7 Isotope geochemistry of continental rocks

Oceanic volcanics, erupted through thin, young lithosphere, provide a window on the asthenosphere and deep mantle. In contrast, continental basalts and mantle xenoliths, emplaced through thick, old lithosphere, may tell us about the nature of the deep crust and the lithospheric mantle, as well as the evolution of magmas during their ascent to the surface. Isotopic data represent a powerful tool for such studies, firstly because of their ability to date geological events, and secondly because of their usefulness as tracers of complex mixing processes.

Unfortunately, continental igneous rocks are difficult to interpret. This is because they can derive an enriched elemental and isotopic signature from three possible sources: mantle plumes, sub-continental lithosphere, and the crust. Resolving these components from one another in continental volcanics and plutons has been a major subject of discussion in geochemistry for several decades. Much progress has been made, but the large number of variables tends to make each case a unique example; or as Read (1948) put it, there are ‘granites and granites’. This makes a generalised approach to continental magmas difficult, and forces us to adopt a case study approach as an attempt to illustrate underlying principles.

Mantle xenoliths provide a more direct means of sampling the sub-continental lithosphere. Their texture provides evidence of a solid source, while the peridotite (i.e. lherzolite) petrology of the commonest types is readily distinguished from crustal xenoliths (which will not be dealt with here). Therefore, our approach in this chapter will be firstly to study the lithospheric mantle by means of xenoliths, secondly to examine crustal contamination processes, and lastly to look at some classic case studies on the genesis and evolution of continental igneous rocks.

7.1 Mantle xenoliths

The sub-continental lithosphere is distinguished from the underlying asthenosphere by its non-convecting, rigid state. Hence it was termed the ‘tectosphere’ by Jordan (1975, 1978). Jordan argued from seismic and heat-flow evidence that this tectosphere was 200)300 km thick under shield areas. Evidence from diamond inclusions in garnets (section 4.2.1) suggests a similar thickness of continental lithosphere in the Archean.

Alkaline magmas, kimberlites and carbonatites in many continental areas bring up peridotite xenoliths (also called nodules) from great depths. On the basis of their mineral chemistry, these must be samples of the mantle rather than the crust. Maaloe and Aoki (1977) analysed the major element composition of numerous such xenoliths in an attempt to estimate the bulk upper mantle composition. They recognised compositional differences between spinel lherzolite xenoliths, derived from Proterozoic and younger lithosphere, and garnet lherzolites, derived from Archean cratons. Both types of xenolith had overlapping ranges of MgO content, but the (Archean) garnet peridotites had distinctly lower FeO contents. In view of their more exotic history, we will direct our main attention to this group.

The world’s classic mantle xenolith suites come from South Africa, where they are obtained as by-products of diamond mining. Within this collection, two main textural types are observed: granular and sheared. Harte (1983) proposed that the former were samples of the lithosphere, whereas the latter, which are more often found around the margins of the Kaapvaal craton, were derived from the convecting asthenosphere. Of the granular types, Harte further sub-divided samples exhibiting obvious or ‘modal’ metasomatism (indicated by hydrous or other exotic minerals) from the more normal garnet peridotites. The latter samples came from the centre of the Kaapvaal craton, at Northern Lesotho and Bultfontein (hence NLB type), and were regarded as typical samples of the mantle lithosphere.

Various explanations have been proposed to account for the differing FeO contents of spinel and garnet peridotites. However, the most satisfactory was developed by Richter (1988). He proposed that garnet peridotites were residues of komatiite extraction in the Archean, and that the large degrees of melting associated with this process caused FeO depletion. This in turn lowered the density of the residuum, relative to fertile mantle, and allowed its stabilisation as sub-continental lithosphere. This material reached sufficient thicknesses (> 150 km) for diamond crystallisation to occur at its base. In contrast, Proterozoic lithosphere was stabilised only by conductive cooling of the upper mantle (a mechanism which would not have been possible in the hotter Archean mantle). Proterozoic mantle lithosphere may be residual from basalt extraction, or may not be depleted by melt extraction at all; hence it has higher levels of FeO and other fertile components. The thickness of lithosphere formed in this way is insufficient to reach the diamond stability field, while its high density makes it susceptible to delamination from the base of the crust during orogenic shortening of the lithosphere.

