Тим не менше, вони зберігають реліктові особливості будови та мінеральні парагенезиси основних маг-
матичних порід нормального ряду лужності. Автори роблять висновок про те, що умови залягання та структурно-тек-
стурні особливості габроїдів ГІТР свідчать про гіпабісальний рівень їх кристалізації. Для тонкої ритмічної шаруватості
габроїдів доводиться первинно-магматичне походження. Реліктові мінеральні парагенезиси дозволяють ідентифікувати
серед них олівінові габро-норити. З огляду на виявлені ознаки кристалізаційної диференціації, що супроводжувалася
кумуляцією ільменіту та титаномагнетиту, звертається увага на потенційну Fe-Ti-V спеціалізацію габроїдів ГІТР.
Ключові слова: геологія Антарктиди, Андійська інтрузивна світа, розшаровані інтрузії, габроїди.
ГЕОЛОГИЧЕСКОЕ СТРОЕНИЕ И ВОЗРАСТ РАССЛОЕННОЙ ГАББРОИДНОЙ
ИНТРУЗИИ ТУКСЕН-РАСМУССЕН (ЗАПАДНАЯ АНТАРКТИКА)
А. В. Митрохин1, В. Г. Бахмутов2, А. Г. Алексеенко1, Л. И. Гаврилив1, Т. В. Митрохина1
1 Киевский национальный университет имени Тараса Шевченко 03022, г. Киев; mitrokhin.a.v@ukr.net;
Государственное учреждение Национальный антарктический научный центр МОН Украины
2 Институт геофизики им. С.И.Субботина НАН Украины 03680, г. Киев; Государственное учреждение
Национальный антарктический научный центр МОН Украины, г. Киев
Реферат. Последние геологические исследования 2017 года показали, что метаморфизированные расслоенные габброи-
ды, которые фрагментарно обнажаются вдоль побережья Антарктического полуострова в районе миса Туксен и острова
Расмуссен, слагают пластоподобное интрузивное тело общей площадью не менее 3 км2. Значительная часть габброидной
интрузии Туксен-Расмуссен (ГИТР) находится под водами залива Ваддингтон. Главной целью исследований было выяс-
нение условий залегания и особенностей строение ГИТР, а также уточнение ее геологического возраста. Кроме полевых
геологических методов исследования, образцы горных пород, собранные в этот и предыдущие годы, изучались с исполь-
зованием методов оптической и электронной микроскопии для определения минералого-петрографических особенно-
стей габброидов. Результаты выполненных исследований подтвердили наблюдения предшественников, согласно кото-
рым ГИТР внедрена в орогенные образования верхнеюрской вулканической группы Антарктического полуострова, обу-
словила их контактовый метаморфизм и подверглась контаминации вулканогенным материалом. Габброиды и вулкани-
ты, в свою очередь, интрудируются гранитоидами позднемелового возраста, относящимися к Андийской интрузивной
серии Земли Греяма. Геологический возраст ГИТР определяется авторами как раннемеловой, предыдущие U-Pb изотоп-
ные датировки габброидов считаются «омоложенными». Установлено, что ГИТР имеет северо-восточное простирание и
крутое северо-западное падение. Исходя из условий залегания, предполагается продолжение интрузивного тела на север-
ном побережье залива Ваддингтон, а также в направлении на юго-запад в сторону островов Барос Рокс. Петрографиче-
ские исследования показали, что габброиды ГИТР подверглись метаморфическим преобразованиям в условиях эпи-
дот-амфиболитовой фации. Тем не менее, они сохраняют реликтовые особенности строения и минеральные парагенези-
сы основных магматических пород нормального ряда щелочности. Авторы делают вывод о том, что условия залегания
и структурно-текстурные особенности габброидов ГИТР свидетельствуют об гипабиссальном уровне их кристаллиза-
ции. Для тонкой ритмической расслоенности габброидов доказывается первично-магматическое происхождение. Релик-
товые минеральные парагенезисы позволяют идентифицировать среди них оливиновые габбро-нориты. Ввиду выявлен-
ных признаков кристаллизационной дифференциации, сопровождаемой кумуляцией ильменита и титаномагнетита, об-
ращается внимание на потенциальную Fe-Ti-V специализацию габброидов ГИТР.
