Fingerprinting sources of orogenic plutonic rocks from Variscan belt with lithium isotopes and possible link to subduction-related origin of some A-type granites
Abstract
Lithium data are presented for plutonic rocks 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.
An exclusively heavy Li isotope signature (δ7Li >4.7‰) found in A-type granites rules out several commonly accepted petrogenetic scenarios. It most probably reflects a derivation of A-type granites from a mantle wedge modified by slab-derived fluids.
Mafic rocks (gabbros and diorites), associated with 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.
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.