10umal of Geosciences, Osaka City University Vol. 42, Art. 8, p. 115-125, March, 1999

Evolution of the Lewisian Complex in South Harris, Northwest Scotland, and its Relation to the North Atlantic Craton in the Palaeoproterozoic (2.0 Ga)

Sotaro BABA

Department of Geosciences, Faculty of Science, Osaka City University, Osaka 558-8585, Japan.

ABSTRACT The evolutionary history of the Lewisian complex in South Harris is inferred from its metamorphic history and geological characteristics. The anticlockwise P-Tpath, involving ultra-high temperature metamorphism of the precur­ sor rocks, suggests that the South Harris complex, especially the rocks in the Leverburgh belt, was formed by plate­ tectonic processes of subduction-magmatism-collision. Similar plate-tectonic histories of subduction and collision in the surrounding North Atlantic regions are dis­ cussed, although their direct correlations are unclear. However, an evolutionary history of South Harris may have been a part of the Palaeoproterozoic assembly of continents proposed by Hoffman (1990).

Key words: Lewisian complex in South Harris, Accretionary prism, Anticlockwise P-T path, UHT metamorphism, Palaeoproterozoic.

2) The precursor rocks of the Leverburgh bel t belonged to an 1. INTRODUCTION accretionary prism. 3) Ultra-high temperature metamorphism related to Protero­ The tectonothermal history of the Lewisian complex in zoic igneous activity. NW Scotland needs revision, based on new age determina­ This geological and tectonothermal history of South Harris tions. Recent age determinations has confirmed two episodes is distinctive from that on the mainland, and provides useful of granulite-facies metamorphism at 2.7 and 2.5 Ga, a 1.7 Ga information for reconstructing the Lewisian complex at 2.0 kyanite-forming amphibolite-facies metamorphism (Zhu et Ga. In this paper, the previous work by the author is summa­ a!., 1997), and two distinct protoliths, which belong to the rized and integrated with new data, and the correlation be­ central and northern regions (Kinny and Friend 1997). These tween the Lewisian complex of the 2.0 Ga tectonic belt with new age determinations have been undertaken on the main­ the North Atlantic region is discussed. land Lewisian complex. On the other hand, the events at 2.1 to 1.8 Ga in the 2. GEOLOGICAL BACKGROUND Lewisian complex have only been recorded in the South Har­ ris complex in the central part of the Outer Hebridean Isles. The Lewisian complex in NW Scotland is a well-known, Cliff et a!. (1998) summarized the evolutionary history of the high-grade gneiss terrane composed of TTG (tonalite­ northern Outer Hebrides based on the Sm-Nd isotopic sys­ trondhjemite-granodiorite) gneisses (Fig. I). On the main­ tem. They concluded that the Leverburgh belt was a post­ land, many geological, petrological and isotopic studies have Archaean component, which had been considered to include been undertaken by many researchers over several decades. an Archaean supracrustal sequence. However, an evolution­ Well-summarized review papers on the Lewisian complex, ary history of South Harris has not been proposed from exist­ despite lacking new age data, have been published by Park ing petrological and geological data. (1987) and Park et a!. (I994). Several new insights are proposed by the author, based The South Harris complex, exposed in the southern part mainly on a petrological and geological study, as follows: of Harris (Fig. I), is composed of metasedimentary and I) An anticlockwise P- T path in the Palaeoproterozoic (2.1 ­ metabasic rocks of the granUlite-facies Leverburgh belt, the 1.8 Ga). amphibolite-facies Langavat belt and the Palaeoproterozoic 116 Evolution ofthe Lewisian Complex in South Harris, Northwest Scotland

r10'W r05'W 7·00'w

57·SO'N

...... • 7 ~8 ~ .... w South Harris

o 57· 45'N

~~menl

8"w

Fig. I. Simplified geological map of the Lewisian complex in the north-west Scotland (after Park et aI., 1994), and the South Harris complex (after Dearnley, 1962; Baba, 1997). I, Leverburgh belt - Benn Obbe Series (A); 2, Leverburgh belt - Chaipaval Pelitic Series; 3, Leverburgh belt - Rodel Series; 4, Leverburgh belt - Benn Obbe Series (B); 5, Langavat belt; 6, South Harris Igneous Complex; 7, Metagabbro; 8, Laxfordian granite and migmatite.

