5.5 Pb–Pb dating and crustal evolution

Because the Pb)Pb whole-rock method depends only on isotopic compositions, it is comparatively resistant to metamorphic re-setting, and can also yield some age information for crustal reservoirs showing complex mixing relationships. Good examples of these uses are provided by Pb)Pb dating studies of Archean crustal evolution in the North Atlantic region, particularly western Greenland.

5.5.1 Archean crustal evolution

A major crust-forming event in the Archean craton of western Greenland is represented by the Nuk gneisses. Gneisses in this association from Fiskanaesset, Nordland and Sukkertopen in western Greenland have Pb compositions (filled circles in Fig. 5.36) which fall on a reference line with slope age of 2900 Myr. If a single stage mantle growth curve is calculated to fit this isochron it yields a :₁ value of 7.5 which is a typical value for the mantle source of juvenile Archean gneiss terranes (Moorbath and Taylor, 1981). This single stage model mantle composition is not expected to represent the real Earth, since this was shown above to be an oversimplification; however, the :₁ value provides a convenient yard-stick for comparison of different crust-forming events.

Fig. 5.36. Pb)Pb isochron diagram for basement gneisses from western Greenland. ( ! ) = Nuk gneisses from Fiskanaesset, Nordland and Sukkertopen; ( " ) = contaminated Nuk gneisses from near Godthaab. M = mantle at 2900 Myr; A = range of Amitsoq gneiss compositions at 2900 Myr. After Taylor et al. (1980).

The Nuk gneisses approximate a two stage Pb isotope evolution model, in which the first stage is in the mantle and the second is in each analysed whole-rock system. However, Taylor et al. proposed that there might be two extra short stages in the middle. The first of these short stages represents basalt extraction from the mantle before subsequent re-melting to form tonalitic magmas, possibly at the base of the crust. The second short stage might occur between the time of tonalite emplacement in the crust and its high-grade metamorphism. The whole process was termed a Crustal Accretion)Differentiation Super-event or CADS by Moorbath and Taylor (1981). More recently, trace element studies have suggested that Archean tonalites may be produced by partial melting of amphibolitised ocean floor basalt in the downgoing slabs of subduction zones (Drummond and Defant, 1990; Foley et al., 2002). In this model, the first short stage would represent the time from oceanic crust formation at an ocean ridge until its consumption in a subduction zone. However, for either of these melting models, and for Pb evolution between magmatism and metamorphism, the short duration of these stages minimises their effect on long term Pb isotope evolution.

Nuk gneisses from near Godthaab (open circles in Fig. 5.36) fall in a scatter below the 2900 Myr isochron. However, Taylor et al. (1980) argued that if these were age-corrected back to 2900 Myr ago, they would lie on a mixing line between 2900 Myr-old mantle (M), and the local crust, represented by Early Archean (3700 Myr-old) Amitsoq gneisses (A). Owing to their low U contents, the Amitsoq gneisses barely changed in Pb isotope ratio between 3700 and 2900 Myr ago. Given these end-members, the distance down the mixing line from ‘M’ to ‘A’ indicates the fraction of crustal Pb incorporated into the magma. The variable pattern of crustal Pb contamination suffered by Late Archean Nuk magmas is consistent with the known extent of Early Archean crust. Thus, while Godthaab is known to lie on Amitsoq gneiss basement, such rocks are not exposed near Fiskanaesset, Nordland or Sukkertopen.

The Qorqut granite is also exposed within the Amitsoq gneiss terrane near Godthaab (Moorbath et al., 1981). Whole-rock samples of this body define a linear array whose slope corresponds to an age of 2580 Myr (Fig. 5.37). However, one attempts to fit a single stage mantle growth curve to these data, an impossibly low :₁ value of 6.23 is obtained, showing that the Qorqut granite cannot be a mantle-derived melt. In fact, the initial Pb isotope ratio of the Qorqut granite coincides closely with the average composition of analysed Amitsoq crust at 2580 Myr, indicating that the Qorqut granite is probably a partial melt of Amitsoq gneiss. It therefore approximates to three stage Pb isotope evolution: stage 1 = mantle; stage 2 = Amitsoq crust; stage 3 = Qorqut granite. The initial ⁸⁷Sr/⁸⁶Sr ratio of the Qorqut granite (0.7083 " 4) supports this model (section 7.3.4).

