COMPOSITIONAL EVOLUTION OF METASOMATIC GARNET IN MELILITIC ROCKS OF THE OSECNA COMPLEX, BOHEMIA

Zoned garnet occurs in melilitic rocks of the subvolcanic alkaline Ose cna complex, in northern Bohemia, Czech Republic. The first generation of garnet (I) consists of dark brown spots of Zr-bearing melanite gra ding into brown melanite. The second generation (II), a pale yellow (O H,F)-bearing titanian andradite, forms overgrowths on garnet I, locall y replaces it, or crystallizes in independent grains. It shows oscilla tory zoning of colorless (OH,F)-bearing Ti poor aluminous andradite, y ellow Ti-enriched (OH,F)-bearing andradite, and brown melanite. Garnet I is primarily dispersed in only slightly metasomatized melilitolite, micromelilitolite and polzenite, and in relics in all rocks of the Os ecna complex. Garnet II predominates in metasomatic derivatives of the melilitolite, i.e., phlogopitite, melilitolitic pegmatite and ijolite , in association with perovskite, titanian magnetite, calzirtite, apat ite, carbonate, zeolites, pectolite and other minerals. Melanite conta ins 9.0 - 12.7 wt.% TiO2, and Zr-bearing melanite has between 1.3 and 9.1 wt.% ZrO2. Titanian (OH,F)-bearing andradite contains 0.1-0.6 wt.% F and 0.9-1.8 wt.% H2O+. Both generations of garnet crystallized from fluids with low a(Si) and initially rather low but increasing f(O-2) and F, in a metasomatic reaction at the expense of melilite, phlogopit e and probably monticellite. An essentially two-phase assemblage of ph logopite + garnet (+/- titanian magnetite, perovskite) was formed in p hlogopitite, whereas a nepheline + calcite + aegirine-augite (+/- garn et, phlogopite) assemblage was generated in ijolite; the melilitolitic pegmatites consist of nepheline + phlogopite + garnet (+/- melilite, apatite, perovskite and calzirtite). The occurrence of the Zr-bearing melanite, Mn-enriched ilmenite and primary magmatic carbonates with de lta(13)C of -3.9 to -7.4 parts per thousand link the Osecna complex to mantle-derived carbonated alkaline ultramafic intrusions of the Maime cha-Kotui province, Magnet Cove and Marathon Dikes kindred.