Ключевые слова: геология Антарктиды, Андийская интрузивная свита, расслоенные интрузии, габброиды.
1. Introduction
Mafic intrusive rocks attract lots of attention among the igneous rocks that are widespread in the vicinity of
Ukrainian Antarctic Akademik Vernadsky station (Vernadsky station). Unlike predominant granitoids and volcanic
rocks, gabbroids do not cover large territories. Nonetheless, small separate outcrops of gabbroids are detected at
numerous islands of Wilhelm Archipelago as well as at the adjacent Antarctic Peninsula coast. Due to inaccessibility
of the scarce outcrops, it is natural that morphology of separate gabbroid bodies, their geological position, age and
ore-bearing potential are still insufficiently studied.
Cape Tuxen, which is situated at the western coast of Graham Land to the south of Waddington Bay, was
discovered and recorded in 1898 by Belgian expedition under Adrien de Gerlache command. This expedition named
Cape Rasmussen, which allegedly limited entrance to the Waddington Bay from the North. Data from the subsequent
aerial survey refuted existence of visible land ledge in the area. Hence, according to the United Kingdom Antarctic
Place-Names Committee decision, Rasmussen Island name stuck to one of the small islands near the northern shore
of Waddington Bay since 1959. R. Сurtis (1966) carried out brief petrographic study of Cape Tuxen gabbros and
referred them to the Andean Intrusive Suite (AIS) of Graham Land (Curtis,1966). In his opinion the AIS formation
included time span from Late-Cretaceous to Early-Tertiary. Obvious flaw of Cape Tuxen geological map represented
in the paper is absence of rocks bedding features. Isotopic dating results on Cape Tuxen and Rasmussen Island can
be found in following papers (Pankhurst, 1982; Tanggeman et al., 1996; Bakhmutov et al., 2013). Gladkochub et al
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O. V. Mytrokhyn, V. G. Bakhmutov, A. G. Aleksieienko, L. I. Gavryliv, T. V. Mytrokhina
GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID INTRUSION (WEST ANTARCTICA)
(2011-12) provided geochemical data on Cape Tuxen gabbros, which assume subduction lithospheric source of mag-
ma generation for the rocks studied. Artemenko et al. (2013) came to conclusion that rhythmic layering was formed
due to minerals crystallization in conditions of rapid moving magma. Magnetites of Cape Tuxen demonstrate high V
concentrations (Artemenko et al., 2011).
During seasonal work in 2017 field geological research of Cape Tuxen and Rasmussen Island gabbroids
took place. The research revealed that layered intrusive body, which is further regarded as Tuxen-Rasmussen
Gabbroid Intrusion (TRGI), lies in this region. Further lab research of the samples taken yielded fundamentally
new data on structural-textural features and matter composition of TRGI. Research conducted was supported by
State Institution National Antarctic Scientific Center, Ministry of Education and Science of Ukrainee in terms of
target scientific-technical program of State Special-Purpose Research Program in Antarctica for 2011-2020. The
research aims the identification of geological position, age, petrographical and mineralogical features of TRGI.
2. Methods and materials
Field research in the area of Cape Tuxen and Rasmussen Island, which was conducted by Mytrokhyn O.V.
and Bakhmutov V.G. during seasonal works in 2017, included studying of petrographic diversity of the rocks, their
bedding and contact relations, outcrops examination and sampling. Further laboratory research was implemented in
Taras Shevchenko National University of Kyiv (Institute of Geology). Mytrokhyn O.V. and Mytrokhyna T. V. studied
thin-sections using polarized light microscopes. Quantitative mineralogical analysis was performed using Andin in-
tegration table. Electron microscopy and electron-microprobe investigations of rock-forming and Fe-Ti oxide-ore
minerals were conducted by Mytrokhyn O.V., Gavryliv L.I. and Aleksieienko A.G. Raster electron microscope
REMMA-202M equipped with energy-dispersion X-ray spectrometer «Link systems» was used.
