Granodiorite – Loch Doon
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Fact sheet

Granodiorite – Loch Doon

This sample of granodiorite from the Loch Doon pluton in Dumfries & Galloway, Scotland formed around 408 million years ago during the Devonian period, late in the Caledonian orogeny. The late Caledonian Loch Doon granitic intrusion ranges in composition from hypersthene diorite at the margin, through quartz diorite, granodiorite and granite to cordierite microgranite at its core. The compositional variation observed in this intrusion indicates that two distinct magmas were involved, and variations were induced by crystallization. As a granodiorite it contains higher proportions of plagioclase feldspar than orthoclase and greater than 20% quartz.

In thin section the rock exhibits a course grained igneous texture including compositionally zoned plagioclase, together with minor amounts of microcline and quartz. The mafic minerals are dominated by biotite which is intergrown with pale green amphibole. Both biotite and amphibole are pleochroic although the amphibole is only weakly pleochroic. Accessory minerals include zircon which forms pleochroic halos in biotite, and apatite.

Additional images
  • granodiorite - width 2 cm
  • granodiorite - width 5 cm
55.073748, -4.401913
Loch Doon, Dumfries & Galloway, Scotland
About this collection

The United Kingdom Virtual Microscope (UKVM) collection consists of igneous, sedimentary and metamorphic rocks from around the UK.

It is intended as a teaching resource, helping to tell the story of the common rock types and how they form, and reflecting the history of the UK at the margins of the continent of Europe. The collection is a series of teaching sets, for example igneous rocks from the North Atlantic Igneous Province and SW England; high-temperature metamorphic rocks from Scotland and low-temperature metamorphic rocks from Wales; and sedimentary rocks, including English limestones and sandstones.

Sample details

Rock-forming mineral
Accessory minerals
Category guide  
Category Guide
Refers to any word or phrase that appears in the individual rock names. Names are generally descriptive; they allow users to search for broad terms like ‘granite’ as well as more specific names such as ‘breccia’. However, the adjacent descriptions of the specimens captures a wider range of general words and phrases and is a more powerful search tool.
Refers to any word or phrase that appears anywhere in the descriptions of the specimens
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
Minerals that make up the bulk of all rock samples and are also the ones used in rock classi?cation.
Selecting one or more period, for example 'Jurassic'.
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).
The owner of the sample that appears in the collection. For example, NASA owns all the samples that appear in the Moon Rocks collection
We would like to thank the following for the use of this sample: