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 delta Li-7, 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 (delta Li-7 >4.7 parts per thousand) 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 delta Li-7 values (<1.2 parts per thousand) 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 parts per thousand), 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 Li-6 compared with Li-7 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. (C) 2010 Elsevier B.V.. All rights reserved.