The major element compositional differences between garnet and spinel peridotite xenoliths, described above, are paralleled by isotopic differences. Figure 7.1 shows a compilation of Sr and Nd isotope data for the two groups (Hawkesworth et al., 1990), which define fairly distinct fields. Spinel peridotite data are derived mainly from separated clinopyroxene (cpx), but garnet peridotite data are based on a combination of separated mineral and whole rock analyses. The latter are less reliable because they are susceptible to contamination by the host magma (usually kimberlite in the case of garnet peridotite).

Fig. 7.1. Nd versus Sr isotope diagram showing the largely distinct compositional fields of spinel peridotite ( " ), and garnet peridotite ( ! ). After Hawkesworth et al. (1990).

Menzies (1989) adopted a terminology for interpreting mantle xenoliths (Fig. 7.2) which was based on the DMM, EMI and EMII end-members proposed for OIB sources by Zindler and Hart (section 6.4.2). He did not propose that the processes which formed these types of lithospheric ‘domains’ were necessarily the same as those which formed OIB end-members, but the use of such a terminology may imply a genetic relationship. Zindler and Hart did in fact propose (section 6.4.2) that the HIMU and EMI components (forming the LoNd array) were derived from recycled mantle lithosphere. However, the EMI component does not necessarily bear a direct relationship to any given segment of lithosphere. Furthermore, such a model does not fit well to the EMII component of the OIB source, which is widely attributed to sediment subduction. Hence, the present author suggests the use of different names for domain types in plume sources and the lithosphere.

Fig. 7.2. Nd versus Sr isotope diagram showing compositional fields for xenolith suites from different provinces, relative to enriched mantle components identified in OIB sources (hatched fields). After Menzies (1989).

7.1.1 Mantle metasomatism

Spinel peridotite data in Fig. 7.1 are generally depleted relative to the Bulk Earth composition. Therefore, they may represent fairly normal samples of the upper mantle. However, garnet peridotites generally fall in the enriched quadrant relative to Bulk Earth, despite the fact that they are interpreted as residues of komatiite extraction. This demands a secondary enrichment process, which could be caused by either silicate melts, or by hydrous or carbonaceous fluids. Only the latter two are examples of metasomatism in the strict sense, but typically, mantle enrichment is regarded as more or less synonymous with mantle metasomatism.

Dawson and Smith (1977) described a suite of mafic xenoliths from kimberlites such as Bultfontein, whose hydrous mineralogy marked them as relics of ancient metasomatising fluids. These nodules, sometimes described as glimmerite, are characterised by the presence of phlogopite mica, together with various other hydrous minerals. Dawson and Smith distinguished an important sub-group of these nodules with a characteristic mineral assemblage of mica–amphibole–rutile–ilmenite–diopside, which they dubbed the ‘MARID’ suite. They suggested that these MARID xenoliths might have crystallised from a pegmatitic magmatic fluid, chemically similar to kimberlite, which would be capable of metasomatising its peridotite wall rocks.

This model was developed by Jones et al. (1982), who suggested that peridotite nodules from Bultfontein had been metasomatised by a fluid which, although not exactly like the parent of the MARID suite, was related to it in some way. This metasomatic process is recorded by different peridotite lithologies, which form a series. Starting from garnet peridotite, this progresses through garnet–pargasite peridotite and phlogopite peridotite to phlogopite– K-richterite peridotite in a suite represented as GP–GPP–PP–PKP (Erlank et al., 1987).

Having established the role of mantle metasomatism in generating the incompatible element enrichments of peridotite xenoliths, another important question is the timing of this process. Kramers (1979) analysed the Pb isotope composition of sulphide inclusions in diamonds (and also cpx from eclogite and peridotite xenoliths) in several Cretaceous kimberlite pipes. Both inclusion and cpx data lay close to a 2.5 Byr isochron line (Fig. 7.3), implying that diamonds and xenoliths are co-genetic, and that mineralogical heterogeneity has been preserved in the South African sub-continental lithosphere since the Archean. In particular, the very unradiogenic composition of the diamonds, which yield Pb model ages of over 2 Byr, would be very difficult to explain in terms of any recent metasomatic event. In contrast, Pb isotope compositions in other ‘fertile’ peridotites and cpx megacrysts were interpreted as evidence of fairly recent disturbance.