Fig. 5.37. Pb)Pb isochron diagram for Qorqut granite samples showing the coincidence of their initial ratio with the average Amitsoq gneiss composition at 2580 Myr. (open symbols omitted from regression). After Moorbath and Taylor (1981).

5.5.2 Paleo-isochrons and metamorphic disturbance

It was argued above that comparatively short periods (less than 200 Myr) between the crustal extraction and metamorphic differentiation of a gneiss complex do not necessarily upset dating of the crustal formation event using Pb)Pb systematics. However if the period between the two events is substantial, then spurious ages may be obtained. A good example is provided by the Vikan gneiss complex from Lofoten ) Vesteralen in NW Norway. If we assume that these rocks behaved as closed systems after their generation from an isotopically homogeneous (mantle?) source, we determine a slope age of 3410 " 70 Myr (Taylor, 1975). However, Nd model age dating yields ages of ca. 2.4 ) 2.7 Byr (Jacobsen and Wasserburg, 1978).

Subsequent examination of present day U/Pb ratios in the gneisses (Griffin et al., 1978) revealed that they were uniformly far too low to ‘support’ the observed range of Pb isotope compositions. Therefore, it is now believed that the Pb data reflect a 2680 Myr-old igneous protolith which suffered high-grade metamorphism ca. 1760 Myr ago. To illustrate this interpretation, Pb isotope compositions for the protolith are shown as a paleo-isochron at the time of metamorphism (Fig. 5.38). If the rocks were depleted in U to a nearly uniform level at 1760 Myr, subsequent U decay would yield a ‘transposed paleo-isochron’ (Griffin et al., 1978; Moorbath and Taylor, 1981) which is almost parallel to the original paleo-isochron.

Fig. 5.38. Pb)Pb isochron diagram showing a ‘transposed paleo-isochron’ defined by Vikan gneisses of NW Norway. These rocks were formed from 2680 Myr-old precursors which were subjected to a granulite-facies uranium-depletion event at 1760 Myr. After Moorbath and Taylor (1981).

The slope of the transposed paleo-isochron approximates Pb evolution from time T (protolith age) to t (metamorphic age). This is described by an equation which is analogous to [5.15] for galena evolution:

(²⁰⁷Pb) (²⁰⁷Pb) [5.16]

())))) ! ()))))

(²⁰⁴Pb)_P (²⁰⁴Pb)_I 1 (e^{8235 T} ! e^{8235 t})

)))))))))))))))) = )))) @ )))))))))))

(²⁰⁶Pb) (²⁰⁶Pb) 137.88 (e^{8238 T} ! e^{8238 t})

())))) ! ()))))

(²⁰⁴Pb)_P (²⁰⁴Pb)_I

In contrast, the simple Pb/Pb isochron equation [5.11], describing evolution from t to the present, yields too large an age because it is based on the lower ²³⁵U/²³⁸U ratio prevailing at the present day compared with that 1760 Myr ago.

Transposed paleo-isochrons can be detected by checking concordancy of Pb with Sr or Nd ages and by checking that observed Pb isotope compositions are adequately supported by the U/Pb ratio in the samples. Another example of this phenomenon was found in upper amphibolite-facies gneisses of the Outer Hebrides, NW Scotland, by Whitehouse (1990). By substituting the 2660 Myr (Badcallian) Pb homogenisation event as T in equation [5.16], and assuming uniform U/Pb ratios after the second event, he was able to estimate the timing (t) of this second event. The calculated age of 1880 " 270 Myr was consistent with the timing of the Laxfordian metamorphic event.

