Pain Mesa

Pain Mesa in Fig. 12.11 is composed of flat-lying layers of Kirkpatrick Basalt much like Tobin Mesa described in the previons section. These basalt flows were investigated by Mensing (1987) and by Mensing et al. (1991). Mount Masley on Pain Mesa is composed 

Basalt. Prohles of the chemical compositions of the flows on Mt. Masley are shown in Fig. 12.12 (Photo by T.M. Mensing) 

The expectation that the age of the high-Ti flows on Pain Mesa is only slightly less than 175 Ma is drawn into question by the whole-rock Rb-Sr dates of the two types of flows. For example, the high-Ti flows in Mills Valley analyzed by Mensing (1987) yield a Rb-Sr date of only 82 9 Ma in Fig. 12.15 with an initial Sr/ Sr ratio of 0.71162 0.00016 compared to 169 17 Ma for the low-Ti flows and an initial Sr/ Sr ratio of 

The evident enrichment of the Kirkpatrick Basalt in radiogenic Sr has been explained by two alternative hypotheses  

The magma originated from heterogeneous sources in the subcontinental mantle (Kyle 1980). 

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Fig. 2.4 During whiteout conditions in the Transantarctic Mountains the horizon becomes invisible and surface definition is lost. In the case shown here, the whiteout conditions resulted from high wind associated with snowfall near the coast of northern Victoria Land. The Scott tent, strU used by geological fieldpar-ties in the Transantarctic Mountains and on the polar plateau, is well suited for use in cold and windy weather, especially when it is set up in well-packed snow or neve. Mills Valley, Pain Mesa, northern Victoria Land, December 1982 (Photo by G. Faure)...

The Exposure Hill Formation occurs at the base of the sheet-like lava flows of Kirkpatrick Basalt that form the Mesa Range of northern Victoria Land and on nearby nunataks most of which are located in the Rennick graben in Fig. 12.6. The deposits of breccia and related pyroclastic rocks occur at Exposure Hill at the southern end of Gair Mesa and at Mt. Fazio and Scarab Peak on Tobin Mesa as well as on Mt. Carson adjacent to the Aeronaut Glacier in Fig. 12.7. hi addition, Elliot et al. (1986a) described a deposit of breccia on Agate Peak located about 26 km east of the northern tip of Pain Mesa (Fig. 12.6). 

Fig. 12.8 Aerial view of the Mesa Range in northern ctoria Land consisting of Gair Mesarn the foreground followed by Tobin and Pain mesas farther north. The Rennick Glacier west of the Mesa Range flows north to the Oates Coast. The flat-lying layers...

Kirkpatrick Basalt interbedded with occasional sedimentary interbeds of lacustrine and volcaniclastic origin. The Mesa Range consists of the Gair, Tobin, and Pain mesas and extends for nearly 80 km from the southern end of Gair Mesa to the northern tip of Pain Mesa. The highest elevation of 3,280 m occurs at the southern tip of Gair Mesa. The elevations of the summit plateaus decrease in a northerly direction toward Mt. Masley on Pain Mesa which has an elevation of 2,605 m. 

The map identifies the location of Mt. Masley on Pain Mesa and of Solo Nunatak which are described in the text (Reproduced from Mensing 1987)... 

Fig. 12.11 Mt. Masley at the northern end of Pain Mesa is composed of a sequence of lava flows of low-Ti tholeiite basalt and the overlying darker high-Ti flows of the Kirkpatrick...

Fig. 12.12 The lava flows that form Mt. Masley of Pain Mesa consist of two suites that have distinctly different major-element concentrations, including SiO and certain trace elements (e.g., zirconium and yttrium). The rocks of both suites have been classified as tholeiitic basalts based on...

Fig. 12.14 The flows of Kirkpatrick Basalt on Pain Mesa have a wide range of silica concentrations from 49.5% to 56.0% which causes them to extend from basalt to basaltic andesite in the chemical classiflcation of volcanic rocks by Cox et al. (1979) and Wilson (1989). The chemical classification based on this diagram is in conflict with the classification based on the mineralogy of the phenocrysts on the basis of which these rocks are tholeiite basalts (Mensing 1987)...

Fig. 12.15 The low-Ti flows of the Kirkpatrick Basalt in Mills Valley of Pain Mesa yield a whole-rock Rb-Sr date of 169 7 Ma (Middle Jurassic) and an initial Sr/ Sr ratio of 0.71041 0.00017, whereas the high-Ti flows that cap the mesa yield a much lower date of 82 9 Ma (Late Cretaceous) and a higher initial Sr/ Sr ratio of 0.71162 0.00016. This unexpected result means either the high-Ti flows were erupted much later than the low-Ti flows (contrary to evidence in the field) or that the isotopic composition of strontium of the high-Ti flows was homogenized by a thermal pulse during the Late Cretaceous as first suggested by Mensing (1987, 1991)...

