Mineral Magnetic Properties of Granodiorite, Metagabbro and Microgabbro of Petermann Island, West Antarctica

CZECH POLAR REPORTS 8 (1): 94-106, 2018 Mineral magnetic properties of granodiorite, metagabbro and microgabbro of Petermann Island, West Antarctica Vitalii Pavlovych Ponomar1, Liubomyr Igorovych Gavryliv2* 1Department of Physics of Mineral Structures and Biomineralogy, Institute of Geochem- istry, Mineralogy and Ore Formation of NAS of Ukraine, 34 Palladina prospect, Kyiv, 03680, Ukraine 2Department of Geochemistry, Mineralogy and Petrography, Institute of Geology, Taras Shevchenko National University of Kyiv, 90 Vasylkivska street, Kyiv, 03022, Ukraine Abstract The research focuses on studying the magnetic properties and mineralogy of iron-bearing minerals of granodiorite, metagabbro, and microgabbro of Petermann Island, West Ant- arctica. The predominant iron-bearing minerals of the rocks are ilmenite, magnetite, and iron sulphides. Magnetite in metagabbro and microgabbro is pointed out to be present as two morphological types with different grain size and morphology. The rocks owe their magnetic properties to the presence of different amounts of magnetite with the Curie temperatures of 570–575°C for granodiorite, 555–560°C for metagabbro and 560–565°C for microgabbro. Magnetite in the rocks is stable under heating to 650°C. A slight de- crease in magnetisation at 350–400°C is attributed to the conversion of maghemite or maghemite-like phase into hematite. Variation of the magnetite content within each sample has a strong expression in the saturation magnetisation. The latter increases in sequence: granodiorite (0.8–1.3 Am2/kg), microgabbro (1.8–3 Am2/kg) and metagabbro (3.1–3.5 Am2/kg). The saturation magnetisation of rocks increases with the increasing content of iron. However, the inverse relation is observed for metagabbro and microgabbro due to the replacement of titanite for magnetite in the latter. The magnetic fraction of microgabbro reveals the wasp-waisted hysteresis loop suggesting bimodal size distribution. According to X-Ray Diffraction, the characteristic peaks (d-spacing) of pure magnetite are identified for magnetic fraction of granodiorite and metagabbro, while magnetite of microgabbro form stable intergrowth with titanite and chlorite. Key words: Graham Land, rock magnetic properties, magnetite, thermomagnetic analysis DOI: 10.5817/CPR2018-1-7 ——— Received January 31, 2018, accepted June 10, 2018. *Corresponding author: L. I. Gavryliv <[email protected]> Acknowledgements: The authors appreciate the access to geological samples collection kindly granted by P. A. Burtnyj and V. G. Bakhmutov from the Institute of Geophysics, National Academy of Sciences of Ukraine. 94 V. P. PONOMAR et L. I. GAVRYLIV Introduction Mineral magnetic properties of the rocks rounded island, lying off the northwest provide outstanding information on geolog- coast of Kiev Peninsula in Graham Land, a ical structure and tectonics of the Earth's short distance south of Booth Island and crust. The magnetic properties of rocks are the Lemaire Channel. The geology of the usually attributed to one or more minerals Island is represented by plutonic and hypa- revealing the magnetic ordering such as byssal igneous rocks (Fig. 1): gabbros, gra- ferrimagnetism, antiferromagnetism, and nodiorites, microgabbros and porphyrites ferromagnetism (Cornell et Schwertmann (a porphyritic rock of diorite composition) 2003). The most abundant minerals, which are common. At the north-west corner of determine rock magnetic properties are the island (Rouch Point) there are expo- magnetite, hematite, maghemite, goethite, sures of the Andean Intrusive Suite horn- titanomagnetite, and ilmenite (Dekkers et blende-gabbros and granodiorites, which Linssen 1989, Frank et Nowaczyk 2008, intrude them. Both granodiorites and gab- Frederichs et al. 2003, Lattard et al. 2006, bros are cut by at least two generations of Strangway et al. 1968). In general, they are mafic dykes: the older porphyritic diabase hardly identified by traditional mineralog- and the younger diabase (Mytrokhyn et al. ical techniques, such as optical microscopy 2017). Rb-Sr whole-rock dating of a late (these minerals are opaque) and electron granodiorite intrusion at Rouch Point, gave microprobe (in a case of very small or skel- 93±8 Ma with the initial ratio 87Sr/86Sr = etal grains with the fine lamellae). 