SOME GEOCHEMICAL FEATURES OF METASOMATIC TOURMALINE RELATED TO PEGMATITES FROM ROMANIA

The present paper deals with a geochemical comparison between the tourmaline hosted by metasomatic veins, crossing some pegmatite bodies from Romania and the so-called proto-pegmatitic tourmaline. In Romania, pegmatite bodies occur as veins and lenses in the medium-grade metamorphic terranes (micaschists, gneisses, migmatites etc.) of Southern, Western and Eastern Carpathians; generally, they have a simple mineralogy as follows: quartz, feldspars, muscovite as principal minerals, biotite, garnets and tourmaline as subordinate minerals. Pegmatites show a granite-like chemical composition; genetically, it seems that the pegmatites formed by both metamorphic differentiation and anatectic processes (MÂRZA, 1980; MURARIU, 2001; STUMBEA, 2001). The study we carried out is based on electron microprobe analysis (major elements) performed on a CAMECA SX 50Link Systems device; the analysis focused on the rim as well as on the core of tourmaline grains. In terms of the variation of major elements from core toward the grain rim, a decrease of SiO2, FeO and Na2O and an increase of Fe2O3 and MgO has been found in metasomatic tourmaline; as for proto-pegmatitic tourmaline, the chemical composition of zoned grains showed a decrease of Al2O3 and FeO and an increase of Fe2O3, MgO and Na2O amounts from core toward the grain rim. The analyses performed on tourmaline grains show also some differences between the chemical features of metasomatic tourmaline cores/rims and those of proto-pegmatite core/rims. Thus, the core of metasomatic tourmaline has higher amounts of SiO2 (36.60%), TiO2 (0.60%), FeO (9.00%), MnO (0.40%), MgO (4.50%) and Na2O (2.70%) than the core of proto-pegmatite tourmaline (35.30% SiO2; 0.28% TiO2; 8.40% FeO; 0.10% MnO; 2.80% MgO and 1.80% Na2O). On the contrary, the amounts of Al2O3 and CaO are lower in the core of metasomatic tourmaline (33% Al2O3 and 0,10% CaO) as compared to the amounts of the same oxides in the core of tourmaline from proto-pegmatites (35.4% Al2O3 and 0.25% CaO). The variation of chemical composition is almost similar when focusing on the rims of tourmaline grains, which proves that the distribution of major elements between the core and the rims of zoned tourmaline is the same, no matter the genesis of tourmaline is. The mineralogical composition of tourmaline grains (mol%) reveals the presence of schorl, dravite, uvite, tsilaisite, ferridravite and alkali-deficient tourmaline end members. In terms of the mineralogical composition of cores vs. rims, both metasomatic and proto-pegmatitic tourmaline show higher amounts of schorl (58.7 mol%) and tsilaisite (1.2 mol%) end members and lower amounts of dravite (5.9 mol%), ferridravite (8.2 mol%) and uvite (4.3 mol%) in cores, than in the rim of grain. On the other hand, the core and the rim of metasomatic tourmaline contain lower amounts of schorl (52% in core, 45% in rim), ferridravite (7% in core, 8% in rim) and uvite (2% in core, 1% in rim) as compared to the core of proto-pegmatitic tourmaline. Structural formulas of tourmalines reveal a smaller deficit in X sites (about 0.15 pfu) as well as in Y sites (about 0.16 pfu) than the proto-pegmatitic tourmaline (about 0.25 pfu in X sites and about 0.21 pfu in Y sites). The present study revealed also three type of chemical substitution: Tschermak substitution – (Mg, Mn, Fe 2+ ) Y + Si T = Al Y + Al T , alumino-buergerite substitution – (Mg, Mn,