Foliations and related structures

In Table 2.1, the average dip angles for foliations measured in the lower basement-derived and upper megablocks sections of the Eyreville-B borehole core are presented (see Appendix 1d for the full list of measurements). Owing to the fact that the true orientation of the borehole core could not be determined, measurements of the dip angle were the only data that could be obtained as there was no strike data available. Other than the massive granite in the upper granite megablock and the pegmatitic granite in the lower basement-derived section, the target rocks of the Eyreville-B borehole core contain at least one foliation. In the lower basement-derived metapelites, structural measurements for the prograde-to-peak mica foliation (S1a;

mean 54 ± 9°; Table 2.1; Figure 2.16a) shows that it dips at a moderate angle from the core axis.

Intrafolial fold hinges defined by either layering or quartz veins are observed in the lower basement-derived metapelites and can reach up to tens of centimetres in some cases (Figure 2.16a). The intrafolial folds tend to be shallowly inclined and have isoclinal to open interlimb angles, with some intrafolial folds in the graphite-rich mica schist containing secondary open

M-folds with wavelengths up to 10 cm in some cases (Figure 2.16a). In the vicinity of these intrafolial fold hinges zones of chaotic foliations are observed (Figure 2.16b). Although superficially resembling breccias, these graphite-rich zones typically contain crenulated minerals (e.g. muscovite) and plagioclase porphyroblasts as well as a highly micaceous matrix (Figure 2.16b and c). The localised nature of these zones (only within fold hinges) combined with the presence of graphite and the absence of significant retrogression of the mica and plagioclase grains suggests that the zones may be related to localised strain partitioning and slip facilitated by graphite and phyllosilicates during the prograde metamorphic path close to peak conditions.

Table 2.1: Average orientation angle of foliations measured from the core axis for the metapelites from the lower basement-derived section (LB), the amphibolite from the upper megablock

A mylonitic foliation (S1b) in the lower basement-derived section, defined by a heterogeneous zone of alternating bands of granite and mica schist, is observed mainly between 1640 m and 1655 m (Figure 2.17a), although mylonitisation is also seen locally above and below this zone.

Dipping at a shallow angle from the core axis (S1b; mean 71 ± 7°; Table 2.1), this foliation is generally observed to be oriented subparallel to the schistose (S1a) foliation (Figure 2.17b) and is associated with an oblique dip-slip mineral lineation. Mylonitised mica schist samples are light brown (Figure 2.17b), owing to the extensive retrogressive replacement of biotite by chlorite and muscovite, whereas mylonitised granite is much more fine-grained compared with pegmatitic veins in other parts of the lower basement-derived section (Figure 2.17c). Lister and Snoke (1984) defined mylonites that form under retrograde conditions and exhibit non-coaxial laminar

Figure 2.16: (a) Portion of corebox 321 from the lower basement-derived section showing the prograde foliation (S1a) as well as intrafolial folds and QV within the MS that is locally intruded by GR. Photomicrographs of sample RG41 showing (a) the locally chaotic foliation adjacent to intrafolial folds (XPL) and (c) the lack of enhanced retrogression despite Gr, Cal and Py occurring in interclast regions (PPL). Abbr.: Cal = calcite; GR = granite vein; Gr = graphite;

MS = mica schist; Ms = muscovite; Pl = plagioclase; PPL – plane polarised light; Py = pyrite; Qz = quartz; Tur = tourmaline; XPL = cross-polarised light.

