Kirkpatrick Basalt Group, Victoria Land
Paleomagnetism and hydrothermal alteration: Kirkpatrick Basalt Group, Victoria Land E.M. CHERRY and H.C. NOLTIMIER Institute of Polar Studies
and
Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210
The Kirkpatrick Basalt Group (KBG) is the extrusive phase of the Jurassic Ferrar Supergroup which crops out along the Transantarctic Mountains from northern Victoria Land to the Pensacola Mountains (Kyle, Elliot, and Sutter 1981). Previous paleomagnetic investigations of Ferrar Supergroup rocks have emphasized the hypabyssal Ferrar Dolerite Group (FDG) and the Forrestal Gabbro Group as summarized by McIntosh et al. (1982). The paleomagnetism of the KBG has been investigated from the Queen Alexandra Range (Ostrander 1971; Hoffman, Narin, and Peterson in press) and from Victoria Land (Cherry and Noltimier 1982; McIntosh et al. 1982). These investigations of the Ferrar Supergroups have used alternating field (AF) demagnetization treatments exclusively in determining their respective virtual geomagnetic pole (vGP) positions. The natural remanent magnetization (NRM) of these rocks has been interpreted to consist of a stable primary thermoremanent magnetization (TRM) of predominantly normal polarity, and a weak viscous remanent magnetization (vRM). Although recent work indicates that the Jurassic contains several reversed and mixed intervals (Steiner 1980), all KBG and FDG results are of normal polarity. We have recently conducted detailed AF and thermal demagnetization experiments on 11 KBG flows from the Mesa Range area, northern Victoria Land (Elliot et al. 1983), and on five KBG flows and one FDG sill from Gorgon Peak, southern Victoria Land. Our results indicate that the NRM contains a significant component of chemical remanent magnetization (cRM). Hydrothermal alteration has resulted in the incomplete low temperature oxidation of titanomagnetite to titanomaghemite. Both of these phases have the spinel structure, and thus, have similar magnetic properties and exhibit a similar response to AF demagnetization. However, titanomaghemite is a metastable phase and inverts to ilmenohematite ± magnetite upon heating. The CRM component is removed during thermal demagnetization by 350 to 450°C, where as the primary TRM in titanomagnetite is not lost until the 500 to 575° range. Bulk susceptibility measurements are used to monitor mineralogic changes (inversion of titanomaghemite, high temperature oxidation, etc.) during laboratory heating. The presence of titanomaghemite is verified through reflected light investigation and by irreversible thermomagnetic behavior (O'Reilly 1984). A comparison of the paleomagnetic response to AF and thermal demagnetization indicates that AF demagnetization is not sufficient to resolve the two components. The degree of maghemitization can be quite variable both between flows and
32
within some of the thickest flows (Cherry and Noltimier 1984). In a few cases, cIivI overprinting has obscured reversed primary TRJvf to the extent that the presence of two components is not apparent from AF treatments (Noltimier and Cherry 1983). Lowtemperature oxidation has been recognized in other Ferrar Supergroup rocks (Funaki 1983; Hoffman et al. in press) and in tholeiites from Queen Maud Land (Lolie 1979). However, these studies have not investigated the possibility of a CRM component being present. It is not likely that the presence of CRM in the Ferrar Supergroup rocks will significantly alter mean Jurassic VGP for the Transantarctic Mountains, because late stage hydrothermal alteration will have the effect of averaging secular variation. However, this CRM component is strong enough to overprint primary reversals in the TRM that are useful in intra- and interregional correlations. In addition, the evaluation of multicomponent paleomagnetic results could prove to be a valuable technique for resolving structural relationships along the Transantarctic Mountains and interpreting the anomalously paleomagnetic results from northern Victoria Land (McIntosh 1983). This research was supported by National Science Foundation grant DPP 80-21401.
References
Cherry, E.M., and H.C. Noltimier. 1982. Paleomagnetic results: Kirkpatrick Basalts from Brimstone Peak and Gorgon Peak, Antarctica. EQS, 63, 309. Cherry, F.M., and H.C. Noltimier. 1982. Paleomagnetic response to hydrothermal alteration in an 86 m basalt flow. EQS. 64, 197. Elliot, D.H., G. Faure, T.M. Mensing, M.A. Siders, M.A. Haban, and E.M. Cherry. 1983. Geological observations on the Kirkpatrick Basalt in the Mesa Range region, northern Victoria Land. Antarctic Journal of the U.S., 18(5) 11 - 12.
