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The Lewisian of Lochinver, Sutherland; the Type Area for the Inverian Metamorphism

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The Lewisian of Lochinver, Sutherland; the Type Area for the Inverian Metamorphism The Lewisian of Lochinver, Sutherland; the type area for the Inverian metamorphism CALVIN R. EVANS & RICHARD ST J. LAMBERT CONTENTS t Petrology and chemistry of the Lewisian of Lochinver ~8 (A) Gneisses . i28 (B) The transition from granulite to amphibolite facies ~29 (c) Pegmatites x3o (D) Mafic and ultramafic dykes x33 2 Structural relationships x33 (A) Badcallian structures . ~34 (S) Inverian structures ~35 (c) Correlation of Inverian folding and metamorphism x38 (D) Intrusion of NW dykes ~38 (~) Laxfordian deformation ~38 3 Geochronology and isotope geology x38 (A) Age of formation of the potash pegmatites ~38 (B) Geochronology of the northern sector of the Lewisian I4I 4 Implications of the Inverian metamorphism and correlations with other shield areas I43 5 References x46 SUMMARY The Lochinver district contains gneisses, and structural features of each sector of the pegmatites and dykes generated in five main complex are described and new whole rock episodes: (I) pre-26oom.y, garnet pyroxene Rb-Sr ages given for the pegmatite suite, granulite facies gneisses with low angle dips, suggesting a maximum age of 254 o m.y. for the formed in their final state by the Badcallian suite (plus intrusion of a further suite at 23io metamorphism; (2) potash-rich pegmatites of m.y. or recrystallisation of the first suite at age range 254o to 23io m.y.; (3) 23 to-22oo 23io m.y.). Other pegmatite ages confirm the m.y. amphibolite facies gneisses occurring in presence ofpre-2ooo m.y. activity NE of Scourie vertical WNW zones, produced by a broadly and at Gruinard. Field and petrographic isochemical metamorphic event; (4) vertical evidence at Lochinver demonstrates the ~w-trending dykes emplaced at 22oo m.y. and existence of amphibolite facies metamorphism perhaps down to 19oo m.y. and (5) epidote- in post pegmatite pre-dyke times, which amphibolite facies gneisses produced in narrow, produced well-defned rock types and w~ew discontinuous Nw shear zones during the suc- vertical structures, and is defined as the ceeding Laxfordian events. The petrographic Inverian metamorphic event. T H F L ~ wI s I AN has been divided by Peach et al. (I 9 o7) and especially by Sutton & Watson (i95I , i962 ) into a pre-dyke (granulite facies) complex 'Scourian', and a post dyke (amphibolite facies) complex 'Laxfordian'. Detailed work in the Scourie area by Watson produced evidence of pre-dyke pegmatite intrusion and amphib- olite facies metamorphism, both of which were ascribed to the closing phase of Scourian metamorphism. Partly on the basis of dyke margins chilled against high- grade gneiss, Sutton and Watson postulated a large time-span between the early Jl geol. Soc. Lond. vol. x3o , i974, pp. x25-I5o , 7 figs. Printed in Northern Ireland. 3 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/130/2/125/4884711/gsjgs.130.2.0125.pdf by guest on 25 September 2021 I26 C. R. Evans & R. St J. Lambert Scourian metamorphism and the later Laxfordian metamorphism. This was later confirmed by Giletti et al. (196 I) who concluded that the Scourian was older than 2540 m.y. and that the Laxfordian was about 165o m.y. old. (All Rb-Sr ages are calculated using a half-life for STRb of 4.85 × lO l° y.) Dearnley (1962 , 1963) has since recognized two post dyke metamorphic events in the Outer Hebrides which he termed 'early Laxfordian' and 'late Laxfordian'. Studies of the NW dykes by O'Hara (1961 , 1962) and Tarney (1963) led to the suggestion that they were emplaced into hot country rock. The recognition of the Inverian metamorphism at Lochinver (Evans 1963, 1965) has caused an evolution of thought about the history of the mainland Lewisian. Bowes (1969) has evolved the view that the Lewisian was developed in three orogenic episodes, Scourian, Inverian and Laxfordian, dated at about 2600 + to 2460 m.y. ago, 2200 to ?~ooo m.y. ago and 16oo to 13oo m.y. ago respectively. Park (i97o) argues against this view, defining the term Badcallian for the meta- morphism which produced the garnet pyroxene granulites and referring the Inverian metamorphism to pre-22oo m.y. time as a major event. Park emphasises the unity of the pre-Nw dyke Inverian event with the dykes themselves and with the Laxfordian and considers that many of the events attributed to the Laxfordian may be Inverian. Evans (1963) and Holland (1966) have both suggested that the Inverian metamorphism affected large areas of the Lewisian. Beach et al. (I973) have described the structure of the area sw of Laxford Bridge and have shown that a set of folds affected the Foindle and Claisfearn zones of Sutton & Watson (195 I) after the pyroxene granulite facies metamorphism and before the intrusion of dykes. They decline to correlate these folds with the Inverian episode, citing a 2540 m.y. pegmatite (sample 