Troctolite from the Rum Central Complex



The information below is taken mainly from the following websites:
(The photo’s belong to the web editor).

[PDF]Golden Rum! Understanding the Forbidden Isle,_Hebridean_Igneous_Province

          The Rum Central Complex is associated with the Hebridean Igneous Province which consists mainly of an outpouring of volcanic rocks which took place about 60 Ma., Tertiary (Palaeogene) times. The Complex includes a series of cyclic layered intrusions formed within a magma chamber, each “layer” consisting of a peridotitic base and a gabbroic to troctolitic upper part. Lawrence Wager, Malcolm Brown, and John Wadsworth suggested that these layered intrusions, later to be termed igneous cumulates, formed by crystal settling processes in a magma chamber which was being regularly replenished by basaltic magmas. Hence the troctolite forms part of a suite of cumulate rocks, which are the typical product of precipitation of solid crystals from a fractionating magma chamber. These accumulations typically occur on the floor of the magma chamber, although they are possible on the roofs if anorthite plagioclase is able to float free of a denser mafic melt.


Polished section of troctolite showing layered structures which vary in scale from tens of metres to millimetres.

Thin section, ppl, of troctolite consisting of plagioclase feldspar (bytownite?). Coloured minerals are mainly olivine, (fayalite?) with subordinate clinopyroxene. The opaques are probably chrome-spinel.

The same section as on left, under crossed polars. The preferred orientation of the feldspars, due to fractionation from the melt and settling, is apparent.

Troctolite (highly magnified) Field of view 2.5 mm

As photo on left, but taken with crossed polars.




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