Direct dating of mid-Cretaceous alteration of the Kirkpatrick Basalt in ...
Direct dating of mid-Cretaceous alteration of the Kirkpatrick Basalt in northern Victoria Land: Argon-40/argon-39 and rubidium/strontium ages of apophyllite THOMAS H. FLEMING, KENNETH A. FOLAND, and DAVID H. ELLIOT, Byrd Polar Research Center and Department of
Geological Sciences, Ohio State University, Columbus, Ohio 43210
rocks have been affected by low-temperature processes, which significantly postdate their eruption and may record an important geologic event during the mid-Cretaceous. Evidence for such an event comes from paleomagnetic, potassium/argon (K/Ar), and rubidium /strontium (Rb/Sr) data from the basalts and dolerites (Fleming et al. 1989, 1993). Detailed study of samples from the uppermost flow unit of the Kirkpatrick Basalt in northern Victoria Land has documented whole-rock strontium isotope compositions which, on an isochron diagram, fall along an array giving a mid-Cretaceous (approximately 100 million years ago) date (Mensing et al. 1988; Fleming et al. 1989, 1992). This date contrasts with 40Ar/ 39Ar incremental heating plateaus from plagioclase separates indicating a Jurassic crystallization age of 176±1.8 million years (Foland et al. 1993). This unit has a markedly homogeneous chemical composition with respect to the less mobile elements but variable calculated initial strontium isotope ratios. These have been interpreted to result from mobility of rubidium, which occurred during alteration of glassy matrix materials about 100 million years ago (Fleming et al. 1989, 1992). The wide range in oxygen isotope ratios (6 18 0 = +5.5 to +8.2) for lava samples also indicates low-temperature interaction with a substantial volume of hydrothermal fluid at some time since their eruption (Fleming et al. 1992). The disturbance of strontium and oxygen isotope systems within this flow unit overprints primary relationships and obscures the petrogenetic significance of the isotope variations in the basalt sequence as a whole. Paleomagnetic pole positions for the basalts and dolerites from northern Victoria Land are more southerly than those for Ferrar rocks from other regions. These anomalous pole positions have been interpreted to reflect overprinting of the original magnetic properties and resetting of the magnetic poles due to low-temperature conversion of magnetite to maghemite (Delisle 1983; Delisle and Fromm 1989). The age of this alteration is inferred to be mid-Cretaceous based upon coincidence of the paleomagnetic pole positions with the apparent polar wander path for that time. Similarly, anomalous paleomagnetic pole positions from northern Victoria Land have been reported for basement rocks of the Bowers Supergroup and have also been interpreted as a mid-Cretaceous overprint (Schmierer and Burmester 1986). Alternative explanations, however, have been proposed for the anomalous poles from the Ferrar Group. These explanations include incomplete averaging of paleosecular variation (McIntosh, Kyle, and Sutter 1986) and transitional phases of the geomagnetic field (Lanza and Zanella 1993).
urassic Tholeiites of the Ferrar Group extend for 3,000 kiloJ meters along the Transantarctic Mountains and represent the youngest rocks of the Gondwana sequence in Antarctica. In northern Victoria Land the Ferrar Group is represented by 780 meters of basalt in the Mesa Range and a number of sills which intrude the Permo-Triassic Beacon sedimentary sequence (Elliot, Siders, and Haban 1986). The crystallization age of the Kirkpatrick Basalt and Ferrar Dolerite is now well constrained by argon-40 / argon-39 ( 40ArI 39Ar) incrementalheating dates for feldspar separated from rocks. These measurements demonstrate a very short period of magmatism 176.7±1.8 million years ago (Heimann et al. in press). In addition, abundant evidence from northern Victoria Land suggests that the glassy or fine-grained components of Ferrar
00 I -_Dufek Massif e3
oow
roo ANTARCTICA
r'
4
r-
900w
±
900E
90°E
+
4r ........... 0. tc Queen Alexandra \ Range I L\V! 0 Beardmore GI.
