Alkaline Rocks and Geodynamics
Origin of A-type alkali feldspar granites is currently the subject of a world-wide debate. Contrasting hypotheses have been proposed, which range from an entirely crustal origin to an almost complete mantle derivation. A-type alkali feldspar granites belong to either unimodal granite (rhyolite)-dominated association, or bimodal gabbro (basalt)-granite (rhyolite) suite. It is argued that (i) the ultimate mantle origin of basic to intermediate rocks is beyond doubt, (ii) highly evolved felsic rocks may be produced by other processes besides crustal involvement through anatexis and (iii) large volumes of felsic rocks are a normal and direct consequence of extensive crystal fractionation processes. After a review of current hypotheses, it is concluded that A-type alkali feldspar granites are likely to be produced from mantle-derived liquids. Consideration of physical parameters (essentially density) of mantle, crust and liquids suggests that crust plays obviously a major role, not as a direct source for liquids, but operates as a density filter for migrating liquids and as a provider of the water required to generate silica oversaturated residual liquid and to fasten kinetics of differentiation processes. Anorthosite, rapakivi magmatism and A-type ring complexes are closely related in a coherent model of magma ascent and differentiation from the upper mantle up to the crustal surface.
Alkaline Rocks and Geodynamics
Origin of A-type alkali feldspar granites is currently the subject of a world-wide debate. Contrasting hypotheses have been proposed, which range from an entirely crustal origin to an almost complete mantle derivation. A-type alkali feldspar granites belong to either unimodal granite (rhyolite)-dominated association, or bimodal gabbro (basalt)-granite (rhyolite) suite. It is argued that (i) the ultimate mantle origin of basic to intermediate rocks is beyond doubt, (ii) highly evolved felsic rocks may be produced by other processes besides crustal involvement through anatexis and (iii) large volumes of felsic rocks are a normal and direct consequence of extensive crystal fractionation processes. After a review of current hypotheses, it is concluded that A-type alkali feldspar granites are likely to be produced from mantle-derived liquids. Consideration of physical parameters (essentially density) of mantle, crust and liquids suggests that crust plays obviously a major role, not as a direct source for liquids, but operates as a density filter for migrating liquids and as a provider of the water required to generate silica oversaturated residual liquid and to fasten kinetics of differentiation processes. Anorthosite, rapakivi magmatism and A-type ring complexes are closely related in a coherent model of magma ascent and differentiation from the upper mantle up to the crustal surface.
