Ferrar Group: Dolerite Sills and the Dufek Intrusion

Abstract

During Robert Scott’s first expedition to Antarctica from 1901–1904, a group of men led by Albert Armitage and including the geologist Hartley Ferrar crossed McMurdo Sound and explored the mountains of Victoria Land. They discovered the Ferrar Glacier and ascended it to the edge of the polar plateau (Section 1.4.1). During this trip, Hartley Ferrar photographed diabase sills that intruded the Beacon sandstones and later described the geology of the region (Ferrar 1907). The diabase samples he collected plus others that were collected during Shackleton’s first expedition from 1907–1909 were later described by Prior (1907) and Benson (1916), respectively. Several other geologists worked in the ice-free valleys during Scott’s “Terra Nova” expedition, but only the report by Smith (1924) provided information about the diabase sills. That was pretty much the extent of the information on the diabase sills of southern Victoria Land until the IGY (1957/58) when several geologists from New Zealand and the USA began to map the geology of the Transantarctic Mountains. British geologists prefer the term “dolerite” for rocks which American geologists call “diabase”. We will use the British term out of respect for the New Zealand geologists who continue to work in Antarctica.

Appendices

Appendices

1.1 Mineralogical Types of Ferrar Dolerite Sills in Southern Victoria Land (Gunn 1966)

Table 8

1.2 Chemical Analyses of Dolerite Sills on Roadend Nunatak, Southern Victoria Land

Table 9

1.3 Rb-Sr Systematics of the Dolerite Sills on Roadend Nunatak at the Confluence of the Touchdown and Darwin Glaciers, Southern Victoria Land

Table 10

1.4 Major-Element Analyses of Whole-Rock Samples, Ferrar Dolerite Sills, Mt. Achernar, Queen Alexandra Range, in Percent by Weight

Table 11

1.5 Rb-Sr Systematics of the Sills of Ferrar Dolerite on Mt. Achernar, Queen Alexandra Range ( 84°12′S, 160°56′E)

Table 12

1.6 δ¹⁸O Values of Plagioclase and Pyroxene in Dolerite Samples of Sill # 2 on Mt. Achernar and Estimates of the Isotope Equilibration Temperature

Table 13

The geothermometry equation of Bottinga and Javoy (1975) is:

$$ {\Delta }_{2}^{1}=\frac{(1.70-1.04b){10}^{6}}{{T}^{2}}$$

(13.4)

where β is the mole fraction of anorthite in the plagioclase and A = (1.70 − 1.04 β). Assuming that the plagioclase in Sill # 2 on Mt. Achernar is labradorite defined as An₅₀ to An₇₀, we set β = 0.6 which yields A = 1.076. Substituting this value into Eq. 13.4 and setting ${\Delta }_{2}^{1}$ = 0.98 (15 m above base) we obtain:

$$ 0.98=\frac{1.076\times {10}^{6}}{{T}^{2}}$$

$$ T={\left(\frac{1.076\times {10}^{6}}{0.98}\right)}^{1/2}=1047.8K$$

T = 1047.8 – 273.15 = 774.6 ∼ 775°C

The resulting temperature estimates in column (1) range from 613°C to 1126°C. Omitting the lowest and the highest values yields a mean of 872 ± 112°C (1σ)

The oxygen-isotope fractionation factors reported by Chiba et al. (1989) for feldspar-pyroxene are: Albite-Diopside = 1.81, Anorthite-Diopside = 0.76. For labradorite (An = 60%, Ab = 40%) the numerical value of A is: A = 0.6 × 0.76 + 0.4 × 1.81 = 1.18 and Eq. 13.4takes the form:

$$ {\Delta }_{2}^{1}=\frac{1.18\times {10}^{6}}{{T}^{2}}$$

Setting ${\Delta }_{2}^{1}$ = 0.98, yields T = 824°C. Omitting the lowest and highest temperature estimates yields an average of 926 ± 117°C (1σ)

