The Metamorphosis of Dolerite into Hornblende-schist

Abstract

THE object of this communication is to describe the gradual change from dolerite to hornblende-schist as it may be observed in two more or less parallel dykes which occur in the Arch~ean gneiss of the northwest of Scotland, near the village of Scourie in Sutherlandshire. The northern promontory of Scourie Bay is named on the Ordnance map Creag a'Mhail. Near this point two dykes of basic igneous rock may be seen. The one to the south is about 30 yards wide ; it has been more affected by denudation than the surrounding gneiss, and its course is thus marked b~' a depression which is occupied by the sea at high tide. • short distance to the north of the actual promontory is another dyke, about 20 or 30 feet wide, of similar character, which terminates westward in a vertical cliff. This may be traced in a south-easterly direction down to a small beach where the southern dyke is again seen, and from this point the two dykes may be followed, running parallel to each other towards the southeast , for a distance of a quarter of a mile or more. The prevalent strike of the gneissie banding in this district is E.N.E. and W.S.W., with a moderate dip to N.N.W. The dykes therefore cut across the strike and cannot possibly be regarded as bands in the gneiss. On the opposite shores of Scourie Lake (Loeh a' :Bhaid Daraich) and close by the side of the road to Laxford Bridge (three quarters of a mile from the Scourie Hotel), the dykes again make their appearance. Prof. ]~lake first recognized the northern dyke at this spot on the occasion of our joint excursion to the northwest of Scotland in the summer of 1883. This dyke is here the wider of the two and forms a portion of the conspicuous hill which overlooks the lake. The southern dyke produces a marked gap to the south of the hill. The distance between the two dykes is only a few yards. The junction-planes of gneiss and dyke are nearly vertical, and the fact that they cut across the gneissic banding is most clearly shown. From this point the dykes may be traced to the southeast for more than a mile. What becomes of them ultimately in this direction I do not know ; but, so far as I followed them, they keep very closely to the same straight line and preserve their lithotogical characters (subject to such exceptional variations as will presently be described) with as much persistence as the dykes in the Carboniferous region of the north of England. The rock forming these dykes occurs in two strongly marked varieties. The one is a moderately coarse-grained, crystalline,

Laramie Peak shear system, central Laramie Mountains, Wyoming, USA: regeneration of the Archean Wyoming province during Palaeoproterozoic accretion

“…S tectonite) and finally to (4) medium-grained, sometimes banded, amphibolite. This mineralogical and textural transformation resembles the classic metamorphic transformation of dolerite to hornblende schist (amphibolite) originally described by Teall (1885) for Scourie dykes affected by Laxfordian shear zones in NW Scotland.…”

Mylonites: lessons from Eriboll

“…But unlike Lapworth, Teall (1885, 1918 did not make a link between such schists and mylonites. Teall (1885) acknowledged his indebtedness to Lapworth for making him aware of the effects of mechanical action on rocks and adds that Lapworth had not seen the sheared Scourie dykes. In his contributions in Peach et al (1907) and in his 1918 paper, Teall stated that he did not believe that the schists formed within the Scourie shears were mylonites because they had not undergone crushing.…”

“…He was also indicating that the deformation process must 'insensibly' change from cataclasis to plastic yielding as the texture of the rock changes from mylonitic to holocrystalline schistose. Teall (1885Teall ( , 1918 and within Peach et al (1907) took the importance of plastic yielding a step further. He concluded that the rocks produced within the shear zones in the Scourie dykes (see above), which we would now regard as mylonitic, had been formed by plastic deformation processes involving two coupled processes, namely deformation induced molecular re-arrangement of component minerals and grain refinement by granulation (recrystallization).…”

“…He concluded that the rocks produced within the shear zones in the Scourie dykes (see above), which we would now regard as mylonitic, had been formed by plastic deformation processes involving two coupled processes, namely deformation induced molecular re-arrangement of component minerals and grain refinement by granulation (recrystallization). He noted that these coupled processes were observed not only in the Scourie dykes but also in the formation of amphibolitic schists elsewhere (see Teall 1885Teall , 1886Teall , 1887Teall , 1918. Later, Read (1931Read ( , 1934Read ( , 1951 used 'granulation' to describe grain refinement by cataclasis, that is, Teall, Geikie and Read used 'granulation' to describe different aspects of mylonite formation and deformation.…”

“…He appears to have been an early adherent to the expanding earth hypothesis or, at least, an expansion of the cold crust due to heating from below, rather than the contracting earth model (see above). Teall (1885) recorded that Heim had earlier attributed grain elongation in naturally deformed calcite to plastic deformation. It is interesting to note that Kilgour, in the discussion of Teall's (1885) paper, noted similarities in aspects of his (Teall's) paper and the behaviour of metals suggesting that cross fertilization between materials science and geology was occurring at this time.…”