South Harris Igneous Complex (granulite-facies) (SHIC). ible into four lithological series, namely, Benn Obbe Series Recently, Sm-Nd age determinations in the northern part of (A), Benn Obbe Series (B), Rodel Series and Chaipaval Pelitic the isles were performed by Cliff et al (1998). They con­ Series (Baba, 1997). Their lithological characteristics are cluded that the Leverburgh belt originally consisted of post­ summarized below. Archaean sediments, and that the post-metamorphic cooling The Benn Obbe Series (A) consists of quartzo-feldspathic, age of the granulite-facies metamorphism was Palaeo­ pelitic, mafic gneisses and rare ultramafic rocks. The quartzo­ proterozoic (1.87-1.83 Ga). Detailed geological information feldspathic gneiss is subdivided into two types, well-banded on the Lewisian complex in the Outer Hebrides and the South and layered, reflecting their field occurrences. In places, well­ Harris complex is summarized by Fettes et al. (1992) and banded gneiss is interbedded with kyanite-bearing pelitic Baba (1997,1998). In this paper, I propose an evolutionary gneiss, hence they probably are of sedimentary origin. The model of the South Harris complex based on the metamor­ pelitic gneiss is also divisible into two types, Fe-AI-rich and phism and tectonic setting of the Leverburgh belt. Na-K-rich; the former is typically rich in iron-oxides and in Fe-rich minerals such as hercynite and staurolite. 3. TECTONIC SETTING OF PRECURSOR ROCKS The Benn Obbe Series (B) consists of layered quartzo­ feldspathic, pelitic, banded quartzo-feldspathic, and mafic 3.1 Lithological assemblage gneisses, with a small amount of leucocratic gneiss (Kfs-rich The constituent rocks of the Leverburgh belt are divis- garnet-bearing quartzofeldspathic gneiss). An occurrence of Sotaro SABA 117