7 this paper; sample I, Giletti et al. I96I ) which they state cuts amphibolized gneiss. However, this pegmatite cuts typical nearly fiat- lying felsic pyroxene granulite, partially retrogressed to hornblende gneiss, with green hornblende pseudomorphing primary pyroxene; it is not in a characteristic vertical shear zone of Inverian type. Consequently we do not regard this peg- matite as evidence of pre-254o m.y. amphibolization and re-affirm the view that the transition zones NF. of Scourie show all the characteristics of an Inverian structure, including structural, petrographic and chemical considerations (for the latter see Holland & Lambert 1973). The Laxfordian metamorphism is now known to have reached its maximum at or before 185o m.y. (Lambert & Holland I972 ) and is polyphase in character (Dash 1969, Beach et al. i973). If the dyke swarm was emplaced in hot country rock closely following on the Inverian metamorphism, then the possibility that the Inverian and Laxfordian events are effectively continuous in the northern Lewisian must be considered seriously. In general, the present authors accord with the views of Park (i97o) on the evolution of the northern sector of the Lewisian. The growing implications of the Inverian event require that the facts concerning the type area be recorded. The area studied (Fig. 1) lies in the central belt of the Lewisian. Previous publications (Evans & Tarney I964; Evans I965; Lambert & Holland i972 ; Pidgeon & Bowes I972) have established the following chronology, which will be assumed throughout: Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/130/2/125/4884711/gsjgs.130.2.0125.pdf by guest on 25 September 2021 The Inverian metamorphism and the Lewisian of Lochinver, NW Scotland 12 7 i I 1 I~ Southern & Northern ~ Ultramafic gneiss pyroxene zones Central zone of amphibolite xxx Agmatite Ultramafic &intermediate -'--Faults dykes' ---- Shear Belt Mafic dykes -~ Lochr~ter Antiform r~Mafic pyroxene gneiss .J.,-:.,~,"Gentle, Steep l I~"~Mafic hornblende gneiss ~ Vertical Dip 24- ~o. ~ 60 5O g 30 L MAP 2o lO N 0 10 20 30 40 50 NW.SCOTLAND I (xx INVER GROUP) 2O PRECAMBRIAN GEOLOGY 19. OF THE LOCHINVER DISTRICT 18- < NATIONAL GRID (Kin) > O6 07 08 09 10 11 12 13 14 I I I I I I I I I FIo. I. Precambriangeology of the Lochinverdistrict. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/130/2/125/4884711/gsjgs.130.2.0125.pdf by guest on 25 September 2021 I28 C. R. Evans & R. St J. Lambert About 275 ° m.y.--granulite facies metamorphic event, 254o-231 o m.y.--potash pegmatites, c. 22oo m.y.--amphibolite facies metamorphism, 2 I9o (-I9OO ?) m.y.--intrusion of marie dykes, c. I85o m.y.hend of major recrystallization at Laxford Bridge, c. t 65o m.y.--end of pegmatite formation at Laxford Bridge, I4OO m.y.hend of Rb-Sr recrystallization of minerals in the north. I. The Petrology of the Lewisian of Lochinver A) GNEISSES The petrology of the garnet pyroxene granulites, found only in the southern and extreme northern parts of the Lochinver area, is similar to that of the granulites of Scourie. Descriptions have been given by Peach et al. (I9O7) , Sutton & Watson (I95I), O'Hara (I96I), Bowes et al. (I964) and Muecke (I969) and so only a few comments are made. The ultramafic rocks of this zone are olivine bronzite or hypersthene diopside edenitic hornblende spinel magnetite rocks, characterized by anhedral minerals with an interlocking, bulbous individual shape. Irregular banding and the meta- morphic texture (Evans i963, Vernon I97o ) do not lend support to the magmatic crystal accumulation theory of Bowes et al. (1964). Marie gneisses occur with or without garnet: the garnetiferous gneisses are of widespread distribution, but one band occurring immediately below the principal ultramafic mass (o9721o) may be genetically connected with it. These marie gneisses contain augite and hypersthene granules with strained, well-twinned andesine (or, rarely, plagioclase near An60), magnetite, ilmenite and garnet. Primary brown hornblende or quartz are characteristic. Analyses of minerals from these rocks (Evans I963 and Muecke pers. comm.) show that the garnets are GrlsSp2AlmsoPyr33; the brown hornblendes are rich in Ti, A1iv and Na + K; and the augites and hypersthenes are highly aluminous (Al~O3 in cpx up to 6. 7 wt.%, Al~O3 in augite/A1203 in hypersthene '~ 1.5, AllY/ A1va averaging 2). These hornblende and garnet analyses are similar to those from the Adirondacks, summarised by de Waard (I965) , but the pyroxenes are much more aluminous. The dominant type of gneiss is felsic, essentially a quartz diorite or tonalite in broad composition (Holland & Lambert I973). It is usually amphibolized, but fresh felsic gneiss occurs south of the Kirkaig River. Fresh leucocratic varieties contain andesine (near An3s) unzoned, antiperthitic and highly strained. Quartz is opalescent blue and highly strained; the ferromagnesian constituents are hyper- sthene and augite, usually showing some replacement by hornblende.
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