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Department of Geology, University of Leicester, Leicester LE1 7RH, UK
School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA
The segmentation of the Lewisian complex which occurred at the end of the Archaean resulted in the juxtaposition of zones representing different crustal levels, including deep crustal granulites. Critical aspects of the geochemistry of Lewisian gneisses are considered in relation to their petrogenesis and tectonic models. Most of the Lewisian is made up of a bimodal suite of tonalitic-trondhjemitic gneisses enclosing mafic-ultramafic enclaves and layered complexes which are often associated with metasediments. These mafic components have a low-pressure petrogenesis and represent tectonic inclusions incorporated into the complex at the time of gneiss generation. They are more abundant in the granulites. The silicic gneisses have a higher pressure petrogenesis and their geochemistry is consistent with hydrous partial melting of amphibolites in a shallow-dipping subducting slab with hornblende as a residual phase.
Granulite metamorphism occurred or peaked some 200 Ma or so after the phase of gneiss generation at 2.9 Ga and terminated about 2.5 Ga. Concentrations of U, Th and Rb are at least an order of magnitude lower in the granulites than in the equivalent amphibolite-facies gneiss zones, and those of K and Pb about 50% lower. Concentrations of other elements are not significantly different. Currently popular models for the removal of these elements are not significantly different. Currently popular models for the removal of these elements from the deep crust through partial melting or transport in a carbonic fluid phase are examined, but each has problems in accounting for all the available data. An alternative model, considering the low contents of heat-producing elements as a primary feature of silicic magmas emplaced into the deep crust, is explored. This relies more on fluid processes removing heat-producing elements from the ocean crust during subduction, followed by hydrous melting of the amphibolite to produce water-saturated magmas which cannot rise to high crustal levels. In contrast to modern subduction zone magmas, which are largely derived from the mantle wedge that has been modified by addition of a fluid-transported, LIL element, ‘subduction component’, Archaean sodic tonalite-trondhjemite magmas result from hydrous melting of amphibolite which has lost that component. Granulite-facies metamorphism is an independent process that is commonly superimposed on deep crustal rocks which are already low in heat-producing elements, and during which further loss of these elements may occur, but granulite-metamorphism may also be superimposed on more normal crustal rocks without significant chemical changes.
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  H. R. Rollinson Tonalite-trondhjemite-granodiorite magmatism and the genesis of Lewisian crust during the Archaean Geological Society, London, Special Publications, 1996; 112: 25 - 42. [Abstract] [PDF]
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