Figure 2.17: Photographs of the S1b foliation in the lower basement-derived mylonite. (a) Corebox 310 showing the mylonite zone with interlayered GR, MS and QV. (b) Hand specimen of sample RG56 showing the S1a and S1b foliations in the retrogressed MS. (c) Hand specimen of sample RG40 showing the S1b foliation in the fine-grained mylonitised Gr. (d) Photomicrograph (PPL) of sample RG57 showing the S1a foliation defined by elongated Pl and Py inclusions in Tur. Abbr.: GR = granite; MS = mica schist; Pl = plagioclase; PPL = plane polarised light; Py = pyrite; QV = quartz vein; Tur = tourmaline; Zrn = zircon.

flow microstructures (e.g. mica fish, S-C fabric; see Section 2.4.3) as Type II S-C mylonites. The fact that the basement granite does not contain the S1a foliation but is affected by the retrogressive mylonitic S1b foliation indicates that the granite intruded the mica schists after peak conditions had mostly passed but before the onset of the retrograde metamorphism in the latter part of D1. This is confirmed by the presence of metasomatic tourmaline aggregates overgrowing the prograde foliation in the wall rocks to granite veins, where elongated inclusions within the tourmaline preserve the S1a foliation (Figure 2.17d).

The dip magnitude of the gneissic foliation in the upper megablock section (mean 60 ± 6° from the core axis; Table 2.1) is similar to that of the lower basement-derived mica foliation (Figure 2.18). A weak mineral lineation (L1b) subparallel to the dip is also observed along fractures (see Section 2.4.2). Rare, thin (<10 cm), mylonitic zones occur locally (e.g. at 1096.59 m in core-box 119 and at 1359.80 m in core-box 216). Unfortunately, the absence of clear evidence that the gneissic foliation developed only during shearing, or that the mylonite formed during a separate event, prevents correlating these foliations with either S1a or S1b in the lower basement-derived section.

The foliation in the granite gneiss is interpreted to be tectonic rather than magmatic based on the presence of aligned biotite and amphibole monomineralic and polymineralic aggregates (more typical of metamorphic foliations; see Section 2.4.3), strain-related recrystallisation and elongation of K-feldspar and quartz grains (see Section 2.4.3) as well as the presence of local mylonite zones also relating to heterogeneous strain conditions (Paterson et al., 1989). The generally massive granite (Figure 2.18), on the other hand, shows a weak foliation defined by K-feldspar and/or biotite adjacent to the gneiss. However, the lack of significant crystal plastic flow in this granite suggests that it is more likely that the weak foliation was not caused by the same process as the gneissic granite but rather by magmatic flow (Paterson et al., 1989).

Figure 2.18: Photograph of corebox 160 showing the gneissic foliation in GN and the massive nature of GR in the upper granite megablock. Abbr.: GN = gneissic megablock granite; GR = massive megablock granite.

The foliation in the biotite schist xenoliths in the upper granite megablock is defined by fine- to medium-grained biotite (Figure 2.19a) and is oriented 48 ± 9° (measured from the core axis;

Table 2.1). This is more steeply dipping than that seen in the gneissic granite variety of the megablock section. The upper amphibolite megablock includes a foliation, oriented at 49 ± 10 (measured from the core axis; Table 2.1; Figure 2.19b) as well as a lineation oriented obliquely to the amphibolite foliation (Figure 2.19c) on dip-slip slickenside fractures (see Section 2.4.2) based on the present orientation of the core. Although the amphibolite is strongly foliated in most of the amphibolite megablock, samples RG03 (1387.51 m depth) and RG06 (1387.24 m depth) are enriched in plagioclase and contain a coarse-grained, phaneritic texture that may have been inherited from the igneous protolith (Figure 2.19d; see Section 2.3.2). These observations correspond with those made for the amphibolite megablock between 1386.93 m and 1387.54 m depth by Horton et al. (2009a), who interpreted the relict igneous texture to be either metagabbroic or metadioritic in nature.

Figure 2.19: Hand specimen photographs of (a) sample RG172 showing the biotite-defined foliation in the biotite schist xenoliths in the upper granite megablock, (b) sample RG02 showing the foliation in the upper amphibolite megablock that is locally cut by Cal and Qz veins, (c) sample RG176 showing the mineral lineation relative to the orientation of the core in the upper amphibolite megablock, and (d) sample RG06 showing the coarse-grained, phaneritic texture preserved in parts of the upper amphibolite megablock. Abbr.: Cal = calcite; Qz = quartz.