Two types of Ti-rich garnet peridotites within Bohemian Massif - Their differences, similarities and implications for terrane structure

Category Mineral processing
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Bakun-Czubarow, Nonna
Holding Date 23 September 2008

Within the Bohemian Massif there can be found Mg-Cr peridotites with Ti-rich garnets which preserved microtextural memory of deep origin. The peridotites are pyrope lherzolites, closely associated with layers of garnet pyroxenites and/or eclogites. They occur in Kutna Hora–Svratka complex (KHS) of Modanubian zone in Czech Republic as well as in Sowie Mountains Block (SMB) of Saxothuringian zone in the Polish Sudetes. In the westernmost part of KHS complex, peridotites under consideration outcrop in migmatitic gneisses. They display inequigranular texture with large (up to 8 mm in diameter) garnet porphyroblasts set in a fine-grained groundmass of olivine, pyroxenes and garnet. In cores of the garnet porphyroblasts there often occur topotaxial exsolutions of rutile, Mg-rich ilmenite, olivine and orthopyroxene, located mainly along {111} planes of the host garnet. The highest volume percentage of the exsolutions in garnet gives approximately 0.4 % of (rutile + Mg-rich ilmenite), 0.3 % of olivine and 0.2 % of orthopyroxene. The precursor for garnet rich in exsolutions was not supersilicic, but contained Al-deficient, Ti-rich molecules: Ca2Na(AlTi)Si3O12 and M3(MgTi)Si3O12, being indicators of ultradeep origin of the garnets. The KHS garnet lherzolites were incorporated into felsic host rocks under pressure of 4.5±0.4 GPa at 1100±30 °C. In SMB of the Polish Sudetes, the garnet lherzolites form small boudins in felsic granulites. The lherzolites contain big (up to 12 mm) garnet porphyroblasts with abundant inclusions in cores, that exsolved along {111} and {110} planes of garnet. The highest volume percentage of exsolved orthopyroxene is 0.4, while that of rutile and Mg-rich ilmenite equals 0.7. Before exsolution the precursor garnet comprised Ti-rich and Al-poor molecules. The SMB garnet lherzolites were incorporated into granulites under conditions of 2.5±0.3 GPa at 1050±40 °C. The KHS and SMB mantle-derived, depleted garnet lherzolites differ in conditions and time of their incorporation into felsic host rocks. For the provenance of the both KHS and SMB garnet lherzolites we can envisage the following scenario: a) the lherzolites were uplifted first from the depths of at least 200 km to the subcontinental litosphere (SCLM) beneath Tepla-Barrandia; b) then, in Devonian, bilateral subduction of Saxothuringia (which started earlier) and Moldanubia took place beneath Tepla-Barrandia; c) SCLM wedge lherzolites beneath Tepla-Barrandia underwent cryptic mantle metasomatism; d) incorporation of SMB lherzolites into felsic granulites and their rapid uplift took place during continental collision about 400 Ma ago, while KHS lherzolites were incorporated into felsic rocks and exhumed in Tournasian, 340 Ma ago. Thus, most likely, the Sowie Mountains Block belongs to the Tepla-Barrandia terrane of the Bohemian Massif. The results of recent paleomagnetic studies of the SMB support also this hypothesis.