Editing Pyroclastic rocks of the Skye Central Complex

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[[File:GSG_SKYE_10.jpg|thumbnail|Figure 10 Geological sketch-map of the Srath na Creitheach pyroclastic deposits (modified from Jassim and Gass 1970)]]
 
 
[[File:GSG_SKYE_11.jpg|thumbnail|Figure 11 Geological sketch-map of the Creagan Dubh and Kilchrist pyroclastic deposits]]
 
[[File:GSG_SKYE_11.jpg|thumbnail|Figure 11 Geological sketch-map of the Creagan Dubh and Kilchrist pyroclastic deposits]]
 
[[File:GSG_SKYE_21.jpg|thumbnail|Figure 19b Excursion  5 Kilchrist. Ignimbrite outcrops, location map]]
 
[[File:GSG_SKYE_21.jpg|thumbnail|Figure 19b Excursion  5 Kilchrist. Ignimbrite outcrops, location map]]
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The acid character of these deposits is only readily identified in the least-altered units, where wispy fragments of rhyolitic material, up to 5mm long, with soft, curved margins, are found set in a fine-grained, fragmental groundmass. Angular crystals of quartz and, less commonly, alkali and plagioclase feldspar, are present, but because these deposits are pyroclastic it is difficult to determine whether or not these minerals are related to the primary acid component. Small xenoliths of various types of country-rock are dispersed throughout these tuffs.
 
The acid character of these deposits is only readily identified in the least-altered units, where wispy fragments of rhyolitic material, up to 5mm long, with soft, curved margins, are found set in a fine-grained, fragmental groundmass. Angular crystals of quartz and, less commonly, alkali and plagioclase feldspar, are present, but because these deposits are pyroclastic it is difficult to determine whether or not these minerals are related to the primary acid component. Small xenoliths of various types of country-rock are dispersed throughout these tuffs.
  
(iv) Ignimbrites in the Eastern Red Hills district were first recorded by Ray (1960), who described material from two localities in the Allt nan Suidheachan-Cnoc nam Fitheach area [[Media:GSG_SKYE_11.jpg|(Figure 11)]]. Both were considered to be intrusive, forming marginal facies of a 'microadamellite' intrusion. Subsequently, this intrusion has been interpreted as a mixed-magma rock (see Section [[The Eastern Red Hills Centre, Skye Central Complex#(B) The Kilchrist Hybrid Ring-dyke|(7B)]] of Chapter 7) and the ignimbrites are considered to be extrusive (see below). In addition to the ignimbrites noted by Ray (1960), three other occurrences from elsewhere within the district [[Media:GSG_SKYE_21.jpg|(Figure 19b)]] have been recorded by B.R. Bell (1985): (1) on top of a rhyolite lava, in the screen between the Inner and Outer Granites (west of Beinn Dearg Mhor); (2) east of Cnoc nam Fitheach, in a tributary of the Allt Cnoc nan Uan; and, (3) in Coire Forsaidh, in a tributary of the Allt Coire Forsaidh [[Media:GSG_SKYE_11.jpg|(Figure 11)]]. These three deposits occur as thin-bedded units within the main sequence of pyroclastic rocks.
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(iv) Ignimbrites in the Eastern Red Hills district were first recorded by Ray (1960), who described material from two localities in the Allt nan Suidheachan-Cnoc nam Fitheach area [[Media:GSG_SKYE_11.jpg|(Figure 11)]]. Both were considered to be intrusive, forming marginal facies of a 'microadamellite' intrusion. Subsequently, this intrusion has been interpreted as a mixed-magma rock (see Section [[The Eastern Red Hills Centree, Skye Central Complex#(B) The Kilchrist Hybrid Ring-dyke|(7B)]] of Chapter 7) and the ignimbrites are considered to be extrusive (see below). In addition to the ignimbrites noted by Ray (1960), three other occurrences from elsewhere within the district [[Media:GSG_SKYE_21.jpg|(Figure 19b)]] have been recorded by B.R. Bell (1985): (1) on top of a rhyolite lava, in the screen between the Inner and Outer Granites (west of Beinn Dearg Mhor); (2) east of Cnoc nam Fitheach, in a tributary of the Allt Cnoc nan Uan; and, (3) in Coire Forsaidh, in a tributary of the Allt Coire Forsaidh [[Media:GSG_SKYE_11.jpg|(Figure 11)]]. These three deposits occur as thin-bedded units within the main sequence of pyroclastic rocks.
  
 
Within the Allt nan Suidheachan-Cnoc nam Fitheach area more detailed mapping by B.R. Bell (1985) identified three discrete outcrops of ignimbrite [[Media:GSG_SKYE_21.jpg|(Figure 19b)]]. One of these outcrops consists of four distinct sheets, with a total thickness of 4.4m, and is exposed in a small gully which flows NW from Cnoc nam Fitheach into the main stream. This ignimbrite is relatively crystal-rich, has a strong eutaxitic texture, and dips at an angle of 35° to the NW.
 
