7.3 Petrogenesis of continental magmas

It is impossible here to attempt a comprehensive review of continental magma suites. Instead, a few case studies will be examined for different magma types which illustrate a variety of approaches to problems of petrogenetic interpretation.

7.3.1 Kimberlites, Carbonatites and Lamproites

Kimberlites, carbonatites and lamproites are highly incompatible-element-enriched magmas which may be genetically related. Experimental evidence suggests that they are all products of very small-degree partial melting in the deep mantle, and that CO₂ plays an important role in their genesis (e.g. Wendlandt and Mysen, 1980). The volatile-rich nature of these magmas causes rapid ascent through the crust. This, coupled with their high incompatible-element concentrations, renders these magmas very resistant to isotopic modification by crustal contamination.

South African kimberlites are divided into two petrological types, basaltic and micaceous (phlogopitic), according to their groundmass mineralogy (Dawson, 1967). These two groups, referred to as types I and II respectively by Smith (1983), have distinct Sr and Nd isotope compositions. Basaltic (Group I) kimberlites have isotopic compositions which cluster just within the depleted quadrant relative to Bulk Earth, whereas micaceous (Group II) kimberlites fall well inside the enriched quadrant (Fig. 7.26).

Kimberlites of western Australia graduate from phlogopite kimberlite to lamproite, and were found by McCulloch et al. (1983) to extend the compositional range of micaceous kimberlites even further into the enriched quadrant in Fig. 7.26. McCulloch et al. calculated T_DM model ages for the source region of these rocks, on the assumption that REE fractionation had not occurred during magma genesis. These were in the range 0.9 ) 1.3 Byr. However, they recognised that the magmas might have been fractionated to more light REE enriched compositions than the source, making these minimum ages which may substantially under-estimate the age of the enrichment event.

Fig. 7.26. Plot of , Nd against Sr isotopic compositions of basaltic and micaceous kimberlites from South Africa ( Q ) and western Australia ( € ), along with Australian lamproites ( ! ). After DePaolo (1988).

The resistance of carbonatites to crustal contamination makes them a potential source of data on the composition of the sub-continental lithosphere. Bell and Blenkinsop (1987) explored this application by the Sr and Nd isotope analysis of carbonatites from Ontario and Quebec, ranging in age from 110 to 2700 Myr. Sr isotope ratios lay along a depleted mantle evolution line, which Bell and Blenkinsop attributed to the sub-continental lithosphere of the Superior Province. However, Nd isotope data for the same samples were more scattered. They comprise part of a larger set of Nd data from different continents (Nelson et al., 1988) which scatter between the depleted mantle and reservoirs at least as enriched as Bulk Earth (Fig. 7.27).

Fig. 7.27. Nd isotope evolution diagram in terms of , Nd against time, showing carbonatite derivation from variably depleted (or mixed) mantle sources. After Nelson et al. (1988).

Nelson et al. argued that the world-wide occurrence of these scattered data militated against an origin in the sub-continental lithosphere, and instead favoured a plume origin similar to that of OIB. This can explain the data in Fig. 7.27, but not the extreme signatures of the Australian lamproite data (Fig. 7.26). Therefore, the mixing model which Nixon et al. (1981) originally proposed for kimberlite genesis can serve as a unifying petrogenetic model for kimberlites, carbonatites and lamproites. In this model, very small-degree partial melts originate in comparatively ‘fertile’ asthenospheric sources (Group I signature); they are subsequently contaminated to different degrees in the LIL-enriched but refractory sub-continental lithosphere (Group II signature).

This mixing model essentially places kimberlites, carbonatites and lamproites on the same footing as other within-plate magmas. These magmas are now generally thought to originate from plume sources, and subsequently undergo variable degrees of contamination in the lithosphere. Therefore, each example must be investigated in its own context, just like other magmas.

One province which has always been important for the study of carbonatites and ultra-alkaline magmas is the East African Rift. Bell and Simonetti (1996) showed that the magmas of this province, including the active carbonatite volcano Oldoinyo Lengai, lie very close to the HIMU–EMI mixing line identified in OIB (Fig. 7.28). Bell and Simonetti (1996) attributed the mixing line to magmas from a HIMU type plume that were subsequently contaminated by sub-continental lithospheric mantle with an isotopic signature resembling that of EMI (section 6.5.3). However, subsequent work on a larger number of carbonate and nephelinite volcanic centres from the East African Rift (Bell and Tilton, 2001) showed that this mixing line was a widespread phenomenon.