Funaki, M. 1983. Paleomagnetic investigation of Ferrar Dolerite in the McMurdo Sound region, Antarctica. Antarctic Record, 77, 20 - 32. Hoffman, J., A.E.M. Narin, and D.N. Peterson. In press. The paleomagnetic investigation of flows and sills from the Queen Alexandra Range, Antarctica. In M.D. Turner and J.F. Splettstoesser, (Eds.), Geology of the Transantarctic Mountains. (Antarctic Research Series, Vol. 36.) Washington, D.C.: American Geophysical Union. Kyle, P.R., D.H. Elliot, and J.F. Sutter. 1981. Jurassic Ferrar Supergroup tholeiites from the Transantarctic Mountains, Antarctica, and their relation to the initial fragmentation of Gondwana. In M. Cresswell and P. Vella., (Eds.), Gondwana V. Lovlie, R. 1979. Mesozoic paleomagnetism in Vestfjella, Dronning Maud Land, East Antarctica. Geophysical Journal Royal Astronomical Society 59, 529 - 537. McIntosh, W.C., Kyle, P.R., Cherry, EM., and Noltimier, H.C. 1982.
Paleomagnetic results from Kirkpatrick Basalt Group, Victoria Land.
Antarctic Journal of the U.S., 17(5), 20 - 22. Noltimier, H.C., and Cherry, E.M. 1983. Reversed TRM in Jurassic
Kirkpatrick Basalts and Ferrar Dolerite at Gorgon Peak, South Victoria Land, Antarctica. Geological Society of America Abstracts with Programs,
15, 654. O'Reilly, W. 1984. Rock and mineral magnetism.
Glasgow: Blackie and Son. Ostrander, J.H. 1971. Paleomagnetic investigations of the Queen Alexandra Range, Antarctica. Antarctic Journal of the U.S., 6, 183 - 185. Steiner, M.B. 1981. Investigation of the geomagnetic field polarity during the Jurassic. Journal of Geophysical Research, 85, 3572 - 3586.
ANTARCTIC JOURNAL
and
Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210
The Kirkpatrick Basalt Group (KBG) is the extrusive phase of the Jurassic Ferrar Supergroup which crops out along the Transantarctic Mountains from northern Victoria Land to the Pensacola Mountains (Kyle, Elliot, and Sutter 1981). Previous paleomagnetic investigations of Ferrar Supergroup rocks have emphasized the hypabyssal Ferrar Dolerite Group (FDG) and the Forrestal Gabbro Group as summarized by McIntosh et al. (1982). The paleomagnetism of the KBG has been investigated from the Queen Alexandra Range (Ostrander 1971; Hoffman, Narin, and Peterson in press) and from Victoria Land (Cherry and Noltimier 1982; McIntosh et al. 1982). These investigations of the Ferrar Supergroups have used alternating field (AF) demagnetization treatments exclusively in determining their respective virtual geomagnetic pole (vGP) positions. The natural remanent magnetization (NRM) of these rocks has been interpreted to consist of a stable primary thermoremanent magnetization (TRM) of predominantly normal polarity, and a weak viscous remanent magnetization (vRM). Although recent work indicates that the Jurassic contains several reversed and mixed intervals (Steiner 1980), all KBG and FDG results are of normal polarity. We have recently conducted detailed AF and thermal demagnetization experiments on 11 KBG flows from the Mesa Range area, northern Victoria Land (Elliot et al. 1983), and on five KBG flows and one FDG sill from Gorgon Peak, southern Victoria Land. Our results indicate that the NRM contains a significant component of chemical remanent magnetization (cRM). Hydrothermal alteration has resulted in the incomplete low temperature oxidation of titanomagnetite to titanomaghemite. Both of these phases have the spinel structure, and thus, have similar magnetic properties and exhibit a similar response to AF demagnetization. However, titanomaghemite is a metastable phase and inverts to ilmenohematite ± magnetite upon heating. The CRM component is removed during thermal demagnetization by 350 to 450°C, where as the primary TRM in titanomagnetite is not lost until the 500 to 575° range. Bulk susceptibility measurements are used to monitor mineralogic changes (inversion of titanomaghemite, high temperature oxidation, etc.) during laboratory heating. The presence of titanomaghemite is verified through reflected light investigation and by irreversible thermomagnetic behavior (O'Reilly 1984). A comparison of the paleomagnetic response to AF and thermal demagnetization indicates that AF demagnetization is not sufficient to resolve the two components. The degree of maghemitization can be quite variable both between flows and