1.7 Chemical Analyses of Rock Samples from a Measured Section of the Sill of Ferrar Dolerite on Portal Rock, Queen Alexandra Range (J.M. Hergt personal communication to G. Faure, April 27, 1987)

Table 14

13.6.7 (continued) Part 2

Table 15

13.6.7 (continued) Part 3

Table 16

13.6.7 (continued) Part 4

Table 17

13.6.7 (continued) Part 5

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1.8 Average Chemical Analyses of the Lexington Granophyre and Other Felsic Differentiates of the Dufek Intrusion in the Forrestal Range and Dufek Massif, in Weight Percent (Ford 1970; Ford and Kistler 1980)

Table 19

1.9 Modal Concentrations of Minerals in the Rocks of the Forrestal Range and the Dufek Massif (Data from Ford et al. 1983)

Table 20

1.10 Concentrations of Metals in Whole-Rock Samples of the Dufek Intrusion (Ford et al. 1983)

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1.11 Concentrations of Vanadium in the Oxide Minerals of the Dufek Intrusion in the Dufek Massif (Ford et al. 1983)

The vanadium concentrations of whole-rock samples of the Dufek intrusion in the Dufek Massif in Fig. 13.49 form a straight line represented by Eq. 13.4:

$$ \text{V}=356+317(\text{pyr})$$

(13.4)

where V is the vanadium concentration of a rock in the Dufek Massif in ppm and (pyr) is the modal concentration of pyroxene in volume percent in that rock. One of the samples (at 1,798 m) in Fig. 13.49 has an anomalously high vanadium concentration presumably because this rock contains oxides as well as pyroxene and plagioclase (Ford et al. 1983).

The vanadium concentration of the oxide minerals can be estimated by means of Eq. 13.5:

$$ {\text{aV}}_{\text{ox}}+{\text{bV}}_{\text{pyr}}+{\text{CV}}_{\text{plag}.}={\text{V}}_{\text{rock}}$$

(13.5)

where a, b, and c are weighting factors expressed as decimal fractions of the abundances of the minerals in weight percent (i.e., a + b + c = 1.0). The modal concentrations of the minerals in volume percent must be converted to percent by weight because the concentrations of vanadium are expressed in weight units. The conversion is based on Eq. 13.6:

$$ \text{Mass}=\text{Volume}\times \text{Density}$$

(13.6)

The modal concentrations of the minerals in volume percent in the anomalous rock sample are: Plagioclase = 67%, pyroxene = 30%, and oxides = 3% (Ford et al. 1983). The densities of these minerals are: Plagioclase = 2.70 g/cm³, pyroxene = 3.30 g/cm³, and oxides = 5.18 g/cm³. Therefore, the masses of the minerals and their abundances in this rock are:

Plagioclase: 0.67 × 2.70 = 1.809 g = 61.23 wt. %

Pyroxene: 0.30 × 3.30 = 0.99 g = 33.50 wt. %

Oxides: 0.03 × 5.18 = 0.1554 g = 5.25 wt. %

Sums = 2.9544 g = 99.98 wt. %

The concentration of plagioclase is 1.809 × 100/2.9544 = 61.23 wt. % and similarly for the other components. By substituting into Eq. 13.5 and solving for V_ox we obtain: 0.0525 V_ox + 0.335 × 320 + 0.6123 × 35.6 = 460

$$ {V}_{ox}=\frac{460-0.335\times 320+0.6123\times 35.6}{0.0525}=6305ppm$$

or V_ox = 0.63% by weight.

The concentrations of chromium (Fig. 13.50a) and nickel (Fig. 13.50b) in the rocks of the Dufek intrusion are also positively correlated with modal pyroxene concentrations and define straight lines:

Cr = –2.06 + 3.11 (pyr),¹ r = 0.9781

Ni = 20.1 + 1.310 (pyr), r = 0.9328 where r = linear correlation coefficient.

¹Forced through the origin by adding 15 zeros.