Palaeomagnetic and geochronological results from the Cambro-Ordovician Granite Harbour Intrusives inland of Terra Nova Bay (Victoria Land, Antarctica)

Palaeomagnetic investigations and Rb-Sr dating were carried out on samples from two plutons from the Granite Harbour Intrusives of the Transantarctic Mountains inland of Terra Nova Bay. The Rb-Sr whole rock-biotite ages from Teall Nunatak (475+/-4, 483+/-4 Ma), a quartz-diorite pluton cropping out t o the south of Priestley Glacier, are older than that from the Mount Keinat h monzogranite (450+/-4 Ma), which is located to the north of the glacier. These results are consistent with the literature data, which suggest that d uring the last phases of the Ross Orogeny the cooling rate of the basement was significantly lower to the north than to the south of Priestley Glacier . The Teall Nunatak: quartz-diorite is characterized by a stable magnetizat ion, whose blocking-temperature spectrum ranges from 530 to 570 degrees C. At one site, the stable magnetization is screened by a large secondary comp onent of opposite polarity, removed by thermal demagnetization below 300 de grees C. The characteristic directions after thermal demagnetization yielde d a southern pole located at lat. 11 degrees S, long. 21 degrees E. The mag netization of Mount Keinath monzogranite consists of several components wit h overlapping stability spectra. A characteristic direction was isolated at one site only, obtained by demagnetizing the specimens in the temperature range from 380 to 460 degrees C. Comparison with the other East Antarctica poles shows that those from Victo ria Land are very well grouped and give a reliable early Ordovician palaeop ole (lat. 5 degrees S, long. 23 degrees E, with K = 196 and A(95) = 3.7 deg rees), whereas the poles from Wilkes, Enderby and Dronning Maud Land are di spersed. We tentatively advance the hypothesis that the dispersion reflects different magnetization ages due to the slow cooling of these regions duri ng the last stages of the Ross Orogeny.