Fig. 7.3. Pb)Pb isochron diagram for nodules from South African kimberlites. ( <> ) = sulphide inclusions in diamonds (F = Finsch mine, K = Kimberly); filled symbols: cpx from peridotite and cpx megacrysts (different symbols signify different mines). After Kramers (1979).

Menzies and Murthy (1980) analysed the Sr and Nd isotope compositions of diopsides in micaceous garnet lherzolite nodules from South African kimberlite pipes (Bultfontein and Kimberley). The diopsides exhibited a strong inverse correlation on the Sr)Nd isotope diagram (Fig. 7.4). This was attributed by Menzies and Murthy to gross mantle heterogeneity, randomly sampled by kimberlite magmas. They suggested that these signatures were generated by an ancient metasomatic event, probably related to an upwelling mantle plume, which caused LIL element enrichment of the mantle lithosphere.

Fig. 7.4. Plot of Nd versus Sr isotope ratios for diopsides from South African kimberlite nodules ( ! ), relative to the mantle array of oceanic basalts. ( Ë ) = whole-rock peridotites. After Menzies and Murthy (1980).

Hawkesworth et al. (1983) estimated from Nd isotope data that the ancient enrichment event postulated by Menzies and Murthy probably occurred ca. 1)4 Byr ago. However, the Rb/Sr ratios of the analysed diopsides (and indeed any mantle diopsides) are much too low to ‘support’ their observed ⁸⁷Sr/⁸⁶Sr compositions (i.e. generate the required extra amount of ⁸⁷Sr by in situ ⁸⁷Rb decay in the required time). This is demonstrated by the clustering of these points near the y axis in Fig. 7.5. Therefore, Hawkesworth et al. argued that the diopsides must have crystallised in a recent event, presumably during secondary metasomatism of the enriched mantle which was generated by the ancient metasomatic event. The enhanced Sr isotope ratios cannot be generated by contamination with the host kimberlite magma itself, because the latter has unradiogenic ⁸⁷Sr.

Fig. 7.5. Rb)Sr isochron diagram for South African kimberlite nodules, showing diopside field (hatched) relative to kimberlite host ( <> ) and nodules of different lithology: ( ! ) = garnet peridotite; ( € ) = garnet)pargasite peridotite; ( Q ) = phlogopite peridotite; ( " ) = phlogopite) K-richterite peridotite. After Hawkesworth et al. (1983).

On the Rb–Sr isochron diagram in Fig. 7.5, whole-rock analyses of (garnet-free) phlogopite-bearing and K-richterite-bearing peridotites (PP and PKP) define a linear array with a slope age of 150 Myr. Since these radiogenic Sr signatures in the nodules could not be derived from the host kimberlite, Hawkesworth et al. attributed the array to a metasomatic event about 150 Myr ago, possibly associated with Karoo flood basalt magmatism. In contrast, phlogopites separated from Bultfontein peridotites yield a well-fitted Rb)Sr mineral isochron with an age of 84 Myr (Kramers et al., 1983), which is close to the emplacement age of 90 Myr determined from U)Pb data. However, this was regarded as a metamorphic age, reflecting the opening of mineral systems during the thermal event associated with kimberlite emplacement.

Kimberlites are actually known to have two distinct isotopic signatures, termed Group I and Group II respectively (section 7.3.1). As noted above, the (Group I) kimberlite host of the peridotite nodules was ruled out as the metasomatising agent because of its unradiogenic Sr signature. Therefore, Erlank et al. (1987) considered the possibility that the Sr signatures found in the peridotite nodules could have been generated by metasomatic fluids related to the more radiogenic Group II kimberlites. A compilation of Sr and Pb isotope data for kimberlites, MARID xenoliths and metasomatised peridotite shows that all three suites form a single array with negative slope, which could be a mixing line (Fig. 7.6). However, several problems led Erlank et al. to reject this model. The most important of these problems was an apparent lack of similarity in trace element signatures between the metasomatised peridotites, MARID xenoliths, and kimberlite magmas.

Fig. 7.6. Plot of initial Sr isotope ratio at 90 Myr (kimberlite emplacement age) against Pb isotope ratio for PKP whole-rocks ( + ), minerals from MARID xenoliths ( " ) and minerals from other Kimberly peridotites ( ! ), compared to the fields for Group I and II kimberlites. After Erlank et al. (1987).