intercalated Fe-AI-rich pelitic gneiss in the banded quartzo­ considered to be very similar to those of the Phanerozoic (see feldspathic gneisses is absent in this series. Layered quartzo­ Condie, 1989). feldspathic gneisses are particularly well exposed. Along the Based on the trace-element pattern as expressed in dis­ western boundary these gneisses are mylonitized, show a crimination diagrams of TiN, Ti/Y, Zr/Y and Ti/Zr, and narrowly-spaced foliation and have retrograde assemblages. MORB-normalized spider diagrams (Baba, 1997), the The Chaipaval Pelitic Series is composed of pelitic, protoliths of the basic metamorphic rocks of the Rodel Series leucocratic, quartz-biotite, and mafic gneisses, with rare, appear to have been derived from MORB (mid-ocean ridge interbanded quartzo-feldspathic gneiss. Pelitic gneiss belongs basalt) and OIT (EMORB generally occur near seamounts or to an Na-K-rich type and is best developed in the Northton ­ oceanic islands), and those of the Benn abbe Series (A) and Toe Head region. Leucocratic gneiss widely occurs as sheets (B) from tAT (island arc tholeiites) and/or CARl (calc-alka­ in the pelitic gneiss on the west side of Chaipaval. line basalts from island arcs). The Chaipaval Pelitic Series The Rodel Series consists mainly of alternating assem­ may be derived from OlT or WPB (within-plate basalt), in blages of strongly foliated garnet-biotite gneiss, biotite-horn­ view of the differing affinities given by the discrimination blende quartzose gneiss, mafic gneiss (garnet + clinopyroxene diagrams and the MORB-normalized patterns. + orthopyroxene, garnet + amphibolite and clinopyroxene + amphibolite assemblages), orthoamphibole-biotite gneiss, 3.3 Tectonic setting sulphide-bearing quartzite and pelitic gneiss. This series, in The tectonic setting of the precursor rock of the contrast to the other main series, shows the following dis­ Leverburgh belt can be interpreted from their lithological as­ tinctive characteristics: 1) it is finely foliated, with common semblages, combined with the geochemical characters of the alternations of rock-types; 2) it consists of a wide variety of basic metamorphic rocks. metamorphic rocks, such as pelitic gneiss, mafic gneiss, Benn Obbe Series (A) and (B): Garnet-pyroxene­ orthoamphibole-bearing gneiss, sulphide-bearing quartzite bearing quartzo-feldspathic gneiss is dominant. The precur­ and marble/calc-silicate rocks, all of which occur as lenses or sors of these quartzo-feldspathic gneisses are considered to thin layers; 3) garnet in pelitic gneiss is generally finer grained be Ca-rich greywackes, by comparing high-temperature ex­ than that in the other series; 4) the presence of marble or calc­ perimental work using greywackes as starting materials silicate blocks and lenses, which occur at several stratigraphic (Vielzeuf and Montel, 1994) with their observed mineral as­ horizons, but appear to be restricted to the Rodel Series. semblage in both series. The well-banded quartzo-feldspathic The characteristics of each series are summarized as fol­ gneisses with minor intercalations of Fe-AI-rich pelitic gneiss lows: the Benn Obbe Series (A) is characterized by quartzo­ occur adjacent to Na-K-rich pelitic gneisses, indicating that feldspathic gneiss with small amounts of two distinct types the rocks were formerly greywacke turbidites alternating with of pelitic gneiss (Na-K-rich and Fe-AI-rich); the Benn Obbe pelites (shales). Greywacke turbidites with a minor pelagic Series (B) is dominated by layered quartzo-feldspathic gneiss, component are considered to have been trench sediments but lacking Fe-AI-rich pelitic gneiss. Pelitic gneisses are (Condie, (989). AI-Fe rich pelitic gneiss, which only occurs notably well developed in the Chaipaval Pelitic Series; these in the Benn abbe Series (A), was deposited as a proximal pelitic gneisses are rich in aluminosilicates (kyanite or silli­ facies at the trench. The basic metamorphic rocks from the manite) and contain leucocratic gneiss sheets. The Rodel Benn abbe Series (A) and (B) show island-arc tholeiitic af­ Series is characterized by abundant and variable, even ex­ finities; therefore, the turbidite sequence was intruded by basic otic, lenticular lithologies, such as marble/calc-silicate rocks, intrusions (sheet, dike or sill) in the island arc or the conti­ pelitic gneiss, mafic gneiss, quartzite and ultramafic pods. nental-margin arc. Boundaries between the four series are typically marked by ChaipavaI Pelitic Series: Kyanite- or sillimanite-bear­ shear zones. ing pelitic gneiss and K-feldspathic gneiss (Ieucocratic gneiss) are dominant, and these gneisses alternate with subsidiary 3.2 Geochemical characteristics of the basic metamor­ biotite- and hornblende-bearing quartzose gneiss. The pre­ phic rocks cursors of the Chaipaval Pelitic Series are considered to be Whole-rock chemistry of mafic gneiss was determined K-Al-rich pelites partly alternating with a small amount of with the aim of estimating the bulk composition of precursor psammitic material. They may have been deposited as clas­ rocks. The characteristic major and trace elements of the meta­ tic sediments in the proximal facies of a trench or as basic rocks give valuable information for estimating the tec­ hemipelagic sediments on the oceanic crust. Basic metamor­ tonic environment of the protolith in comparison with mod­ phic rocks are sparse, but the few sampled rocks from this ern plate-tectonic systems. The plate-tectonic environments series show TMORB(transitional MORB) and and related lithological associations in the Proterozoic are EMORB(enriched MORB) affinities. The basic rocks occur 118 Evolution ofthe Lewisiall Complex ill South Harris, Northwest Scotland

Table. I. Summary of precursor rock of the Leverburgh belt.

Rodel Series

Lithology: Lack of continuous bedding, and various inclusion of rock fragments of mafic gneiss (basalt), meta-ultramafic gneiss (ophiolite fragment?), marble/calcsilicate (impure lime­ stone), quartz-feldspathic gneiss (greywacker). Basic rock: MORB or OIT(oceanic island or seamount) Possible tectonic setting: melange

Benn Obbe Series (A) and (B)

Lithology: Quartzo-feldspatic gneiss with alternating with pelite (greywackes turbidites with a minor pelagic component) Basic rock type: IAT (island arc tholeiitic basalt) or CABI (island arc calcalkaline basalt) Possible tectonic setting: trench sediment

