The amphibolites from the lower basement-derived section and the upper megablock show chemically distinct characteristics from each other. This trend is illustrated in Figure 3.18a where the amphibolites and calc-silicate of the Eyreville-B borehole core were plotted on the CaO-MgO-FeO ternary diagram proposed by Walker et al. (1960) to discriminate between those amphibolites with an igneous (ortho-) source and those with a sedimentary (para-) source
(Figure 3.18). In Chapter 4, Section 4.1.3, mineral chemistry reveals that the lower basement-derived amphibolite contains pargasitic amphibole, Na-enriched plagioclase and alkali feldspar compared to that of the megablock, and is closely associated with the calc-silicate, which is interpreted to be a metamarl (see Section 3.3.2), and it was suggested that the lower basement-derived amphibolite could be a para-amphibolite (Chapter 2, Section 2.5). Figure 3.18 confirms this suggestion, where samples from the lower basement-derived section plot within the para-amphibolite field.
Figure 3.18. Ternary diagram indicating CaO-MgO-FeO abundances for the discrimination between ortho- and para-amphibolites for the megablock and basement amphibolites and calc-silicate (after Walker et al., 1960) of the Eyreville-B borehole core. FeO was calculated using the CIPW norm.
Based on these findings, the basement para-amphibolite and calc-silicate rock units were plotted with the mica schists of the Eyreville-B borehole core in Figures 3.16 and 3.17. The basement amphibolite plots within the wacke protolith field in Figure 3.16a, consistent with observations that protoliths for para-amphibolites are often greywackes generated by turbiditic currents off continental shelves (Pettijohn, 1957; Bhatia and Crook, 1986; Winter, 2001). Figure 3.16b shows that sample RG51 plots in the active continental margin field while sample RG179 plots in the passive margin field. Unfortunately, this diagram does not clearly indicate the possible tectonic setting in which the para-amphibolites were generated (Figure 3.16b).
However, Figure 3.17 shows that the para-amphibolite does have an intermediate igneous source similar to that seen in the basement mica schists, suggesting either a mature island arc setting or immature continental magmatic arc setting (Roser and Korsch, 1988).
Possible protoliths for the calc-silicate vary from an Fe-rich shale to an Fe-rich sandstone (Figure 3.16a), corresponding with the interpretation that the calc-silicate is a metamarl based on the classification for sediments by Pettijohn (1957). In Figure 3.16b, the calc-silicate plots primarily in the island arc setting, although single samples also plot in the active (W134) and passive (RG50) margin fields. In Section 3.3.4 and Figure 3.9, the calc-silicate was shown to be the most highly altered rock unit within the Eyreville-B borehole core with an average CIA value of 92, where a value of 100 represents the most altered material (Nesbitt and Young, 1982).
Furthermore, as discussed in Section 3.6.1, inference for protolith and provenance requires caution where samples are observed to be altered. In an attempt to overcome the errors associated with alteration effects, the calc-silicate was plotted in Figure 3.17b for which the discriminant functions are calculated to exclude biogenic CaCO3 and SiO2 (Roser and Korsch, 1988). The calc-silicate plots exclusively in the mafic igneous provenance field in Figure 3.17b in contrast to the intermediate igneous provenance in Figure 3.17a. Although the findings of Figure 3.17b are preferred, both provenances confirm an island arc depositional environment (Roser and Korsch, 1988).
Figure 3.18 indicates that the megablock amphibolite was derived from an igneous origin, in contrast to that of the lower basement-derived section. Although sample RG02 plots within the zone in which both fields overlap, and sample RG06 plots within the para-amphibolite field, these samples do not differ sufficiently to conclude that they do not form part of the same sequence as the other megablock amphibolite samples. These deviations could be attributed to alteration by quartz and calcite veining (see Chapter 2, Section 2.4.2).
Using the TAS classification scheme (after Middlemost, 1994), total alkalis (Na2O + K2O) for the megablock amphibolite were plotted against SiO2 to determine the possible igneous protoliths
for this rock unit (Figure 3.19). Based on textural observations in Chapter 2, Section 2.4.1, the megablock amphibolite was plotted using intrusive igneous protoliths. Most samples plot within the gabbro protolith field and tend to be more alkalic in nature; the only significant outliers in Figure 3.19 are samples RG03 and RG06. Petrographic analysis for these two samples revealed a relict igneous texture resembling what would be expected in diorite (see Chapter 2, Section 2.3.2). Although this corresponds with geochemistry of sample RG06 (Figure 3.19), petrographic analysis also showed that sample RG03 to be particularly rich in pyrite and quartz veins (Appendix 1b), which would skew the accuracy of this sample’s results. These two outliers are both located within the deepest two metres of the amphibolite megablock (Appendix 1b), suggesting this portion of the megablock was primarily dioritic, whereas the other ~11 m of the megablock is metamorphosed gabbro (Figure 3.19).
Figure 3.19: Bi-variate TAS diagram illustrating possible protoliths for the megablock amphibolite rock unit (after Middlemost, 1994) of the Eyreville-B borehole core. All data in wt%.
Determining the nature of the magma can assist with identifying the tectonic setting in which an igneous rock was derived. The megablock amphibolites have been plotted on an igneous AFM ternary diagram in Figure 3.20, in which A represents the alkalis, Na2O+K2O, F represents Fe2O3(total) and M represents MgO (Winter, 2001). The AFM diagrams in Figure 3.20 are formulated for bulk rock compositions and are different from those based on mineral chemistry as devised by Thompson (1957). Irving and Baragar (1971) observed that fields denoting magma series type can be plotted on this AFM diagram. In Figure 3.20, it is observed that most samples plot within the tholeiitic magma series field.
Figure 3.20. Ternary diagrams indicating AFM abundances (wt%) for the megablock amphibolite rock units rock unit of the Eyreville-B borehole core. Black dashed line shows the chemical boundary between tholeiitic and calc-alkaline magma series (after Irving and Baragar, 1971). Abbr.:
A (alkalis) = Na2O + K2O; F = Fe2O3(total); M = MgO.
The megablock amphibolite was plotted on Zr/Y versus Zr (Figure 3.21a; after Pearce et al., 1981) and TiO2 versus Zr (Figure 3.21b; after Pearce and Norry, 1979) diagrams to determine the possible tectonic setting in which the magma was formed. Although the samples do show much scatter, the trace element geochemistry suggests the upper megablock amphibolite formed within an island arc tectonic setting.
Figure 3.21: Discriminant diagrams showing the tectonic setting for the amphibolite rock units of the Eyreville-B borehole core. (a) TiO2 versus Zr diagram (after Pearce et al., 1981), and (b) Zr/Y versus Zr diagram (after Pearce and Norry, 1979). Abbr.: IAB = island arc basalt; OFB = ocean floor basalt; WPB = within-plate basalt; MORB = mid-oceanic ridge basalt.