This problem was revisited by Gregoire et al. (2002), who identified another subgroup of richterite (amphibole)-free glimmerites with an assemblage characterised by phlogopite mica, minor rutile, ilmenite, and diopsitic cpx. Gregoire et al. named these ‘PIC’ (phlogopite–ilmenite–cpx) xenoliths, although ‘MID’ (mica–ilmenite–diopside) would have been more consistent with the established term MARID. However, the important point is that the PIC xenoliths have isotope signatures resembling those of Group I kimberlites. Gregoire et al. also argued that there were sufficient trace element resemblances between the two respective suites of peridotites, glimmerites and kimberlites to suggest that mantle metasomatism in the Kimberly area was caused by fluids related to the two recognised kimberlite magma groups rather than the Karoo volcanism. In addition to the resemblances in Fig. 7.6, these similarities can also be seen on a Sr–Nd isotope plot (Fig. 7.7). It remains to be seen whether this simple unified model will bear detailed scrutiny.

Fig. 7.7. Plot of ,Nd versus ,Sr at 90 Myr to show resemblances between the isotope signatures of different types of kimberlite magmas and glimmerite nodules. After Gregoire et al. (2002).

References

Barfod, D. N., Ballentine, C. J., Halliday, A. N. and Fitton, J. G. (1999). Noble gases in the Cameroon line and the He, Ne, and Ar isotopic composition of high mu (HIMU) mantle. J. Geophys. Res. 104, 29509–27.

Bell, K. and Blenkinsop, J. (1987). Archean depleted mantle: evidence from Nd and Sr initial isotopic ratios of carbonatites. Geochim. Cosmochim. Acta 51, 291)8.

Bell, K. and Simonetti, A. (1996). Carbonatite magmatism and plume activity: implications from the Nd, Pb and Sr isotope systematics of Oldoinyo Lengai. J. Petrol. 37, 1321–39.

Bell, K. and Tilton, G. R. (2001). Nd, Pn and Sr isotopic compositions of East African carbonatites: evidence for mantle mixing and plume heterogeneity. J. Petrol. 42, 1927–45.

Brandon, A. D. and Goles, G. G. (1988). A Miocene subcontinental plume in the Pacific Northwest: geochemical evidence. Earth Planet. Sci. Lett. 88, 273–83.

Brandon, A. D. and Goles, G. G. (1995). Assessing subcontinental lithospheric mantle sources for basalts: Neogene volcanism in the Pacific Northwest, USA as a test case. Contrib. Mineral. Petrol. 121, 364–79.

Briqueu, L. and Lancelot, J. R. (1979). Rb)Sr systematics and crustal contamination models for calc-alkaline igneous rocks. Earth Planet. Sci. Lett. 43, 385)96.

Carlson, R. W. and Hart, W. K. (1988). Flood basalt volcanism in the northwestern United States. In: MacDougall, J. D. (Ed.), Continental Flood Basalts. Kluwer, pp. 35)62.

Carlson, R. W., Lugmair, G. W. and MacDougall, J. D. (1981). Columbia River volcanism: the question of mantle heterogeneity or crustal contamination. Geochim. Cosmochim. Acta 45, 2483)99.

Chamberlain, V. E. and Lambert, R. St. J. (1994). Lead isotopes and the sources of the Columbia River Basalt Group. J. Geophys. Res. 99, 11 805–17.

Chappell, B. W. and White, A. J. R. (1974). Two contrasting granite types. Pacific Geol. 8, 173)4.

Chappell, B. W. and White, A. J. R. (1992). I- and S-type granites in the Lachlan Fold Belt. Trans. Roy. Soc. Edin. : Earth Sci. 83, 1)26.

Chappell, B. W., White, A. J. R., Williams, I. S., Wyborn, D., Hergt, J. M. and Woodhead, J. D. (1999). Discussion and Reply: Evaluation of petrogenetic models for Lachlan Fold belt granitoids: implications for crustal architecture and tectonic models. Australian J. Earth Sci. 46, 827–36.

Chesley, J. T. and Ruiz, J. (1998). Crust–mantle interaction in large igneous provinces: implications from the Re–Os isotope systematics of the Columbia River flood basalts. Earth Planet. Sci. Lett. 154, 1–11.