The discovery that the mixing line originally observed at Oldoinyo Lengai was also seen in carbonatites from elsewhere in the East African Rift caused Bell and co-workers to change their ideas about the origin of these components. The widespread occurrence of relatively coherent two-component mixing throughout the rift made a source in the lithosphere less likely, since this is expected to be laterally heterogeneous. Therefore, Bell and Tilton (2001) proposed that the signature of HIMU–EMI mixing in East African carbonatites reflected heterogeneity in the plume source itself, as seen in OIB.

Fig. 7.28. Nd–Pb isotope plot showing carbonatites from the East African Rift ( ! ) relative to the end-members invoked to explain the compositions of OIB. After Bell and Tilton (2001).

Additional support for the importance of plume sources in carbonatite magmatism is provided by rare gas evidence from carbonatites of the Kola Peninsula. For example, neon isotope data provide clear evidence for the involvement of a plume type lower mantle source (Tolstikhin et al., 2002). However, individual provinces of kimberlite, carbonatite and lamproite magmatism are likely to show variable degrees of contamination of plume-derived melts by enriched sources in the sub-continental lithosphere.

7.3.2 Alkali basalts

An interesting location to study alkali basalt genesis is provided by the Cameroon Line of western Africa. This volcanic chain, composed dominantly of alkali basalts with subordinate tholeiites, stretches from the Atlantic island of Pagalu (700 km SW of the Niger delta) to the Biu Plateau (800 km inland). Despite the fact that the volcanic chain is situated half on young oceanic crust and half on ancient continental crust, the range of trace element contents and Sr isotope ratios in the two sections of the line are identical (Fitton and Dunlop, 1985). Given that oceanic and continental lithosphere would be expected to have different signatures, Fitton and Dunlop argued that the magma source must lie below the lithosphere.

The Cameroon Line shows no evidence of age progression along its length, and must therefore represent a ‘hot zone’ rather than a hot-spot trail generated by plate motion over a small plume. Fitton and Dunlop argued that because there is no evidence of migration of the area of volcanism over its 65-Myr history (despite movement of the African plate), ‘the mantle source must be coupled to the lithosphere’, rather than originating from a deep mantle plume that tracked across the overlying plate. This was a surprising result, since the only explanation (at that time) for isotopic heterogeneity inherited from a shallow mantle source was disequilibrium melting (section 6.1.2).

To investigate this problem further, Halliday et al. (1988) performed a more detailed isotopic study on the Cameroon Line, including Pb and Nd isotope measurements (Fig. 7.29). A few samples were found to show evidence of contamination by continental basement; however, after excluding these samples, the most distinctive feature of the data was the very radiogenic Pb isotope compositions displayed by basic lavas from the continent)ocean boundary, about half-way along the volcanic chain. These compositions approach those of the St Helena hot spot, but volcanics on either side (within the oceanic and continental segments) are less radiogenic.

Fig. 7.29. Plot of Nd versus Pb isotope compositions for Cameroon Line volcanics. Solid symbols: continental lavas; open symbols: oceanic; half-filled: continental edge. A, I and P = Atlantic, Indian and Pacific MORB. After Halliday et al. (1988).

Halliday et al. (1988) attributed these features to the ‘impregnation’ of the upper mantle under the Cameroon Line by material from the St Helena plume. This plume played a major role in promoting the initial opening of the South Atlantic, about 120 Myr ago. It was probably responsible for the actual location of rifting, which subsequently became the continental edge. With time, the African plate moved away from the St Helena plume, but a ‘blob’ of hot plume material became incorporated into the lithospheric mantle under Cameroon as the continental margin cooled after the rifting event. As the plume component was gradually dispersed laterally along the volcanic chain, its compositional effect was seen at volcanic centres progressively further from the continental edge.