The apparent Late Cretaceous isotopic homogenization of strontium of the high-Ti flows that cap Pain Mesa is a separate issue that is related to the tectonic history of northern Victoria. The most likely explanation for this phenomenon is illustrated in Fig. 12.16. The idea is that the high-Ti flows were extruded soon after the eruption of the low-Ti basalt at about 170 Ma and that the anomalously low date of these flows is the result of isotopic homogenization of strontium during... 

Excluding the Lower Black Marker on Mt. Masley of Pain Mesa... 

Table 12.5 Summary of Rb-Sr dates and initial Sr/ Sr ratios of Kirkpatrick Basalt in Pain Mesa, Mesa Range, NVL (Data and interpretation by Mensing 1987, 1991)...

Fig. 12.16 The strontium in the high-Ti flows at Mills Valley of Pain Mesa, northern Victoria Land, could have evolved from magma that was erupted at 170 Ma with an initial Sr ratio of 0.70940. Subsequently, the Sr/ Sr ratio in each flow increased at rates depending on their Rb/ Sr (i.e., Rb/ Sr) ratios. At 84 Ma, when the average Sr/ Sr ratio of the high-Ti flows had reached 0.71162, the Sr/ Sr ratios of all high-Ti flows were equilibrated (reset) to a value of 0.71162, thereby yielding the errorchron shown in Fig. 12.15 (Mensing 1987, 1991)...

The majority of the flows in Fig. 12.19 have nearly uniform chemical compositions (e.g., SiO - 53%, MgO + CaO 18%). These profiles lack the discontinuities caused by the differences in cfamical composition of the low-Ti and high-Ti flows on Pain Mesa in Fig. 12.12. Therefore, all of the flows on Solo Nunatak belong to the low-Ti suite of the Kiikpatrick Basalt with concentrations of TiOj between 0.54% and 0.79% (Mensing 1987). 

The initial Sr/ Sr ratios of the low-Ti flows on Solo Nunatak reported by Mensing et al. (1984) range from 0.70985 to 0.71207 with an average of 0.71069 0.00001 (Ict) for 37 samples from 24 flows recalculated to 175 Ma using = 1.42 x 10 " year. The average initial Sr/ Sr ratio of the Kirkpatrick Basalt on Solo Nunatak is virtually identical to the average initial St/ Sr ratios of the low-Ti flows on Pain Mesa,... 

Fig. 12.17 Solo Nunatak at 72°50 S and 163°35 E is an erosional remnant located 22 km east northeast of the northern tip of Pain Mesa (Fig. 12.9). Is it composed of 23 flows of the Kirkpatrick Basalt of the low-Ti suite. Mensing...

Fig. 12.18 On the alkali-silica diagram of Cox et al. (1979) and Wilson (1989) the flows of Kirkpatrick Basalt of Solo Nunatak extend from basalt to basaltic andesite much like the lava flows on Pain Mesa (Fig. 12.14) and on Tobin Mesa (Fig. 12.10). However, the mineral composition of the phenocrysts cause all of the flows to be classified as subalkalic tholeiite basalt. In addition, all of the flows are composed of low-Ti basalt because the high-Ti basalt flows that cap Tobin and Pain mesas of the Mesa Range are not present and may have been eroded (Mensing 1987)...

Fig. 12.28 The concentrations of TiO of the Kirkpatrick Basalt on Storm Peak in the Marshall Mountains range from greater than 0.4% to less that 2.4% but are not as sharply divided into low-H and high-H suites as the basalt flows on Solo Nunatak and Pain Mesa of northern Victoria Land (Data from EUiot 1970a, 1972 Faure et al. 1982 Mensing et al. 1984 Mensing 1987)...

The high-Ti flows appear to be conformable to the underlying low-Ti flows on Pain Mesa and there is no evidence for an extended hiatus between the eruption of the two sets of flows. In addition, the ArPAr plateau dates of plagioclase in the high-Ti flow on Tobin Mesa demonstrate that the age of this flow is Middle Jurassic and not Cretaceous. The loss of Ar and the homogeni-... 

Mensing TM (1987) Geology and petrogenesis of the Kirkpatrick Basalt, Pain Mesa and Solo Nunatak, northern Victoria Land, Antarctica Ph.D. dissertation. The Ohio State University, Columbus, OH... 

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