ON THE POSSIBILITY OF OCCURRENCE OF MAJORITIC-LIKE GARNETS IN VARIOUS LITHOLOGIES OF UHPM COMPLEXES: EXAMPLES FROM BOHEMIAN MASSIF OF CENTRAL EUROPEAN VARISCIDES AND FROM SULU TERRANE OF EASTERN CHINA

Within the upper mantle, the pyroxene-›garnet transformation starting at the depths of about 200 km results in formation of majorite being new class of supersilicic garnet end-members that have one fourth of silicon in octahedral coordination (Ringwood & Major, 1971). Majoritic garnets are indicators of ultradeep origin (UDO) of their host rocks, that were equilibrated at deep levels of the upper mantle and within transition zone. During decompression majoritic garnets topotaxially exsolve rods of pyroxenes. The first majoritic garnets were discovered in ultramafic xenoliths of S. Africa kimberlites (Haggerty & Sautter, 1990), then they were recognized in UHPM peridotites in Western Gneiss Region of Norway (Van Roermund et al., 2000). Apart from the pyroxene exsolutions in UHP garnets there have been also recognized exsolutions of oxides (rutile, ilmenite, spinel) and apatite (YE et al., 2000). In the Bohemian Massif, majoritic-like garnets can be found in various lithologies of UHPM complexes: in small boudins of garnet lherzolites incorporated into HT/HP acid granulites of Sowie Mountains Block (SMB) in the Sudetes and in diamondiferous gneisses (Massonne, 2001) of Central Erzgebirge (CE). In SMB garnet lherzolites there can be met big (up to 12 mm) garnet porphyroclasts with abundant needles of rutiles and ilmenites as well as laths of pyroxenes in cores, topotaxially exsolved along the {111} planes of garnet. The former majoritic-like garnet had composition Pyr₇₁Alm₁₆Ca-mol₁₃ and showed enrichment in Ti (0.09 at. pfu) as compared to the garnet devoid of exsolutions (0.01 Ti pfu). In CE, similar exsolutions of TiO₂ phase and pyroxene (?) in cores of diamondiferous garnets can be observed. The Chijiadian (CH) garnet lherzolite with coesite eclogite pod outcrops in NE part of the Sulu UHP terrane. Sporadically, in garnet cores of this lherzolite the exsolutions of tiny Cr-spinels and orthopyroxenes can be observed. These exsolutions together with oriented rods of apatite found in CH eclogitic garnet point to the pre-existance of majoritic-like garnets. At present, for better understanding of extreme conditions of equilibration of UDO rocks containing majoritic-like garnets, new UHP experimental works in peridotitic systems with excess of TiO₂, Cr₂O₃, Al₂O₃ and P₂O₅ are needed (Zhang et al. 2003).