Halliday et al. (1990) revised this model as a result of new observations on the Pb isotope data. These revealed that the radiogenic ²⁰⁶Pb/²⁰⁴Pb signatures at the continental edge were not accompanied by high enough ²⁰⁷Pb/²⁰⁴Pb ratios to represent direct mixing with the St Helena plume. Instead, a positive correlation between ²⁰⁶Pb/²⁰⁴Pb and U/Pb ratios was observed (Fig. 7.30), which Halliday et al. interpreted as a 200 Myr-old erupted isochron. However, it was shown in Fig. 7.29 that ²⁰⁶Pb/²⁰⁴Pb correlates with Nd isotope ratio, which cannot develop large variations by Sm decay over periods of only 200 Myr. Therefore the arrays of Pb isotope ratio are probably mixing lines. The radiogenic end-member must represent young lithosphere whose high ²⁰⁶Pb/²⁰⁴Pb signature was generated by magmas with high U/Pb ratios (:) at the time of continental rifting. Mixing between this component and local asthenospheric upper mantle can explain the isotopic mixing process, which also satisfies more recent rare gas evidence from the Cameroon Line (Barfod et al., 1999).

Fig. 7.30. U)Pb isochron diagram for young Cameroon line lavas with over 4% MgO, yielding an apparent age of ca. 200 Myr. Half-open symbols denote individual volcanos from the continental edge. Other symbols indicate continental ( " ) and oceanic segments ( # ). After Halliday et al. (1990).

This work has two important conclusions. Firstly, continental rifting episodes can replace old sub-continental lithosphere with young lithosphere which may have an exotic composition. Secondly, U is more incompatible than Pb in magmatic processes, so that preferential extraction of Pb from the mantle cannot be invoked to explain the lead paradox (section 6.3.1).

7.3.3 Flood basalts

The northwestern USA displays one of the world’s major flood basalt provinces, the Columbia River Basalt Group (CRBG). The controversies about the petrogenesis of these lavas serve very well to illustrate the complexities of modelling the genesis of flood basalts.

Early Nd isotope data on the Columbia River basalts clustered near , Nd = 0, leading DePaolo and Wasserburg (1976) to propose an undepleted (primordial type) source for these magmas, in contrast to the depleted mantle source of MORB (, Nd = +10). This model was supported by DePaolo (1983) on the basis of a volume-weighted histogram of initial , Nd values measured on the Columbia River basalts (Fig. 7.31). DePaolo argued that the marked concentration of data at slightly positive , values, and the sharp cut off at , Nd = 0 constituted evidence for a chondritic source for the most voluminous Grand Ronde group of lavas, merging into a depleted mantle source for the Imnaha and Picture Gorge basalts. However, the abundance peak at , Nd = 0 in Fig. 7.31 was partly a product of adding the data of DePaolo and Wasserburg (1976, 1979b) to those of Carlson et al. (1981), which caused ‘double sampling’ of some of the same flows.

Fig. 7.31. Histogram of , Nd compositions for Columbia River basalts, weighted according to eruptive volume. Double-hatched data are from DePaolo and Wasserburg (1976, 1979b). Modified after DePaolo (1983).

A compilation of Sr and Nd isotope data for several basaltic suites from the northwestern USA appears to present a rather different picture. The samples display a very strong, almost continuous, curved trend, which fans out somewhat in the enriched quadrant (Fig. 7.32). These data imply that a relatively simple mixing process, such as crustal contamination, was involved in the genesis of the lavas (Carlson et al., 1981). However, it has become apparent over the last few years that radiogenic isotopes alone may not be able to distinguish between enriched mantle and crustal sources. Incompatible element ratios and stable isotope data may be needed to assist in this distinction.

Fig. 7.32. Plot of Nd versus Sr isotope ratio for basalts from the northwestern US. ( € ) = Grand Ronde; ( ! ) = Picture Gorge; ( Q ) = Wanapum; ( > ) = Steens Mtn; ( Î ) = Saddle Mountains; ( ' ) = HAOT; ( <> ) = SROT. C1 to C3 are possible sources discussed in the text. Data from the main diagram define the shaded field on the inset. After Carlson and Hart (1988).

Carlson and Hart (1988) argued that the ratio of a highly incompatible element against a high-field-strength element (e.g. K₂O/P₂O₅) can be used as an index of (specifically) crustal contamination. This index is plotted against Sr isotope ratio in Fig. 7.33. Some Picture Gorge and Grand Ronde basalts of the CRBG, together with Steens Mountain basalts from the Oregon Plateau, have quite elevated K₂O/P₂O₅ ratios, despite having low-to-intermediate Sr isotope ratios. Carlson and Hart attributed this pattern to contamination of magmas from a ‘C-1’ mantle source by crustal units with a variety of ages. The C-1 source was identified as typical asthenospheric upper mantle, whose melting was probably caused by mantle convection behind the Cascades arc. In contrast to the above lavas, some Saddle Mountains CRBG flows, high-Al olivine tholeiites (HAOT) from the Oregon Plateau, and Snake River olivine tholeiites (SROT) have ⁸⁷Sr/⁸⁶Sr ratios up to 0.708, but low K₂O/P₂O₅ ratios. Carlson and Hart attributed these signatures to a lithospheric mantle source (‘C-3’).