Another example where high grade metamorphism occurred much later than protolith formation is provided by the Grenville Province of eastern North America. In this case, high grade metamorphism (at ca. 1.1 Byr) occurred up to 1.6 Byr after crustal formation. To investigate Pb isotope evolution in Grenville crust, DeWolf and Mezger (1994) measured Pb isotope ratios on K feldspars from various rock types in the Grenville Province of Ontario and New York State. Data for Mid Proterozoic rocks of the Adirondacks and Central Meta-sedimentary Belt formed a compact array of shallow slope, whereas data from the Central Gneiss Belt (CGB) defined a steeper but more scattered array (Fig. 5.39).

DeWolf and Mezger attempted to use the Pb–Pb data set to test Nd isotope mapping of the extent of Archean crust in the CGB. However, the susceptibility of Pb isotope data to open system behaviour during high grade metamorphism makes Pb a less sensitive tool than Nd for mapping crustal formation ages in the Grenville Province. A combination of Nd isotope and U–Pb data indicates the existence of a juvenile Early Proterozoic terrane in the CGB. However, meta-igneous rocks in this terrane have ambiguous Pb isotope signatures that cannot be resolved from the Archean or Mid Proterozoic arrays. This applies both to the ²⁰⁷Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb diagram (Fig. 5.39) and to the ²⁰⁸Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb diagram (not shown).

Fig. 5.39. Pb–Pb isotope diagram for Grenvillian gneisses with older crustal formation ages: ( ! ) = Archean; ( + ) = Mid Proterozoic; ( " ) = Early Proterozoic ages with ambiguous Pb isotope signatures. ( <> ) = Archean, Superior Province. Growth curve from Doe and Zartman (1979). Data from DeWolf and Mezger (1994).

The two case studies discussed above show that Pb–Pb isotope systems can be quite unreliable when a high grade metamorphic event occurred some time after crustal formation. However, these were both cases in which a Late Archean crustal terrane was subjected to Metamorphism in the Early to Mid Proterozoic. In contrast, Pb isotope systematics allow much tighter constraints on crustal evolution for Early Archean rocks, due to the rapid evolution of ²⁰⁷Pb during early Earth history. Therefore, for Early Archean rocks, even Pb model ages can be used to test Pb–Pb isochron ages for metamorphic disturbance.

Kamber and Moorbath (1998) used this approach to test a Pb–Pb regression age for 83 Amitsoq gneisses from the coastal Godthabsfjord area, south of Nuk in western Greenland (Fig. 5.40). Because of the large size of the data set, a relatively precise age of 3.65 " 0.07 Byr (2F) was obtained. However, the regression gave a large MSWD of 18, suggesting either initial ratio heterogeneity or metamorphic disturbance. Both of these effects could have subtly influenced the errorchron slope to produce a meaningless age. Therefore, to test this possibility, the Amitsoq Pb–Pb regression line was compared with a depleted mantle growth curve to determine a Pb model age for the rocks.

Fig. 5.40. Pb–Pb isochron for Amitsoq gneisses from the Godthabsfjord area of western Greenland showing intersection with the depleted mantle growth curve at a model Pb age of 3.66 Byr, in good agreement with the regression age. ( ! ) = whole-rocks; ( " ) = Feldspar. After Kamber and Moorbath (1998).

The growth curve used was that of Kramers and Tolstikhin (1997), but the curve of Stacey and Kramers (1975) gives almost identical results. The regression line was shown to intersect the growth curve at a point corresponding to a model Pb age of 3.66 Byr, in excellent agreement with the regression age. The inclusion of leached feldspar analyses in the data set makes the model age particularly robust because these are very close to initial Pb isotope ratios. Hence, these data confirm that the Pb–Pb regression age gives the true age of crustal formation from a typical mantle Pb source. This suggests that in the coastal Godthabsfjord area of western Greenland, there is no significant crustal prehistory for the Amitsoq gneisses before 3.66 Byr. However, this does not rule out such a prehistory for Amitsoq gneisses from the area further inland, near Isua (section 4.4.4).

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