Editing Migmatites, early igneous activity within the Caledonides, Northern Highlands of Scotland

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'''From: Johnstone, G S and Mykura, W. 1989. [[British regional geology: Northern Highlands of Scotland|British regional geology: the Northern Highlands of Scotland]] (4th edition). (Nottingham: British Geological Survey.)'''
 
== Migmatites, introduction ==
 
== Migmatites, introduction ==
[[File:P916024.jpg|thumbnail|Migmatitic rocks: lit-par-lit gneiss. Loch Shiel Migmatite Complex, Coire nan Gall, Sgurr na Ciche area. P916024.]]
 
[[File:P219057.jpg|thumbnail|Granite-gneiss with neosome lits bounded by black biotite selvages. P219057.]]
 
[[File:P916026.jpg|thumbnail|Migmatitic rocks and pegmatites: lit-par-lit gneiss in an extreme form, with thick pegmatite bands showing a vague palimpsest banding. P916026.]]
 
[[File:P915476.jpg|thumbnail|Major intrusions, granite gneiss and migmatites within the Caledonides of the Northern Highlands. P915476.]]
 
 
The term migmatite is applied to a rock comprising an intimate mixture of a schistose host (normally metasedimentary) referred to as the palaeosome (Menhert, 1971) and granitic (usually pegmatitic) material, the neosome, which has been derived from the host by a process of partial melting (anatexis) or by a process of segregation involving volatile components, some of which may have been introduced from an extraneous source. Migmatites are the products of high- grade metamorphism which required the rock to be at or about sillimanite grade (c.700°C). The resulting rocks are lithologically very variable.
 
The term migmatite is applied to a rock comprising an intimate mixture of a schistose host (normally metasedimentary) referred to as the palaeosome (Menhert, 1971) and granitic (usually pegmatitic) material, the neosome, which has been derived from the host by a process of partial melting (anatexis) or by a process of segregation involving volatile components, some of which may have been introduced from an extraneous source. Migmatites are the products of high- grade metamorphism which required the rock to be at or about sillimanite grade (c.700°C). The resulting rocks are lithologically very variable.
  
The typical rock of the migmatite complexes of the Northern Highlands Moines is a pelitic or semipelitic ''lit-par-lit ''gneiss, in which the palaeosome and neosome are intimately interbanded parallel to the foliation ([[Media:P916024.jpg|P916024]]). As the migmatisation process preferentially concentrates quartz and feldspar in the neosome (leucosome) the relative increase in the biotite content of the palaeosome commonly renders it a matter of difficulty to decide whether the host rock was originally a pelitic or a semipelitic metasediment. The neosome commonly has a black selvedge (melanosome) or restite, of black biotite (Plate 18). The body of the host rock is a much coarser grained than that of similar rocks outside the area of migmatisation, and there is a more distinct separation of mica and quartzofeldspathic material, the latter tending to aggregate into minute blebs or eyes (augen) rather than layers ([[Media:P219057.jpg|P219057]]). (This more-or-less uniform material is sometimes referred to as ‘permeation gneiss’; however, this term is best avoided because of certain genetic complications.) Within it, the quartzofeldspathic component aggregates gradually until it forms ''‘lits’ ''of coarse-grained granitic material (some of which are continuous along the foliation for several metres) giving a gneissic, banded (stromatic) appearance to the rock. Other concentrations are more distinctly lensoid, forming chains of small or large augen along the foliation. The spacing of the lits or augen chains is irregular.
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The typical rock of the migmatite complexes of the Northern Highlands Moines is a pelitic or semipelitic ''lit-par-lit ''gneiss, in which the palaeosome and neosome are intimately interbanded parallel to the foliation (Plate 17). As the migmatisation process preferentially concentrates quartz and feldspar in the neosome (leucosome) the relative increase in the biotite content of the palaeosome commonly renders it a matter of difficulty to decide whether the host rock was originally a pelitic or a semipelitic metasediment. The neosome commonly has a black selvedge (melanosome) or restite, of black biotite (Plate 18). The body of the host rock is a much coarser grained than that of similar rocks outside the area of migmatisation, and there is a more distinct separation of mica and quartzofeldspathic material, the latter tending to aggregate into minute blebs or eyes (augen) rather than layers ([[Media:P219057.jpg|P219057]]). (This more-or-less uniform material is sometimes referred to as ‘permeation gneiss’; however, this term is best avoided because of certain genetic complications.) Within it, the quartzofeldspathic component aggregates gradually until it forms ''‘lits’ ''of coarse-grained granitic material (some of which are continuous along the foliation for several metres) giving a gneissic, banded (stromatic) appearance to the rock. Other concentrations are more distinctly lensoid, forming chains of small or large augen along the foliation. The spacing of the lits or augen chains is irregular.
  
