geol. bull. turkey Türkiye Jeoloji Bülteni 1016-9164 2564-6745 TMMOB Jeoloji Mühendisleri Odası 10.25288/tjb.1849204 Mineralogy- Petrography Igneous and Metamorphic Petrology Mineraloji-Petrografi Magmatik ve Metamorfik Petroloji A Comparative Example of Multi-Level Mafic Magma Emplacement Along The South Shetland Arc (West Antarctica): The Dee and Cecilia Intrusions Güney Shetland Yayı (Batı Antarktika) Boyunca Çok Seviyeli Mafik Magma Yerleşimine Karşılaştırmalı Bir Örnek: Dee ve Cecilia İntrüzyonları / A Comparative Example of Multi-Level Mafic Magma Emplacement Along The South Shetland Arc (West Antarctica): The Dee and Cecilia Intrusions https://orcid.org/0000-0002-5078-9244 Ünal Alp ISTANBUL TECHNICAL UNIVERSITY 04 28 2026 69 2 203 224 12 25 2025 04 02 2026 Copyright © 1947, Geological Bulletin of Turkey 1947 Geological Bulletin of Turkey

This study presents a comparative investigation of the emplacement conditions for gabbroic intrusions exposed on Dee and Cecilia islands, located along the South Shetland Islands to the north of the Antarctic Peninsula in Western Antarctica. Crystallization temperatures and emplacement pressures were calculated using two-pyroxene geothermobarometry, and oxygen fugacity conditions were additionally constrained based on Fe–Ti oxide pairs.Petrographically, the Dee intrusions are characterized by fine-grained, holocrystalline porphyritic textures composed of plagioclase, clinopyroxene, orthopyroxene, olivine, and opaque minerals. The Cecilia intrusion displays broadly similar mineral assemblage but coarser-grained holocrystalline granular textures. Plagioclase in the Dee intrusions predominantly exhibits normal zoning, whereas plagioclase crystals in the Cecilia intrusion commonly show oscillatory and locally reverse zoning, together with sieve textures and embayment structures. Two-pyroxene geothermobarometric calculations suggest that the Dee intrusions crystallized at temperatures of approximately 1,030–1,090 ℃ and pressures of 2.5–3.5 kbar, corresponding to relatively shallow upper-crustal emplacement. In contrast, the Cecilia intrusion yields crystallization temperatures of 1,000–1,170 ℃ and pressures of 4.5–6.5 kbar, consistent with deeper emplacement at mid- to lower-crustal levels. Fe–Ti oxide barometry indicates that both intrusions crystallized under oxidizing redox conditions, with the Cecilia intrusion recording systematically higher oxygen fugacity values relative to the Dee intrusions.When field, petrographic, and mineral chemistry data are evaluated together, the Dee and Cecilia intrusions are interpreted to represent mafic magmatic bodies emplaced at different crustal levels within the South Shetland Arc magmatic system. Future geochronological studies will lead the investigation of the timing of emplacement and further assess the relationship between the crustal evolution of these mafic intrusions and the regional tectonic framework.

