GRANITE MAGMAS - THEIR SOURCES, INITIATION AND CONSEQUENCES OF EMPLACEMENT

Geochemical and other evidence shows that granite magmas are generally derived from a mixture of melted crust and mantle material and the existence of a spectrum of types varying from largely mantle-derived to mainly crustal-derived is emphasized. Each pluton possesses a unique mix or range of mixtures which preserves an adumbrate history of the deeper levels of the crust and mantle at the particular location. Four illustrative granite-forming provinces are considered: (1) Archaean cratons with their voluminous tonalites, (2) magmatic arcs as exemplified by the western Americas, (3) the British Tertiary Province as an example of the results of extensional faulting and (4) the British and Irish Caledonide granites, the main phase (440-390 Ma) of which was produced during major Acadian strike-slip faulting. All these granites provide evidence of mixing of source materials. Geochemistry, especially of the Rare Earth Elements, U-Pb ages and isotopic studies of Sr, Nd, Pb, O, H and in some volcanic rocks Be, is beginning to enable quantification of the source components. The importance of major fracturing of the crust and mantle in providing magma triggers, magma focusses and magma consolidation sites is stressed; major batholiths probably occupy holes created by pulling apart the crust. Geophysical evidence does not suggest that granite plutons are distributed throughout the thickness of the crust, but rather that they are concentrated relatively near the surface. The extant elevation of the whole of the north British crust, its altitude over the last 400 Ma and even in places the present shape of the coastline, are significantly influenced by Caledonian granites, the tops of which are generally within about 3 km of the present landsurface. Thus relatively little erosion of most of these bodies has taken place and the dominant sources of post-Silurian clastic sediments in the area were stripped off volcanic cover rocks and recycled sediments and metasediments. By inference, other continental areas with numerous exposed granite pluton roofs had a similar history. The process of granite formation is an important means of density-stabilization of the crust with high-level emplacement of light sialic granite refined out of the mantle and lower, middle and even upper crust. The refractory mafic residues generally become denser and sink or stay down in the density-stratified crust but the division into residue and magma is not sharp with some granite magmas including significant quantites of restite.