Granitic Rocks of Ophiolites

Aldiss, Donald Tresham (1978). Granitic Rocks of Ophiolites. PhD thesis The Open University.



The field relationships, primary and secondary petrography and chemical composition of granitic rocks (plagiogranites) and associated formations in five ophiolites are described. In addition, the occurrence of similar rocks in other ophiolites, some island arcs and in the ocean crust is reviewed. Plagiogranites in ophiolites can be distinguished from all other granitic rocks by their almost complete lack of potassium feldspar, low alumina content, high K/Rb ratio and their LREdepleted, flat or "dished" rare earth profiles and negative europium anomalies. The hypothesis that ophiolites represent oceanic lithosphere fragments is supported by the strong resemblance of some granitic rocks d~edged from the seafloor to those in ophiolites.

Granitic rocks in ophiolites differentiate from subalkaline tholeiitic magmas at spreading axes in mid-ocean and in marginal ocean basins. These magmas evolved on a trend of ironenrichment unless FeTioxides appeared as fractionating phases, which seems to have followed saturation of the magma by seawater. The magma then became progressively enriched in silica and sodium, evolving by the removal of plagioclase, FeTioxides, apatite and zircon. However, potassium was partitioned into the magmatic volatile phase and so entered the open hydrothermal system at the spreading axis and was thereby not enriched in the plagiogranites.

Plagiogranites in ophiolites are all deuterically altered to assemblages typical or upper greenschist or lower amphibolite racies metamorphism. Hydrothermal activity in the vicinity or each magma cell beneath the spreading axis ceased soon after the last of the silicate liquid froze and there was little repenetration of the ophiolite plutonic complex by hydrothermal fluids.

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