The basic rocks of the Shetland Ophiolite Complex and their bearing on its genesis
Authors: Flinn D. 
Published: January 2010

Synopsis

The paper presents some of the results of a recent survey of the whole ophiolite complex and the adjacent basement. It combines previously published work with new field, petrographic and chemical results obtained principally from the mafic components of the ophiolite and offers an outline account of the evolution of the complex. The Shetland Ophiolite Complex occupies the largest part of the islands of Unst and Fetlar, part of the Shetland Islands lying north of Scotland. The complex is composed of two ophiolitic nappes, a lower nappe and an overlying upper nappe represented by erosional remnant klippen. The nappes are separated by and underlain by, imbricate zones of basement gneisses, ophiolitic rocks, metavolcanic and ophiolite-debris-bearing metasedimentary rocks. The lower nappe presents a 7 km continuous section through the ophiolite succession from a 2 km thick layer of mantle to the base of the sheeted dyke zone. The mantle layer has been uniformly and pervasively lizardite serpentinized (metaharzburgite), but has later been recrystallized to antigorite in a 100 m zone immediately above the basal obduction thrust. It is considered that obduction took place cold (<500°). Immediately below the obduction thrust are hornblende schists in a relationship to the nappe usually interpreted as a dynamothermal aureole in underlying volcanic rocks caused by obduction over them of hot nappe. However the Shetland hornblende schist is seen to be of MORB composition and to have been intruded up the obduction thrust into the serpentinized nappe, where it became hydrated and crystallized as hornblende schist. Above the metaharzburgite layer are successive layers of metadunite and metagabbro in igneous contact and each up to 2.5 km thick. The y are uniformly and pervasively hydrated to lizardite serpentinite and to amphibole and saussurite respectively. The absence of an intervening cumulate layered unit implies the lack of a magma chamber so that sheeted dykes and metamicrogabbros forming the uppermost levels in the lower nappe cannot have been derived from within the nappe. Also they are of compositions distinctly different to that of the metagabbro. Some of the dykes are of boninitic composition and more primitive than the metagabbro. Hornblende from the hornblende schists in the obduction thrust has given an age of
c.
500 Ma for the obduction. The uniformity and complete nature of the hydration of the nappe and the absence of any evidence that hydration was based on fractures needed to allow sea-water penetration is considered to preclude hydration when the ophiolitic slab formed sea-floor. It is suggested that the slab was first subducted to the east and hydrated by water driven upwards within the slab from lower levels and that the magmas forming the sheeted dykes and metamicrogabbro were created by water escaping from the slab into the overlying mantle wedge, followed by their intrusion into the slab from above. Closure of Baltica against Laurentia reversed the subduction and obducted the slab westwards onto Laurentia at about the same time that the older western Norwegian ophiolites were obducted eastwards onto Baltica. A period of subaerial erosion and deposition followed, during which sediments containing ophiolitic debris and volcanic rocks were deposited on the surface of the obducted nappe. Then further pressure from the east thrust a section of the already obducted slab westwards over the lower nappe and its cover of sediments to form the upper nappe, dragging with it slices of underlying ophiolite and basement to form the imbricate zone between the two nappes. The nappe pile was thrust further west-wards to form an imbricate zone beneath the pile and fold, deform and metamorphose the rocks of the imbricate zones. It is possible that the emplacement of the upper nappe coincided with the final closure of Baltica against Laurentia and the eastwards obduction of the younger west Norwegian ophiolites during the Scandian orogeny.