X
+ 80°S
Prince Albert Mountains 0 Kirkpatrick Basalt
ci
• Ferrer Dolerite and Dufek Gabbro( 0 Undifferentiated bedrock NVL. North Victoria Land
i_Mt Frustum
-Mesa
Range
Edge of ice shelf 0 500 km 1800
Figure 1. Geological sketch map of the Transantarctic Mountains showing the distribution of the Ferrar Group. The apophyllite sample discussed in the text was collected from Mount Frustum in northern Victoria Land.
ANTARCTIC JOURNAL - REVIEW 1993 38
Conventional K/Ar ages on whole-rock samples of the basalts in northern Victoria Land range from about 90 to 180 million years (Kreuzer et al. 1981; Elliot and Foland 1986; Brotzu et al. 1988; Faure and Mensing 1993). The range of young ages is now recognized to reflect some event during which partial argon-loss occurred (Fleming et al. 1989, 1993). As a result, the K/Ar whole-rock dates range between the time of argon loss (in the event of total loss) and the age of crystallization (no loss). The minimum ages of 90 to 120 million years provide an estimate for the time of argon loss. Similarly, low-temperature steps of 40Ar/ 39Ar incremental heating experiments record dates between 100 and 120 million years. These dates have also been used to estimate the age of alteration (Fleming et al. 1993; Foland et al. 1993). These minimum ages agree in general with the time of alteration inferred by other methods. In an effort to establish the age of alteration in northern Victoria Land more definitively, a sample of apophyllite collected from Mount Frustum (figure 1) was studied by 40Ar/39Ar and Rb/Sr dating techniques. The apophyllite, a potassiumbearing, hydrous, sheet silicate, commonly occurs in vugs and vesicles in the lavas and was precipitated, along with a variety of other minerals including quartz, calcite, stilbite, heulandite, and natrolite, from hydrothermal fluids which passed through the rocks. To our knowledge, this is the first attempt to use apophyllite for isotopic age determinations. The apophyllite has a very high Rb/Sr ratio, as illustrated in figure 2. As a result, an Rb/Sr model age can be derived from the present-day isotope compositions by assuming that strontium incorporated into the apophyllite had a strontium87/strontium-86 ( 87Sr/ 86Sr) ratio similar to its host basalt
(approximately 0.710). The date calculated in this manner is 94±1 million years and is insensitive to any reasonable values chosen for the initial ratio of 87Sr/ 86Sr incorporated into the apophyllite. 40Ar/ 39Ar incremental-heating data for the apophyllite are shown in figure 3. Most of the argon is released at relatively high temperatures (greater than 600 0C), similar to the laboratory release found for most coarse-grained micas. The loss of a large portion of argon between about 750°C and 820°C probably reflects breakdown of the mineral structure. The variation in apparent age among different temperature increments is significant but relatively minor. This discordance, or most of it, may be the result of recoil redistribution of 39Ar produced by potassium during sample irradiation (see Foland et al. 1993). The age of the vast majority of heating steps falls between 90 and 100 million years with a weighted average (or total gas) age of 96±1 million years. The increments over the interval 650°C to 790°C show a plateau of the same age. The Rb/Sr and 40Ar/ 39Ar dates are in good agreement and suggest formation of the apophyllite 95 million years ago. On the basis of these