The thickness of the neosome is variable, from barely discernable to several metres; it is evident that the thickest neosomes cannot be entirely of local origin. Possibly neosome material migrated from its source rock to aggregate in thicker bands. In places the neosome may make up large areas of country in which the host rock is only represented by relict streaks. The rock has become a nebulite. In the pelitic ''lit-par-lit ''gneisses the neosome (and the larger pegmatites probably derived from them) tend to be oligoclase-bearing (trondhjemitic) ([[Media:P916026.jpg|P916026]]).
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The thickness of the neosome is variable, from barely discernable to several metres; it is evident that the thickest neosomes cannot be entirely of local origin. Possibly neosome material migrated from its source rock to aggregate in thicker bands. In places the neosome may make up large areas of country in which the host rock is only represented by relict streaks. The rock has become a nebulite. In the pelitic ''lit-par-lit ''gneisses the neosome (and the larger pegmatites probably derived from them) tend to be oligoclase-bearing (trondhjemitic) (Plate 19).
  
 
''Lit-par-lit ''gneisses with a psammitic host are less common. As in the case of the pelite-hosted gneisses, the neosomes which have developed along the micaceous foliae commonly have biotite selvedges, but they are granitic rather then trondhjemitic in composition. The palaeosome may be much recrystallised to a coarse- grained rock; small porphyroblastic augen of potash feldspar are commonly found within it.
 
''Lit-par-lit ''gneisses with a psammitic host are less common. As in the case of the pelite-hosted gneisses, the neosomes which have developed along the micaceous foliae commonly have biotite selvedges, but they are granitic rather then trondhjemitic in composition. The palaeosome may be much recrystallised to a coarse- grained rock; small porphyroblastic augen of potash feldspar are commonly found within it.
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The ''lit-par-lit ''gneisses could be the products of isochemical anatexis (Stevenson, 1971; Butler, 1965), but Brown (1967) and Cheng (1944) have shown that their formation requires the metasomatic introduction of sodium. In the Loch Coire Complex, Brown has followed Read in identifying the sheet-like form of the migmatites with an upward transition (seen on Ben Klibreck) from unmigmatised metasediments to structurally overlying migmatised rocks. The progression is through a zone of veins and sills distinct from the transected rock into a zone in which the two are intimately mixed to form banded migmatites, which themselves are cut by other veins and sheets. These veins are of oligoclase granite or pegmatite. Masses of oligoclase-bearing granite lie within the complex, of which the Loch Coire granite is the largest. It is weakly foliated. The rocks of the complex are cut by seams of pink aplite, probably of separate, later generation.
 
The ''lit-par-lit ''gneisses could be the products of isochemical anatexis (Stevenson, 1971; Butler, 1965), but Brown (1967) and Cheng (1944) have shown that their formation requires the metasomatic introduction of sodium. In the Loch Coire Complex, Brown has followed Read in identifying the sheet-like form of the migmatites with an upward transition (seen on Ben Klibreck) from unmigmatised metasediments to structurally overlying migmatised rocks. The progression is through a zone of veins and sills distinct from the transected rock into a zone in which the two are intimately mixed to form banded migmatites, which themselves are cut by other veins and sheets. These veins are of oligoclase granite or pegmatite. Masses of oligoclase-bearing granite lie within the complex, of which the Loch Coire granite is the largest. It is weakly foliated. The rocks of the complex are cut by seams of pink aplite, probably of separate, later generation.
  
Although the migmatitisation phenomena of the north coast of Scotland were reported by Home and Greenly as long ago as 1898, little regional descriptive work has been published on the Strath Halladale migmatite complex. It appears that this comprises lit-par-lit migmatite of regional extent, with a more local migmatite associated with the early emplacement of a soda-rich granite found around the main outcrop of the Strath Halladale granite. This granite shows a foliation, and is considered by McCourt (1980) to be the same as that which forms the sheets and veins of the Loch Coire Complex. McCourt, however, cites evidence of feldspar zoning and quartz-plagioclase reaction, which he considers indicative of a magmatic origin for the rock. The migmatite and the sodic granites are folded, but the main mass of the Strath Halladale granite (previously classed with the migmatites) is of a later generation and is a true magmatic biotite granite. It is affected only by late-stage minor folding, and cannot be correlated with the migmatite complex.
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Although the migmatitisation phenomena of the north coast of Scotland were reported by Home and Greenly as long ago as 1898, little regional descriptive work has been published on the Strath Halladale migmatite complex. It appears that this comprises lit-par-lit migmatite of regional extent, with a more local ''Early igneous activity within the Caledonides ''91 migmatite associated with the early emplacement of a soda-rich granite found around the main outcrop of the Strath Halladale granite. This granite shows a foliation, and is considered by McCourt (1980) to be the same as that which forms the sheets and veins of the Loch Coire Complex. McCourt, however, cites evidence of feldspar zoning and quartz-plagioclase reaction, which he considers indicative of a magmatic origin for the rock. The migmatite and the sodic granites are folded, but the main mass of the Strath Halladale granite (previously classed with the migmatites) is of a later generation and is a true magmatic biotite granite. It is affected only by late-stage minor folding, and cannot be correlated with the migmatite complex.
  
 
== Ardgour Granite Gneiss  ==
 
== Ardgour Granite Gneiss  ==

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