Fig. 7.33. Plot of K₂O/P₂O₅ against Sr isotope ratio for basalts from the northwestern US showing mixing models between C-1 magmas and three crustal contaminants with different Sr isotope ratios. Symbols as in Fig. 7.29. After Carlson and Hart (1988).

A plot of * ¹⁸O against Sr isotope ratio supports this model (Fig. 7.34). Steep vectors result from contamination of basaltic magmas by typical crustal units. In contrast, sub-horizontal vectors could be produced by mixing with old ⁸⁷Sr-enriched mantle, or possibly by recent contamination of the Sr-poor mantle source by subducted sediment. Such a distinction between source and magma contamination vectors on the oxygen–strontium isotope diagram was investigated in detail by Taylor (1980) for granitic rocks (section 7.3.5).

Fig. 7.34. Plot of *¹⁸O against Sr isotope ratio for basalts from the northwestern U.S. Curves show effects of mixing with crust of a given Sr and ¹⁸O composition. Steep mixing lines model contamination of magmas ( " = 10% increments); shallow mixing line models contamination of MORB-type source with subducted sediment ( x = 1% increments). Symbols as in Fig. 7.32. After Carlson and Hart (1988).

Pb isotope data reveal one more level of complexity in this picture. Basalts with ⁸⁷Sr/⁸⁶Sr ratios below 0.708 display a triangular distribution on plots of Sr or Nd isotope ratio against ²⁰⁶Pb/²⁰⁴Pb (Fig. 7.35). On this diagram, many Grand Ronde lavas trend towards an end-member (C-2) with radiogenic Pb which is distinct from the C-1 and C-3 mantle end-members recognised from other evidence. Carlson and Hart speculated that the C-2 source may have been derived by contamination of C-1 depleted mantle by subducted sediment.

Fig. 7.35 Isotope compositions of basalts from the northwestern US: a) Nd versus Pb isotope plot; b) Sr versus Pb isotope plot. Three distinct mantle sources are resolved (C-1 to C-3). Symbols as in Fig. 7.24. After Carlson and Hart (1988).

Unlike DePaolo, Carlson and Hart did not invoke any lower mantle plume source for the Columbia River basalts, which therefore makes this an unusual model for a Flood Basalt province. In contrast, several subsequent studies of the Columbia River Basalt Province have invoked a plume as one of the end-members. The first of these studies (Brandon and Goles, 1988) involved trace element analysis only, whereas three subsequent studies used radiogenic isotope tracers. However, the analytical results obtained in these later isotope studies were largely the same as Carlson and Hart... just the conclusions differed. Therefore, these models will be discussed using the older isotope data in Fig. 7.35.

Hooper and Hawkesworth (1993) and Brandon and Goles (1995) used multiple isotope tracers (Pb–Sr–Nd) in their investigation, as well as elemental data. They broadly agreed with Carlson and Hart (1988) that C-1 is asthenospheric upper mantle and C-3 is lithospheric mantle. However, both groups reinterpreted C-2 as a plume source, identified as part of the track of the present-day Yellowstone plume. In contrast, Chamberlain and Lambert (1994) used Pb isotope only, and also divided both C-1 and C-2 into two sub-component reservoirs (R1-R2 and R4-R5 respectively). However, the main difference was that they identified the C-3 component (their R3) as the plume source and the C-2 component (i.e. R4-R5) as crustal.

These competing models were tested by Dodson et al. (1997) using helium and neon isotope evidence. Two samples were analysed, an Imnaha basalt (close to the C-2 end-member) and a Wanapum basalt (close to the C-3/R3 end-member). The Imnaha (C-2) basalt was found to have a much stronger plume (undegassed lower mantle) signature than the Wanapum basalt, thus confirming the models of Hooper and Hawkesworth (1993) and Brandon and Goles (1995). In so doing, this work also vindicated to some extent the early studies of DePaolo and Wasserburg, by showing that the Columbia River basalts do indeed contain a lower mantle component. However, these studies also demonstrate that no single isotope tracer (either Nd or Pb) is sufficient alone to reliably distinguish between sources in such a complex multi-component system.

