Crustal melting processes and the formation of granulites and granites : a study based on the Lewisian complex, NW Scotland
The Lewisian complex of northwest Scotland contains the oldest known rocks of the British Isles and is commonly cited as a classical example of a high-grade gneiss terrane. The grey gneisses consist of a suite of tonalite, trondhjemite and granodiorite (TTG) rocks. The mainland Lewisian is divided into three - the northern, central and southern regions. The central region is known to have experienced a high-grade metamorphic event at 2490 Ma but may also have been affected by earlier events. The northern and southern regions only attained amphibolite grade. LILE depletion suggests that the Lewisian gneisses have partially melted.Additionally, published geothermometric and geobarometric estimates far exceed conditions required for anatexis of metatonalites, even in the absence of a fluid phase. There is also a suite of 'early' granite, trondhjemite and tonalite sheets (some of which contain garnet) in higher-grade parts of the complex that have been proposed as the products of this anatexis. A further suite of granitic rocks (late granite sheets) commonly occurs in the northern region. A series of fluid-absent, partial-melting experiments was carried out using three amphibolite-facies tonalitic and dioritic starting materials at P-T conditions thought to have been equivalent to the granulite-facies metamorphism (0.8-1.2 GPa and 800-1000 °C) in order to investigate the origins of these felsic sheets and the compositions of such partial melts. A series of H[sub]2 O-saturated experiments was carried out at temperatures around the wet solidus and 0.6 GPa. A further two experiments were carried out using a NaCI-H[sub]2 O fluid at 0.6 GPa to determine the affects that such a fluid would have at high grades. These experiments have been carried out in conjunction with petrographic studies into retrogression in the central region gneisses and a geochemical investigation into the compositions many rock types typical of the Lewisian. The experiments have produced thermometric estimates for the major granulite-facies and partial-melt-forming event of 950 ± 50 °c. Garnet stability in the restite leads to an estimate of maximum P of 1.1 ± 0.1 GPa. If the origin of the garnet in some of the early sheets can be shown to be the solid product of biotite breakdown then 0.9 < P < 1.1 GPa. For the Gruinard Bay area P < 0.9 GPa. The experiments and geochemistry show that it is unlikely that the early sheets are the products of partial melting of tonalitic protoliths. The limited extent of pristine granulites and the very high temperatures shown to have been attained suggest that the granulite-facies metamorphism was not a truly regional event and that temperature was locally higher where granulite-facies assemblages are preserved, probably in response to the introduction of mafic material. The late sheets are shown not to be the products of H[sub]2 O-saturated partial melting of tonalitic material similar to that exposed at the surface and it is likely that a layer containing significant quantities of K-feldspar lies beneath the northern region.