measurements, apophyllite appears to provide useful isotopic ages by both methods. Dating of apophyllite could be very useful in determining the time of volcanism and hydrothermal activity at many locations. The 95-million-year age is interpreted to reflect the time of actual formation of apophyllite and associated minerals in the lava sequence. The possibility that the 95-million-year age reflects resetting of both isotopic systems in the mid-Cretaceous is difficult to evaluate because nothing is known about the behavior of apophyllite isotopic systems. The concordance of ages by two isotopic geo chronometers, however, suggests that resetting is unlikely because of the very different chemical behavior of the daughter elements (strontium and argon). Moreover, the ages demonstrate mid-Cretaceous hydrothermal activity in northern Victoria Land and support the conclusion that the K/Ar dates of less than 177 million years from the basalts result primarily from episodic argon loss in the Cretaceous rather than by slow leakage over geologic time. The dates documented for apophyllite are considered to be the best estimate for the age of alteration of the basalts in northern Victoria Land because the apophyllite is a direct product of the hydrothermal activity. Previous estimates for the age of alteration are less definitive because they have relied upon whole rocks which will not have had their K/Ar or Rb/Sr systems completely reset during alteration (see Foland et al. 1993) and because the underlying assumptions for isotopic age determinations are not strictly valid. Considering these factors, the slightly older apparent ages from whole rocks, which form the bases of previous estimates for the timing of alteration, are consistent with a 95-million-year age of alteration. Mid-Cretaceous dates provided by a variety of methods document a hydrothermal event that affected rocks of northern Victoria Land. The nature and cause of this event is not well understood. It could be related to the mid-Cretaceous (95±5 million years ago; Veevers 1986) separation of Australia
No 5 I Mt. Frustum
Apophyllite
C') 94±1 Ma .(J)
2
1 c_ Basalts (0.710) 0 1000 2000 3000 87Rb/86Sr Figure 2. Rb/Sr isochron diagram illustrating the relationship between apophyllite from Mount Frustum and its host basalt. The apophyllite has a much higher Rb/Sr ratio than the basalt; the higher ratio allows calculation of a model age of 94±1 million years.
ANTARCTIC JOURNAL - REVIEW 1993 39
0.26
Elliot, D.H., M.A. Siders, and M.A. Haban. 1986. Jurassic tholeiites in the region of the upper Rennick Glacier, north Victoria Land. In E.
CZ 0.24
Stump (Ed.), Geological investigations in northern Victoria Land
(Antarctic Research Series, Vol. 46). Washington, D.C.: American Geophysical Union. Faure, G., and T.M. Mensing. 1993. K-Ar dates and paleomagnetic evidence for Cretaceous alteration of Jurassic basaltic lava flows, Mesa Range, northern Victoria Land, Antarctica. Chemical Geology (Isotope Geoscience Section), 109(1/4), 305-315. Fleming, T.H., D.H. Elliot, K.A. Foland, L.M. Jones, and J.R. Bowman. 1993. Disturbance of Rb-Sr and K-Ar isotopic systems in the Kirkpatrick Basalt, north Victoria Land, Antarctica: Implications for middle Cretaceous tectonism. In R.H. Findlay, R. Unrug, M.R.
0. 22 0.20 co
120
Mt. Frustum 450 Apophyllite
300
CD 100 -
750 780 790 800 [_ 650
80
fuse
950
Banks, and J.J. Veevers (Eds.), Gondwana 8-Assembly, evolution and dispersal. Rotterdam: Balkema.
