Fingerprinting sources of orogenic plutonic rocks from Variscan belt with lithium isotopes and possible link to subduction-related origin of some A-type granites

Lithium (Li) elemental and isotope data are presented for a suite of plutonic rocks (I-, S-, A- and SS-type, coexisting mafic bodies) from the Western Carpathians, Slovakia, that were generated throughout the complete Variscan orogenic cycle. I-type granites show a limited range of δ7Li, contrasting with the large variations found in S-type granites and orthogneisses of distinct ages. This contradicts previous reports of predominantly light-Li S-type granites, sourcing metasedimentary lithologies, and isotopically heavier I-type granites, derived from meta-igneous sources. An almost exclusively heavy Li isotope signature (δ7Li > 4.7‰) found in four out of five A-type granites rules out several commonly accepted petrogenetic scenarios such as remelting of granulitic residue after formation of I-type granitic melts, anatexis of calc-alkaline meta-igneous crust or extensive closed-system fractional crystallization of mantle-derived magmas. Instead, it most probably reflects a derivation of A-type granites from a mantle wedge modified by slab-derived fluids during an Early Variscan or Pan-African subduction episode. In contrast, the distinct and lower δ7Li values (< 1.2‰) in the contemporaneous SS-type granites can be related to their likely pelitic parentage.Mafic rocks (gabbros and diorites), associated with several occurrences of granites, are uniformly Li-rich and isotopically light (<−0.5‰), precluding a direct derivation from the mantle. These signatures testify to their cumulate origin whereby kinetic effects may be a viable explanation for the light Li isotope compositions, associated with diffusive redistribution of Li between mantle-derived mafic melts and acid magmas. This corroborates recent studies on faster diffusion of 6Li compared with 7Li in natural systems.Taken together, the presumed dichotomy between the sources and processes leading to generation of S- and I-type granitic magmas does not seem to be reflected by Li isotope signatures in a simple and globally valid manner. Interpretation of Li isotope compositions thus needs to be paralleled by other available information on petrology, whole-rock geochemistry and the magmatic context.