More recent investigation of the CRBP using the osmium isotope tracer has further quantified the degree of crustal contamination in the origin of these basalts (Chesley and Ruiz, 1998). These data do not distinguish between the alternative sources in the upper and lower mantle (C-1 and C-2), but they throw additional light on the identity of the C-3 source. Contrary to the arguments of Carlson and Hart (1988) based on trace element and oxygen isotope data, Chesley and Ruiz argued that the C-3 end-member was represented by crustal rather than mantle lithosphere. After all of the argumentation above, this seems a surprising conclusion. However, Chesley and Ruiz claimed that initial osmium signatures in lavas from the Grande Ronde, Wanapum and Saddle Mountains groups were so radiogenic that only a crustal source could explain them (see section 8.3 for background).

Additional evidence for this interpretation comes from dykes of the Wanapum group that are intruded across a suture boundary between Precambrian and Mesozoic terranes. Analyses of lithophile isotope ratios revealed no change across this boundary, but initial ¹⁸⁷Os/¹⁸⁸Os ratios changed from 0.2 on the Mesozoic side to around 3 on the Precambrian side. Hence this provides strong support for a relatively shallow contamination process, attributed by Chesley and Ruiz to mafic lower crust. This material would have low values of * ¹⁸O and possibly also low K₂O/P₂O₅ ratios, thus confounding the evidence from these tracers. This should serve as a warning that the composition of the lower continental crust is still not well understood, even for isotope tracer systems which have supposedly reached a mature stage of understanding.

7.3.4 Precambrian granitoids

One of the most fundamental questions about the continental crust is the extent to which any given block of sialic basement is the product of juvenile separation from the mantle or reworking of older cratonic material. Sr isotope data were originally applied to this problem on the grounds that crustal reservoirs, which have high Rb/Sr ratios, develop higher ⁸⁷Sr/⁸⁶Sr ratios over geological time than the low-Rb/Sr mantle. Calculation of the initial Sr isotope composition of a plutonic crustal segment should then indicate whether it has a mantle or crustal source. The evolution line for Sr in the depleted mantle is constructed by drawing a linear growth curve from the ‘basaltic achondrite best initial’ (BABI) value of 0.69899 " 5 (section 3.2.4) to the ⁸⁷Sr/⁸⁶Sr composition of recent ocean ridge basalts in the range 0.702)0.704. Data for specific crustal provinces can then be compared with this evolution line to assess their petrogenesis.

A classic example of the application of the Sr isotope evolution diagram to the provenance of crustal basement is provided by studies of the Archean and Proterozoic gneisses of West Greenland by Moorbath and Pankhurst (1976). Figure 7.36 shows average growth lines for 3.7 Byr-old Amitsoq gneisses from four localities, 2.8 ) 2.9 Byr-old Nuk gneisses from five localities, 1.8 Byr-old Ketilidian gneisses from two localities (in South Greenland), and the 2.52 Byr-old Qorqut granite. The initial ratios of these terranes are compared in Fig. 7.36 with a hypothetical linear upper mantle growth line drawn between BABI and MORB.

Fig. 7.36. Sr isotope evolution diagram showing the development of four crustal suites relative to the depleted mantle evolution line. The Qorqut granite (Q) is attributed to crustal anatexis. After Moorbath and Taylor (1981).

Moorbath and Pankhurst argued that the Nuk (and Ketilidian) gneisses could not be derived by reworking of older (e.g. Amitsoq) gneiss, since the growth lines of the Amitsoq samples are much too steep to generate products with initial ratios of only 0.702 – 0.703. Instead they concluded that the igneous precursors of the Nuk gneisses represented a massive addition of juvenile calc-alkaline crust to the Archean basement of West Greenland. The slight elevation of the calculated initial ratios above the upper mantle evolution line was attributed to a period of crustal Sr isotope evolution, lasting perhaps 100 ) 200 Myr, between the separation of the igneous precursors from the mantle and their subjection to granulite-facies metamorphism (see section 5.5). In contrast, Moorbath and Pankhurst recognised the Qorqut granite as a good candidate for a pluton derived by re-working of older crust, since its initial ratio of 0.709 " 0.007 is well within error of the compositions of Amitsoq gneisses at that time.