Collins, W. J. (1998). Evaluation of petrogenetic models for Lachlan Fold belt granitoids: implications for crustal architecture and tectonic models. Australian J. Earth Sci. 45, 483–500.

Davidson, J. P. and Tepley, F. J. (1997). Recharge in volcanic systems; evidence from isotopic profiles of phenocrysts. Science 275, 826–9.

Davidson, J., Tepley, F., Palacz, Z. and Meffan-Main, S. (2001). Magma recharge, contamination and residence times revealed by in situ laser ablation isotopic analysis of feldspar in volcanic rocks. Earth Planet. Sci. Lett. 184, 427–42.

Davis, W. J., Gariepy, C. and van Breemen, O. (1996). Pb isotopic composition of late Archean granites and the extent of recycling early Archean crust in the Slave Province, northwest Canada. Chem. Geol. 130, 255–69.

Dawson, J. B. (1967). A review of the geology of kimberlite. In: Wyllie, P. J. (Ed.), Ultramafic and Related Rocks. Wiley, pp. 241)51.

Dawson, J. B. and Smith, J. V. (1977). The MARID (mica–amphibole–rutile–ilmenite–diopside) suite of xenoliths in kimberlite. Geochim. Cosmochim. Acta 41, 309–23.

DePaolo, D. J. (1981a). Trace element and isotopic effects of combined wallrock assimilation and fractional crystallisation. Earth Planet. Sci. Lett. 53, 189)202.

DePaolo, D. J. (1981b). A neodymium and strontium isotopic study of the Mesozoic calc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California. J. Geophys. Res. 86, 10470)88.

DePaolo, D. J. (1983). Comment on ‘Columbia River volcanism: the question of mantle heterogeneity or crustal contamination’ by R. W. Carlson, G. W. Lugmair and J. D. Macdougall. Geochim. Cosmochim. Acta 47, 841)4.

DePaolo, D. J. (1988). Neodymium Isotopes in Geology. Springer-Verlag, 187 p.

DePaolo, D. J. and Wasserburg, G. J. (1976). Nd isotopic variations and petrogenetic models. Geophys. Res. Lett. 3, 249)52.

DePaolo, D. J. and Wasserburg, G. J. (1979a). Petrogenetic mixing models and Nd)Sr isotopic patterns. Geochim. Cosmochim. Acta 43, 615)27.

DePaolo, D. J. and Wasserburg, G. J. (1979b). Neodymium isotopes in flood basalts from the Siberian Platform and inferences about their mantle sources. Proc. Nat. Acad. Sci. USA 76, 3056)60.

Dickin, A. P. (1981). Isotope geochemistry of Tertiary igneous rocks from the Isle of Skye, N. W. Scotland. J. Petrol. 22, 155)89.

Dickin, A. P., Brown, J. L., Thompson, R. N., Halliday, A. N. and Morrison, M. A. (1984). Crustal contamination and the granite problem in the British Tertiary Volcanic Province. Phil. Trans. Roy. Soc. Lond. A 310, 755)80.

Dodson, A., Kennedy, B. M. and DePaolo, D. J. (1997). Helium and neon isotopes in the Imnaha Basalt, Columbia River Basalt Group: evidence for a Yellowstone plume source. Earth Planet. Sci. Lett. 150, 443–51.

Ellam, R. M. and Cox, K. G. (1989). A Proterozoic lithospheric source for Karoo magmatism: evidence from the Nuanetsi picrites. Earth Planet. Sci. Lett. 92,

207–18.

Ellam, R. M. and Cox, K. G. (1991). An interpretation of Karoo picrite basalts in terms of interaction between asthenospheric magmas and the mantle lithosphere. Earth Planet. Sci. Lett. 105, 330–42.

Erlank, A. J., Waters, F. G., Hawkesworth, C. J., Haggerty, S. E., Allsopp, H. L., Rickard, R. S. and Menzies, M. A. (1987). Evidence for mantle metasomatism in peridotite nodules from the Kimberly pipes, South Africa. In: Menzies, M. A. and Hawkesworth, C. J. (Eds.), Mantle Metasomatism. Academic Press, pp. 221–311.

Fitton, J. G. and Dunlop, H. M. (1985). The Cameroon line, West Africa, and its bearing on the origin of oceanic and continental alkali basalt. Earth Planet. Sci. Lett. 72, 23)38.