Chaipaval Pelitic Series

Lithology: Aluminosilicate-rich pelitic gneiss and K-feldspar rich pelitic gneiss (pelite) Basic rock type: OIT or WPB (within plate basalt) Possible tectonic setting: proximal facies of trench sediment or hemipelagic sediment as lenses and pods, suggesting that they may have been taken core, and have overgrowths of kyanite in the rims. up by sediments as small blocks during the subduction of 2) Garnet in the pelitic gneiss shows a progressive increase oceanic crust. in grossular content from outer core to rim. Rode] Series: This series is characterized by a lack of 3) The Al v1 / A(lv ratio of clinopyroxene in mafic gneiss in­ continuous bedding and the inclusions of fragments of im­ creases from core to rim. pure limestone (marble/calc-silicate rocks), greywacke 4) Reaction coronas of cordierite and hercynite + cordierite (quartzo-feldspathic gneiss), mudstone (pelitic gneiss), chert are formed between garnet and kyanite, and orthopyroxene (sulphide-bearing quartzite?), basalt (basic gneiss) and + cordierite and orthopyroxene + plagioclase reaction co­ ophiolite (meta-ultramafic rock and garnetiferous mafic ronas occur between garnet and quartz. gneiss). These occurrences indicate typical melange assem­ 5) A P-T path is deduced from inclusion assemblages in gar­ blages, hence the Rodel Series is interpreted as a possible net and from staurolite-breakdown reactions to produce accreted melange in an accretionary prism. The meta-basic garnet + sillimanite and garnet + sillimanite + hercynite rocks show NMORB-, TMORB-, and EMORB-like affini­ with increasing temperature. ties. Rock assemblages and the geochemistry of the meta­ 6) In sheared and strongly foliated rocks, hydrous minerals basic rocks from the Rodel Series suggest that it was formed such as biotite, muscovite and hornblende form a folia­ from oceanic materials related to the subduction of oceanic tion, modifying pre-existing textures. lithosphere. The inferred metamorphic history of the Leverburgh belt The possible tectonic setting of the precursor rocks of is divided into 4 stages, as follows: (M I) prograde metamor­ each series is summarized in Table I. phism with increasing temperature; (M2) metamorphism with increasing pressure; (M3) retrograde decompressional meta­ 4. METAMORPHISM morphism with decreasing pressure and temperature; and, (M4) retrograde metamorphism accompanied by shearing. 4.1. Anti-clockwise poT path The P-T conditions of the MI and M2 stages are 830-880 °C, The metamorphic history has been determi ned from the 8-10 kbar and 800±30 °C, 13 - 14 kbar, respectively. The M3 following mineral textures and compositions observed in stage metamorphic condition is almost 550-650 °C, 5-7 kbar. pelitic, quartzo-feldspathic and mafic gneisses, especially in A pressure increase from M 1 to M2 and the P-Tconditions at pelitic gneisses from the Leverburgh belt. each stage indicate that the metamorphic P-T path was I) Some coarse-grained garnets in the pelitic gneiss include anticlockwise. Further detailed discussions are summarized biotite and quartz in the inner core, sillimanite in the outer in Baba (1998). Sotaro BABA 119

