Garnet gneiss - Roneval
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Fact sheet
Description: 

This garnet-rich, high-grade Lewisian rock comes from Roneval on South Harris in the Western Isles of Scotland. The rock was subjected to deep burial and temperatures exceeding 800 °C during the Proterozoic Laxfordian orogeny. The origin of this rock is difficult to determine but it may have been a granite intruded into the Archean Lewisian prior to regional metamorphism. 

In thin section the rock is dominated by garnet, amphibole and plagioclase feldspar, with some scapolite. The garnet grains are surrounded by particularly good examples of symplectite (an intergrowth of two minerals at a scale of a few microns formed when another mineral breaks down) texture, and are strongly denuded by reactions that occurred as the rock travelled from its deepest point towards the surface, cooling and decompressing to form plagioclase and pyroxene (later altered to amphibole).

In Place
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Location description: 
Abhainn Thorro, SW side of Roneval (=Roinebhal), South Harris, Western Isles, Scotland
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Poor
Timescale:
Timescale: 
Archean
Ma = Millions of years ago
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metamorphic
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