SW13 - Layered microgabbro
Click the microscope button to view a thin section for this sample.
Click the microscope button to view a thin section for this sample.

Fact sheet

SW13 - Layered microgabbro

A microgabbro from the same intrusion as SW4, with prominent carbonate veins that impose a form of layering on the rock. This sample was collected from exposures near the bottom of Highbridge Road steps, at the same level as the canal, close to Brewin's Cutting.

This fine-grained igneous rock shows a fine igneous texture of interlocking crystals, including randomly-oriented plagioclase laths, some of which show lamellar twinning under XPL. The predominant greenish-brown colour of the thin section is caused by abundant serpentine minerals, notably replacing olivine and pyroxene crystals throughout the rock; you can still see the typical globular or oval shapes of the original olivine crystals. Opaque iron oxide grains are scattered across the whole section; these were likely magnetite in the original igneous assemblage, though in this sample much of the opaque material is very fine grained. At high magnifications, small interstitial patches of a colourless mineral occur, which under crossed polars can be seen to be composed of radiating fibrous crystals with first-order grey interference colours. This may be the zeolite mineral analcime, precipitated from diagenetic fluids in voids after the rock crystallized.

Irregular veins and smaller patches of pale crystalline material, with very high ('washed out') interference colours, are also common. These represent calcite precipitated from fluids passing through the rock, possibly the same pervasive fluids that caused the replacement of olivine by serpentine. Curiously, the diagonal veins seem to delineate portions of the rock that show different degrees of alteration. This could be due to several factors: perhaps the veins predated the fluid percolation and acted as a partially impermeable barrier to the fluids, reducing their effect in some layers; or perhaps the veins formed at the same time as fluid percolation and focused the fluid flow into narrower zones, leaving the rock nearby less heavily altered.

This microgabbro was intruded into the surrounding Carboniferous rocks about 307 million years ago, towards the end of the Carboniferous - perhaps not all that long after the surrounding sediments were laid down; there is evidence nearby that the magma interacted with still poorly consolidated, wet sediments. It is likely related to other examples of mafic intrusions and volcanic rocks scattered across central England, notably the volcanic tuffs and agglomerates at nearby Barrow Hill and Tansey Green quarries, less than 3 km to the north-west. Tansey Green is known for preserving trunks of conifer trees in growing position, which appear to have been rapidly buried by volcaniclastic deposits during eruptions at this time. The microgabbro at Brewin's Cutting was probably one of the subsurface conduits feeding these volcanoes.

This sample was collected as part of the 'Macro to Micro' project.

Additional images
  • Hand specimen of microgabbro on black background
52.486911, -2.094883
Exposures near Highbridge Steps, at canalside
About this collection

This Collection showcases the geodiversity of a classic geological site: the Saltwells National Nature Reserve in the West Midlands.

As well as displaying thin section and hand specimen views along with information setting them in the context of their landscapes, we also include perspectives and creative responses to the geological heritage of the sites from the local community.

Explore the stories of the rock layers at Saltwells and Wren's Nest NNRs, designed by students at King Edward VI School, Stourbridge:

This Collection was made possible by funding awarded to the 'Macro to Micro' project by the Natural Environment Research Council (NERC) under their 'Growing Roots' scheme.

Sample details

Collection: Saltwells
Rock-forming mineral
Category guide  
Category Guide
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Accessory minerals
Minerals that occur in very low abundance in a rock. They are usually not visible with the naked eye and contribute perhapssver, they often dominate the rare elements such as platinum group metals.
Rock-forming minerals
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A term used to group together related samples that are not already gathered into a single Collection. For instance, there is a ‘SW England granites’ theme that includes such rock types as granite, hydrothermal breccia, skarn and vein samples.
A general term used to label a rock sample. It is a useful way of grouping similar samples throughout a collection. Category names are often, but not exclusively, common rock names (e.g. granite, basalt, dolerite, gabbro, greisen, skarn, gneiss, amphibolite, limestone, sandstone).
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We would like to thank the following for the use of this sample: