Isotopic tracking of the Western Carpathians Hercynian granitic rocks sources

The present-day structure of the Western Carpathians was derived from Late Jurassic to Tertiary (Alpine) orogenic processes connected with the evolution of the Tethys Ocean, in a long mobile belt sandwiched between the stable North European Plate and continental fragments of African origin. A typical feature of this mobile belt is the presence of huge reworked slices of Hercynian crystalline basement within the Mesozoic and Cenozoic sedimentary successions that have been deformed into large-scale nappe structures. Granitic rocks of various origins form an important constituent of these basement fragments. The Hercynian granitic rocks were related to distinct sources and/or geotectonic position. They present spatially developed granitic suites from subduction-related I-type, through syncollisional S-type to late- and post-orogenic A-type granites. The genesis and history of the local crust sampled by the granitoid rocks can be traced back to the Early Palaeozoic and/or Neoproterozoic times, consistent with a derivation from the north-Gondwanan margin. A complex study integrating petrological, geochemical and/or isotope data have been performed during last decades, resulting in distinguishing of following rock suites: a) the older sheared granitic rocks – orthogneisses (OG) with intrusive age 495–475 Ma; b) related mafic suite gabbros & diorites rocks (M-s) intrusive age 370 Ma; c) biotite granodiorite to hornblendebiotite tonalite (I-s) with intrusive age 365–355 Ma; d) two micas granodiorites to granites (Ss) intrusive age 360–350 Ma; e) biotite granodiorite to granite (A-s) intrusive age 270–260 Ma; and f) suite of specialised ore-bearing, biotite granodiorite to biotite-muscovite granite (S_s-s) intrusive age 265–250 Ma. The Sr isotopes with ⁸⁷Sr/⁸⁶Sr_(i) values 0.707–0.720 (OG), 0.702–0.706 (M-s), 0.704–0.709 (I-s), 0.706–0.714 (S-s), 0.705–0.709 (A-s), and 0.715–0.730 for S_s-suite suggest for significant crustal recycling and mantle related influence for mafic varieties of Carpathians granites. Similarly Nd isotopic characteristics with εNd_(i) values -7.4 to -2.0 (OG), +0.9 to +5.8 (M-s), -2.8 to +2.2 (I-s), -7.0 to -1.3 (S-s), -3.1 to +1.9 (A-s), -4.4 to -0.2 (S_s-s) indicate recycling of vertically zoned lower and/or middle crust with significant contribution from basic metaigneous rocks. The stable isotopes with δ¹⁸O_SMOW (in ‰) for OG = 11.0–11.7; M-s = 6.6–8.4; I-s = 7.6–9.9; S-s = 9.0–11.3; A-s = 7.8–8.0; and Ss-s = 9.9–11.5 together with δ³⁴S_CDT (in ‰) for M-s = +0.3 to +0.8; I-s = -2.9 to +2.6; S-s = -1.0 to +5.7; A-s = -2.0 to -0.7; and Ss-s = +4.5 suggest for mixed sources in metasedimentary and basic metaigneous rocks. The OG suite has δ⁷Li = -4.5 to +1.6‰ indicating crustal source. Mafic rocks (gabbros and diorites), associated with several occurrences of granites, are uniformly Li-rich and isotopically light (δ⁷Li = -0.5 ~ -3.7 ‰), precluding a direct derivation from the mantle, and require an explanation invoking an initial loss of original Li inventory, followed by a secondary enrichment in light Li via ingress of diffusing or percolating fluids. The Carpathian I-suite granites (δ⁷Li = -1.2 to +0.5 ‰) are on average isotopically lighter and show minimal scatter pointing to a homogeneous meta-igneous source; the S-type granites on the other hand (δ⁷Li = -3.2 to +7.0 ‰) testifying to highly variable meta-sedimentary/igneous precursors. The A-type granites are systematically heavier than the other types or even Earth’s mantle (δ⁷Li = +4.7 ~ +6.6 ‰), which could hint to a significant role of a material processed in a subduction event modified by slab-derived fluids. The S_S-suite of the orebearing granites with δ⁷Li = -0.42 to +1.22 ‰ looks to have a metapelitic parentage. The zircon Hf isotope study of the Western Carpathians granitic and related rocks brings following average εHf_(t) values: OG = -4.50 ±1.38 (St.dev.); M-s gabbro = +0.54 ±2.1; I-suite tonalites = -0.34 ±2.18; S-suite granites = -1.69 ±2.64; A-type granites = +0.55 ±1.65 indicating substantial crustal recycling and/or significant participation of mantle material as potential source for M-s, I-s, and A-s rocks types. Noteworthy, that mantle contribution to their genesis has rather character of melted mantle derived mafic lower crust than fresh input of mantle melt to the Devonian (Permian) subduction zone what suggest the Hf model ages of
zircons from these rocks.
   Acknowledgment: This work was supported by the Slovak Research and Development Agency under the contract No. APVV-0549-07, and partly by a grant of the Czech Science Foundation (GAČR 205/07/0992).