3. Results and discussion
3.1. Bedding, contact relations and age of TRGI
Cape Tuxen (65°16’ S, 64°07’ W) is a small rocky cape at the western coast of Antarctic Peninsula. The cape is
situated at 6.9 km distance to the east from UAS at Galindez Island, from which it is separated by Penola Strait (Fig.1).
Fig.1. Geological position of Tuxen-Rasmussen Gabbroid Intrusion. Legend: 1 – TRGI gabbroids; 2 – AIS granitoids;
3 – UJVG volcanites; 4 – mafic dikes; 5 – strike of the magmatic layering with bedding features respectively.
The map was compiled using (Curtis, 1966) with author’s corrections and additions.
Cape Tuxen is limited by Waddington Bay from the north. Its central part is occupied by Mount Demaria 638
m high. Lumiere Peak 1066 m is situated to the east of it. Collins Bay cuts into the coast to the south of Cape Tuxen.
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GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID INTRUSION (WEST ANTARCTICA)
Gabbroids compose most northwestern ridge of Cape Tuxen and are traced for more than 1.5 km along its western
coast (Fig.2). East of Mount Demaria steep slopes gabbroids contact with volcanic rocks, which are traditionally
referred to Upper Jurassic Volcanic Group (UJVG). South of Mount Demaria gabbroids and volcanic rocks are in-
truded by granitoids, which are described as a part of Andian Intrusive Suite (AIS) of Graham Land. During 2017
seasonal works gabbroid outcrops were also detected at Rasmussen Island, which is situated in Waddington Bay 2
km north the northernmost point of Cape Tuxen. Similarities in the bedding of Cape Tuxen and Rasmussen Island
gabbroids as well as their petrographic identity suggest that they belong to a large single intrusive body, main part of
which is submerged under Waddigton Bay. The intrusive body might possess flattened form, northeastern strike and
steep northwestern dip. Tuxen-Rusmussen gabbroid intrusion (TRGI) is at least 3-4 km long and 1-1.5 km wide.
More accurate estimation of dimensions requires further outcrops research of the northern Waddington Bay shore
and Barros Rocks Islands.
Fig.2. Bedding of the Cape Tuxen rocks: a – look from the west from Penola Straight; b – look from the north
from the Waddington Bay. Legend: TRGI – Tuxen-Rasmussen Gabbroid Intrusion; UJVG – Upper Jurassic Volcanic Group;
AIS – granitoids of the Andean Intrusive Suite.
Characteristic feature of TRGI is its original igneous layering (Fig.3). Layered structure is distinguished by
variation of 10-20 cm thick layers, which differ by mafic and sialic minerals ratios and texture. Medium-grained
mesocratic gabbroids with hypidiomorphic cumulate texture are dominant. Layers of leucocratic gabbroids have
larger grain size sometimes show porphyritic texture. Tabular phenocrysts of plagioclase and elongated mafic min-
erals demonstrate weak orientation pattern parallel to layering surface. At the most northwestern point of Cape Tuxen
TRGI outcrop (65°16’3,4’’ S, 64°06’56,2’’ W) gabbros layering strikes NE80° and dips NW at angle of 65°. Farther
south at the Mount Demaria slopes layering strike changes to NE55°, NW dip at 55° angle. Layered gabbroids from
southern part of Rasmussen Island (65°15’29,6’’ S, 64°04’48,3’’ W) strike NE75-80° and dip subvertical, which
correlates with Cape Tuxen gabbroids.
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GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID INTRUSION (WEST ANTARCTICA)
Fig.3. Layered structures of TRGI gabbroids: a – Cape Tuxen, most northwestern point; b – Cape Tuxen,
northern slope of Mount Demaria, layered gabbroids near the contact with UJVG; c – Rasmussen Island,
microlayered gabbroids intruded by aplite vein.
