Paleomagnetic study of Antarctic Peninsula Volcanic Group, Danco Coast. NW Antarctic Peninsula.

VILAS, J. F.; LIPPAI, H.; TASSONE, A.; PERONI, J. I.

19-20 de noviembre. Santiago. Chile

Simposio; International Geological Congress on the Southern Hemisfere (Geosur 2).; 2007

GEOSUR

The paleogeographic location of the Antarctic Peninsula (AP) before Gondwana fragmentation, as well as its correlation with southernmost South America, are actual matter of debate. A paleomagnetic survey has been carried on in NW AP in order to achieve new insights into this topic. Two critical areas were chosen: one corresponds to the Primavera Base area and the other to San Martín Base. The former has been the locus of geologic and geophysic surveys during the 2005-06 and 2006-07 Antarctic summers. Magnetic susceptibility, paleomagnetic and ASM data were obtained in Danco Coast outcrops; the geophysical survey was complemented with sampling directed to petrological, structural and geochronological studies. Upper Cretaceous to Tertiary volcanic rocks intruded by a granodioritic pluton outcropping in coastal areas, belong to the Mesozoic magmatic arc. Leucocratic and melanocratic andesitic dykes hosted in the granodioritic pluton at Cierva creek bear sulphide mineralization. Paleomagnetic survey included 43 sampling sites. The magnetic susceptibility was measured with a Fugro susceptibilimeter at each sampling site. Sample processing at INGEODAV included the measurement of natural remanent magnetism (NRM); the coherence in the MRN directions among different specimens of each site was verified. In order to analyze the MRN stability, demagnetization stages by high temperatures (100 to 560 ºC) were applied and the residual remanent magnetism (RRM) was measured at each stage. 8 to 10 demagnetization stages were measured for each sample. Two blocking temperature ranges have been recognized: a low (300-400ºC) and a high (500-580ºC) Curie temperature rank. The mineral carriers of magnetic remanence with low blocking temperatures are Ti-rich titanomagnetites and those with high Curie temperatures are magnetite and Ti-poor titanomagnetite. The magnetization directions of different samples from a single site show a strong internal coherence within the sites of granodiorite rocks, whereas volcanic rocks display two polarities (normal and reverse). The characteristic remanent magnetization (MRC) was obtained from each specimen. The vectorial analysis of magnetic components was performed with the Zijderlveld orthogonal graphs. The MRC displays a better clustering than the MRN when represented on stereographic projection which indicates that demagnetization treatments have been effective to eliminate secondary remanent magnetizations. The age of the granodiorite and volcanic rocks has been considered as Upper Cretaceous to Tertiary (lowermost levels of Antarctic Peninsula Volcanic Group, Rinaldi et al., 1999). The correlation of the obtained paleomagnetic data with the table of reversions of the Earth´s magnetic field suggests an Upper Cretaceous or younger magnetic age. Furthermore, given that the Q Koenigsberger coefficient overestimates the NRM in relation to the NIM, it could be inferred that the negative anomalies correspond with the alignment of reverse remanent magnetizations which might account for the negative magnetic anomalies measured in the Hughes bay area.