A to Z of North Wales Geology
D is also for..............DOLERITE
Palaeozoic dolerite dyke, Point Lynas, Anglesey
Palaeozoic dyke intruding schists of the
New Harbour Group, Point Lynas, Anglesey
Photo © Rob Crossley 2005,
Petrographic Section by Fugro-Robertson Ltd.
Tertiary dolerite dyke, Holland Arms, Anglesey
Tertiary dyke intruding schists of the
Berw Shear Zone, Anglesey
Photo © Rob Crossley 2005,
Petrographic Section by Jonathan Wilkins.

Dykes formed from a basic magma (similar to a basalt lava) are commonly described as being of DOLERITE, or DIABASE according to some authors. Because they were able to cool more slowly than erupted lavas they have generally coarser and more complex textures, and frequently are found as GABBRO which is just a fancy name for a coarse-grained igneous rock of basic (silica-poor) composition. Logically, Dolerite should be called MICROGABBRO, but the old name stuck. Such a rock comprises essential CLINOPYROXENE, iron-oxides and a calcium-rich PLAGIOCLASE feldspar - note that OLIVINE is not essential, but will often be present if the overall composition is sufficiently basic.

The right-hand image above is therefore an OLIVINE DOLERITE and is a quite superb example. Olivine shows brilliant turquoise colours and thick black crack marks where iron-oxide (magnetite) has been ejected from the crystal as it has cooled in the magma. Clinopyroxene is pale grey in this picture but has a distinctive rectangular cleavage. Close examination shows that it encloses plagioclase (showing dark-grey 'humbug' stripes). This is an ophitic texture - which is a defining feature of dolerite to some authors. The irregular black blobs are more iron oxides (magnetite) which are opaque. Note the generally fresh appearance of the mineral grains.

In contrast, the left-hand image above is frankly a mess. None of the original minerals are recognisable here, or in the thin-section come to that! The only clear feature is a vein of quartz running across diagonally, and that only indicates that the rock has suffered deformation long after it cooled. All of the iron and magnesium-bearing minerals have been broken down to a mess of chlorite and clay and the feldspar has been degraded to a sericite with abundant iron oxides. The key reason for this is that it has suffered a low-grade metamorphism - insufficient to cause the growth of new high- temperature minerals, but sufficient to cause the breakdown of the original magmatic assemblage. Clearly this rock is old enough to have experienced the Caledonian Orogeny, while the first is not. The other key observation is that the first dyke can be seen to intrude the Carboniferous Limestone of southern Anglesey, which makes it post-Caledonian.

There - that wasn't too hard, was it? Unlike the boulder from Holland Arms, which took my Dad nearly an hour to break into as a first step to making the thin-section..............................

REFRENCES
- Petrology of the Igneous Rocks (13th Edition), Hatch, Wells & Wells, 1972

©Jonathan Wilkins, 03.2006