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Hagdale crushing circle. Image courtesy of Steve & Heather Darlington

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Hagdale quarry and Nikka Vord. Image courtesy of Steve & Heather Darlington

 

Unst ophiolites

Significance: Ophiolites (from the Greek words "ophis" --snake, and "lithos"--rock) are fragments of the oceanic crust and mantle that are found preserved in the Earth's mountain belts. They provide valuable information about how ocean crust develops at spreading centers. In addition, because no ocean crust is older than about 200 million years, ophiolites represent the only source of information available about the oceanic crust prior to that time. Ophiolites are found throughout the world, including such places as Alaska, Argentina, the Balkans, California, China, Cyprus, Greece, Japan, New Guinea, Newfoundland, Oman, Taiwan, Tibet, and Turkey.

Ophiolites are geological windows into the history of the Earth and Earth processes. They provide important clues to how ocean basins formed and disappeared in the past and how the dynamic planet Earth's paleogeography (distribution of continental masses and oceans) looked many millions of years ago. The discovery of copper in ophiolitic volcanic rocks in the Mediterranean region ushered in the Bronze age in the history of human civilizations. Studies of ophiolites have advanced the methods and theories of geology for more than 200 years.

The Unst ophiolite has been regarded as fitting most of the characteristics of the ‘classic’ ophiolite model including magma chamber derived ‘sheeted dykes’ and the petrography of a supra-subduction zone. Recent work however (Flinn 2001), has shown that the Unst ophiolite is deficient in some aspects of the classic model and suggests a different provenance for this ophiolite.

Locality 1. Mu Ness. HP 638 013. Quasi-sheeted dykes. Coastal exposures.

            Mu Ness and Ham Ness consist of metagabbro and meta-microgabbro at the top of the Lower Nappe. The hill on Mu Mess is a klippe of Upper Nappe mantle metaharzburgite-serpentinite. A zone of meta-microgabbro cut by parallel and subparallel ‘sheeted’ mafic dykes outcrop along the coast of Mu Ness and Ham Ness.

            Most of the dykes range in thickness between 30cm and 60cm and appear to have more of a parallel aspect to the boundary of the metagabbro layer than a normal one expected of ‘classic’ ophiolites. In ‘classic’ ophiolite sequences sheeted dykes form at the top of the magma chamber to cut the gabbros above.

Flinn (2001) suggests there is no evidence of a magma chamber on Unst and has shown that the composition of many of the dykes is too primitive (boninitic composition) to have been derived from it. Most of these dykes have a composition pointing to partial melting of mantle material but no evidence has been found for a source of these in the mantle rocks of the Lower Nappe. From this it has been concluded that since the dykes cannot have been intruded into the upper metagabbro from below they must have been intruded from above.

During subduction of basin crust, water driven upwards from the descending slab into the hanging wall of mantle in the plate above caused the partial melting of the mantle wedge. The magmas so generated were boninitic or island-arc type and were intruded into the descending slab from above to form the sheeted dykes we see today.

Locality 2. The Taing of Norwick. HP 652 147. Obduction Thrust of the Upper Nappe.

The Taing is the obvious promontory within the bay at Norwick lying close to the obduction thrust contact between the mantle metaharzburgite of the ophiolite to the east (upper nappe) and the Dalradian metasediments to the west. The Taing forms part of a narrow band of serpentinite between the obduction thrust and the metasediments.

            Near the Taing a lamprophyre dyke cuts the garnetiferous Norwick Phyllite and shows well developed boudinage structures.

In the cliffs on northern shore of the bay is the thrust between the Skaw granite and the Dalradian schists. Loose boulders of the Skaw Granite may be seen on the beach. The Skaw Granite block is in sheared contact to the west with Dalradian metasediments. The granite itself is a deformed augen-granite in which red feldspar crystals up to 8 cm in length are set in a granulitic matrix of quartz and mica. Contained within the granite are xenoliths of metasediment of different provenance than those with which the granite is now in contact.

Locality 3. Hagdale Chromite Quarry. HP 639 103. Massive Chromite in dunite. Restored Crushing Circle (Horse Mill).

            Hagdale Quarry, once almost 100 feet deep, is now almost completely filled in and grassed over, however a small part of the northern and western quarry faces are still exposed as well as a few spoil heaps. Just to the west of the quarry is a recently restored example of a crushing circle dating from the early periods of chromite working on Unst. The eastern exit of the quarry contains the rusty remains of the quarry's more modern milling plant abandoned when ore extraction became un-economic.

            Professor Robert Jamieson may have been the first person to recognise the presence of chromite on Unst in about 1795. In 1817 the geologist Samuel Hibbert discovered commercial quantities on the hillsides north of Baltasound and a small shipment was made in 1823.

            Quarrying started in earnest in 1824 and ended in 1873 in which time about 50,000 tons of chromite ore was extracted, mainly from Hagdale Quarry which first opened in 1839 and was the largest chromite deposit worked in Britain.

            The horse powered crushing circle had been installed by 1850. This mill consists of a circular crushing stone of imported gritstone fitted with an iron rim that was held vertically by, and rotated around the end of, a horizontal shaft that was mounted on the central pillar. A horse harnessed to the other end of the shaft walked in a circle driving the vertical crushing wheel around a circular pan into which mined ore was shovelled for crushing. Water was channelled through the pan to flush away the lighter rock leaving the heavier chromite for recovery. Below the mill there may have been settling ponds to collect any chromite washed away.

            Quarries re-opened in 1908 and up until the First World War small quantities, the combined output of several scattered quarries, was exported from Unst for use by the chemical industry. After the war Blackwell Sandison Chrome Mines was formed and installed a milling plant from Cornish tin mines in a vain attempt to produce 45-50% chromite from waste and low grade ore. The plant included a Marsden fine crusher, a pair of Broadbent rollers, a four-compartment Hartz type jig and Record vanning tables. The mill was powered by reducer gas, produced from a coal retort, which fired an internal combustion engine. The engine drove the belts for vanning tables and the crushers as well as driving a generator for lighting.

            The ore, which had too high a silica content for the production of ferro-chrome and gave high rates of wear on machinery, became un-economic to ship due to a slump in freight rates which made higher grade foreign ores more competitive and the mill ceased operations in 1927. During this period of operation about 6,300 tons of ore were shipped from Unst.

            In 1938 there was a resumption in quarrying and both chrome and serpentine were shipped up until 1944, thereafter only serpentine was shipped with this period of production ceasing in 1969. During this period over 55,000 tons of serpentine had been produced for making refractory bricks for steel furnaces.

Locality 5. Nikka Vord Chromite Quarries. HP 625 104. Chromite concentrations in dunite.

            The track leads up the southern slope of Nikka Vord where approximately 8 small quarries are dotted over the hillside. Although small in area (about 8 by 4 metres) most are deep and flooded. The quarries are often at the end of trenches; these were dug to follow seams of chromite that terminated in pods of massive chromite in the dunite. The spoil heaps by the quarries contain chromite ore and other minerals. Seams of chromite can be found in outcrops around and about the quarries.

            The break of the slope, which runs SW-NE above the quarries, approximates to the boundary between the mantle material (harzburgite) and the dunite i.e. the Moho. If the metadunite was magma chamber rocks, as believed by some researchers, then this area would represent the base of the magma chamber.