Bu çalışma, Batı Antarktika’da Antarktik Yarımada’nın kuzeyinde yer alan Güney Shetland Adaları boyunca konumlanan Dee ve Cecilia adalarında yüzeyleyen gabroik intrüzyonların yerleşim koşullarını karşılaştırmalı olarak incelemektedir. İntrüzyonların kristallenme sıcaklıkları ve yerleşim basınçları, iki-piroksen jeotermobarometresi kullanılarak hesaplanmış ve bununla beraber Fe–Ti oksit çiftlerinden oksijen fugasitesi koşulları belirlenmiştir.Petrografik olarak Dee intrüzyonları, plajiyoklaz, klinopiroksen, ortopiroksen ve olivin ve opak minerallerden oluşan, ince taneli ve holokristalen porfrik dokular ile temsil edilmektedir. Buna karşılık Cecilia İntrüzyonu, benzer mineralojik bileşime sahip olmakla birlikte, daha iri taneli holokristalen granüler dokular sergilemektedir. Dee intrüzyonlarında plajiyoklazlar baskın olarak normal zonlanma gösterirken, Cecilia İntrüzyonu’nda plajiyoklaz kristallerinde osilasyonlu ve yer yer ters zonlanmalar ile birlikte elek dokuları ve körfez yapıları izlenmektedir. Yapılan iki-piroksen jeotermobarometre hesaplamaları, Dee intrüzyonlarının yaklaşık 1030–1090 ℃ kristallenme sıcaklıkları ve 2,5–3,5 kbar basınç aralığı ile üst kabuk seviyelerinde görece sığ yerleşimine işaret etmektedir. Buna karşılık Cecilia İntrüzyonu, 1000–1170 ℃ aralığındaki kristallenme sıcaklıkları ve 4,5–6,5 kbar basınç değerleri ile orta–alt kabuk seviyelerinde daha derin bir sokuluma karşılık gelmektedir. Fe–Ti oksit barometresi sonuçları, her iki intrüzyonun da oksitleyici redoks koşulları altında kristallendiğini; Cecilia İntrüzyonu’nun Dee’ye kıyasla daha yüksek oksijen fugasitesi değerleri sergilediğini göstermektedir. Saha, petrografi ve mineral kimyası verileri birlikte değerlendirildiğinde, Dee ve Cecilia intrüzyonlarının Güney Shetland Yayı boyunca gelişen mafik magmatizmanın kabukta farklı seviyelere yerleşmiş olan üyelerini temsil ettikleri görülmektedir. İleride yapılacak olan yaşlandırma çalışmaları ile bu mafik intrüzyonların kabuk içerisindeki evriminin bölge tektonizması ile ilişkisinin araştırılması mümkün olabilecektir.

 Güney Shetland Adaları Gabro Petrografi Mineral Kimyası Jeotermobarometre South Shetland Islands Gabbro Petrography Mineral Chemistry Geothermobarometry TÜBİTAK 125Y065 Altunkaynak, Ş., Aldanmaz, E., Güraslan, I. N., Çalışkanoğlu, A. Z., Ünal, A. & Nývlt, D. (2018). Lithostratigraphy and petrology of Lachman Crags and Cape Lachman lava-fed deltas, Ulu Peninsula, James Ross Island, north-eastern Antarctic Peninsula: Preliminary results. Czech Polar Reports, 8(1). https://doi.org/10.5817/CPR2018-1-5 Altunkaynak, Ş., Ünal, A., Howarth, G. H., Aldanmaz, E. & Nývlt, D. (2019). The origin of low-Ca olivine from ultramafic xenoliths and host basaltic lavas in a back-arc setting, James Ross Island, Antarctic Peninsula. Lithos, 342-343, 276-287. https://doi.org/10.1016/j.lithos.2019.05.039 Altunkaynak, Ş., Aldanmaz, E. & Nývlt, D. (2022). Effects of mantle hybridization by interaction with slab derived melts in the genesis of alkaline lavas across the back-arc region of South Shetland subduction system. Journal of Petrology, 63(11), Article egac111. https://doi.org/10.1093/petrology/egac111 Altunkaynak, Ş., Aldanmaz, E., Velev, S., Forster, M. & Yang, J. H. (2025). Early Cretaceous to Eocene magmatic evolution of Livingston Island, South Shetland Archipelago: Geochronological and isotope geochemical constraints from intrusive suites. Lithos, 492-493, Article 107877. https://doi.org/10.1016/j.lithos.2024.107877 Annen, C., Blundy, J. D., Leuthold, J. & Sparks, R. S. J. (2015). Construction