XenolithsThis is a specimen from the oldest rock suite in Britain, the 3 Ga Lewisian Gneiss group an

XenolithsThis is a specimen from the oldest rock suite in Britain, the 3 Ga Lewisian Gneiss group and was found near Ullapool in the Scottish Highlands. Compared with most specimens of Lewisian gneiss, this specimen is of particular interest. After its initial formation the gneiss was intruded by mafic magmas, part of the Scourie Dyke intrusions(dated around 2.4Ga) with temperatures up to ~1300°C. At the margins of the dykes the cooler country rock (gneiss) fractured and separated, becoming incorporated into the liquid magma where it was locked in place as the magma cooled.These pieces are known as xenoliths – inclusions of compositionally different rock within an igneous body.Xenoliths can provide important information to geologists, such as samples for radiometric dating to acting as depth indicators. This particular xenolith tells us that the dyke likely intruded into cold rock before cooling rapidly as the fragment does not appear to have degraded relative to the source rock, a fact supported by the small crystal size within the dyke. If intruded into a hot rock, this xenolith would be more likely to partially melt or mix with the dyke, clouding its original characteristics.More information about the Lewisian group and the rocks of NW Scotlandhttp://bit.ly/1CTV8DePetrology of Scourie Dykeshttp://bit.ly/1HHj2R6Image: The author (RJW) -- source link
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