NM6224 : Pelite Xenolith in Gabbro

taken 7 years ago, near to Lochbuie, Argyll And Bute, Great Britain

Pelite Xenolith in Gabbro
Pelite Xenolith in Gabbro
This outcrop tells an interesting story to a geologist.

The surrounding rock is gabbro, which is an igneous rock formed from molten magma which has cooled slowly inside the crust of the Earth, and is therefore coarse-grained, because there was time for crystals to grow before it solidified. (If a magma of similar chemical composition cools quickly, the result is a fine-grained rock which is termed basalt.)

The brownish pod in the middle is pelite, which is a metamorphic rock formed by heat and pressure from muddy sediment.

A xenolith is a piece of existing rock which has got caught up in magma.

There are two specific features of this xenolith, and its fellows nearby, which are of note.

First, it has a pale rim round it. This has been formed by aluminium diffusing out from the pelite and calcium diffusing inwards from the gabbro. These combine with silica, which is present in most rocks, to form a type of feldspar called anorthite. Rock formed mainly of anorthite is termed anorthosite. (By way of a digression, the paler parts of the surface of the Moon are anorthosite).

Second, the rims of most of these xenoliths are unbroken. This means that they were not being rolled around by currents within the magma, but cooled and formed in situ, so by inference the magma was static as it cooled.
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NM6224, 21 images   (more nearby )
Photographer
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Date Taken
Tuesday, 29 May, 2012   (more nearby)
Submitted
Wednesday, 6 June, 2012
Geographical Context
Geological interest 
Subject Location
OSGB36: geotagged! NM 6231 2437 [10m precision]
WGS84: 56:21.1074N 5:50.9225W
Camera Location
OSGB36: geotagged! NM 6231 2437
View Direction
NORTH (about 0 degrees)
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Other Tags
Pelite  Anorthite  Anorthosite  Gabbro  Basalt  Xenolith  Magma  Silica  Mudstone  Aluminium  Calcium  Feldspar 

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