Magnetic Petrology of the Ross Orogen in Oates Land (Antarctica)

F. Talarico¹*, E. Armadillo², F. Ferraccioli^2,3 & N. Rastelli⁴

¹Dipartimento Scienze della Terra, Università di Siena, Via del Laterino 8, 53100 Siena - Italy
²Dipartimento per lo Studio del Territorio e delle sue Risorse, Settore Geofisica, Università di Genova,
V.le Benedetto XV 5, 16132 Genova - Italy
³British Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge CB3 0ET- United Kingdom
⁴Museo Nazionale dell’Antartide, Sezione di Scienze della Terra, Via del Laterino 8, 53100 Siena – Italy

Received 13 December 2002; accepted in revised form 29 October 2003
 

Abstract - This paper presents the results of a pilot study on the magnetic petrology of basement rock units in the Ross Orogen in Oates Land (Antarctica). Magnetic susceptibilities of migmatitic gneisses – the dominant lithology - are generally low (ilmenite+/-graphite-bearing rocks), with the exception of two occurrences (magnetite+/-green spinel-bearing rocks) at Harald Bay and Burnside Ridge. Similar low values are typical for most Granite Harbour Intrusives, except for the variably mylonitic Exiles Nunataks granite and the Archangel Nunataks gabbros and pyroxenites, which  are among the most magnetic rock types in the area. Metamorphosed mafic and ultramafic rocks, occurring in outcrop as volumetrically minor bodies (lenses of 1 dm to 10 m size), are variably but generally about one order of magnitude more highly magnetic than country gneisses.
Petrological investigations on representative samples indicate that nearly pure or low-Ti magnetite is the only ferrimagnetic phase, irrespective of lithologic type, and magnetic susceptibility values are always positively correlated with the modal amount of magnetite. The rare occurrence of magnetite in the opaque mineralogy of migmatitic gneisses indicates that appropriate chemical and fO₂ conditions were only locally attained in the region, and microstructural evidence indicate that magnetite possibly formed at different metamorphic stages during the post–peak decompressional path at high T and/or the retrograde path. In variably magnetic ultramafic and mafic lenses magnetite (almost pure to low-Ti) typically occurs as very fine grained opaque inclusions in secondary hornblende and/or chlorite. This evidence indicates that a high proportion of magnetite is of secondary origin and related to amphibolite grade or lower grade metamorphic processes. Titanomagnetite and exolution features are documented in the gabbros from Archangel Nunataks, in which the decrease of magnetite content is inversely correlated with the extent of sub-solidus late-magmatic re-equilibration (replacement of pyroxene by hornblende). Similar microstructural evidence of re-equilibration during cooling was also found in the highly magnetic magnetite-bearing mylonitic hornblende-biotite granites  from Exiles Nunataks.
All highly magnetic rock units represent potential petrologic sources of regional crustal magnetic anomalies in the Oates Land region as revealed by  aeromagnetic surveys. In particular, the Exiles Nunataks magnetite-bearing granites are clearly marked by aeromagnetic anomalies forming a small part of  the “Matusevich Anomaly”, a prominent magnetic anomaly, co-linear with the Matusevich Glacier. The gabbros as exposed in the southestermost Archangel Nunataks are correlated with a distinct high-amplitude anomaly that occurs at this location and delineates the extent of the gabbroic intrusion itself. Significant volumes of metamorphosed mafic/ultramafic rocks may account for the high-frequency anomaly chains flanking the main part of the Matusevich Anomaly in the Lazarev Mountains.


 
*Corresponding author (talarico@unisi.it)