Editing Glen Bay Gabbro - St. Kilda: an illustrated account of the geology

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[[File:P991510.jpg|thumbnail|Figure 13A Chilled margin of the Glen Bay Gabbro. Xenocrysts of plagioclase and olivine lie in a groundmass of plagioclase, pyroxene and opaque minerals. Both xenocrysts are rounded and corroded; the plagioclase has provided a nucleus for the growth of more sodic plagioclase (pale rim), and pyroxene has nucleated on the olivine. (S64879A), 150m south of tunnel at Gob na h-Airde; cross polarised light; area shown represents 4 mm across.]]
 
[[File:P991510.jpg|thumbnail|Figure 13A Chilled margin of the Glen Bay Gabbro. Xenocrysts of plagioclase and olivine lie in a groundmass of plagioclase, pyroxene and opaque minerals. Both xenocrysts are rounded and corroded; the plagioclase has provided a nucleus for the growth of more sodic plagioclase (pale rim), and pyroxene has nucleated on the olivine. (S64879A), 150m south of tunnel at Gob na h-Airde; cross polarised light; area shown represents 4 mm across.]]
 
[[File:P991511.jpg|thumbnail|Figure 13B Gabbro consisting largely of plagioclase (2 mm long), rounded olivine enclosed in a mass of smaller pyroxenes (pale brown) and opaque minerals. Brown biotite mantles some of the opaque grains and prismatic apatite lies adjacent to the subhedral plagioclase. 565208, 300 m south of tunnel at Gob na h-Airde; plane-polarised light.]]
 
[[File:P991511.jpg|thumbnail|Figure 13B Gabbro consisting largely of plagioclase (2 mm long), rounded olivine enclosed in a mass of smaller pyroxenes (pale brown) and opaque minerals. Brown biotite mantles some of the opaque grains and prismatic apatite lies adjacent to the subhedral plagioclase. 565208, 300 m south of tunnel at Gob na h-Airde; plane-polarised light.]]
[[File:P991512.jpg|thumbnail|Figure 13C Gabbro rich in magnetite, ilmenite (opaque) and augite pyroxene (blue and yellow interference colours). Apatite is abundant as inclusions (grey) in the opaque minerals and its hexagonal (dark) outlines are visible next to pyroxene. Sinuous lines of crushed material penetrate the rock. Although this comes from an isolated block on Mullach Sgar, it is the same as some facies of the gabbro on the western side of Glen Bay. (S68268), cross-polarised light.]]
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[[File:P991512.jpg|thumbnail|Figure 13C Gabbro rich in magnetite, ilmenite (opaque) and augite pyroxene (blue and yellow interference colours). Apatite is abundant as inclusions (grey) in the opaque minerals and its hexagonal (dark) outlines are visible next to pyroxene. Sinuous lines of crushed material penetrate the rock. Although this comes from an isolated block on Mullach Sgar, it is the same as some facies of the gabbro on the western side of Glen Bay. 568268, cross-polarised light.]]
  
 
== Chapter 7 Glen Bay Gabbro EG ==
 
== Chapter 7 Glen Bay Gabbro EG ==
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Chemically, the Glen Bay Gabbro is richer in TiO<sub>2</sub>, K<sub>2</sub>O, P<sub>2</sub>O<sub>5</sub> and rare earth elements than are earlier gabbros and dolerites on St Kilda (E<sup>''W''</sup>, D and EK), and it almost certainly has a different source. It marks a separate stage in the evolution of the St Kilda volcanic centre and may represent part of a magma chamber developed at a relatively high level in the earth's crust. One way of explaining the unusually thick chilled margin of the Gabbro is by postulating that a large block or segment of cold gabbro-dolerite breccia (EK) collapsed into such a magma chamber, following perhaps a particularly violent surface eruption.
 