Pb isotope analysis of the Nuk gneisses revealed a more complex picture than the Sr isotope data alone, by demonstrating that Nuk magmas emplaced into areas of Amitsoq crust had suffered significant contamination with old crustal Pb (section 5.5). In view of the lack of obvious crustal Sr contamination, selective contamination by Pb was invoked to explain these observations (Taylor et al., 1980). In this situation, the application of Nd isotope analysis provides an ideal tool to test petrogenetic models for the Nuk gneisses.

Taylor et al. (1984) analysed a selection of both Pb-contaminated and Pb-uncontaminated gneisses for Nd isotope composition (Fig. 7.37). The data display a good correlation between , Nd and Rb, an incompatible trace element expected to be enriched in the Amitsoq gneisses. Taylor et al. attributed these results to contamination of mantle-derived Nuk magmas by partial melts of Amitsoq gneiss in the lower crust. However, , Nd does not correlate well with the degree of Pb isotopic contamination (represented by the index ²⁰⁴Pb_contaminant/²⁰⁴Pb_total). This suggests that additional selective Pb contamination occurred, due to Pb-enriched fluids generated by crustal dehydration.

The paradox whereby substantial Pb and Nd contamination of the Nuk magmas was not accompanied by observable Sr isotope disturbance must be attributed to the stratified nature of the Amitsoq crust. Taylor et al. argued that the deep crust responsible for Pb and Nd contamination must have had lower Rb/Sr ratios than those analysed from the surface outcrops, presumably due to the flushing out of Rb from the lower crust during granulite-facies metamorphism. Hence it did not develop elevated Sr isotope ratios over geological time. It is concluded from this evidence and other studies that Sr isotope data often cannot readily distinguish between mantle and lower crustal source regions. In this situation, Nd isotopes are a more powerful petrogenetic tracer because Sm/Nd is fractionated during crustal extraction from the mantle but is not significantly fractionated by intra-crustal processes (section 4.3).

Fig. 7.37. Variation of , Nd in Nuk gneisses, compared with (a) the fraction of isotopic contamination by Amitsoq Pb (from section 5.5); and (b) Rb content. After Taylor et al. (1984).

The Pb isotope system comprises two coupled dating systems, whereas Sm–Nd normally offers only one. Therefore Pb–Pb data should theoretically provide more control than Nd data on mixing processes between crustal reservoirs of different ages. However, U–Pb isotope systems are quite susceptible to open system behaviour over geological history, whereas whole-rock Sm–Nd systems are very resistant to such effects. Therefore, when Pb–Pb data are used to study ancient mixing events, the measured isotope ratios cannot normally be corrected to unique initial ratios. Instead, model initial ²⁰⁷Pb/²⁰⁴Pb ratios are usually determined by projecting the Pb–Pb data back along an isochron line corresponding to the age of the mixing event (e.g. Fig. 5.36). Hence, we are left with a single isotopic tracer, analogous to ¹⁴³Nd/¹⁴⁴Nd.

Davis et al. (1996) compared the use of initial ²⁰⁷Pb/²⁰⁴Pb and ¹⁴³Nd/¹⁴⁴Nd ratios in late Archean granitoids as tracers of the extent of Early–Mid Archean basement in the Slave Province of NW Canada. They found a relatively good correlation between the two tracers (Fig. 7.38), supporting a model of two-component mixing between juvenile (recently mantle-derived) and old crustal end-members. The mixing line is somewhat curved, indicating a Pb/Nd ratio in the old crustal end-member about three times higher than the juvenile end-member. In principle this makes Pb a more sensitive tracer of hidden crust than Nd. However, evidence from Greenland (Fig. 7.37) suggests that Pb may behave less reproducibly. Hence, the scatter in Fig. 7.38 is probably due to non-stoichiometric mixing of Pb rather than Nd.

Fig. 7.38. Plot of calculated initial ¹⁴³Nd/¹⁴⁴Nd versus ²⁰⁷Pb/²⁰⁴Pb compositions of Late Archean granitoid magmas, explained by mixing of juvenile and ancient crustal end-members. After Davis et al. (1996).

