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2.6 South Harris Anorthosite

[NG 059 849]

A Livingstone

Introduction

Scottish Lewisian rocks, including the Palaeoproterozoic South Harris Igneous Complex (SHIC), of which anorthosite forms the bulk, originated in the Laurentian Shield and possess multi-stage complex histories. On the pre-drift re-construction of Bullard et al. (1965) for the northern hemisphere, Herz (1969) recognised a belt of concordant massif-type anorthosites extending across a pre-existing Precambrian continent of Northern Europe, Canada and America. Herz (1969) noted the age similarities of these anorthosite complexes in Proterozoic terranes and regarded them a product of an unique event during Earth’s early history.

The SHIC is bordered by the Archaean Leverburgh and Langavat metasedimentary belts (1,890–1,87 Ma) and includes the highest point Roineabhal ([NG 043 863], 460 m) of the anorthosite massif. The area has been the subject of numerous regional, structural, geochronological, geochemical and petrological studies. Salient works cover more than eight decades including those by Jehu and Craig (1927), Dougal (1928), Davidson (1943), Dearnley (1963), Witty (1975), Fettes et al. (1992), Baba (1997, 1998, 1999) and Mendum (2009).

The Roineabhal anorthosite massif, with marginal ultramafites, layered gabbros and extensive anorthosite zones forms an elongated NW–SE 6 x 0.75–2.5 km triangular outcrop. Approximately 50 percent of the outcrop area constitutes the Roineabhal site (Mendum, 2009). The small selected anorthosite site, sharing in part a common boundary with the above site, exposes readily accessible, exemplary Roineabhal massif characteristics, namely ‘pure’ anorthosite, gabbro layering, schlieren, and metamorphic reaction products. Additionally, at the site, the vast anorthosite volume relative to that of the very minor layered gabbro differentiate reflects the internationally renowned anorthosite problem.

Ancient continents stabilised between 2,500–1,60 Ma and various mechanisms and components of plate tectonic models have been proposed to account for the complex geology of the Rodel–Leverburgh–Langavat area. The Lewisian of NW Scotland was part of a Proterozic orogeny and that continent to continent collision occurred (Mason, 2012, 2016).

Description

The study by Witty (1975) of the anorthosite massif (Fig. 16) revealed that the sheet-like intrusion, banded throughout, with marginal ultramafites and gabbros, passes into lower and middle gabbro–anorthosite zones followed by an upper leucogabbro–anorthosite zone. During later tectonic movements the anorthosite was folded into a tight upright antiform with the result that the banding is largely sub-vertical. At the site three small quarries, closely located, are within the leucogabbro anorthosite zone and two expose predominately massive ‘pure’ anorthosite with minor ferromagnesian aggregates, schlieren (Fig. 17) and gabbro layering (Fig. 18). The most westerly quarry resulted from an exploratory operation during the 1990s for reflective road stone whereas the third historic quarry worked white saussuritized anorthosite.

On a fresh surface, in the exploratory quarry, the granoblastic anorthosite is pale greyish–purplish although weathered anorthosite exhibits a pinkish–white hard coating. The anorthosite is uniformly medium-grained with glassy plagioclase ranging from 2 to 5 mm approximately. For the upper zone Witty (1975) ascertained a plagioclase composition of An 69–72. The anorthosite is never pure, containing up to approximately 95% plagioclase but with small amounts of deep olive-green amphibole. Also present are mafic schlieren and segregations, variable in size, with or without visible garnet (Fig. 17). When present in plagioclase-rich areas the garnet, up to 15 mm, possesses a striking green amphibole corona. Accessory minerals include clinopyroxene, scapolite, epidote, clinozoisite, biotite and chlorite. The second and smallest quarry, accessed immediately west off the Rodel-Finsbay road, exposes an example of narrow primary igneous layering (Fig. 18). On the opposite side of the road, in the historic workings, the third quarry, with a single worked bench, reveals white saussuritzed anorthosite devoid of mafic streaks. The quarry is situated on the northwestern edge of an extensive saussuritized zone, within which Davidson (1943) and Mendum (2009) reported zoisite, clinozoisite, epidote, paragonite, sericite and quartz. The former author reported that this saussuritized anorthosite is directly comparable with saussuritized anorthosites from Norway described by Kolderup (1904).

Interpretation

The SHIC magma comprises ultramafites, norite, layered gabbros, diorite and tonalite, and a vastly greater volume of anorthosite containing a basic calcium plagioclase. Early differentiation, due to heavy mafic mineral gravity separation, generated magma becoming progressively enriched in lighter plagioclase. A chronological sequence of events, changing pressure-temperature regimes followed by retrogression surprisingly left the anorthosite relatively unchanged.