Fleming, T.H., D.H. Elliot, L.M. Jones, and J.R. Bowman. 1989. Secondary alteration of iron-rich tholeiitic rocks of the Kirkpatrick Basalt, northern Victoria Land. Antarctic Journal of the U.S., 24(5), 37-40. Fleming, T.H., D.H. Elliot, L.M. Jones, J.R. Bowman, and Siders. 1992. Chemical and isotopic variations in an iron-rich lava flow from the Kirkpatrick Basalt, north Victoria Land: Implications for low-tem-
1100
ttg = 96 ± 1 Ma
60
(Run#47M2)
perature alteration. Contributions to Mineralogy and Petrology,
111(4),440-457. Fitzgerald, P.G., and E. Stump. 1991. Uplift history of the Transantarctic Mountains from Victoria Land ( ..70°S) to Scott Glacier (-86°S): Evidence from fission track analysis. Abstracts-Sixth International Symposium on Antarctic Earth Sciences. Saitama, Japan: Ranzan-machi. Foland, K.A., T.H. Fleming, A. Heimann, and D.H. Elliot. 1993. Potassium-argon dating of fine-grained basalts with massive Ar loss: Application of the 40Ar/ 39Ar technique to plagioclase and glass from the Kirkpatrick Basalt, Antarctica. Chemical Geology (Isotope Geoscience section), 107(1/2), 173-190. Heimann, A., T.H. Fleming, D.H. Elliot, and K.A. Foland. In press. A short interval of Jurassic continental flood basalt volcanism in Antarctica as demonstrated by 40Ar/ 39Ar geochronology. Earth
0 20 40 60 80 100 %
39
Ar cumulative release
Figure 3. 40Ar/39Ar incremental-heating diagram for apophyllite from Mount Frustum. Temperatures for each of the heating steps are indicated. The total-gas age (t tg), calculated by integrating all increments, is 96±1 million years. and Antarctica and may record hydrothermal activity in the Rennick Graben (Fleming et al "I - 993) The graben may repre sent a failed rift arm formed during continental break-up. Alternatively, the dates could reflect the movement of groundwaters during initial stages of uplift of the Transantarctic Mountains. Fission-track studies suggest the uplift may have begun as early as the Cretaceous (Fitzgerald and Stump 1991; Stump and- Fitzgerald 1992). The authors would like to express thanks to F.A. Hubacher and J.S. Linder for vital laboratory support with isotopic analyses, M.A. Siders for contributing the sample of apophyllite, and A. Heimann for discussions on Kirkpatrick basalt geochronology. This work has been supported in part by National Science Foundation grants OPP 80-21401, OPP 8318957, and OPP 90-22603.
and. Planetary Science Letters
Kreuzer, H., A. Hohndorf, H. Lenz, U. Vetter, F. Tessensohn, P. Muller, u:Jcrdan, W. Harre, and C. Besang. 1981. K-Ar and Rb-Sr dating of igneous rocks from north Victoria Land. Geologisches Jahrbuch, B41,267-273. Lanza, R., and E. Zanella. 1993. Paleomagnetism of the Ferrar Dolerite in the northern Prince Albert Mountains, Victoria Land, Antarctica. Geophysical Journal International, 114(3), 501-511. McIntosh, W.C., P.R. Kyle, and J.F. Sutter. 1986. Paleomagnetic results from the Kirkpatrick Basalt Group, Mesa Range, north Victoria Land. In E. Stump (Ed.), Geological investigations in northern Victoria Land (Antarctic Research Series, Vol. 46). Washington, D.C.:
American Geophysical Union. Mensing, T.M., G. Faure, L.M. Jones, and J. Hoefs. 1988. Evidence for Cretaceous volcanism in northern Victoria Land, Antarctica, and implications for a mantle enrichment event during the Ross Orogeny based on the chemical and isotopic data from the Kirk-
References
patrick Basalt in the Mesa Range. Abstracts-Seventh International Symposium on Gondwana, Sao Paulo, Brazil. Sao Paulo: Instituto
Brotzu, P., G. Capaldi, L. Civetti, L. Melluso, and G. Orsi. 1988. Jurassic Ferrar Dolerites and Kirpatrick Basalts in northern Victoria Land (Antarctica): Stratigraphy, geochronology and petrology. Memorie Della Societa Geologica Italiana, 43, 97-116. Delisle, G. 1983. Results of paleomagnetic investigations in northern Victoria Land. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science. Cambridge: Cambridge University Press. Delisle, G., and K. Fromm. 1989. Further evidence for a Cretaceous thermal event in north Victoria Land, Geologisches Jahrbuch, E38, 143-151. Elliot, D.H., and K.A. Foland. 1986. K-Ar age determinations of the Kirkpatrick Basalt, Mesa Range. In E. Stump (Ed.), Geological investigations in northern Victoria Land (Antarctic Research Series, Vol. 46). Washington, D.C.: American Geophysical Union.
de Geociências. Schmierer, K., and R. Burmester. 1986. Paleomagnetic results from the Cambro- Ordovician Bowers Supergroup, northern Victoria Land, Antarctica. In E. Stump (Ed.), Geological investigations in northern Victoria Land (Antarctic Research Series, Vol. 46). Washington, D.C.: American Geophysical Union. Stump, E., and P.G. Fitzgerald. 1992. Episodic uplift of the Transantarctic Mountains. Geology, 20(2), 161-164. Veevers, J.J. 1986. Breakup of Australia and Antarctica estimated as mid-Cretaceous (95±5 Ma) from magnetic and seismic data at the continental margin. Earth and Planetary Science Letters, 77(1), 91-99.