32
within some of the thickest flows (Cherry and Noltimier 1984). In a few cases, cIivI overprinting has obscured reversed primary TRJvf to the extent that the presence of two components is not apparent from AF treatments (Noltimier and Cherry 1983). Lowtemperature oxidation has been recognized in other Ferrar Supergroup rocks (Funaki 1983; Hoffman et al. in press) and in tholeiites from Queen Maud Land (Lolie 1979). However, these studies have not investigated the possibility of a CRM component being present. It is not likely that the presence of CRM in the Ferrar Supergroup rocks will significantly alter mean Jurassic VGP for the Transantarctic Mountains, because late stage hydrothermal alteration will have the effect of averaging secular variation. However, this CRM component is strong enough to overprint primary reversals in the TRM that are useful in intra- and interregional correlations. In addition, the evaluation of multicomponent paleomagnetic results could prove to be a valuable technique for resolving structural relationships along the Transantarctic Mountains and interpreting the anomalously paleomagnetic results from northern Victoria Land (McIntosh 1983). This research was supported by National Science Foundation grant DPP 80-21401.
References
Cherry, E.M., and H.C. Noltimier. 1982. Paleomagnetic results: Kirkpatrick Basalts from Brimstone Peak and Gorgon Peak, Antarctica. EQS, 63, 309. Cherry, F.M., and H.C. Noltimier. 1982. Paleomagnetic response to hydrothermal alteration in an 86 m basalt flow. EQS. 64, 197. Elliot, D.H., G. Faure, T.M. Mensing, M.A. Siders, M.A. Haban, and E.M. Cherry. 1983. Geological observations on the Kirkpatrick Basalt in the Mesa Range region, northern Victoria Land. Antarctic Journal of the U.S., 18(5) 11 - 12.
Funaki, M. 1983. Paleomagnetic investigation of Ferrar Dolerite in the McMurdo Sound region, Antarctica. Antarctic Record, 77, 20 - 32. Hoffman, J., A.E.M. Narin, and D.N. Peterson. In press. The paleomagnetic investigation of flows and sills from the Queen Alexandra Range, Antarctica. In M.D. Turner and J.F. Splettstoesser, (Eds.), Geology of the Transantarctic Mountains. (Antarctic Research Series, Vol. 36.) Washington, D.C.: American Geophysical Union. Kyle, P.R., D.H. Elliot, and J.F. Sutter. 1981. Jurassic Ferrar Supergroup tholeiites from the Transantarctic Mountains, Antarctica, and their relation to the initial fragmentation of Gondwana. In M. Cresswell and P. Vella., (Eds.), Gondwana V. Lovlie, R. 1979. Mesozoic paleomagnetism in Vestfjella, Dronning Maud Land, East Antarctica. Geophysical Journal Royal Astronomical Society 59, 529 - 537. McIntosh, W.C., Kyle, P.R., Cherry, EM., and Noltimier, H.C. 1982.
Paleomagnetic results from Kirkpatrick Basalt Group, Victoria Land.
Antarctic Journal of the U.S., 17(5), 20 - 22. Noltimier, H.C., and Cherry, E.M. 1983. Reversed TRM in Jurassic
Kirkpatrick Basalts and Ferrar Dolerite at Gorgon Peak, South Victoria Land, Antarctica. Geological Society of America Abstracts with Programs,
15, 654. O'Reilly, W. 1984. Rock and mineral magnetism.
Glasgow: Blackie and Son. Ostrander, J.H. 1971. Paleomagnetic investigations of the Queen Alexandra Range, Antarctica. Antarctic Journal of the U.S., 6, 183 - 185. Steiner, M.B. 1981. Investigation of the geomagnetic field polarity during the Jurassic. Journal of Geophysical Research, 85, 3572 - 3586.
ANTARCTIC JOURNAL
