The dual origin of I-type granites: the contribution from experiments

New laboratory experiments using granulite xenoliths support a dual origin for I-type granites as primary and secondary. Primary I-type granites represent fractionated liquids from intermediate magma systems of broadly andesitic composition. Fluid-fluxed melting of igneous rocks that resided in the continental crust generates secondary I-type granites. The former are directly related to subduction, with Cordilleran batholiths as the most characteristic examples. Experiments with lower crust granulite sources, in the presence of water, show that amphibole is formed by a water-fluxed peritectic rehydration melting reaction. Entrainment of only 10% of restites composed of amphibole, pyroxene, plagioclase and magnetite, is sufficient to account for discrepancies in aluminium saturation index and maficity in secondary I-type granites. Lower crust granulite xenoliths, attached to a sanukitoid containing 6 wt% water, have been used in two-layer capsules to test fluid-fluxed melting reactions as the origin of secondary I-type granites. It is proposed that sanukitoid magmas act as water donors that trigger extensive melting of the lower crust, giving rise to granodioritic liquids. Because primary granites are related to coeval subduction, and secondary ones are crustal melts from older subduction-related rocks, the distinction between both I-types is essential in tectonic reconstructions of ancient orogenic belts.

The Lundy granite: a geochemical and petrogenetic comparison with Hercynian and Tertiary granites

New chemical data show that the two main granite types (G1 and G2) cannot be discriminated, but that microgranite sheets/dykes (G3) are significantly different and more evolved, largely as a result of biotite, accessory mineral, and plagioclase fractionation. The Lundy granite is similar to other Tertiary granites of Scotland and Ireland, in age, setting, possible high-temperature mineralogy, relationship to basic magmatism, and REE patterns. These features and a highly evolved chemistry suggest derivation from an unexposed more ‘primitive’ granite that, in turn, had a basaltic parentage. However, similarities with the nearby S-type Hercynian granites, such as high aluminium saturation index (and normative corundum), high trace alkali, Nb, and F contents, low Zr, and high initial Sr ratio suggest a significant crustal component. The problem is resolved by proposing either mixing of silicic magma derived by strong fractionation of basaltic magma with anatectic magma from a pelitic/semi-pelitic crustal source, or fractionation of basaltic magma heavily contaminated by assimilated crustal material. Both origins would yield the high REE contents and fiat REE patterns of a ‘primitive’ granite magma. Fractionation, perhaps of hornblende initially, and later, of biotite and accessory minerals together with feldspars, would produce the small volume of highly fractionated Lundy granite.