4.2. Ultra-high temperature metamorphism indicated by prograde reactions of staurolite-breakdown and Ultra-high temperature (> 900 °C, UHT) metamorphism other dehydration reactions seen in inclusions within garnet (Harley, 1998), has only been recognized in mainland and clinopyroxene porphyroblasts. Lewisian complex (2.7 or 2.5 Ga) by the use of several The Ml metamorphism, probably an isobaric heating geothermobarometers (e.g. Cartwright, 1990 and references metamorphism, was most likely caused by the emplacement therein), although the mineral association of sapphirine + of the SHIC as a heat source (Baba, 1998). The ultra-high orthopyroxene + aluminosilicate + quartz, a possible indica­ temperature conditions, nearly 950 ± 30°C, characterized by tor of UHT metamorphism, has not been observed in Lewisian the aluminous orthopyroxene + sillimanite + sapphirine + complex. quartz assemblage, were probably formed during this local Two types of sapphirine-bearing orthopyroxene-silli­ temperature rise due to the emplacement of the SHIC before manite (Opx-Sil ) and orthopyroxene-kyanite (Opx-Ky) the formation of kyanite (Baba, in press). The idea of em­ granulites were found in the Leverburgh belt. Both granu­ placement of SHlC at Ml and the thermal effect of UHT lites indicate ultra-high temperature conditions (Baba, in press metamorphism are consistent with the previous observations. and submitted), which are interpreted as Palaeoproterozoic Because igneous equilibration temperatures of 1135 to 1315 UHT metamorphism, rather than lateArchaean metamorphism °C (± 70 0c) were obtained by Witty (in Fettes et aI., 1992), events at 2.7 Ga and 2.5 Ga. the age of Ml might coincide with the age of intrusion of the In the Opx-Ky granulites, the orthopyroxene and kyanite SHIC. However, the emplacement of the SHIC was thought are intergrown in a stable mineral assemblage, which indi­ to have occurred in several stages during 2.2 Ga to 1.8 Ga, cates metamorphic conditions >12 kbar at 800 - 900 0c. Silli­ with the anorthosite at first followed by the gabbro and dior­ manite inclusions within orthopyroxene, which is surrounded ite (Cliff et aI., 1983). The intrusion age of about 2.18 to 1.86 by kyanite, suggest that sillimanite formed earlier and kyan­ Ga may be regarded as the age of the M 1 metamorphism in ite was stable later. Conditions of 950 ± 30°C at 10 kbar are response to continuous heating by multiple intrusions. estimated from orthopyroxene isopleths (Harley, 1998) for In the M2 stage of metamorphism, the pressure increased aluminous-orthopyroxene porphyroblasts « 9.7 wt %). In progressively. M2 pressure conditions are consistent with a the Opx-Sil granulite, the orthopyroxene + sillimanite + gar­ crustal depth of 45 - 50 km, and the pressure increase from net + sapphirine assemblage is stable at the peak metamor­ the M1 to M2 metamorphism might reflect thrusting of a con­ phic stage, indicating P-T conditions of 930-950 °C, > 8 kbar, tinental crust over the South Harris belt. According to Sm­ according to the FMAS petrogenetic grid (Hensen and Harley, Nd studies of the granulite-facies mineral isochrons, high­ 1990). Similar conditions were obtained by using pressure granulite facies metamorphism (M2 stage) took place orthopyroxene-garnet geothermobarometers (Baba, in sub­ at least earlier than 1.87 - 1.83 Ga (Cliff et aI., 1983, Cliff et mitted). aI., 1998). Unfortunately, precise age determinations of the These two types of orthopyroxene-aluminosilicate granu­ Ml and M2 stages are not possible from the previous data. lites indicate that the peak metamorphic conditions were over The retrograde path after peak-M2 involved a decompres­ 900 °C, and this is comparable with ultra-high temperature sion path in a static state. Isothermal decompression paths metamorphism. These granulites were formed during the have generally been considered to be due to rapid exhuma­ Palaeoproterozoic metamorphism due to the emplacement of tion of overthickened crust as a result of isostatic rebound the SHIC at ca. 2.2-1.87 Ga. (Ellis, 1987), or to the extensional thinning of thickened crust (Harley, 1989). Based on mineral textures, Baba (1998) con­ 4.3. Metamorphic history cl uded that the M3 stage probably underwent non­ The deduced P-Tpath for the metamorphic evolution of deformational static conditions reflecting isostatic rebound. the granulites in the Leverburgh belt is shown in Fig. 2. The The M3 stage extends from the M2 stage down to conditions P-T path is determined by the evidence for an early prograde of 550 - 650 °C and 5 - 7 kbar, with the formation of different history deduced from mineral textures and mineral composi­ types of corona and the replacement of kyanite by sillimanite. tions, and by estimates of P-T conditions for the two stages In the sheared and foliated rocks, biotite and hornblende crys­ of prograde and two stages of retrograde metamorphic as­ tals were produced by further retrograde hydration during M4, semblages. and they form a distinct new foliation, which truncates the At first, the granulites in the Leverburgh belt experienced M3 metamorphic texture (e.g., orthopyroxene + plagioclase a prograde path with increasing temperature within the silli­ corona). This foliation probably resulted from tectonic move­ manite stability field, and attained a thermal peak at M 1 (at ments after the M3 stage. However, the P-T conditions in­ regional P-Tconditions of830 - 880 °C, 8 - 10 kbar, in places ferred from cordierite corona of M3 and the secondary bi­ over 900 0C). The prograde part of the inferred P-T path is otite of M4 are similar, and hence, M3 and M4 metamor- 120 Evolution ofthe Lewisian Complex in South Harris, Northwest Scotland

