Melting and metasomatic history of the lithospheric mantle in the Erzgebirge UHP terrane: Constraints from trace elements, Sr–Nd–Hf–Os isotopes and Lu-Hf dating

Garnet peridotites provide a unique insight into the early processes in collisional belts as well as crust-mantle interactions through their elemental and Sr–Nd–Hf–Os isotopic signatures. Here, we focus on (ultra)mafic rocks (peridotite, pyroxenite, eclogite and garnetite) from the Erzgebirge Crystalline Complex, Bohemian Massif, associated with diamond and coesite-bearing crustal rocks. We combine data from petrography, mineralogy, multiphase solid inclusions, major and trace element composition of rocks and minerals as well as Sr–Nd–Hf–Os compositions. We demonstrate that each peridotite type reveals a distinct process of the complex mantle evolution in terms of melting and depletion, superposed metasomatism/refertilization and its timing. Garnet peridotites show SCLM affinity as demonstrated by the presence of amphibole and its composition, high Cr# values of spinel, as well as bulk rock LILE enrichment, HFSE depletion, and flat to LREE-enriched REE patterns. The peridotite from Oberlochmühle records depletion after about 20–30 % of partial melting, while its present fertile character reflects superposed extensive modal metasomatism/refertilization by hydrous alkaline basaltic melts in supra-subduction zone setting. In comparison, the nearby Zöblitz peridotite has undergone about 10–20 % of melt extraction. In addition, the peridotite next to pyroxenite was subject to metasomatism by carbonated silicate (basaltic) melts expressed by the precipitation of garnet, clinopyroxene and phlogopite. Moreover, both lithologies show enrichment in incompatible elements (LREE, LILE – Li, Cs, Ba), U, Th, Pb, and negative HFSE (Nb, Ta, Ti) and Eu anomalies, and contain multiphase solid inclusions composed of silicates (amphibole, barian mica and olivine), carbonates (magnesite, dolomite) and LREE, U and Th-rich phases (apatite, monazite, thorianite). The radiogenic Sr-Os, but negative ε_Nd (down to −1.6) values detected in both pyroxenite and garnetite from Zöblitz indicate the presence of recycled crustal component in the source of metasomatizing basaltic melt. The Voigtsdorf eclogite composition corresponds to subalkaline basaltic rocks with radiogenic ε_Nd and ε_Hf values of clinopyroxene plotting close to mantle array. The Lu-Hf age of ⁓390 Ma obtained for Oberlochmühle peridotite reflects Devonian burial and mantle refertilization, while the ⁓370 Ma age for Zöblitz pyroxenite dates carbonated metasomatism close to UHP conditions. The latter provides evidence for the pre-Carboniferous deep crustal subduction and crust-mantle interaction.