Within the Allt nan Suidheachan-Cnoc nam Fitheach area more detailed mapping by B.R. Bell (1985) identified three discrete outcrops of ignimbrite [[Media:GSG_SKYE_21.jpg|(Figure 19b)]]. One of these outcrops consists of four distinct sheets, with a total thickness of 4.4m, and is exposed in a small gully which flows NW from Cnoc nam Fitheach into the main stream. This ignimbrite is relatively crystal-rich, has a strong eutaxitic texture, and dips at an angle of 35° to the NW.
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== (E) The Creagan Dubh and Kilchrist deposits: origin ==
 
== (E) The Creagan Dubh and Kilchrist deposits: origin ==
  
From the descriptions presented in Section [[Pyroclastic rocks of the Skye Central Complex#(D) The Creagan Dubh and Kilchrist deposits: description|(8D)]], above, it is evident that the volcanic products of the Eastern Red Hills district can be readily distinguished and that it is not appropriate to group such highly variable pyroclastic accumulations under the term 'agglomerate'. Furthermore, it has been concluded that the field relationships at Kilchrist can be better explained in terms of a ring-dyke intrusion (see Section [[Pyroclastic rocks of the Skye Central Complex#(D) The Creagan Dubh and Kilchrist deposits: description|(8D)]], above, and Section [[The Eastern Red Hills Centre, Skye Central Complex#(B) The Kilchrist Hybrid Ring-dyke|(7B)]] of Chapter 7), together with its central, down-dropped block of pyroclastic rocks [[Media:GSG_SKYE_11.jpg|(Figure 11)]]. The evidence from the different types of material which have been described strongly suggests that surface processes—either subaqueous or subaerial—have dominated the mode of deposition of the pyroclastic rocks and that within-vent processes, as suggested by Harker (1904), were not involved.
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From the descriptions presented in Section [[Pyroclastic rocks of the Skye Central Complex#(D) The Creagan Dubh and Kilchrist deposits: description|(8D)]], above, it is evident that the volcanic products of the Eastern Red Hills district can be readily distinguished and that it is not appropriate to group such highly variable pyroclastic accumulations under the term 'agglomerate'. Furthermore, it has been concluded that the field relationships at Kilchrist can be better explained in terms of a ring-dyke intrusion (see Section [[Pyroclastic rocks of the Skye Central Complex#(D) The Creagan Dubh and Kilchrist deposits: description|(8D)]], above, and Section [[The Eastern Red Hills Centree, Skye Central Complex#(B) The Kilchrist Hybrid Ring-dyke|(7B)]] of Chapter 7), together with its central, down-dropped block of pyroclastic rocks [[Media:GSG_SKYE_11.jpg|(Figure 11)]]. The evidence from the different types of material which have been described strongly suggests that surface processes—either subaqueous or subaerial—have dominated the mode of deposition of the pyroclastic rocks and that within-vent processes, as suggested by Harker (1904), were not involved.
  
 
The numerous acid tuffs, for example, together with the ignimbrites, are clearly extrusive in form. They were possibly erupted as a result of magma-mixing processes in subvolcanic chambers which developed during the evolution of the Skye Centre (see Section [[The Western Red Hills Centre, Skye Central Complex#(K) Mechanisms of intrusion and volcanic processes|(6K)]] of Chapter 6). Deposition of these volcanic products would have been on relatively irregular surfaces, strongly controlled by the voluminous agglomerate accumulations. The aspect ratios of the fiamme in the ignimbrites and the wispy rhyolitic fragments in the tuffs also suggest large degrees of compaction. It has been shown by Wolff and Wright (1981) that rheomorphism, in the form of secondary mass flowage, can occur in such deposits, and that deposition on slopes is a critical aspect of features such as flow folds, cusps and slump structures. Magma of the peralkaline type appears to favour the formation of such structures.
 
The numerous acid tuffs, for example, together with the ignimbrites, are clearly extrusive in form. They were possibly erupted as a result of magma-mixing processes in subvolcanic chambers which developed during the evolution of the Skye Centre (see Section [[The Western Red Hills Centre, Skye Central Complex#(K) Mechanisms of intrusion and volcanic processes|(6K)]] of Chapter 6). Deposition of these volcanic products would have been on relatively irregular surfaces, strongly controlled by the voluminous agglomerate accumulations. The aspect ratios of the fiamme in the ignimbrites and the wispy rhyolitic fragments in the tuffs also suggest large degrees of compaction. It has been shown by Wolff and Wright (1981) that rheomorphism, in the form of secondary mass flowage, can occur in such deposits, and that deposition on slopes is a critical aspect of features such as flow folds, cusps and slump structures. Magma of the peralkaline type appears to favour the formation of such structures.

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