Structural correlation between serpentinite and rodingite dykes
The foliation trajectory map (Fig. 2) constitutes the chronological reference frame for the relative timing of foliation development; grids of successive foliations transecting lithologic boundaries played a relevant role for the correlation of mineral assemblages within different bulk chemistry (e.g. rodingites and serpentinites), since the whole deformation history is recorded both in serpentinites and their rodingitised gabbro dykes.
The orientation of rodingite dykes is mainly parallel to the pervasive S2 or composite S1/S2 foliations (Fig. 3), as dykes are transposed into these foliation attitudes. Usually the dominant structure in the dykes is the S2 foliation, which is physically continuous from serpentinites to rodingites (Figs. 5a, b). Only locally rodingites record an S1 foliation that can be overprinted by an S2 crenulation foliation (Fig. 5c). Late-D2 shear zones affect the margin of rodingite dykes (Fig. 5d). Locally D3 is responsible for an intense folding of the dykes producing an S3 crenulation cleavage that from serpentinites extends into rodingites or affects only the reaction rims between the two rock types (Fig. 5e). D4 may be sporadically responsible for a gentle folding of the dykes. Generally, structures extending from serpentinites to rodingites can be continuously followed and correlated along outcrops up to about 50 m long and the regional scale correlation is supported by the 1:5,000 scale mapping, synthesised in Fig. 2. Significant examples are the detailed structural sections on the slopes above Singlin Dèsot and Pesontsé (*1 and *2 respectively in Fig. 6), whose location is shown on the map of Fig. 2. The stereoplots on Fig. 6 show that orientation data of planar and linear structures in serpentinite are coherent with those in rodingites.
Figure 5. Structures affecting serpentinite and rodingite
a) Rodingite dyke isoclinally folded by D2. North of Pesontsé. b) Close up showing the pervasive S2 foliation continuous from serpentinite to rodingite. c) S1 in rodingite dyke overprinted by S2 and folded by D3. South of Dzilles. d) Late-D2 normal shear zone involving a rodingite dyke. Slope above Singlin Dèsot. e) Refracted S3 crenulation cleavage across serpentinite - rodingite reaction rim. S3 is not recorded in the rodingite dyke core. Slope above Singlin Dèsot.
Figure 6. The shared structural history of serpentinites and rodingites
Natural structural cross-sections *1 and *2 (located in the regional scale map of Fig. 2) objectively showing close ups of the common structural history of serpentinites and rodingites. Lower hemisphere equal area stereo plots report in detail the orientation data of planar and linear structures. Foliation trajectories and fold axial plane traces are in black and grey respectively, with 1, 2 or 3 dots indicating their relative chronology. The position of the field photos (Figs. 5d and 5e) is indicated in the cross-section *1.