- Sm-Nd dating of Fig Tree clay minerals of the Barberton greenstone belt, South Africa
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Figure 5 shows the variations in some calculated parameters, plotted against SiO 2 or MgO. The only unit that is actually metaluminous is the quartz monzonite of Stage 3.
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This, along with its low SiO 2 , high MgO, separate trends on the variation diagrams and potassic, alkaline character mark the magmas of this unit as fundamentally different from the others in the batholith. For Mg-number [mol. For the rest of the units, the data points define either uncorrelated clouds or near-vertical arrays. This sort of variation is unlike that expected for rocks that belong to a differentiation series produced by crystal fractionation, or a series produced through magma mixing. This reinforces the already cited evidence that these processes were not important for the Heerenveen magmas.
The other important observation in this diagram is that the units belonging to the earlier emplacement stages Stages 1 and 2 tend to be more magnesian higher Mg-number , despite their lower MgO contents Fig. Partial melting experiments on all types of protoliths have demonstrated that the MgO contents of melts increase with T. This is true whether the experiments are fluid-absent or fluid-present.
Thus, it seems more likely that the protoliths for the earlier stages contained more mafic rocks. Combined with their peraluminous character, it seems likely that the magmas from these early stages of batholith construction were produced through melting reactions that left substantial quantities of hornblende behind in the residual assemblages see, e.
Sm-Nd dating of Fig Tree clay minerals of the Barberton greenstone belt, South Africa
The inferred presence of large modal quantities of residual hornblende in the restitic source rocks suggests that the melting reactions may have been fluid-present, as fluid-absent partial melting would have destroyed the amphibole, leaving residual, anhydrous pyroxenes. Figure 5 c Mg-number plotted against MgO content shows that there is no correlation between these two parameters. This is instructive because it rules out production of the series of magma compositions by any kind of mixing between a mafic magma high MgO and Mg-number and a felsic magma low MgO and Mg-number.
The same is true for CaO variation with MgO not shown ; the lack of correlation here also rules out mafic—felsic magma mixing. Indeed, fractionation of mafic minerals normally results in a decrease in Mg-number as MgO contents decrease. Thus, the lack of any such positive correlation in Fig. As can be seen in Fig. Even if this mechanism did operate here, it would have driven the Heerenveen magma compositions along vectors that significantly depart from the variations that are actually observed. This conclusion is reinforced if one considers the trace-element variations shown in Fig.
Thus, it seems clear that wall-rock assimilation played no significant role in producing the variations in the Heerenveen batholith. Additionally, none of the forms of variation shown among the major elements, and discussed above, is similar to the trends that would be expected for magmas undergoing differentiation by crystal fractionation, crystal unmixing processes or modification by magma mixing.
We therefore need to examine the trace-element variations to shed light on the origins of the geochemical heterogeneity in the Heerenveen rocks. Harker diagrams for selected trace elements in the main intrusive units of the batholith; a V; b Pb; c Sr; d Zr; e Ba; f La. Data for the Badplaas TTG pluton small open circles are also plotted, together with a dashed line of best fit. Trace-element variations for Heerenveen are illustrated in the Harker diagrams of Fig.
The plot for V shows an overall linear variation, suggesting control by minerals such as hornblende and Fe—Ti oxides. The only rocks that contain hornblende are the quartz monzonites of Stage 3. These are indeed the rocks with the highest V contents and, as pointed out above, they are the only rocks in the batholith that are actually metaluminous see Fig. Pb contents are highly scattered. There are possible negative trends with SiO 2 for some units e. The whole Heerenveen batholith is very Sr-rich; even the most SiO 2 -rich leucogranites contain over ppm Sr.
The general lack of REE evidence for plagioclase fractionation see below and the paucity of hornblende in the batholith suggest that the CaO and Sr abundances may have been controlled by residual Cpx or Hbl in the source, rather than having been produced through fractionation. As for Sr, Zr shows a rudimentary negative quasi-linear correlation with SiO 2.
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The overall tendency, however, appears to be a composite of clusters of data points, rather than a single overall variation trend. As for many elements and oxides, there is considerable scatter within the high-SiO 2 group. The Ba contents of the quartz monzonite and some low-SiO 2 Stage 4 pink granites are significantly elevated — ppm , suggesting a shoshonitic affinity, and indeed these rocks plot near or within the shoshonite field in Fig.
The data for the Badplaas TTG rocks are also plotted in these diagrams, along with the best-fit linear trends defined by the Badplaas data.
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As for the major-element diagrams, these trace-element variations especially for Pb, Sr, Zr, Ba and La confirm that assimilation of wall-rocks probably had no role in producing the geochemical variations in the Heerenveen batholith. Figure 7 shows chondrite-normalized REE patterns for the Heerenveen rocks, with separate plots for each of the four emplacement stages. Also, the flatter HREE patterns for the two rocks with negative Eu anomalies suggest different protoliths for these.
Taken together, these features suggest that the Stage 2 rocks are not related to each other by fractionation processes.
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Chondrite-normalized REE diagrams for the intrusive stages of the Heerenveen batholith; a Stage 1 leucogranite 1 ; b Stage 2 dark grey line, porphyritic granite; black line, grey granite ; c Stage 3 black dashed line, leucogranite 2; black line, quartz monzonite; dark grey line, pink granite 1; light grey line, granodiorite ; d Stage 4 pink granite 2. A further similarity lies in the general absence of negative Eu anomalies in many of the rocks analysed.