Gibson, S. A., Thompson, R. N., Dickin, A. P. and Leonardos, O. H. (1996). High-Ti and low-Ti mafic potassic magmas: key to plume–lithosphere interactions and continental flood-basalt genesis. Earth Planet. Sci. Lett. 141, 325–41.

Gray, C. M. (1984). An isotopic mixing model for the origin of granitic rocks in southeastern Australia. Earth Planet. Sci. Lett. 70, 47)60.

Gray, C. M. (1990). A strontium isotopic traverse across the granitic rocks of southeastern Australia: petrogenetic and tectonic implications. Aust. J. Earth Sci. 37, 331)49.

Gregoire, M., Bell, D. R. and Le Roex, A. P. (2002). Trace element geochemistry of phlogopite-rich mafic mantle xenoliths: their classification and their relationship to phlogopite-bearing peridotites and kimberlites revisited. Contrib. Mineral. Petrol. 142, 603–25.

Halliday, A. N., Davidson, J. P., Holden, P., DeWolf, C., Lee, D-C, and Fitton, J. G. (1990). Trace-element fractionation in plumes and the origin of HIMU mantle beneath the Cameroon line. Nature 347, 523)8.

Halliday, A. N., Dickin, A. P., Fallick, A. E. and Fitton, J. G. (1988). Mantle dynamics: a Nd, Sr, Pb and O isotopic study of the Cameroon line volcanic chain. J. Petrol. 29, 181)211.

Hammouda, T., Pichavant, M. and Chaussidon, M. (1996). Isotopic equilibration during partial melting: an experimental test of the behaviour of Sr. Earth Planet. Sci. Lett. 144, 109–21.

Harte, B. (1983). Mantle peridotites and processes ) the kimberlite sample. In: Hawkesworth, C. J. and Norry, M. J. (Eds), Continental Basalts and Mantle Xenoliths. Shiva, pp. 46)91.

Hawkesworth, C. J., Erlank, A. J., Marsh, J. S., Menzies, M. A. and van Calsteren, P. W. C. (1983). Evolution of the continental lithosphere: evidence from volcanics and xenoliths in Southern Africa. In: Hawkesworth, C. J. and Norry, M. J. (Eds), Continental Basalts and Mantle Xenoliths. Shiva, pp. 111)38.

Hawkesworth, C. J., Gallagher, K., Kelley, S., Mantovani, M. S. M., Peate, D. W., Regelous, M. and Rogers, N. W. (1992). Parana magmatism and the opening of the South Atlantic. Geol. Soc. Lond. Spec. Pub. 68, 221–40.

Hawkesworth, C. J., Kempton, P. D., Rogers, N. W., Ellam, R. M. and van Calsteren, P. W. C. (1990). Continental mantle lithosphere, and shallow level enrichment processes in the Earth’s mantle. Earth Planet. Sci. Lett. 96, 256)68.

Hawkesworth, C. J., Rogers, N. W., van Calsteren, P. W. C. and Menzies, M. A. (1984). Mantle enrichment processes. Nature 311, 331)3.

Hergt, J. M., Peate, D. W. and Hawkesworth, C. J. (1991). The petrogenesis of Mesozoic Gondwana low-Ti flood basalts. Earth Planet. Sci. Lett. 105, 134–48.

Hoefs, J. (1987). Stable Isotope Geology. 3rd Edn, Springer-Verlag, 241 p.

Hooper, P. R. and Hawkesworth, C. J. (1993). Isotopic and geochemical constraints on the origin and evolution of the Columbia River basalt. J. Petrol. 34, 1203–46.

Huppert, H. E. and Sparks, R. S. J. (1985). Cooling and contamination of mafic and ultramafic magmas during ascent through continental crust. Earth Planet. Sci. Lett. 74, 371)86.

Hurley, P. M., Bateman, P. C., Fairbairn, H. W. and Pinson, W. H. (1965). Investigation of initial Sr⁸⁷/Sr⁸⁶ ratios in the Sierra Nevada plutonic province. Bull. Geol. Soc. Amer. 76, 165)74.

Jones, A. P., Smith, J. V. and Dawson, J. B. (1982). Mantle metasomatism in 14 veined peridotites from Bultfontein Mine, South Africa. J. Geol. 90, 435–53.

Jordan, T. H. (1975). The continental tectosphere. Rev. Geophys. Space Phys. 13 (3), 1)12.

