Lithium and boron in late-orogenic granites - Isotopic fingerprints for the source of crustal melts?

Geochemically diverse late-and post-Variscan granites of the Erzgebirge-Vogtland, the Saxon Granulite Massif, and Thuringia (Germany) formed by anatectic melting of Palaeozoic sedimentary successions and associated mafic to felsic volcanic rocks. The compositional diversity of the least evolved of these granites is largely inherited from the protoliths. We present Li and B-isotopic data of these granites and compare them with the isotopic composition of their protoliths, to investigate whether (i) there exist systematic differences in the Li and B-isotopic composition among different granite types and (ii) Li and B-isotopic compositions provide information on the granite sources complementary to information from the isotopic composition of Sr, Nd, and Pb and the trace-element signatures. Low-F biotite and two-mica granite types have flat upper continental crust (UCC)-normalized trace-element pattern with variable enrichments in Li, Rb, Cs, Sn, and W and depletions in Sr, Ba, and Eu. These signatures are least pronounced for the Niederbobritzsch biotite granite, which has the largest contribution of mafic material, and most pronounced for the two-mica granites. The granites show a relatively narrow range of delta Li-7 values (-3.0 to -0.5) and a broad range of delta B-11 values (-13.4 to +20.1). The delta B-11 values are lower in rocks with distinctly higher contents of Li, Rb, Cs, and Sn. The high delta B-11 of the Niederbobritzsch granite may be explained by the melting of former altered oceanic crust in its source. Relative to UCC, intermediate-F to high-F low-P granites show strong depletions in Sr, Ba, Eu as well as Zr and Hf, strong enrichments in Li, Rb, and Cs as well as Nb, Sn, Ta, and W, and REE pattern with stronger enrichments for HREE than for LREE. These granites show narrow ranges of delta Li-7 (-2.0 to +1.6) and delta B-11 values (-14.7 to -9.1), reflecting the smaller variability of the Li and B-isotopic composition in their source lithologies. The anomalously high delta Li-7 value (14.7) of one granite sample (Burgberg), which is similar to delta Li-7 values of its wall rocks (up to 14.5), may indicate late-magmatic fluid-rock interaction with external, wall rock-derived fluids. Because of the small compositional range of most source lithologies, the Li and B-isotopic variation in the granites is also small indicating that the isotopic composition of Li and B does not represent a particularly sensitive source tracer, with the exception of source lithologies characterized by extreme delta Li-7 or delta B-11 values. (C) 2014 Elsevier Ltd. All rights reserved.