ANTARCTIC JOURNAL - REVIEW 1993 40
Geological Sciences, Ohio State University, Columbus, Ohio 43210
rocks have been affected by low-temperature processes, which significantly postdate their eruption and may record an important geologic event during the mid-Cretaceous. Evidence for such an event comes from paleomagnetic, potassium/argon (K/Ar), and rubidium /strontium (Rb/Sr) data from the basalts and dolerites (Fleming et al. 1989, 1993). Detailed study of samples from the uppermost flow unit of the Kirkpatrick Basalt in northern Victoria Land has documented whole-rock strontium isotope compositions which, on an isochron diagram, fall along an array giving a mid-Cretaceous (approximately 100 million years ago) date (Mensing et al. 1988; Fleming et al. 1989, 1992). This date contrasts with 40Ar/ 39Ar incremental heating plateaus from plagioclase separates indicating a Jurassic crystallization age of 176±1.8 million years (Foland et al. 1993). This unit has a markedly homogeneous chemical composition with respect to the less mobile elements but variable calculated initial strontium isotope ratios. These have been interpreted to result from mobility of rubidium, which occurred during alteration of glassy matrix materials about 100 million years ago (Fleming et al. 1989, 1992). The wide range in oxygen isotope ratios (6 18 0 = +5.5 to +8.2) for lava samples also indicates low-temperature interaction with a substantial volume of hydrothermal fluid at some time since their eruption (Fleming et al. 1992). The disturbance of strontium and oxygen isotope systems within this flow unit overprints primary relationships and obscures the petrogenetic significance of the isotope variations in the basalt sequence as a whole. Paleomagnetic pole positions for the basalts and dolerites from northern Victoria Land are more southerly than those for Ferrar rocks from other regions. These anomalous pole positions have been interpreted to reflect overprinting of the original magnetic properties and resetting of the magnetic poles due to low-temperature conversion of magnetite to maghemite (Delisle 1983; Delisle and Fromm 1989). The age of this alteration is inferred to be mid-Cretaceous based upon coincidence of the paleomagnetic pole positions with the apparent polar wander path for that time. Similarly, anomalous paleomagnetic pole positions from northern Victoria Land have been reported for basement rocks of the Bowers Supergroup and have also been interpreted as a mid-Cretaceous overprint (Schmierer and Burmester 1986). Alternative explanations, however, have been proposed for the anomalous poles from the Ferrar Group. These explanations include incomplete averaging of paleosecular variation (McIntosh, Kyle, and Sutter 1986) and transitional phases of the geomagnetic field (Lanza and Zanella 1993).
urassic Tholeiites of the Ferrar Group extend for 3,000 kiloJ meters along the Transantarctic Mountains and represent the youngest rocks of the Gondwana sequence in Antarctica. In northern Victoria Land the Ferrar Group is represented by 780 meters of basalt in the Mesa Range and a number of sills which intrude the Permo-Triassic Beacon sedimentary sequence (Elliot, Siders, and Haban 1986). The crystallization age of the Kirkpatrick Basalt and Ferrar Dolerite is now well constrained by argon-40 / argon-39 ( 40ArI 39Ar) incrementalheating dates for feldspar separated from rocks. These measurements demonstrate a very short period of magmatism 176.7±1.8 million years ago (Heimann et al. in press). In addition, abundant evidence from northern Victoria Land suggests that the glassy or fine-grained components of Ferrar
00 I -_Dufek Massif e3
oow
roo ANTARCTICA
r'
4
r-
900w
±
900E
90°E
+
4r ........... 0. tc Queen Alexandra \ Range I L\V! 0 Beardmore GI.