and evolution of igneous bodies: Towards an integrated perspective of crustal magmatism. Lithos, 230, 206–221. https://doi.org/10.1016/j.lithos.2015.05.008 Bacon, C. R. & Hirschmann, M. M. (1988). Mg/Mn partitioning as a test for equilibrium between coexisting Fe–Ti oxides. American Mineralogist, 73(1–2), 57–61. Bastías, J., Chew, D., Villanueva, C., Riley, T., Manfroi, J., Trevisan, C. … & Jaña, R. (2023). The South Shetland Islands, Antarctica: Lithostratigraphy and geological map. Frontiers in Earth Science, 10, Article 1002760. https://doi.org/10.3389/feart.2022.1002760 Berrocoso, M., Fernández-Ros, A., Prates, G., García, A. & Kraus, S. (2016). Geodetic implications on block formation and geodynamic domains in the South Shetland Islands, Antarctic Peninsula. Tectonophysics, 666, 211–219. https://doi.org/10.1016/j.tecto.2015.10.023 Brey, G. P. & Köhler, T. (1990). Geothermobarometry in four-phase Lherzolites II. New thermobarometers, and practical assessment of existing thermobarometers. Journal of Petrology, 31(6), 1353–1378. https://doi.org/10.1093/petrology/31.6.1353 Brown, M. (2013). Granite: From genesis to emplacement. Geological Society of America Bulletin, 125(7–8), 1079–1113. https://doi.org/10.1130/B30877.1 Cao, M., Hollings, P., Cooke, D. R., Evans, N. J., McInnes, B. I., Qin, K. & Baker, M. (2018). Physicochemical processes in the magma chamber under the Black Mountain porphyry Cu–Au deposit, Philippines: Insights from mineral chemistry and implications for mineralization. Economic Geology, 113(1), 63–82. https://doi.org/10.5382/econgeo.2018.4544 Dalziel, I. W. D. & Elliot, D. H. (1982). West Antarctica: Problem child of Gondwanaland. Tectonics, 1(1), 3–19. https://doi.org/10.1029/TC001i001p00003 de Silva, S. L., Riggs, N. R. & Barth, A. P. (2015). Quickening the pulse: Fractal tempos in continental arc magmatism. Elements, 11(2), 113–118. https://doi.org/10.2113/gselements.11.2.113 Ducea, M. N., Paterson, S. R. & DeCelles, P. G. (2015). High-volume magmatic events in subduction systems. Elements, 11(2), 99–104. https://doi.org/10.2113/gselements.11.2.99 Ducea, M. N., Bergantz, G. W., Crowley, J. L. & Otamendi, J. (2017). Ultrafast magmatic buildup and diversification to produce continental crust during subduction. Geology, 45(3), 235–238. https://doi.org/10.1130/G38726.1 Dumont, J. F., Santana, E., Hervé, F. & Zapata, C. (2006). Regional structures and geodynamic evolution of North Greenwich (Fort Williams Point) and Dee Islands, South Shetland Islands. In Fütterer, D.K., Damaske, D., Kleinschmidt, G., Miller, H. & Tessensohn, F. (Eds.), Antarctica, Contributions to Global Earth Sciences (pp.: 255–260). Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-32934-X_31 Fitzgerald, P. G. (2002). Tectonics and landscape evolution of the Antarctic plate since the breakup of Gondwana, with an emphasis on the West Antarctic Rift System and the Transantarctic Mountains. In J. A. Gamble, D. N. B. Skinner & S. Henry (Eds.), Antarctica at the Close of a Millennium, Royal Society of New Zealand Bulletin, 35, 453–469. Guterch, A., Grad, M., Janik, T., Perchuć, E. & Pajchel, J. (1985). Seismic studies of the crustal structure in West Antarctica 1979–1980—preliminary results. Tectonophysics, 114(1–4), 411–429. https://doi.org/10.1016/0040-1951(85)90024-1 Haase, K. M., Beier, C., Fretzdorff, S., Smellie, J. L. & Garbe-Schönberg, D. (2012). Magmatic evolution of the South Shetland Islands, Antarctica, and implications for continental crust formation. Contributions to Mineralogy and