7.3.5 Phanerozoic batholiths

Hurley et al. (1965) made a strontium isotope study of the Sierra Nevada batholith and concluded that most of the intrusive bodies making up the batholith had initial ⁸⁷Sr/⁸⁶Sr ratios of ca. 0.7073 " 0.001. They recognised that this value was intermediate between expected upper mantle and Precambrian crustal values of ca. 0.703 ) 0.705 and 0.71 ) 0.73, respectively. However they were unable to determine on the basis of the Sr isotope evidence whether the Sierra Nevada batholith represented mantle-derived magmas subsequently contaminated by the crust, or simply partial melting of geosynclinal sediments and volcanics.

DePaolo (1981b) made a combined Sr and Nd isotope study of both the Sierra Nevada and Peninsular Ranges batholiths in a further attempt to resolve the genesis of these bodies. The data define hyperbolic arrays on , Sr versus , Nd diagrams (Fig. 7.39), running from the island-arc basalt field towards the composition of a nearby Precambrian schist. The latter was regarded as representative of the source area which yielded the Paleozoic)Mesozoic geosynclinal sediments into which the batholiths are intruded. DePaolo recognised that the western Peninsular Ranges samples closely conformed to the Sr)Nd mantle array, and that they could therefore be products of a heterogeneous mantle without crustal contamination. However, in the context of the Sierra Nevada data, crustal contamination of magmas within the island-arc field seems much more likely.

Fig. 7.39. Plots of , Nd against , Sr for granitoids ( ! ) from the Peninsular Ranges and the Sierra Nevada. Compositions of crustal reservoirs and the effect of seawater alteration are also shown. After DePaolo (1981b).

This interpretation is supported by a comparison of strontium and oxygen isotope data (Taylor and Silver, 1978; DePaolo, 1981b) which together form another powerful tool for studies of granite petrogenesis. (For background to stable isotope geology, see Hoefs, 1987). Sierra Nevada and Peninsular Range granitoids form a hyperbolic array on the , Sr versus *¹⁸O diagram (Fig. 7.40), between mantle-derived and Paleozoic sediment end-members. The shape of the hyperbola is determined by the relative strontium/oxygen concentrations in the two end-members, and is consistent with a simple mixture of high *¹⁸O sedimentary crustal melts with basic magmas.

Three alternative models can all be ruled out because they would cause vertical vectors in Fig. 7.40, in which Sr isotope increases would not be accompanied by appreciable change in *¹⁸O. These models are the following:

1. Sr (and Nd) isotopic enrichment of a mantle source along the mantle array in Fig. 7.39

2. Contamination of the mantle source by sediment subduction. The much lower strontium content of mantle, relative to basic magmas would make it much more susceptible to contamination by subducted sedimentary or seawater Sr, whereas the oxygen content of the mantle is the same as that of basic magmas. In other words, the mantle has a lower Sr/O ratio than basic magma, which would yield a mixing hyperbola of steeper slope in Fig. 7.40.

3. Contamination of magmas with a hypothetical lower crustal component with low *¹⁸O values.

Fig. 7.40. Plot of , Sr against *¹⁸O showing data for the Peninsula Ranges ( ! ) and Sierra Nevada ( € ) batholiths, relative to various possible magma sources and models. After DePaolo (1981b).

Neither Fig. 7.39 nor Fig. 7.40 can distinguish between genesis of the Peninsula Ranges batholith as a direct mantle-derived differentiate or as a re-melt of young basic igneous rock at the base of the crustal geosyncline. However, the San Marcos gabbro unit of the Peninsular Ranges must be a direct mantle melt because of its basic major element composition. Additional mantle-derived melts must have been present at depth to cause crustal melting. Therefore, the simplest, but not exclusive, model is that these melts contributed fractionated magmas to the rest of the batholith.

As important products of Phanerozoic crustal evolution, the California batholiths are paralleled by the Berridale and Kosciusko batholiths of the Lachlan fold belt of SE Australia. However, the genesis of these granitoid suites has proved particularly controversial. This debate began when Chappell and White (1974) distinguished two major types of granite on the basis of chemical and mineralogical criteria. ‘S-type’ granites with low Ca contents and a tendency to per-aluminous character (Al₂O₃/[Na₂O+K₂O+CaO]>1.05) were regarded as partial melts of sedimentary rocks; ‘I-type’ granites with high Ca contents and Al₂O₃/[Na₂O+K₂O+CaO]<1.05 were regarded as partial melts of young igneous crustal rocks.