0.7045 (Pankhurst 1982). Isotopic dating In the field of geology, quantitative of zircon grains of granodiorites by U-Pb identification of magnetic minerals as well gave 95.9±1.0 and 96.1±0.7 Ma (Bakhmu- as investigation of their properties is crit- tov et al. 2013). The southern part of Pe- ical for deciphering the process of mag- termann I., near Port Circumcision, is com- matic crystallization, ore body emplace- posed entirely of granitoids of the Andean ment, secondary alteration as well as struc- Intrusive Suite (Curtis 1966) with a grano- tural analysis of almost all rock types. On diorite being the most typical rock among the other hand, the mineral magnetic prop- them. These granitoids host a significant erties can serve as sensitive indicators of amount of volcanic and gabbroic xeno- geochemical conditions the rock formed liths. Furthermore, the granitoids are in- and developed in. truded by mafic dykes, similar to those Graham Land is the part of the Ant- occurring at the northern part of the island: arctic Peninsula that lies north of a line porphyritic and diabase dykes are common joining Cape Jeremy and Cape Agassiz. here (Mytrokhyn et al. 2017). Guenthner et Systematic geological investigations of the al. (2010) performed thermochronometric Andean Intrusive Suite of Graham Land, dating of the granodiorite from the southern Antarctic Peninsula, were performed by the part of the Petermann I. Thus, apatite British Graham Land Expedition (BGLE) (U‐Th)/He (apatite He) with a closure during 1934–1937 and later by Falkland temperature of ∼50–70°C gave 11.1±0.9 Islands Dependencies Survey (FIDS). The Ma; apatite fission track (apatite FT) with latter was renamed British Antarctic Survey a closure temperature of ∼100–120°C gave (BAS) in 1962. Since 1996 the Ukrainian 30.6±9.6 Ma; zircon (U‐Th)/He (zircon Antarctic Center has commenced geolog- He) with a closure temperature of ∼170– ical and geophysical investigations of the 200°C gave 42.8±0.9 Ma; zircon fission Argentine Islands and the adjacent area. track (zircon FT) with a closure tempera- Petermann Island is a small, low and ture of ∼220–260°C gave 35.6±4.2 Ma. 95 IRON-BEARING MINERALS Fig. 1. Geological map of Petermann Island, Graham Coast. The map is developed using a topographic surface map of BAS/4/05; frame numbers 244-247 (acquired January 2005) and the existent geological sketch map of Petermann Island after Mytrokhyn et al. (2017). Brief petrographic descriptions of horn- trusion and attributed it to post-tectonic blende gabbros of the north-western part hydrothermal quartz veins and epigenetic and the tonalites and granodiorites of the processes in the tectonic faults. Artemenko southern part of the Petermann I could be et al. (2013) identified the presence of com- found in Curtis (1966). The author con- positional layering in the late intrusive cluded that there are two types of gab- phase of Petermann I. gabbroids: it is claim- broids: the first type with ophitic augite ed to consist of light (up to 20 mm) and (6 mm) and high content of magnetic min- dark (up to 30 mm) layers composed of dif- erals, and the second type with the rem- ferent amounts of pyroxene and magnetic nants of augite around big crystals of am- minerals. The author concluded that these phibole (1–5 mm) and less content of mag- rocks host up to 20% of iron minerals and netic minerals. iron mineral “dust”. Iron magnetic miner- More recently, Artemenko et al. (2011) als occur near pyroxene crystals, which are identified the mineralization of pyrite, chal- altered to green hornblende on its edges. copyrite and magnetite in the granitic in- 96 V. P. PONOMAR et L. I. GAVRYLIV However, mineralogy and mineral mag- sal rocks of Petermann I., West Antarctica, netic properties of the rocks of Petermann and to describe the mineral magnetic prop- I. remain virtually unstudied. Since geolog- erties of these rocks by thermomagnetic, ical position and geodynamic conditions of X-Ray Diffraction, optical microscopy, mag- the formation and secondary alteration of netometry methods, and electron micro- these rocks is a controversial piece of data, probe analysis. The proceedings of this re- new mineralogical and magnetic findings search may lead to a more comprehensive may partially fill the gap in the existing understanding of the geology and genesis data. of the intrusive igneous rocks found on The aim of this work is to identify the Graham Land. magnetic minerals of plutonic and hypabys- Material and Methods The research is performed based on the tion on temperature and the dependence of samples collected during Ukrainian Ant- magnetisation on an applied field). Sam- arctic expeditions (1998–2008). In total, ples are crushed to the size <5 µm at the 11 samples from Petermann I are used in Laboratory of Mineralogical and Geochem- our research (Fig. 1): 5 samples of grano- ical studies of the Institute of Geology, diorite (10/133, 10/136, 10/144, 10/145, Taras Shevchenko National University of 10/146) from the southern part of the Kyiv, Ukraine. island; 2 samples of metagabbro from gab- Petrographic examination of samples broids (B-133, B-137) of the northeastern was carried out both in transmitted and part of the island; 2 samples of porphyritic reflected light using NIKON ECLIPSE diabase dyke (B-127, B-129) which cuts LV100 POL. The samples were polished, the gabbroids at the northwestern part; 2 graphite-coated and analyzed with a micro- samples of microgabbro from diabase dyke probe X-ray spectral microanalysis on a (B-153/12, B-159/12) which cuts the por- raster electron microscope REMMA-202M phyritic diabase dyke and gabbroids at the with an energy-dispersive X-ray spectro- northwestern part.

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