Tourmaline Chemistry In Diamond-Bearing UHP Pelitic Gneiss

Tourmaline is a minor but a ubiquitous mineral that often preserves metamorphic history. Because of its chemical diversity and refractory property, metamorphic tourmaline often shows zonings reflecting P-T and fluid conditions and bulk chemistry. Recently we found `K-rich tourmaline' coexisting with microdiamond in a tourmaline-rich quartzofelspathic rock from the Kokchetav Massif, Kazakhstan (Shimizu and Ogasawara, 2005). This suggests that K- tourmaline was stable under the diamond stability field, and its compositions are potential UHP indicator. So here we reconsider chemistry of tourmaline in Kokchetav UHP gneisses. Tourmaline in the tourmaline-rich quartzofeldspathic rock (no. A6) displays discontinuous chemical zonation. The core part of tourmaline exclusively contains diamond inclusions and shows K-dominant composition (K > Na + Ca). The K2O contents in the core are up to 2.75 wt%, which is extremely high among metamorphic tourmalines. The graphite-bearing domains (mantle and rim) of tourmaline in this rock contain lower amounts of potassium; mantle: 0.53 and rim: 0.16 wt%. Pelitic gneisses in the Kokchetav Massif have various mineral assemblages. Constituent minerals of diamond- bearing gneisses are Qtz, Grt, and Dia ± Cpx, Ky, Phe, Bt, Tur, Kfs, Pl, Zo, Rt, Ttn, Zrn, and several secondary minerals. Microdiamonds are included mainly in garnet, zircon, clinopyroxene, and kyanite. The other UHP evidence is coesite in zircon and phlogopite lamellae in clinopyroxene. Tourmaline shows several occurrences; 1) subhedral to euhedral porphyroblast, 2) well developed prism in leucocratic veins, and 3) anhedral crystal at grain boundaries of garnet and quartz. The former two types have similar dravitic compositions (Mg/Mg+Fe = 0.7-0.9) and zoning patterns. Core parts have considerably K-rich composition (0.4-0.2 wt% K2O, i.e. 0.08-0.05 apfu) and rim parts have less than 0.15 wt% K2O. CaO increases at the rims up to 2.0 wt%. These two types are also characterized by relatively high amounts of Ti (0.14 apfu) and low X-site vacancies. Thus, the zoning patterns and chemical property of these tourmalines are similar to those of mantle-rim of K-rich tourmaline in the tourmaline-rich quartzofeldspathic rock (A6). On the other hand, Anhedral tourmaline (type 3) does not contain potassium and have intermediate compositions between dravite- schorl-uvite (Ca- and Fe-rich compared to potassium-bearing tourmaline). Microdiamond is currently not found in all tourmalines in pelitic gneisses. Quartz inclusions are abundant in tourmaline. Based on inclusion contents and chemical compositions, tourmaline in gneisses probably formed in graphite stability (i.e. during and/or after exhumation). The zoning of tourmaline may show retrograde patterns as in the K-rich tourmaline. However, stability and phase relation of K-bearing tourmaline are still unclear. Whereas Schertl et al. (1991) and Reinecke (1991) report tourmaline coexisting with coesite from the Western Alps, their K2O contents are negligible (< 0.1 wt%). This suggests that control factors for K2O contents in tourmaline are not only pressure. Timing and condition of formation of tourmaline also remain a mystery. The above questions will be discussed by comparing chemical and textural information. Although further research is required, tourmaline in UHP gneiss may give implication to behavior of boron in deeply subducted crustal material.