Southdean Law

Southdean Law is approximately 7.5 miles south of Jedburgh on the A6088, the Carter Bar to Bonchester Bridge road. Quarrying here for road stone has opened up the elliptically shaped volcanic plug and its vertical contacts with the local Old Red Sandstone. There are two abandoned quarries, an earlier one on the western side and a later, more accessible one by the road on the southern side.

Map showing locations, sample points and rock types around Southdean Law
Key to the map of Catcleugh Shin, Southdean Law, Bonchester Hill

The structure of the plug is interesting on account of two distinct sets of jointing identified by S.I. Tomkeieff. and described in his 1949 paper to the Edinburgh Geological Society. He observed a vertical set of joints that radiated from the centre of the vent and a second set that followed the form of a bell-like paraboloid up through the vent. The two sets together were said to give the intrusion the appearance of a ‘convex black wall’ with curved jointing that was intersected by vertical projections with vertical joints. A photograph in the same paper illustrated this.
Tomkeieff argued the ‘convex of paraboloid to top’ (hanging bell) jointing of Southdean Law’s intrusion means that we are seeing the lower region of a plug because, in his experience, jointing in the the upper regions of plugs tends to have a ‘convex of paraboloid to bottom’ (upturned bell) configuration. Further, he noted that the level of the plug at Southdean is well below the level of the area’s coeval alkali lavas.
Currently it is hard to see these two distinct jointing systems. Instead, jointing seems to conform to a roughly radial pattern set at a steep angle inwards.

Southdean Law south quarry c.1949

Southdean Law south quarry c.1949
This photograph does seem to show a combination of curved and vertical jointing systems in this Southdean rock.

Southdean Law (South quarry) NT 634093

Southdean Law south quarry, 2017
Two distinct jointing systems are not so apparent now. Jointing appears to radiate conically at a steep angle from the centre of the plug.

The rock is interesting because it is nepheline basanite, a type of rock similar to the pyroxene-rich Hillhouse type of basalt. Both rocks are almost equally alkali but the basanite is far poorer in silica resulting in the occurrence of low-silica feldspathoids in the basanite but not in the basalts.
Basanites are most commonly associated with continental rift and ocean island magmatism. They are sometimes parental magmas to a crystal fractionation series that leads to phonolites.

QAPF diagram showing the relationship of basanite’s composition to other extrusive rocks.

QAPF Basanite

Petrography of the Southdean Law nepheline basanite

Southdean Law’s basanite is a fine-grained alkaline igneous rock with olivine and pyroxene phenocrysts. Its groundmass is reported to be pyroxene, feldspar, nepheline and iron ore.

Nepheline basanite - dark, fine-grained basaltic rock containing  nepheline, olivine, pyroxene, plagioclase and iron oxides, Southdene S. Quarry (centre) NT634093 RL 40mm across

Nepheline basanite from Southdean Law, south quarry NT634093
Prepared hand specimen measuring 40mm across, viewed in reflected light.

Nepheline basanite, Southdene Quarry PPL

Nepheline basanite, Southdean Quarry
Thin section from the same sample viewed in plane polarised light

Nepheline basanite, Southdean Law south quarry XP

Nepheline basanite, Southdean Law south quarry
The same sample viewed with crossed polarising filters

The poorly formed or partly resorbed olivine crystals in the Southdean rock have been wholly or partly altered to clay materials that include serpentine.

Altered olivine with biotite in an fine-grained groundmass. Thin section viewed in plane polarised light. (FoV 1.2 x 0.8 mm)

Altered olivine with biotite in an fine-grained groundmass
Thin section viewed in plane polarised light. (FoV 1.2 x 0.8 mm)

Altered olivine, biotite in a fine-grained groundmass viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area of the thin section viewed with crossed polarising filters

Altered olivine in nepheline basanite,  in feldspar and nepheline groundmass, Section viewed in plane polarised light.  (FoV 4.6 x 2.9 mm)

Altered olivine the in feldspar and nepheline groundmass
Crystals in the same section viewed in plane polarised light. (FoV 4.6 x 2.9 mm)

Altered olivine in nepheline basanite,  in feldspar and nepheline groundmass, Section viewed with crossed polarising filters  (FoV 4.6 x 2.9 mm)

The same area viewed with crossed polarising filters

Altered olivine in nepheline basanite with scattered biotite.  Sample from a second thin section viewed in plane polarised light. (FoV 2.3 x 1.5 mm)

Altered olivine in nepheline basanite with scattered biotite
Sample from a second thin section viewed in plane polarised light. (FoV 2.3 x 1.5 mm)

Altered olivine in nepheline basanite with scattered biotite.  Sample from a second thin section viewed with crossed polarising filters. (FoV 2.3 x 1.5 mm)

The same crystals viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

Pyroxenes crystals often occur in glomerocrysts. They tend to be prismatic, twinned and often show zoning.