At several outcrops on Cape Tuxen thin porphyritic and diabase dikes intrude the gabbroids (Fig.4a). Such
dikes rarely exceed 20-30 cm in thickness, strike NE25° or NW310° and dip subvertical. Dikes have sharp intrusive
contacts with angular unconformity with gabbro layering. Chilled margins that are composed by aphanitic diabase
are common, which suggests significantly younger age for the dikes in relation to TRGI. Some of the dikes are even
younger than granite veins that cut gabbroids.
Geological data suggests the Mesozoic age of TRGI. Lower age limit is based on intrusive contact between
Cape Tuxen gabbroids and UJVG. Xenoliths of UJVG in gabbroids as well as contact metamorphism of volcanic
rocks at the contact with gabbroids described by R. Curtis (1966) prove Post-Jurassic age of TRGI. Upper age limit
for TGRI formation is controlled by aforementioned AIS granitoid intrusion inside TRGI. Numerous granite dikes
and veins intrude gabbroids along the western shore of Cape Tuxen at the steep slopes of Mount Demaria (Fig.4b-c).
One of the biggest dikes was studied at 65°16’24,1’’S, 64°07’37,1’’W outcrop. Gabbroids demonstrate
microrythmical layering that strikes NE25° and dips NW at 75° angle. 1 m thick granite dike strikes NE81° and dips
SE at angle of 80°. The dike has sharp straight cutting contacts with gabbro layering. Central part of the dike has
slightly better crystallinity relatively to the peripheral aplite parts, which, probably, indicates that granite magma
intruded in completely cooled gabbroids. Granitic apophysises intrude gabbros at both sides of the dike and form
thin aplite veins 1-10 cm thick. Numerous xenoliths of gabbro are presented in this and in other aplite veins and
sometimes form eruptive breccia.
According to Tanggeman et al. (1996), U-Pb age of zircons from Mount Demaria granitoids is 84.5±0,9
Ma, which corresponds to Upper-Cretaceous period. Two samples of amphibolized gabbroids from Cape Tuxen
and Mount Demaria that were mistakenly named quartz-diorites (Tanggeman et al., 1996) yielded almost identical
results of U-Pb dating 85,2±0,7 Ma – sample AP90-11H and 84,8±0,5 Ma – sample AP90-11J. One can easily
identify those as gabbros according to published chemical analysis data (Tanggeman et al., 1996). Close to the
mentioned U-Pb isotope data was obtained for zircons of Cape Tuxen gabbroids (Bakhmutov et al., 2013).
Nevertheless, despite of seeming evidence of the isotope datings Upper-Cretaceous age of the TRGI is doubtful.
First, numerous geological observations prove older age of gabbroids relatively to granitoids, which doesn’t
demonstrate isotope dating. Second, petrographic research points out metamorphic alteration of TRGI, which has
certainly affected isotope ratios and reduced the age. At last, granites of Rasmussen Island, which demonstrate
post-gabbro geological age, yielded 117,0±0,8 Ma using U-Pb zircon dating (Tanggeman et al., 1996) and
128,0±3,0 Ma – Rb-Sr whole-rock estimations (Pankhurst, 1982). Hence, the most probable age of TRGI is Early
Cretaceous or even Late Jurassic.
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GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID INTRUSION (WEST ANTARCTICA)
Fig.4. Dikes and veins in TRGI gabbroids: a – Cape Tuxen gabbroids are intruded by vertical NE-striking diabase dike;
b – numerous granite veins in gabbroids on Mount Demaria slope; c – NE-striking granite dike intrudes Cape Tuxen gabbroids.
3.2. Petrographic characteristic of TRGI gabbroids
Characteristic feature of TRGI gabbroids is metamorphic alteration these rocks underwent in different limits.