Jordan, T. H. (1978). Composition and development of the continental tectosphere. Nature 274, 544)8.

Kaczor, S. M., Hanson, G. N. and Peterman, Z. E. (1988). Disequilibrium melting of granite at the contact with a basic plug: a geochemical and petrographic study. J. Geol. 96, 61–78.

Keay, S., Collins, W. J. and McCulloch, M. T. (1997). A three-component Sr–Nd isotopic mixing model for granitoid genesis, Lachlan fold belt, eastern Australia. Geology 25, 307–10.

Kerr, A. C., Kempton, P. D. and Thompson, R. N. (1995). Crustal assimilation during turbulent magma ascent (ATA); new isotopic evidence from the Mull Tertiary lava succession, N. W. Scotland. Contrib. Mineral. Petrol. 119, 142–54.

Kille, I. C., Thompson, R. N., Morrison, M. A. and Thompson, R. F. (1986). Field evidence for turbulence during flow of a basalt magma through conduits from southwest Mull. Geol. Mag. 123, 693–7.

Knesel, K. M. and Davidson, J. P. (1999). Sr isotope systematics during melt generation by intrusion of basalt into continental crust. Contrib. Mineral. Petrol. 136, 285–95.

Knesel, K. M. and Davidson, J. P. (2002). Insights into collisional magmatism from isotopic fingerprints of melting reactions. Science 296, 2206–8.

Kramers, J. D. (1979). Lead, uranium, strontium, potassium and rubidium in inclusion-bearing diamonds and mantle-derived xenoliths from southern Africa. Earth Planet. Sci. Lett. 42, 58)70.

Kramers, J. D., Roddick, J. C. M. and Dawson, J. B. (1983). Trace element and isotopic studies on veined, metasomatic and ‘MARID’ xenoliths from Bultfontein, South Africa. Earth Planet. Sci. Lett. 65, 90–106.

Lambert, D. D., Walker, R. J., Morgan, J. W., Shirey, S. B., Carlson, R. W., Zientek, M. L., Lipin, B. R., Koski, M. S., and Cooper, R. L. (1994). Re–Os and Sm–Nd isotope geochemistry of the Stillwater Complex, Montana: implications for the petrogenesis of the J–M reef. J. Petrol. 35, 1717–35.

Maaloe, S. and Aoki, K. (1977). The major element composition of the upper mantle estimated from the composition of lherzolites. Contrib. Mineral. Petrol. 63, 161)73.

Mantovani, M. S. M. and Hawkesworth, C. J. (1990). An inversion approach to assimilation and fractional crystallisation processes. Contrib. Mineral. Petrol. 105, 289)302.

Maury, R. C. and Bizouard, H. (1974). Melting of acid xenoliths into a basanite: an approach to the possible mechanisms of crustal contamination. Contrib. Mineral. Petrol. 48, 275–86.

McCulloch, M. T. and Chappell, B. W. (1982). Nd isotopic characteristics of S- and I-type granites. Earth Planet. Sci. Lett. 58, 51)64.

McCulloch, M. T., Jaques, A. L., Nelson, D. R. and Lewis, J. D. (1983). Nd and Sr isotopes in kimberlites and lamproites from Western Australia: an enriched mantle origin. Nature 302, 400)3.

McKenzie, D. and Bickle, M. J. (1988). The volume and composition of melt generated by extension of the lithosphere. J. Petrol. 29, 625–79.

Menzies, M. A. (1989). Cratonic, circumcratonic and oceanic mantle domains beneath the Western United States. J. Geophys. Res. 94, 7899)915.

Menzies, M. A. and Murthy, V. R. (1980). Enriched mantle: Nd and Sr isotopes in diopsides from kimberlite nodules. Nature 283, 634)6.

Moorbath, S. and Pankhurst, R. J. (1976). Further rubidium)strontium age and isotope evidence for the nature of the late Archean plutonic event in West Greenland. Nature 262, 124)6.

Moorbath, S. and Taylor, P. N. (1981). Isotopic evidence for continental growth in the Precambrian. In: Kroner, A. (Ed.), Precambrian Plate Tectonics. Elsevier, pp. 491)525.

