Lower crustal granulite xenoliths from the Pannonian Basin, Hungary, Part 2: Sr-Nd-Pb-Hf and O isotope evidence for formation of continental lower crust by tectonic emplacement of oceanic crust

Mafic granulite xenoliths from the lower crust of the Pannonian Basin are dominated by LREE-depleted bulk-rock compositions. Many of these have MORB-like Nd-143/Nd-144 but Sr-87/Sr-86 is elevated relative to most MORBs. Their delta(18)O values cover a wide range from + 3.8 to + 9.5parts per thousand. A group of LREE-enriched mafic granulites have higher Sr-87/Sr-86 (0.704-0.708) and lower Nd-143/Nd-144 (0.5128-0.5124), with higher delta(18)O values on average (+ 7.8 to + 10.6parts per thousand) than the LREE-depleted granulites. The LREE-enriched granulites are, however, isotopically similar to newly discovered metasedimentary granulite xenoliths. A sublinear correlation in epsilonHf-epsilon Nd isotope space has a shallower slope than the crust-mantle array, with the metasedimentary rocks forming the low epsilonHf end member; the radiogenic end is restricted to the LREE-depleted granulites and these overlap the field of MORB. Pb isotopes for the LREE-depleted samples are less radiogenic on average than those of the LREE-en-riched and metasedimentary xenoliths, and metasedimentary granulites have consistently higher Pb-208/Pb-204. The wide range in delta(18)O over a restricted range in Nd and Sr isotope values, in combination with the predominance of LREE-depleted trace-element compositions, is consistent with an origin as a package of hydrothermally altered oceanic crust. The existence of delta(18)O values lower than average MORB and/or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature, presumably in the oceanic lower crust. The low Nd-143/Nd-144 of the LREE-enriched mafic granulites cannot be explained by simple mixing between a LREE-depleted melt and an enriched component, represented by the recovered metasediments. Instead, we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments. A possible model is accretion of oceanic crustal slices during subduction and convergence, followed by high-grade metamorphism during the Alpine orogeny.