M01. Amphibolite - Lough Nagilly, Donegal
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Fact sheet
Description: 

An amphibolite raft in the Thor Granite (Caledonian). The sample was collected from a huge xenolith, or raft, enclosed within the Thor pluton, which is one of the component plutons of the Donegal Granite batholith. The random orientation of the hornblende and plagioclase grains is consistent with thermal metamorphism, with no directed stress, and the brown colour of the hornblende suggests that the conditions were at the high-temperature end of the spectrum of amphibolite-facies conditions. The protolith was a basic intrusion from about 600 Ma which was one of many such intrusions invading the Dalradian Supergroup sedimentary rocks before they were first metamorphosed during the Grampian Orogeny (470 Ma). This amphibolite was totally recrystallized by the heat from the surrounding granite about 400 Ma. Rotation 1 - Best cross section of hornblende with two sharp cleavage traces and symmetric extinction. Rotation 2 - Twinned hornblende 15° extinction to twin plane. Also twinned plagioclase.

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Timescale: 
Proterozoic
Ma = Millions of years ago
In Focus
Type:
Type: 
metamorphic
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