Sector-zoned clinopyroxene with olivine and biotite in nepheline basanite.  Section viewed in plane polarised light. (FoV 4.5 x 3.0 mm)

Sector-zoned clinopyroxene with olivine and biotite in nepheline basanite
Section viewed in plane polarised light (FoV 4.5 x 3.0 mm)

Sector-zoned clinopyroxene with olivine and biotite nepheline basanite. Section viewed with crossed polarising filters.

The same area viewed with crossed polarising filters (FoV 4.5 x 3.0 mm)

Pyroxene occurs in the groundmass as very small and stumpy prisms. Plagioclase feldspars tend to be oligoclase to andesite in composition occurring as laths and plates formed in close association with the pyroxenes. Tiny grains of iron oxides are scattered throughout the groundmass . Apatite occurs sparsely.

A pyroxene and feldspar glomerocryst enclosing a clear, polygonal-shaped area with many inclusions. Section viewed in plane polarised light. (FoV 1.2 x 0.8 mm)

A pyroxene and feldspar glomerocryst enclosing a clear, polygonal-shaped area with many inclusions
Section viewed in plane polarised light. (FoV 1.2 x 0.8 mm)

The dark, complex-twinned,  polygonal-shaped crystal at the centre of this glomerocryst is full of inclusions and has a feldspar phenocryst at its centre. We think it is most probably leucite although  it could be analcime. Section viewed with crossed polarising filters. (FoV 1.2 x 0.8 mm)

The dark, complex-twinned, polygonal-shaped crystal at the centre of this glomerocryst is full of inclusions and has a feldspar phenocryst at its centre
We think it is most probably leucite although it could be analcime. Section viewed with crossed polarising filters.(FoV 1.2 x 0.8 mm)

The last portion of the rock to crystallise, the mesostasis, occurs in patches. It is made up of hexagonal crystals and larger plates of clear to turbid nepheline that can poikilitically enclose crystals of pyroxene, feldspar and iron ore.
We have found one very small area of what we think is leucite. It certainly has leucite’s typically low birefringence, crystal shape and tartan twinning but we read that analcime can occasionally present the same properties.

Leucite in Southdean Law’s nepheline basanite
The video shows tartan twinning appearing in the very low birefringent mineral occupying the area at the centre of the frame as the microscope stage is rotated.

In his 1949 paper, Tomkeieff referred to the work of another early investigator of Scottish Borders geology, Rachel Workman McRobert. He duly noted her correct identification of Southdean Law rock as nepheline basanite but pointed out that she was mistaken in thinking that the intrusion was a sill. He also questioned her identification of biotite in the rock;
‘She… mentions altered olivine moulded by biotite. In my collection of slides no biotite appears in this rock and it may be that it was iddingsite, which is sometimes observed as a product of alteration of olivine.’
It is late in the day, but here are three photomicrographs that would seem to support McRobert’s observation.

Biotite in nepheline basanite showing maximum pleochroic colour parrallel to polariser. Viewed in plane polarised light (FoV 1.2 x 0.8mm)

Biotite in nepheline basanite showing maximum pleochroic colour parallel to polariser
Viewed in plane polarised light (FoV 1.2 x 0.8mm)

Biotite showing minimum pleochroic colour at 45 degrees to the polariser. Viewed in plane polarised light (FoV 1.2 x 0.8mm)

Biotite showing minimum pleochroic colour at 45o to the polariser
Viewed in plane polarised light (FoV 1.2 x 0.8mm)

Biotite showing typical interference colours. Viewed with crossed polarising filters (FoV 1.2 x 0.8mm)

Biotite showing typical interference colours
Viewed with crossed polarising filters (FoV 1.2 x 0.8mm)

Chlorite occurs in all of our samples of Southdean rock. Vesicles are not so common in our samples but perhaps they are more common at the edge of the intrusion where we didn’t take samples. The vesicles we do see are often filled with fibrous minerals as shown below.

Chlorite in nepheline basanite viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Chlorite in nepheline basanite viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Chlorite in nepheline basanite viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

The same patch of chlorite viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

Vesicle in nepheline basanite viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Vesicle in nepheline basanite viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Vesicle in nepheline basanite viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

The same vesicle viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

References

MacAdam A.D., Clarkson E.N.K., Stone, P. (editors) 1993 .Scottish Borders geology: an excursion guide. Edinburgh, Scottish Academic Press.

Tomkeieff S.L. 1949. Nepheline-Basanite of Southdean. Transactions of the Edinburgh Geological Society, pp349-359. Oliver and Boyd, Ltd. Edinburgh, 1952.

BGS online Geology of Britain http://mapapps.bgs.ac.uk/geologyofbritain/home.html

No vestige of a beginning, – no prospect of an end

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