X
+ 80°S
Prince Albert Mountains 0 Kirkpatrick Basalt
ci
• Ferrer Dolerite and Dufek Gabbro( 0 Undifferentiated bedrock NVL. North Victoria Land
i_Mt Frustum
-Mesa
Range
Edge of ice shelf 0 500 km 1800
Figure 1. Geological sketch map of the Transantarctic Mountains showing the distribution of the Ferrar Group. The apophyllite sample discussed in the text was collected from Mount Frustum in northern Victoria Land.
ANTARCTIC JOURNAL - REVIEW 1993 38
Conventional K/Ar ages on whole-rock samples of the basalts in northern Victoria Land range from about 90 to 180 million years (Kreuzer et al. 1981; Elliot and Foland 1986; Brotzu et al. 1988; Faure and Mensing 1993). The range of young ages is now recognized to reflect some event during which partial argon-loss occurred (Fleming et al. 1989, 1993). As a result, the K/Ar whole-rock dates range between the time of argon loss (in the event of total loss) and the age of crystallization (no loss). The minimum ages of 90 to 120 million years provide an estimate for the time of argon loss. Similarly, low-temperature steps of 40Ar/ 39Ar incremental heating experiments record dates between 100 and 120 million years. These dates have also been used to estimate the age of alteration (Fleming et al. 1993; Foland et al. 1993). These minimum ages agree in general with the time of alteration inferred by other methods. In an effort to establish the age of alteration in northern Victoria Land more definitively, a sample of apophyllite collected from Mount Frustum (figure 1) was studied by 40Ar/39Ar and Rb/Sr dating techniques. The apophyllite, a potassiumbearing, hydrous, sheet silicate, commonly occurs in vugs and vesicles in the lavas and was precipitated, along with a variety of other minerals including quartz, calcite, stilbite, heulandite, and natrolite, from hydrothermal fluids which passed through the rocks. To our knowledge, this is the first attempt to use apophyllite for isotopic age determinations. The apophyllite has a very high Rb/Sr ratio, as illustrated in figure 2. As a result, an Rb/Sr model age can be derived from the present-day isotope compositions by assuming that strontium incorporated into the apophyllite had a strontium87/strontium-86 ( 87Sr/ 86Sr) ratio similar to its host basalt
(approximately 0.710). The date calculated in this manner is 94±1 million years and is insensitive to any reasonable values chosen for the initial ratio of 87Sr/ 86Sr incorporated into the apophyllite. 40Ar/ 39Ar incremental-heating data for the apophyllite are shown in figure 3. Most of the argon is released at relatively high temperatures (greater than 600 0C), similar to the laboratory release found for most coarse-grained micas. The loss of a large portion of argon between about 750°C and 820°C probably reflects breakdown of the mineral structure. The variation in apparent age among different temperature increments is significant but relatively minor. This discordance, or most of it, may be the result of recoil redistribution of 39Ar produced by potassium during sample irradiation (see Foland et al. 1993). The age of the vast majority of heating steps falls between 90 and 100 million years with a weighted average (or total gas) age of 96±1 million years. The increments over the interval 650°C to 790°C show a plateau of the same age. The Rb/Sr and 40Ar/ 39Ar dates are in good agreement and suggest formation of the apophyllite 95 million years ago. On the basis of these