Thus, the steep REE patterns suggest either high-pressure partial melting of crustal protoliths, leaving garnet in the restite, or partial melting of older TTG rocks at lower pressure, with the steep patterns inherited from the protoliths. Although, from the geochemical viewpoint, some of the Heerenveen magmas could have been produced by partial melting of TTG rocks, others with higher REE abundances and flatter HREE patterns must have been derived from crustal protoliths other than TTGs.
The question of whether TTG rocks were the protoliths for any of the Heerenveen magmas can be examined using the results of experimental petrology. Watkins et al. Plagioclase was abundant and garnet very scarce in the residual assemblages. They concluded that whatever the protoliths were, they must have been more mafic and more potassic than any of the exposed TTG rocks. Thus, irrespective of the nature of the melting reactions that may have occurred, TTG rocks are not suitable candidates for the Heerenveen protoliths.
From the outset, it should be recognized that in the Heerenveen batholith there are very few or no potential magmatic lineages that extend beyond the identified rock units.
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The scatter in geochemical plots such as Figs 4 e and 5 b, for example, demonstrates this lack of kinship between the magmas. The geochemical features, described above, suggest that heterogeneity within and between units belonging to a particular stage cannot be attributed to either fractionation or magma mixing processes. Thus, as suggested by Clemens et al.
Where there are potential intra-unit lineages e. As discussed above, apart from crystal fractionation in general , other variation mechanisms that can be ruled out include emplacement-level assimilation of wall-rocks, mafic—felsic magma mixing and crystal e. Because most of the units contain ilmenite as the only Fe—Ti oxide accessory phase, much of the batholith would be classified as ilmenite-series Ishihara, , although the voluminous central porphyritic granite is clearly magnetite-series, presumably reflecting a more oxidized protolith.
This further demonstrates the likely heterogeneity in the source region that gave rise to the Heerenveen magmas. If these rocks had crustal protoliths, as seems likely for most of the units, they would have been a variety of meta-igneous rocks or feebly peraluminous greywackes, ranging from intermediate to felsic in composition.
However, as demonstrated above, the wall-rock TTG gneisses were not involved, either as protoliths or assimilants. The Heerenveen batholith thus provides us with a window into the possible lithological make-up of the Archaean crust of the region. The likely protoliths for the Heerenveen magmas are discussed below, under the headings of the two major geochemical groups—the high- and low-SiO 2 types.
As discussed above, partial melting of TTG rocks is not a good model for producing the relatively potassic magmas of the low-SiO 2 Heerenveen group. Alternatively, the low-SiO 2 rocks could potentially represent cumulates, formed by the concentrations of crystals from magmas similar to the high-SiO 2 rocks.
However, this hypothesis can be rejected on the grounds that there is very little textural evidence for a cumulate character, and that there do not appear to be evolutionary trends linking the low- and high-SiO 2 groups in any of the variation diagrams e. Figs 4—6. It has already been shown that magma mixing is most unlikely to have played a significant role in formation of the magmas of the batholith.
Thus, the spread of compositions between the low- and high-SiO 2 groups is most likely to be the result of protolith heterogeneity. For the low-SiO 2 group, we propose an origin involving partial melting of potassic, intermediate crustal protoliths, as has previously been suggested by Anhaeusser et al. As noted above, the generally scattered distributions of data points for the high-SiO 2 group on many variation diagrams, and the extreme variation in concentrations of some oxides and trace elements e.
K 2 O, Sr, Ba, Zr and Pb over very small ranges of SiO 2 content Figs 4 and 6 represent powerful arguments against differentiation processes having been involved in producing the geochemical diversity.
The directions of any rough trends that are shown for some elements are also incompatible with an origin involving magma mixing e. Using the same arguments as for the low-SiO 2 magmas, wall-rock assimilation can also be discounted. Thus, as for the low-SiO 2 group, these high-SiO 2 magmas were most probably generated through partial melting of heterogeneous crustal rocks.
Again, like the low-SiO 2 group, these magmas range from sodic, superficially TTG-like to highly potassic, shoshonitic compositions, which implies a large range in protolith chemistry. Thus, it seems unlikely that the peraluminous Heerenveen rocks gained their character through the involvement of highly aluminous metasedimentary rocks in the protolith.
The REE evidence, presented above, also appears to rule out TTG rocks as possible protoliths, unless they were present at depths of at least 70 km. There is no evidence of this sort of massive crustal thickening at this time in the terrane, so a TTG source for the Heerenveen magmas is ruled out.
As mentioned above, there is a good deal of evidence that hornblende must have been a prominent mineral in the residues of partial melting that created the magmas. This is the most probable explanation for the peraluminous character of many of the rocks see Abbott, However, provided that partial melting took place in normal-thickness crust, the small or absent Eu anomalies in the granitic rocks also rule out mafic protoliths, as these too would produce restitic plagioclase at crustal pressures e.
It is true that, in very high- T melting, plagioclase stability could be exceeded, especially in a relatively sodic protolith. However, this would also eliminate hornblende from the residual assemblage. Since hornblende appears to be a necessary residual phase, such very high- T melting can also be discounted.