Nevertheless, all of the samples studied preserve relicts of original structural-textural features and mineral associa-
tions, which make it possible to use igneous plutonic rocks classification and nomenclature. Gabbroids from Rasmussen
Island underwent minimum metamorphism. The most “fresh” sample 17-118A-1 was identified as gabbronorite. It is
unevenly striped gray with microrythmic structure that is formed by 0.5-4 cm thick layers, which differ in mafic min-
erals content. Two thin layers represent leucogabbronorite, other – mesocratic. Texture is phanerytic, equigranular,
medium-grained, grain orientation is identified only in thin sections. Tabular plagioclases 1-2 mm in size are more
idiomorphic than mafic minerals. Under binocular loupe dark-gray pyroxene, greenish columnar amphibole and steel-
gray magnetite are easily distinguished among mafic and ore minerals. In thin sections TRGI gabbroids demonstrate
hypidiomorphic granular gabbro-ophitic texture (Fig.5), which is characterized by subhedral plagioclase relatively to
the mafic minerals. Mineral composition: plagioclase (46-89%), amphiboles (20-47%), pyroxenes (0-12%), Fe-Ti
oxides (4-5%), quartz (1-2%). Biotite, chlorite and talk were present in some samples. Accessory minerals are repre-
sented by apatite, sphene, zircon and baddeleyite. The last two were diagnosed using REMMA.
Fig. 5. Microscopic features of Rasmussen Island gabbronorite, sample 17-118A-1 in thin section under polarized microscope.
Main rock-forming minerals are represented by plagioclase and pyroxenes. Plagioclase demonstrates idiomorphism in relation
to pyroxenes. Pyroxenes form small cumulative aggregates up to several grains. Parallel-fibrous actinolite replaces partly
pyroxene grains.
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GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID INTRUSION (WEST ANTARCTICA)
Calcium plagioclase is the main primary magmatic mineral of all studied samples. It crystalizes as chaot-
ic-oriented tabular grains 0.5-2 mm size. In polarized light the plagioclases demonstrate both simple and polysyn-
thetic twinning, in some places it has zonned extinction. Block extinction and other signs of brittle deformations can
also be found. At high magnification rate one can distinguish numerous oriented inclusions of opaque minerals
within plagioclases. Some grains underwent mild sericitization. Electron microprobe analysis of the least altered 17-
118A-1 sample yielded bytownite-anorthite An72-93 for the core parts and andesine An39-45 at the rims. Less basic
plagioclases that are found in more altered TRGI samples probably have secondary genesis. Secondary genesis is
also suggested for quartz, which crystallizes in small anhedral grains and granoblastic aggregates between pla-
gioclases.
Fibrous amphiboles are the most common metamorphic minerals of the studied rocks. They are mostly repre-
sented by pale-green actinolite or sometimes by colorless cummingtonite, precise diagnostics of which were per-
formed using REMMA. The amphiboles usually crystalize as parallel-fibrous aggregates that pseudomorphicaly re-
place original pyroxenes. Less common are matted-fibrous aggregates. Dimensions of the aggregate reach 2-3 mm
with single fibers 1-2 mm thick. Pyroxenes that are undoubtedly typical igneous minerals can be found only in the
least altered samples of TRGI. They are represented by both clinopyroxene and orthopyroxene. According to the
electron microprobe analysis in 17-118A-1 sample clinopyroxenes are represented by diopside Wo46-47En40-41, when
orthopyroxenes are enstatites Wo2-3En66-67. Pyroxenes usually crystalize as euhedral grains 1-4 mm size or form cu-
mulate aggregates. Inside the biggest pyroxene grains magnetite-talc aggregates can be found, which are likely to be
fully pseudomorphosed olivines that preserve previous “bay-like” configuration of crystals. In some cases these
pseudomorphoses are altered into fibrous amphiboles.