McCulloch and Chappell (1982) tested this model by analysing a suite of samples from the Berridale and Kosciusko batholiths for Sr and Nd isotope compositions. The data formed two overlapping fields which together define a hyperbolic array in the lower right quadrant of the , Nd versus , Sr diagram (Fig. 7.41). McCulloch and Chappell interpreted these data in support of the crustal melting model of Chappell and White (1974). However, Gray (1984, 1990) attributed them to contamination of mantle-derived basic magmas by mixing with a sedimentary crustal component. Possible end-members are represented by young basic rocks with a mantle-like signature, and Ordovician flysch with a model Nd age of ca. 1400 Myr. The left end of the array projects back to a depleted mantle-like end-member with , Nd of +6. The existence of ‘rare gabbros’ in the vicinity of the batholiths demonstrates that such magmas were available in the crust; their rarity at the surface can be attributed to the ‘density problem’ of raising basic magma through a felsic crust. The crustal end-member is well represented by the Cooma granodiorite, which displays strong structural evidence of being an in situ melt of Ordovician flysch.

Fig. 7.41. Diagram of , Nd against , Sr for ‘I type’ ( ! ) and ‘S type’ ( + ) granites and crustal xenoliths ( <> ) from SE Australia. A best-fit mixing line is shown between hypothetical crustal and mantle-derived end-members. After McCulloch and Chappell (1982).

Gray supported his model with major-element variation diagrams and by examining Sr isotope compositions on a Rb)Sr isochron diagram (Fig. 7.42). Average initial ⁸⁷Sr/⁸⁶Sr and Rb/Sr ratios are plotted for two gabbros and for several plutons from the major ‘S-type’ and I-type’ batholiths. Most of the data form a cone-shaped array, which Gray attributed to mixing between a low Rb/Sr basaltic or andesitic end-member (⁸⁷Sr/⁸⁶Sr = ca. 0.703)0.704) and a somewhat heterogeneous crustal end-member, typified by the crustally-derived Cooma granodiorite. Compositions to the right of this array were attributed to plagioclase fractionation subsequent to mixing, which would yield horizontal displacements.

Fig. 7.42. Rb)Sr pseudo-isochron diagram for granites of SE Australia ( ! ) showing possible ‘mixing fan’. ( + ) = gabbros; ( o ) = granites argued to have fractionated plagioclase after contamination. After Gray (1984).

McCulloch and Chappell (1982) and Chappell and White (1992) acknowledged that the isotopic data for the Lachlan Fold Belt could be explained by two-component mixing between mafic and sedimentary end-members. However, they rejected this model on the grounds that mixing of basic igneous and greywacke components could not explain the major element signatures of the rocks (White and Chappell, 1988).

This problem was solved by a more detailed study of the most mafic granitoids of the Lachlan Fold Belt, comprising the Moruya granitoid suite (Keay et al., 1997). A variety of rock types from this suite defined an elongate distribution with , Nd values from +8 to +4, lying between the fields for Paleozoic mantle derived magmas and Cambrian greenstones (Fig. 7.43) believed to be an important deep-crustal component of the fold belt. Hence, Keay et al. attributed these rocks to mixing between a mantle-derived basic magma and tonalitic partial melts of the Cambrian greenstones. These mixed magmas then provide one end-member for an additional mixing process involving melts of Ordovician turbidites, leading overall to a three-component mixing model. This model also explains the relatively greater abundance of the S type granites (containing a large sedimentary component) in the western part of the belt, since the turbidite sequence is thickest in the west and dies out eastwards (Collins, 1998).

Fig. 7.43. Plot of , Nd versus initial Sr isotope ratio for granitoids of the Lachlan Fold Belt, along with mantle-derived and crustal end-members (shaded) involved in a three component mixing model. After Keay et al. (1997).

In response to this revised contamination model, Chappell et al. (1999) continued to assert one aspect of their earlier crustal melting model, involving the importance of restite (the solid residue of crustal melting) as a component in granite genesis. However, this is largely a second order problem because all workers agree that restite is present in the granitoids of the Lachlan Fold belt; the disagreement is now about the relative importance of this solid component in the relatively late stage evolution of granitoid magmas.

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