Age of the Conachair Granite, by M. Brook - St. Kilda: an illustrated account of the geology

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From: Harding, R.R. and Nancarrow, P.H.A. 1984. St. Kilda: an illustrated account of the geology. BGS Report Vol. 16, No. 7. Keyworth: British Geological Survey.].
Figure 40 Diagram showing Rb and Sr ratios of St Kilda intrusions

Chapter 22 The age of the Conachair Granite[edit]

Keywords: Rb–Sr isotopes, initial 87Sr/86Sr ratios, granite, gabbro

The islands of St Kilda are made up of a series of intrusive igneous rocks whose relative ages can be determined by examining their mutual contacts, but the boundary between the whole complex and the surrounding country rock has so far proved inaccessible beneath more than 50 m of water. So in this case one cannot assign an older age limit to the complex by identifying the country rock, and, similarly, a younger limit cannot be assigned from field mapping methods because (apart from glacial deposits) neither lavas, sediments nor any other country rock remains of the cover that must have overlain the complex. The intrusions have been considered Tertiary solely on the basis of their similarity with those in Mull, Skye and other Tertiary centres.

One way to determine rock age uses the radioactive decay of part of the trace element rubidium (Rb). Rubidium consists of two isotopes with different masses, 85Rb and 87Rb; the latter breaks down to 87Sr by losing an electron. The rate of this breakdown or decay is constant, so if the amount of Rb in the rock and the amount of Sr that has formed as a result of the decay are known, the time taken to form the Sr can be calculated. This radiogenic 87Sr joins the isotopes of Sr (88, 87, 86 and 84) initially present in the rocks, the extra amount being most conveniently measured as an increase in the 87Sr/86Sr ratio. The parts of the rock that contain high Rb will, through time, gain more 87Sr than those parts with low Rb and this is best illustrated by measuring and plotting the 87Rb/86Sr and 87Sr/86Sr isotope ratios on a graph known as an isochron diagram.

Ideally, the slope of the isochron is proportional to the age of the rock. Each sample of the rock should lie on the isochron but in practice this rarely happens. First because there are small experimental errors in measuring the isotopic ratios and secondly because the isotopic systems may be disturbed and small amounts of radiogenic strontium may be lost from certain samples by weathering and other processes of alteration. Isotopic measurements were made on 33 samples from St Kilda. The results are tabulated in (Table 41). When all the data are plotted on an isochron diagram, the scatter about the best fit line is considerable as indicated by the high value of 31 for the MSWD (mean square weighted deviates, an estimate of the goodness of fit of the points to a single line). When MSWD has a value of 2.5 or less, all the scattering about the line can be attributed to analytical error, but if the number is greater than 2.5, then there is a geological reason for the scattering and the errors on the age of the rock must be increased to account for this. Two samples from St Kilda fall statistically a long way from the line which passes through all the other points: an aplite vein, which is probably younger than the other intrusive bodies; and a marginal sample of the Conachair Granite which may be contaminated. When these samples are removed from the plot, the MSWD becomes 7 (Figure 40) and when the errors on the age are enhanced to allow for the small excess scatter, the age of the St Kilda intrusive complex becomes 55 ± 1 Ma, in good agreement with the conclusions drawn from the Palaeomagnetism results in the previous section.

On this basis one can calculate the apparent initial 87Sr/86Sr ratios for each point and evaluate the differences in these for each phase of intrusion and thus suggest possible geochemical relationships between the different phases. The Conachair Granite shows the widest range in possible initial strontium ratios which may reflect varying source contamination of the magma or later disturbance of the Rb-Sr systematics ((Table 41)). The calculated possible initial strontium ratios for the Mullach Sgar Diorite, the Mullach Sgar Microgranite and the Glen Bay Granite show remarkable internal consistency. The means of the initial ratios for each phase are significantly different and suggest varying contamination of the source magma for each distinct intrusion. However, the total range in initial 87Sr/86Sr ratios is very small and may indicate derivation from a common magma reservoir. The sample of aplite, No. 327A, appears to be younger than the main intrusive phases. For an age of 55 Ma, its calculated apparent initial ratio is unacceptably low. With the more realistic value for the initial 87Sr/86Sr ratio of 0.7039, the age calculated for this phase becomes 50 Ma.

In the context of Tertiary igneous rocks of the North Atlantic region, the initial 87Sr/86Sr ratios are amongst the lowest yet recorded.

(Table 41) Rb-Sr data for intrusive rocks on St Kilda[edit]

Sample Rb(ppm Sr(ppm) 87Rb/86Sr 87Sr/86Sr 87Sr/86Sro Calculated assuming an age of 55 +1 Ma Mean 87Sr/86Sr ±2 s.e.
234 102 7.9 38.534

0.73390 0.70379
322 100 8 38.690

0.73395 0.70372
Conachair Granite 326A 78 79 2.8665 0.70310 0.70302
206A 123 5.2 68.3850 0.75816 0.70473 0.70410 ± 65
90 96 7.9 35.7900 0.73323 0.70527
232 103 7.3 41.2150 0.73626 0.70406
298A 31 291 0.3091 0.70443 0.70419
Mullach Sgar Complex 298B 29 290 0.2889 0.70429 0.70406
299A 24 267 0.2607 0.70431 0.70411
300B 35 249 0.4096 0.70451 0.70419 0.70414 ± 5
Dun Passage 301A 31 263 0.3463 0.70446 0.70419
Diorite 302A 24 311 0.2276 0.70427 0.70409
302B 24 310 0.2208 0.70429 0.70412
303 56 131 1.2349 0.70526 0.70430
305 61 123 1.4345 0.70535 0.70423
Mullach Sgar Complex 306 53 136 1.1309 0.70516 0.70428
307 56 123 1.3054 0.70526 0.70424
308 58 130 1.2825 0.70525 0.70425 0.70426 ± 3
Na h-Eagan Microgranite 309 47 129 1.0565 0.70512 0.70429
310 54 137 1.1516 0.70515 0.70425
181B 71 95 2.1730 0.70608 0.70438
113 72 40 5.3950 0.70800 0.70378
237 55 98 1.6409 0.70512 0.70384
238 60 104 1.6728 0.70501 0.70370
Glen Bay Granite 239 58 108 1.5723 0.70504 0.70381
240 57 105 1.5895 0.70500 0.70376 0.70379 ± 5
241 59 100 1.6926 0.70511 0.70379
242 57 108 1.5215 0.70498 0.70379
244 56 109 1.5030 0.70506 0.70389
Boreray Gabbro 369B 6.7 104 0.1867 0.70525 0.70510
Aplite 327A 97 2.4 113.45 0.78488 0.69624
Glen Bay Gabbro H7387 15 263 0.1622 0.70423 0.70410
Western Gabbro H7640 7 160 0.1270 0.70311 0.70302


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