Fe-Ti-oxide ore minerals of TRGI gabbroids are present both as primary magmatic, and secondary metamor-
phic varieties. Primary ones are represented by ilmenite and Ti-magnetite, which occur in both individual grain and
cumulate aggregate 1-2 mm in size forms (Fig.6). Such aggregates are rare but form sideronitic-like texture when
present. Primary magmatic ilmenites crystallized earlier than Ti-magnetite and usually are more euhedral. Such il-
menites form tabular crystals that include lamellar exsolution inclusions of hematite parallel to the (0001) crystallo-
graphic plane. Some of the ilmenite grains also contain numerous sphene inclusions or demonstrate slight leucoxe-
nization. On the contrary, primary Ti-magnetite graines are anhedral. They contain tabular exsolution inclusions of
ilmenite that are oriented parallel to the (111) crystallographic plane. Microprobe analysis of Ti-magnetites demon-
strated 1-6% of TiO2 and 0.7-0.9% V2O3. Except primary magmatic Ti-magnetites secondary metamorphic magne-
tites are present in examined rocks. Their dusty grains are located within fibrous amphibole aggregates. The second-
ary magnetites contain no ilmenite inclusions as well as any other chemical impurity of TiO2.
Fig.6. Fe-Ti-oxide ore minerals of TRGI gabbroids under electron microscope: a – cumulate aggregate of Ti-magnetite and
ilmenite, sample 17-118A-1; b – individual ilmenite grain with exsolution inclusions of hematite, sample 16-2.
Anhedral laths and scaly aggregates of red-brown biotite are unevenly distributed in the studied rocks. They
grow around Fe-Ti-oxide ore minerals or at the mafic mineral to plagioclase margins. Microprobe analysis detected
significant TiO2 and Cl impurities. Biotite is often replaced by chlorite with “drops” surplus titanium as fine sphene.
Microcrystal aggregates of sphene are also found within fibrous amphiboles. The most significant accessory mineral
of TRGI gabbroids is apatite. It is crystalized as small elongated-prismatic crystals 0.1-0.3 mm in size, which com-
monly tend to the mafic minerals aggregations. Microprobe analysis detected high content of Cl in the apatites.
4. Conslusions
On the basis of conducted research following conclusions concerning bedding, geological age and petro-
graphic features of the studied rocks were made.
1. Metamorphosed gabbroids that fragmentary crop out on Cape Tuxen and Rasmussen Island form sheet-like
intrusive body at least 3 km2. Tuxen-Rasmussen Gabbroid Intrusion intruded into orogenic rocks of Upper Jurassic
Volcanic Group, metamorphosed them and underwent contamination by volcanic material itself. Gabbroids and
UJVG rocks were subsequenty intruded by Upper Cretaceous granites of Andean Intrusive Suite. Geological age of
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GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID INTRUSION (WEST ANTARCTICA)
TRGI is estimated as Lower Cretaceous. Previous U-Pb isotope datings of zircon from the gabbroids are believed to
be “rejuvenated”.
2. TRGI strikes NE and dips steep in SW direction. Authors suggest that intrusive body continues underwater
and can crop out at the northern shore of the Waddington Bay and in Barros Rocks direction to the southwest.
Microrythmic layering of TRGI gabbroids has original igneous origin. Cryptic layering, which implies variation in
the chemical composition of rock-forming minerals with stratigraphic height in a layered sequence, is possible and
requires additional research.
3. TRGI gabbroids underwent metamorphism in epidote-amphibolite facies. The most common alteration
process is amphibolization. They preserve, nevertheless, relict structural-textural features and mineral associations
of mafic igneous rocks. Geological position and petrographical features of TRGI correspond to hypabyssal level of
crystallization. Relict mineral associations allow to identify the rocks as olivine gabbro-norites. Potential Fe-Ti-V
ore specialization is emphasized according to the revealed patterns of crystal fractionation and accumulation of il-
menite and magnetite.
5. Acknowledgements
Large impact on organization and execution of field works conducted belongs to the winterers of XXI
Ukrainian Antarctic Expedition namely M.P. Starinec, V.V. Khrapach et al., whose work is highly acknowledged by
the authors. Some rock samples for research were courteously provided by P.A.Burtnuy which the authors sincerely
appreciate.
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mineralization outcrops in Antarctic Peninsula Batholith rocks]. Ukrainskij Antarktychnij Zhurnal [Ukrainian Antarctic
Journal], 10-11. 13-25. (In Russian).
2. Artemenko, G.V., Bakhmuntov, V.G., Samborskaya, I.A., Bakhmutova, L.N., Shpyra, V.V. 2013. Magmatich-
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