Petrology, 163(6), 1103–1119. https://doi.org/10.1007/s00410-012-0719-7 Hawkesworth, C. J., Blake, S., Evans, P., Hughes, R., Macdonald, R., Thomas, L. E., Turner, S.P. & Zellmer, G. (2000). Time scales of crystal fractionation in magma chambers—integrating physical, isotopic and geochemical perspectives. Journal of Petrology, 41(7), 991-1006. https://doi.org/10.1093/petrology/41.7.991 Jankowski, E. J. & Drewry, D. J. (1981). The structure of West Antarctica from geophysical studies. Nature, 291, 17–21. https://doi.org/10.1038/291017a0 Jiang, W. P., E, D. C., Zhan, B. W. & Liu, Y. W. (2009). New model of Antarctic plate motion and its analysis. Chinese Journal of Geophysics, 52(1), 23–32. https://doi.org/10.1002/cjg2.1323 Kamenov, B. K. (2008). Multiepisodic dyke systems in Hurd Peninsula, Livingston Island, South Shetland Islands Volcanic Arc (Antarctica): Petrological and geochemical implications for their magma evolution. Geochemistry, Mineralogy and Petrology, 46, 103–142. Karslı, O., Moghadam, H. S., Kandemir, R., Aydın, F., Santos, J. F. & Demir, Y. (2025). Cretaceous arc magmatism in the Antarctic subduction system: Insights from major and trace element geochemistry and Sr–Nd–Hf–Pb isotopes. Lithos, 516-517, Article 108277. https://doi.org/10.1016/j.lithos.2025.108277 Keller, R. A., Fisk, M. R., White, W. M. & Birkenmajer, K. (1992). Isotopic and trace element constraints on mixing and melting models of marginal basin volcanism, Bransfield Strait, Antarctica. Earth and Planetary Science Letters, 111(2–4), 287–303. https://doi.org/10.1016/0012-821X(92)90185-X La Spina, G., Burton, M., de’ Michieli Vitturi, M. & Arzilli, F. (2016). Role of syn-eruptive plagioclase disequilibrium crystallization in basaltic magma ascent dynamics. Nature Communications, 7, Article 13402. https://doi.org/10.1038/ncomms13402 Leat, P. T., Scarrow, J. H. & Millar, I. L. (1995). On the Antarctic Peninsula batholith. Geological Magazine, 132(4), 399–412. https://doi.org/10.1017/S0016756800021464 Machado, A., Lima, E. F., Chemale Jr., F., Morata, D., Oteíza, O., Almeida, D. P. M. & Urrutia, J. L. (2005). Geochemistry constraints of Mesozoic–Cenozoic calc-alkaline magmatism in the South Shetland arc, Antarctica. Journal of South American Earth Sciences, 18(3–4), 407–425. https://doi.org/10.1016/j.jsames.2004.11.011 Moreno-Alfonso, S. C., Okumura, S., Kilgour, G., Ubide, T., Iizuka, Y., Yasutake, M., Uesugi K., Jegal Y., Mead S. & Zellmer, G. F. (2026). Short crystal residence times and fast magma ascent prior to effusive eruptions at Mount Ruapehu, New Zealand. Journal of Petrology, 67(2), Article egag005. https://doi.org/10.1093/petrology/egag005 Nixon, G. T. (1988). Petrology of the younger andesites and dacites of Iztaccíhuatl volcano, Mexico: II. Chemical stratigraphy, magma mixing, and the composition of basaltic magma influx. Journal of Petrology, 29(2), 265–303. https://doi.org/10.1093/petrology/29.2.265 Pankhurst, R. J. & Smellie, J. L. (1983). K–Ar geochronology of the South Shetland Islands, Lesser Antarctica: Apparent lateral migration of Jurassic to Quaternary island arc volcanism. Earth and Planetary Science Letters, 66, 214–222. https://doi.org/10.1016/0012-821X(83)90137-1 Paterson, S. R. & Ducea, M. N. (2015). Arc magmatic tempos: Gathering the evidence. Elements, 11(2), 91–98. https://doi.org/10.2113/gselements.11.2.91 Paterson, S. R., Memeti, V., Mundil, R. & Žák, J. (2016). Repeated, multiscale, magmatic erosion and recycling in an upper-crustal pluton: Implications for magma chamber dynamics and magma volume