Moorbath, S. and Thompson, R. N. (1980). Strontium isotope geochemistry and petrogenesis of the early Tertiary lava pile of the Isle of Skye, Scotland and other basic rocks of the British Tertiary Province: an example of magma crust interaction. J. Petrol. 21, 217)231.

Moorbath, S. and Welke, H. (1969). Lead isotope studies on igneous rocks from the Isle of Skye, Northwest Scotland. Earth Planet. Sci. Lett. 5, 217)30.

Morrison, M. A., Thompson, R. N. and Dickin, A. P. (1985). Geochemical evidence for complex magmatic plumbing during development of a continental volcanic center. Geology 13, 581–4.

Nelson, D. R., Chivas, A. R., Chappell, B. W. and McCulloch, M. T. (1988). Geochemical and isotopic systematics in carbonatites and implications for the evolution of ocean-island sources. Geochim. Cosmochim. Acta 52, 1)17.

Nixon, P. H., Rogers, N. W., Gibson, I. L. and Grey, A. (1981). Depleted and fertile mantle xenoliths from southern African kimberlites. Ann. Rev. Earth Planet. Sci. 9, 285)309.

Read, H. H. (1948). Granites and granites. In: Gilluly, J. (Ed.), Origin of Granite. Geol. Soc. Amer. Mem. 28, 1)19.

Richter, F. M. (1988). A major change in the thermal state of the Earth at the Archean)Proterozoic boundary: consequences for the nature and preservation of continental lithosphere. J. Petrol. Spec. Vol., 39)52.

Smith, C. B. (1983). Pb, Sr and Nd isotopic evidence for sources of southern African Cretaceous kimberlites. Nature 304, 51)4.

Taylor, H. P. (1980). The effects of assimilation of country rocks by magmas on ¹⁸O/¹⁶O and ⁸⁷Sr/⁸⁶Sr systematics in igneous rocks. Earth Planet. Sci. Lett. 47, 243)54.

Taylor, H. P. and Silver, L. T. (1978). Oxygen isotope relationships in plutonic igneous rocks of the Peninsula Ranges Batholith, southern and Baja California. US Geol. Surv. Open File Rep. 79)701, 423)6.

Taylor, P. N., Jones, N. W. and Moorbath, S. (1984). Isotopic assessment of relative contributions from crust and mantle sources to the magma genesis of Precambrian granitoid rocks. Phil. Trans. Roy. Soc. Lond. A 310, 605)25.

Taylor, P. N., Moorbath, S., Goodwin, R. and Petrykowski, A. C. (1980). Crustal contamination as an indicator of the extent of Early Archean continental crust: Pb isotopic evidence from the Late Archean gneisses of West Greenland. Geochim. Cosmochim. Acta 44, 1437)53.

Thirlwall, M. F. and Jones, N. W. (1983). Isotope geochemistry and contamination mechanisms of Tertiary lavas from Skye, northwest Scotland. In: Hawkesworth, C. J. and Norry, M. J. (Eds), Continental Basalts and Mantle Xenoliths. Shiva, pp. 186)208.

Thompson, R. N. (1982). Magmatism of the British Tertiary Volcanic Province. Scott. J. Geol. 18, 49)107.

Thompson, R. N., Dickin, A. P., Gibson, I. L. and Morrison, M. A. (1982). Elemental fingerprints of isotopic contamination of Hebridean Palaeocene mantle derived magmas by Archean sial. Contrib. Mineral. Petrol. 79, 159–68.

Tolstikhin, I. N., Kamensky, I. L., Marty, B. and 8 others. (2002). Rare gas isotopes and parent trace elements in ultrabasic–alkaline–carbonatite complexes, Kola Peninsula: identification of lower mantle plume component. Geochim. Cosmochim. Acta 66, 881–901.

Tommasini, S. and Davies, G. R. (1997). Isotope disequilibrium during anatexis: a case study of contact melting, Sierra Nevada, California. Earth Planet. Sci. Lett. 148, 273–85.

Wendlandt, R. F. and Mysen, B. O. (1980). Melting phase relations of natural peridotite + CO₂ as a function of degree of partial melting at 15 and 30 kbar. Amer. Mineral. 65, 37)44.

White, A. J. R. and Chappell, B. W. (1988). Some supracrustal (S-type) granites of the Lachlan Fold Belt. Trans. Roy. Soc. Edin.: Earth Sci. 79, 169)81.