P (kbar) M2 Pelitic Series). The various rocks showing different protoliths are observed in a narrow region, and are similar to those of 14 modern arc-trench systems. The idea that Archaean and Palaeoproterozoic rocks are similar to those in modern arc­ 12 trench systems has been well documented by several work­ ers (e.g., Helmstaedt and Scott, 1992; Kimura et aI., 1993; Windley, 1993 and 1995). Archaean oceanic assemblages 10 have comparable HFS/REE relative to MORB and oceanic plateaus, except for their being generally richer in FeO than 8 modern MORB (e.g., Storey et aI., 1991), and this supports the tectonic setting deduced from the chemical compositions 6 of mafic gneiss and the lithological constitutions discussed above. Cliff et al. (1998) concluded that the Leverburgh metasediments belonged to the Proterozoic supracrustal belt 4 of the Loch Maree Group, rather than to an Archaean 700 800 900 1000 400 500 600 metasedimentary component, as exposed in the mainland. In T("e) addition, the SHIC is tholeiitic - calcalkaline and alkaline in Fig. 2. Metamorphic P-T path of the Leverburgh belt (after Saba, nature, resembling that of igneous rocks in magmatic arcs on

1998; Saba, in press). AI 2 SiOs triple point is after continental margins (Fettes et aI., 1992). It is concluded that .Holdaway and Mukhopadhyay (1993). MI with "regional" the Leverburgh belt represents an Palaeoproterozoic accre­ and "in part" in the brackets indicates the regional peak tionary prism which formed during the subduction of an oce­ P-Tcondition (Saba, 1998) and maximum temperature estimated from Opx-Ky granulites (Saba, in press). Other anic plate in the magmatic arcs prior to regional metamor­ conditions of M2 and M3 are after Saba (1998). phism and deformation. The anti-clockwise P-T path for the Leverburgh belt is different from anti-clockwise terranes which involve isobaric phisms may be in part contemporaneous. Thus, there is an cooling with a pressure condition of 5 - 8 kbar (Bohlen, 1991; alternative possibility that these corona textures developed Harley, 1989), because the pressure increased along the pro­ only in a low-strain regime in the large deformational zone grade path up to 14 kbar. Furthermore, the observed retro­ related to the extensional uplift, rather than to simple tec­ grade reaction coronas are similar to those of typical tonic rebound. In the zone where deformation was enhanced, overthickened granulite terranes formed by continental colli­ hydrous minerals, which characterize the M4 stage, were de­ sion (Harley, 1989). The inferred P-Tpath can be explained veloped. Further detailed study for the interaction of the M3 by significant or substantial loading (collision) after and M4 stages is needed. magmatism, resulting in the South Harris granulites preserv­ Nevertheless, the M4 stage, which involved late-M3, is ing mineral textures consistent with an isothermal considered to be part of a later collisional event (e.g., second decompressional history which is typical of overthickened orogenic event; Ellis, 1987) associated with deformation, granulite terranes formed by continent-continent collisions. which enhanced exhumation and uplift of the South Harris An anti-clockwise P- T path in a granulite terrane is likely to high-pressure granulites. be characteristic of magmatically thickened and heated crust, such as a continental arc (Bohlen, 1987). I propose that the 5. TECTONIC MODEL OF THE SOUTH HARRIS South Harris anti-clockwise metamorphism reflects COMPLEX magmatism at a continental margin or island arc, followed by overthrusting during continental collision. The tectonic setting of precursors, the metamorphic P-T The evolutionary history of South Harris deduced from path of the Leverburgh belt, and the nature of the SHIC are the sedimentary precursors, igneous activity and metamor­ the keys to understanding the tectonic evolutionary history phic history, is as follows. of the South Harris complex. I) The South Harris complex was a Proterozoic mag­ The rock assemblages in the Leverburgh belt can be clas­ matic arc on a continental margin (Modell) or an island arc sified into the following types: melange (Rodel Series) with (Model 2), which was formed on a continental block above a oceanic-type volcanics; trench sediments with arc-type subducting oceanic plate; the different continental blocks volcanics (Benn Obbe Series (A) and (B)); and trench or eventually collided along the South Harris granulite belt af­ hemipelagic sediments with ocean-type volcanics (Chaipaval ter a temperature increase at the M 1 stage, due to the em- Sotaro SABA 121

------Subduetlon-- - magmatism Model 1: Magmatism Accretion ~. ~130km . SHIC s N Model 2: Accretion Magmatism

'" ~1i!ii!iiii!!iii!f(~130km

------Collision­ Model 1: Collision of small continent ~ "~145-50km

K-rich granitlc magma? s N Model 2: Collision of small continent ~ ~145-50km

K-rich granitic magma?