The large volume of anorthosite at the site reflects the late stages in magmatic processes which commonly occurred in ancient shield areas, but were not generated in the classic Paleogene layered gabbro centres of Mull, Skye and Rum (Emeleus and Gyopari, 1992) emphasising that the South Harris anorthosite massif has more in common with Proterozoic anorthosite massifs around the world. However, in the Mull basic sheets a thick anorthosite reaction rim formed around mullite-bearing glassy xenoliths derived from melted aluminous pelitic crustal rocks. The anorthosite rims grew where an aluminous liquid became engulfed by basic basaltic magma (Dempster et al., 1999). On a large-scale Dempster et al. (op. cit.) postulated that when lower crustal aluminous lithologies, came in contact with basic magmas derived from melted mantle, a large plagioclase volume would be precipitated. Intrusion into higher levels of the large plagioclase precipitate during the Proterozoic would result in an anorthosite massif.

Anorthosite zircons yielded a U–Pb age of 2,491±31–2 Ma thus demonstrating that the anorthosite is considerably older than the adjacent meta-igneous bodies dated at 1,890–1,88 Ma (Mason et al., 2004). Using Sm–Nd dating Cliff et al. (1983) obtained ages of 2,180±6 Ma and 1,870±4 Ma when cooling occurred after granulite facies metamorphism. Baba (2004) predicted that the high-pressure metamorphism of the SHIC occurred shortly after emplacement. Further work by Baba (2012) on the Leverburgh psammitic rocks and anorthosite concluded that the 2,18 Ma age represented the date of emplacement whereas 1,87 Ma was the time of peak metamorphism.

Subsequent prograde granulite facies metamorphism generated garnet–pyroxene assemblages in the basic gabbroic variants and layered accumulates. Possible reaction routes are anorthite + olivine = garnet: and basic plagioclase + olivine + diopside = garnet + clinopyroxene. The association garnet–clinopyroxene in high-grade regional metamorphic terranes is a critical signature assemblage for granulite and eclogite facies conditions. From sapphirine–orthopyroxene–kyanite and orthopyroxene–sillimanite assemblages in the Rodel–Leverburgh metasedimentary belt Baba (1999) deduced peak metamorphism occurred at lower crustal depths of 25–30 km, under temperatures of 930–950O°C and pressures of 12 kbar. Additionally, he postulated a further downward movement of 10–15 km (3–5–kbar) due to continental collision. This additional temperature-pressure regime would, conceivably, transform a second granulite facies metamorphism into eclogite facies conditions (Davidson, 1943; Livingstone, 1967, 1976). Subsequent uplift retrogressed anorthosite assemblages to amphibolite and greenschist facies and was followed by Laxfordian hydrous metasomatic changes. Remarkably, apart from the saussuritized anorthosite zone, the massif shows relatively few effects of the intense metamorphic episodes.

The above temperature and pressure data lends credence to the view that the SHIC anorthosite formed during an arc-subduction episode as suggested by Garson and Livingstone (1973), Hollis et al. (2006) and Mendum (2009). Ashwal (2011) reviewed data from worldwide anorthosite massifs, including that of South Harris, and his conclusions supported the well-established view that the origin of anorthosite massifs is a ‘petrogenetic enigma’.

Conclusions

The small Rodel anorthosite site is an excellent mineralogical representative of an almost pure plagioclase rock which is very rare elsewhere throughout the UK. Minor amphibole–garnet bands, and clots, are present in the anorthosite. The South Harris anorthosite can be correlated with anorthosite massifs around the world. The anorthosite developed from a magma which, for reasons not yet understood, produced vast volumes of plagioclase rock rather than volcanic rocks. The magma resulted from continental collision and subduction mechanisms. The South Harris rocks were thrust deep into the crust to a depth of 35–45 km and subjected to high temperatures and pressures. Subsequent uplift to more hydrous levels and lower temperature/pressure regimes altered the minerals forming the bands and clots. Remarkably, the anorthosite plagioclase component remained virtually unaltered throughout all the changes.

The Rodel–Leverburgh–Langavat area, which embraces the anorthosite site, and Roineabhal massif, is an outstanding national and international example of Earth’s very early plate tectonic collision belt mechanisms.

Glossary

Schlieren: Tabular bodies up to tens of feet long in plutonic rocks. They possess the same minerals, but in a considerably different concentration than the surrounding host rock and may be darker or lighter than the host.