Evidence of Two Different Components in a Hercynian Peraluminous Cordierite-bearing Granite: the San Basilio Intrusion (Central Sardinia, Italy)

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Abstract
Detailed petrographic and geochemical data and Sr and Nd isotope compositions of enclaves and host-granite are reported for one of the largest strongly peraluminous cordierite-bearing intrusions of the Hercynian Sardinia-Corsica Batholith: the San Basilio Granite. Compared with other peraluminous series, the San Basilio Granite has a ‘non-minimum melt’ composition and shows variations primarily owing to fractionation of early-crystallized plagioclase, quartz and biotite. Crystallization age is constrained at ∼ 305 Ma, by Rb-Sr whole-rock age [305±23 Ma with (87Sr/86Sr)i = 0.71105±0.00041], and occurred during late Hercynian tectonic events. ∈Nd(305Ma) values range from −7.8 to −7.5. The San Basilio Granite contains both magmatic and metamorphic enclaves. Magmatic enclaves, similar to mafic microgranular enclaves common in calc-alkaline granitoids, are tonalitic in composition and show a variation in silica content from 60.3 to 67.7 wt % correlating with a variation in (87Sr/86 Sr) (305Ma) and ∈Nd (305 Ma) from 0.7092 to 0.7109 and from −6.6 to −7.4, respectively. Together with petrographic and other geochemical data, the Sr and Nd isotopic data record different stages in a complex homogenization process of an unrelated mafic magma with a crustal melt. A process of simple mixing may account for the variations of nonalkali elements and, to some extent, of Sr and Nd isotopes, whereas the distribution of alkali elements requires diffusioncontrolled mass transfer. Petrographic and mineralogical data on metamorphic enclaves and geochemical modelling for trace elements in granite indicate melt generation by high-degree partial melting involving biotite breakdown of a dominantly quartzo-feldspathic protolith at about T>750–800°C and P>6 kbar leaving a granulite facies garnet-bearing residue, followed by emplacement at ∼ 3 kbar. ∈Nd(305Ma) values of the granite fall within the range defined by the pre-existing metamorphic rocks but (87Sr/86Sr) (305Ma) ratios are lower, indicating involvement of at least two distinct components: a dominant crustal component and a minor well-mixed mafic end-member. These data point to a decoupling between the Sr-Nd isotope systematics and major and trace element compositions, suggesting that the effect of the mafic component was minor on granite major and trace element concentrations, but significant on Sr and Nd isotopes. The study of the magmatic enclaves and the isotopic evidence demonstrate that unrelated mafic magmas, probably derived from the mantle, had a close spatial and temporal association with the production of ‘on-minimum melt’ strongly peraluminous granites, and support the proposal that heat from the mafic magma contributed to crustal melting.

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