------Upllnft'---

s /' Erosion or extension N ~'''''~~I20''

Laxfordian granite?

------Uplift--- (shearing)

s S and N Uist Secondary collision Lewis N ~~

~ Pre-existing continent 1 D Granite and migmatite complex

~ Pre-existing continent 2 lIlIl Pre-existing continent 3

Fig.3. Two plate-tectonic models for the evolution of the Lewisian complex in the Outer Hebrides. Model I: Leverburgh belt underlies the continental margin and SHIC emplaced in the continental margin. Model 2: Leverburgh belt accretes the island arc and SHIC emplaced in the island arc. Possible pre-existing continent consisting of grey gneiss: I, grey gneiss complex which has U-Pb age of 2.77 Ga (Pidgeon and Aftalion, 1972) dominant in Lewis; 2, grey gneiss complex Pb-Pb age of 2.64 ± 0.12 Ga (Moorbath et aI., 1975) and Sm-Nd (CHUR age of 2.62 - 2.68 Ga (Whitehouse, 1990) dominant in Southand North Uist; 3, speculative grey gneiss complex not exposed now by erosion. 122 Evolution ojthe Lewisian Complex in South Harris, Northwest Scotland placement of the SHIC (Fig. 3. subduction-magmatism). comparable with the Nagssugtoqidian of both East and West There is no room here for detailed discussion about which Greenland, and the West Nain province of Labrador (Park et models to adopt, i.e., whether the magmatism occurred on a a!., i994; Myers. 1987; Korstgard et a!., 1987). There are continental margin or on an island arc, because the tectonic several important aspects for comparison, such as: 1) the se­ setting of the magmatism of SHIC was estimated mainly on quence of early shear zones followed by dyke-swarm em­ the characteristics of the major elements, not on the geochem­ placement, and then by main deformation, is common to all istry of the trace and rare-earth elements. four regions; 2) the rarity of early Proterozoic granitoid plu­ 2) During the collision, thrusting of continental crust over tons;3) a similar d.eformation history between the Nag­ the South Harris granulites caused the M2 high-pressure meta­ ssugtoqidian belt of east Greenland and the Lewisian com­ morphism at around ;2; 1.83 - 1.87 (Fig. 3. collision). plex (Nagssugtoqidian I and 11 events correspond to the 3) Following the collision, exhumation of overthickened Inverian event and to the Laxfordian 01 and 2 events respec­ crust occurred as a result of isostatic rebound or extensional tively). These correlations were established using the his­ uplift (Fig. 3). Sooner or later, a second collisional event tory of the mainland Lewisian complex. However, distinc­ occurred, and then exhumation and uplift of South Harris tive geological significances can be summarized as follows, complex took place (Fig. 3 shearing). especially in the Outer Hebrides,. This Palaeoproterozoic Lewisian history in South Har­ i) Very little information is available from the Outer ris, subduction - magmatism - continent-continent collision, Hebrides concerning the early sequence of the Inverian and is quite different from that of the mainland Lewisian. Laxfordian 01 events recognized on the mainland. 2) The Lewisian metasedimentary rocks of South Har­ 6. CORRELATION WITH THE NORTH ATLANTIC CRATON ris, Coli, Tiree and lona, more closely resemble the early Pro­ terozoic rocks in the Rinkian belt of NW Greenland and the The Lewisian complex has been considered to be closely metasedimentary rocks within the Nagssugtoqidian mobile

Archaean craton ~ Palaeoproterozoic ~;;';-':.:::":':':~ Okier belts (1.9-1.65 Ga)

TIS (Trans-Scandinavian igneous belt)