measurements, apophyllite appears to provide useful isotopic ages by both methods. Dating of apophyllite could be very useful in determining the time of volcanism and hydrothermal activity at many locations. The 95-million-year age is interpreted to reflect the time of actual formation of apophyllite and associated minerals in the lava sequence. The possibility that the 95-million-year age reflects resetting of both isotopic systems in the mid-Cretaceous is difficult to evaluate because nothing is known about the behavior of apophyllite isotopic systems. The concordance of ages by two isotopic geo chronometers, however, suggests that resetting is unlikely because of the very different chemical behavior of the daughter elements (strontium and argon). Moreover, the ages demonstrate mid-Cretaceous hydrothermal activity in northern Victoria Land and support the conclusion that the K/Ar dates of less than 177 million years from the basalts result primarily from episodic argon loss in the Cretaceous rather than by slow leakage over geologic time. The dates documented for apophyllite are considered to be the best estimate for the age of alteration of the basalts in northern Victoria Land because the apophyllite is a direct product of the hydrothermal activity. Previous estimates for the age of alteration are less definitive because they have relied upon whole rocks which will not have had their K/Ar or Rb/Sr systems completely reset during alteration (see Foland et al. 1993) and because the underlying assumptions for isotopic age determinations are not strictly valid. Considering these factors, the slightly older apparent ages from whole rocks, which form the bases of previous estimates for the timing of alteration, are consistent with a 95-million-year age of alteration. Mid-Cretaceous dates provided by a variety of methods document a hydrothermal event that affected rocks of northern Victoria Land. The nature and cause of this event is not well understood. It could be related to the mid-Cretaceous (95±5 million years ago; Veevers 1986) separation of Australia
No 5 I Mt. Frustum
Apophyllite
C') 94±1 Ma .(J)
2
1 c_ Basalts (0.710) 0 1000 2000 3000 87Rb/86Sr Figure 2. Rb/Sr isochron diagram illustrating the relationship between apophyllite from Mount Frustum and its host basalt. The apophyllite has a much higher Rb/Sr ratio than the basalt; the higher ratio allows calculation of a model age of 94±1 million years.
ANTARCTIC JOURNAL - REVIEW 1993 39
0.26
Elliot, D.H., M.A. Siders, and M.A. Haban. 1986. Jurassic tholeiites in the region of the upper Rennick Glacier, north Victoria Land. In E.
CZ 0.24
Stump (Ed.), Geological investigations in northern Victoria Land
(Antarctic Research Series, Vol. 46). Washington, D.C.: American Geophysical Union. Faure, G., and T.M. Mensing. 1993. K-Ar dates and paleomagnetic evidence for Cretaceous alteration of Jurassic basaltic lava flows, Mesa Range, northern Victoria Land, Antarctica. Chemical Geology (Isotope Geoscience Section), 109(1/4), 305-315. Fleming, T.H., D.H. Elliot, K.A. Foland, L.M. Jones, and J.R. Bowman. 1993. Disturbance of Rb-Sr and K-Ar isotopic systems in the Kirkpatrick Basalt, north Victoria Land, Antarctica: Implications for middle Cretaceous tectonism. In R.H. Findlay, R. Unrug, M.R.
0. 22 0.20 co
120
Mt. Frustum 450 Apophyllite
300
CD 100 -
750 780 790 800 [_ 650
80
fuse
950
Banks, and J.J. Veevers (Eds.), Gondwana 8-Assembly, evolution and dispersal. Rotterdam: Balkema.