Chemically, the Glen Bay Gabbro is richer in TiO<sub>2</sub>, K<sub>2</sub>O, P<sub>2</sub>O<sub>5</sub> and rare earth elements than are earlier gabbros and dolerites on St Kilda (E<sup>''W''</sup>, D and EK), and it almost certainly has a different source. It marks a separate stage in the evolution of the St Kilda volcanic centre and may represent part of a magma chamber developed at a relatively high level in the earth's crust. One way of explaining the unusually thick chilled margin of the Gabbro is by postulating that a large block or segment of cold gabbro-dolerite breccia (EK) collapsed into such a magma chamber, following perhaps a particularly violent surface eruption.
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{| class="wikitable"  
 
{| class="wikitable"  
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|-  
 
|-  
 
|  | Major elements Oxide wt %
 
|  | Major elements Oxide wt %
 +
||
 
||  
 
||  
 
|  | Minor elements (ppm)
 
|  | Minor elements (ppm)
 +
||
 
||  
 
||  
 
|-  
 
|-  
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|  | Li
 
|  | Li
 
|  | 7
 
|  | 7
 +
|  |
 
|-  
 
|-  
 
|| TiO<sub>2</sub>
 
|| TiO<sub>2</sub>
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|  | F
 
|  | F
 
|  | 550
 
|  | 550
 +
|  |
 
|-  
 
|-  
 
|| A1<sub>2</sub>O<sub>3</sub>
 
|| A1<sub>2</sub>O<sub>3</sub>
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|  | S
 
|  | S
 
|  | 650
 
|  | 650
 +
|  |
 
|-  
 
|-  
 
|| Fe<sub>2</sub>O<sub>3</sub>
 
|| Fe<sub>2</sub>O<sub>3</sub>
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|  | V
 
|  | V
 
|  | 340
 
|  | 340
 +
|  |
 
|-  
 
|-  
 
|  | FeO
 
|  | FeO
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|  | Cr
 
|  | Cr
 
|  | 25
 
|  | 25
 +
|  |
 
|-  
 
|-  
 
|  | MnO
 
|  | MnO
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|  | Co
 
|  | Co
 
|  | 52
 
|  | 52
 +
|  |
 
|-  
 
|-  
 
|  | MgO
 
|  | MgO
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|  | Ni
 
|  | Ni
 
|  | 16
 
|  | 16
 +
|  |
 
|-  
 
|-  
 
|  | CaO
 
|  | CaO
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|  | Cu
 
|  | Cu
 
|  | 24
 
|  | 24
 +
|  |
 
|-  
 
|-  
 
|| Na<sub>2</sub>O
 
|| Na<sub>2</sub>O
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|  | Zn
 
|  | Zn
 
|  | 160
 
|  | 160
 +
|  |
 
|-  
 
|-  
 
|| K<sub>2</sub>O
 
|| K<sub>2</sub>O
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|  | Rb
 
|  | Rb
 
|  | 20
 
|  | 20
 +
|  |
 
|-  
 
|-  
 
|| H<sub>2</sub>O+110
 
|| H<sub>2</sub>O+110
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|  | Sr
 
|  | Sr
 
|  | 250
 
|  | 250
 +
|  |
 
|-
 
|-
 
|  | H<sub>2</sub>O-110
 
|  | H<sub>2</sub>O-110
 
|  | 0.1
 
|  | 0.1
 
|  |  
 
|  |  
|  | Y
+
|  |  
 
|  | 54
 
|  | 54
 +
|  |
 
|-  
 
|-  
 
|| P<sub>2</sub>O<sub>5</sub>
 
|| P<sub>2</sub>O<sub>5</sub>
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|  | Zr
 
|  | Zr
 
|  | 265
 
|  | 265
 +
|  |
 
|-
 
|-
 
|  |  
 
|  |  
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|  | Nb
 
|  | Nb
 
|  | 30
 
|  | 30
 +
|  |
 
|-
 
|-
 
|  |  
 
|  |  
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|  | Ba
 
|  | Ba
 
|  | 590
 
|  | 590
 +
|  |
 
|-
 
|-
 
|  |  
 
|  |  
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|  | Pb
 
|  | Pb
 
|  | 15
 
|  | 15
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|  |
 
|-  
 
|-  
 
| colspan="6" | * Analyses by members of BGS Analytical Chemistry Unit,
 
| colspan="6" | * Analyses by members of BGS Analytical Chemistry Unit,
 
* Laboratory of the Government Chemist, and R. R. Harding
 
* Laboratory of the Government Chemist, and R. R. Harding
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|-
 
|-
 
|}
 
|}
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== [[St. Kilda: an illustrated account of the geology#References|References]] ==
 
== [[St. Kilda: an illustrated account of the geology#References|References]] ==
 
{{EGwalks}}
 
{{EGwalks}}
 
[[Category: 2. Northern Highlands]]
 
[[Category: 2. Northern Highlands]]

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