~ Younger belts (1.83-1.55 Ga) 500km

FigA. Simplified reconstruction of eastern Laurentia and Baltica for the period ca. 1.9-1.5 Ga (modified after Park, 1995). Abbreviations: NAC, North Atlantic craton; SUP, Superior craton; NQ, New Quebec belt; TN, Torngat belt; WNag, west Nagssugtoqidian belt; ENag, east Nagssugtoqidian belt; Ket, Ketilidian belt; Mak, Makkovik belt; Lap-Kola, Lapland-Kola belt. Arrows represent presumed main directions of upward tectonic transport within active belts at 1.9-1.85 Ga. Sotaro BABA 123

belt, rather than the supracrustal rocks of the Archaean gneiss plate-tectonic history for the regions between ca. 2.6 and 1.5 complex of Greenland. Ga. (I) 2.6-2.4 Ga: development of a conjugate shear-zone 3) The anorthosite complexes of South Harris, Ness and system in the North Atlantic Craton; (2) 2.4-2.0 Ga: rifting lona, do not resemble the widespread, distinctive Archaean and dyke emplacement in the older craton; creation of oce­ anorthosite complexes of Greenland (Bridgewater et al.,1976). anic and intracontinental basins; (3) 2.0-1.8 Ga: subduction 4) There do not appear to be any traces of Laxfordian­ at active margins of older cratons with creation of magmatic style granites and migmatites (Myers, 1971) in the arcs; collision of cratons accompanied by closure of intra­ Nagssugtoqidian mobile belt of East Greenland, nor are there cratonic basins and accretion of arc terranes; (4) 1.8-1.5: de­ equivalents of the 15.5 Ga post-tectonic, high-level intrusions velopment of a new active margin discordant to the previous of East Greenland in the Lewisian. ones, with significant change in convergence direction within 5) South Harris lacks a major tectonic event, such as the the amalgamated continental assembly. The 2.0-1.8 Ga event identification of any obvious representative of the Scourie was interpreted in terms of subduction and collision in dike suite, so that the classic stratigraphic marker from the Laurentia and Baltica (see Park, 1995). Furthermore, at 2.4 ­ mainland, which separates the two metamorphic episodes 2.0 Ga, ophiolite and mafic volcanics (ocean-floor material) (Peach et aI., 1907; Sutton and Watson, 1951), is absent (Fettes and passive-margin sequences and/or accretionary prisms et aI., 1992). were recognized in the new Quebec and the Lapland-Kola These observations imply that previous evolutionary belts. These events and their ages are very similar to the models for the Lewisian complex (e.g. Park and Tarney, early part of the evolutionary model for South Harris pro­ 1987), which involved reworking of Archaean sediments in posed here. From the viewpoint of the relation between the Palaeoproterozoic, are not applicable to the evolutionary Laurentia and Baltica, not from the Lewisian history, the sub­ history of the South Harris complex. In addition, the South duction and collisional event for South Harris is consistent Harris complex was formed during the Palaeoproterozoic, at with regional Palaeoproterozoic tectonics. least ca. < 2.4 Ga (e.g., Cliff et aI., 1998). Consequently, the Direct correlation between South Harris and other North Palaeoproterozoic tectonic history, deduced from the Atlantic regions can not be determined here, because many anticlockwise P-T history and precursor rocks of the similarities and dissimilarities are present. However, the plate­ Leverburgh belt, suggests that the South Harris complex tectonic processes of subduction - magmatism - collisions should be considered to occur in a suture zone, which has a for South Harris may relate to the Palaeoproterozoic assem­ different evolutionary history with a different P-T path, com­ bly of the various cratons of Laurentia, as proposed by pared with the mainland Lewisian complex. Hoffman (1990). Therefore, the postulated tectonothermal According to recent isotope studies, the Inverian event history of South Harris, including the Outer Hebrides, will recognized in the mainland took place at much higher granu­ be a key to clarifying the evolutionary history of the Lewisian lite-facies metamorphic conditions, than estimated previously complex, extending to the North Atlantic Craton, during (Corfu et aI., 1994; Friend and Kinny, 1995; Zhu et aI., 1997). Palaeoproterozoic times. Zhu et al. (1997) reported 1.7 Ga high-pressure metamor­ phism for metasedimentary rocks in the mainland Lewisian. ACKNOWLEDGEMENTS This metamorphic episode might have been related to similar tectonics in South Harris, such as a suture zone, although This paper is mainly based on my three published pa­ details are not clear. The Lewisian metasedimentary rocks in pers. I am grateful for reviewers who have improved the the mainland have been considered to be comparable with previous papers. I would like to thank M. Yoshida and col­ Archaean metasediments in the Nagssugtoqidian belt of NW leagues of Osaka City University for their encouragement. Greenland, although this needs careful re-comparison involv­ B.E Windley and Y. Hiroi are thanked for their constructive ing the P-T-t path and tectonic setting. 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Manuscript received October 29, 1998. Revised manuscript accepted February 9, 1999.