Fleming, T.H., D.H. Elliot, L.M. Jones, and J.R. Bowman. 1989. Secondary alteration of iron-rich tholeiitic rocks of the Kirkpatrick Basalt, northern Victoria Land. Antarctic Journal of the U.S., 24(5), 37-40. Fleming, T.H., D.H. Elliot, L.M. Jones, J.R. Bowman, and Siders. 1992. Chemical and isotopic variations in an iron-rich lava flow from the Kirkpatrick Basalt, north Victoria Land: Implications for low-tem-
1100
ttg = 96 ± 1 Ma
60
(Run#47M2)
perature alteration. Contributions to Mineralogy and Petrology,
111(4),440-457. Fitzgerald, P.G., and E. Stump. 1991. Uplift history of the Transantarctic Mountains from Victoria Land ( ..70°S) to Scott Glacier (-86°S): Evidence from fission track analysis. Abstracts-Sixth International Symposium on Antarctic Earth Sciences. Saitama, Japan: Ranzan-machi. Foland, K.A., T.H. Fleming, A. Heimann, and D.H. Elliot. 1993. Potassium-argon dating of fine-grained basalts with massive Ar loss: Application of the 40Ar/ 39Ar technique to plagioclase and glass from the Kirkpatrick Basalt, Antarctica. Chemical Geology (Isotope Geoscience section), 107(1/2), 173-190. Heimann, A., T.H. Fleming, D.H. Elliot, and K.A. Foland. In press. A short interval of Jurassic continental flood basalt volcanism in Antarctica as demonstrated by 40Ar/ 39Ar geochronology. Earth
0 20 40 60 80 100 %
39
Ar cumulative release
Figure 3. 40Ar/39Ar incremental-heating diagram for apophyllite from Mount Frustum. Temperatures for each of the heating steps are indicated. The total-gas age (t tg), calculated by integrating all increments, is 96±1 million years. and Antarctica and may record hydrothermal activity in the Rennick Graben (Fleming et al "I - 993) The graben may repre sent a failed rift arm formed during continental break-up. Alternatively, the dates could reflect the movement of groundwaters during initial stages of uplift of the Transantarctic Mountains. Fission-track studies suggest the uplift may have begun as early as the Cretaceous (Fitzgerald and Stump 1991; Stump and- Fitzgerald 1992). The authors would like to express thanks to F.A. Hubacher and J.S. Linder for vital laboratory support with isotopic analyses, M.A. Siders for contributing the sample of apophyllite, and A. Heimann for discussions on Kirkpatrick basalt geochronology. This work has been supported in part by National Science Foundation grants OPP 80-21401, OPP 8318957, and OPP 90-22603.
and. Planetary Science Letters
Kreuzer, H., A. Hohndorf, H. Lenz, U. Vetter, F. Tessensohn, P. Muller, u:Jcrdan, W. Harre, and C. Besang. 1981. K-Ar and Rb-Sr dating of igneous rocks from north Victoria Land. Geologisches Jahrbuch, B41,267-273. Lanza, R., and E. Zanella. 1993. Paleomagnetism of the Ferrar Dolerite in the northern Prince Albert Mountains, Victoria Land, Antarctica. Geophysical Journal International, 114(3), 501-511. McIntosh, W.C., P.R. Kyle, and J.F. Sutter. 1986. Paleomagnetic results from the Kirkpatrick Basalt Group, Mesa Range, north Victoria Land. In E. Stump (Ed.), Geological investigations in northern Victoria Land (Antarctic Research Series, Vol. 46). Washington, D.C.:
American Geophysical Union. Mensing, T.M., G. Faure, L.M. Jones, and J. Hoefs. 1988. Evidence for Cretaceous volcanism in northern Victoria Land, Antarctica, and implications for a mantle enrichment event during the Ross Orogeny based on the chemical and isotopic data from the Kirk-
References
patrick Basalt in the Mesa Range. Abstracts-Seventh International Symposium on Gondwana, Sao Paulo, Brazil. Sao Paulo: Instituto
Brotzu, P., G. Capaldi, L. Civetti, L. Melluso, and G. Orsi. 1988. Jurassic Ferrar Dolerites and Kirpatrick Basalts in northern Victoria Land (Antarctica): Stratigraphy, geochronology and petrology. Memorie Della Societa Geologica Italiana, 43, 97-116. Delisle, G. 1983. Results of paleomagnetic investigations in northern Victoria Land. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science. Cambridge: Cambridge University Press. Delisle, G., and K. Fromm. 1989. Further evidence for a Cretaceous thermal event in north Victoria Land, Geologisches Jahrbuch, E38, 143-151. Elliot, D.H., and K.A. Foland. 1986. K-Ar age determinations of the Kirkpatrick Basalt, Mesa Range. In E. Stump (Ed.), Geological investigations in northern Victoria Land (Antarctic Research Series, Vol. 46). Washington, D.C.: American Geophysical Union.
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