estimates. American Mineralogist, 101(10), 2176–2198. https://doi.org/10.2138/am-2016-5576 Petford, N., Cruden, A. R., McCaffrey, K. J. W. & Vigneresse, J. L. (2000). Granite magma formation, transport and emplacement in the Earth's crust. Nature, 408(6813), 669–673. https://doi.org/10.1038/35047000 Petsch, C., Rosa, K. K. D., Oliveira, M. A. D., Velho, L. F., Silva, S. L., Sotille, M. E. & Simões, J. C. (2022). An inventory of glacial lakes in the South Shetland Islands (Antarctica): Temporal variation and environmental patterns. Anais da Academia Brasileira de Ciências, 94(Suppl. 1), Article e20210683. https://doi.org/10.1590/0001-37652022020210683 Putirka, K. D. (2008). Thermometers and barometers for volcanic systems. Reviews in Mineralogy and Geochemistry, 69(1), 61–120. https://doi.org/10.2138/rmg.2008.69.3 Riley, T. R., Burton-Johnson, A., Flowerdew, M. J. & Whitehouse, M. J. (2018). Episodicity within a mid-Cretaceous magmatic flare-up in West Antarctica: U–Pb ages of the Lassiter Coast intrusive suite, Antarctic Peninsula, and correlations along the Gondwana margin. Geological Society of America Bulletin, 130(7–8), 1177–1196. https://doi.org/10.1130/B31800.1 Sauerzapf, U., Lattard, D., Burchard, M. & Engelmann, R. (2008). The titanomagnetite–ilmenite equilibrium: New experimental data and thermo-oxybarometric application to the crystallization of basic to intermediate rocks. Journal of Petrology, 49(6), 1161–1185. https://doi.org/10.1093/petrology/egn021 Schärer, U., Kornprobst, J., Beslier, M. O., Boillot, G. & Girardeau, J. (1995). Gabbro and related rock emplacement beneath rifting continental crust: U–Pb geochronological and geochemical constraints for the Galicia passive margin (Spain). Earth and Planetary Science Letters, 130(1–4), 187–200. https://doi.org/10.1016/0012-821X(94)00261-V Smellie, J. L., Pankhurst, R. J., Thomson, M. R. A. & Davies, R. E. S. (1984). The geology of the South Shetland Islands: VI. Stratigraphy, geochemistry and evolution. British Antarctic Survey Scientific Reports, 87, 1–85. Smellie, J. L. (2020). The role of volcanism in the making of Antarctica. In Oliva, M, & Ruiz-Fernández, J. (Eds.), Past Antarctica, Paleoclimatology and Climate Change (ss. 69–87). Academic Press. https://doi.org/10.1016/B978-0-12-817925-3.00004-5 Smellie, J. L., Panter, K. S. & Geyer, A. (2021). Volcanism in Antarctica: 200 million years of subduction, rifting and continental break-up. Geological Society of London Memoirs, 55, 1–6. https://doi.org/10.1144/M55 Smith, J. V. & Brown, W. L. (1988). Feldspar minerals (2. Edt.). Springer-Verlag. Stern, R. J. (2015). Magmatism at convergent plate boundaries. In Encyclopedia of Marine Geosciences (ss. 1–12). Springer. Streck, M. J. (2008). Mineral textures and zoning as evidence for open system processes. Reviews in Mineralogy and Geochemistry, 69(1), 595-622. https://doi.org/10.2138/rmg.2008.69.15 Stimac, J. A. & Pearce, T. H. (1992). Textural evidence of mafic–felsic magma interaction in dacite lavas, Clear Lake, California. American Mineralogist, 77(7–8), 795–809. Ünal, A. (2025). Preliminary results on the geology and petrography of the Burro Peaks and Cecilia intrusions, South Shetland Islands (Antarctica). In EGU General Assembly Conference Abstracts (EGU25-10432). Zheng, G. G., Liu, X., Pei, J., Zhao, Y., Chen, H. & Li, J. (2022). Early Palaeogene mafic–intermediate dykes, Robert Island, West Antarctica: Petrogenesis, zircon U–Pb geochronology, and tectonic significance. Geological Journal, 57(6), 2209–2220. https://doi.org/10.1002/gj.4402