The mineralogy of a unique baratovite- and miserite-bearing quartz albite aegirine rock from the Dara-i-Pioz complex, northern Tajikistan

Metasomatic rocks of the Dara-i-Pioz alkaline complex in northern Tajikista n are known to contain a great variety of rare silicates enriched in Li, B, F and incompatible elements. One example is a quartz - albite - aegirine r ock from the Upper Dara-i-Pioz intrusion, which is interpreted as a product of metasomatic alteration of an alkaline syenite parent. The bulk of the r ock, an assemblage of aegirine-hedenbergite, albite, microcline and fluorap atite, formed during an albitization event. Unusual K-Ca silicates that cry stallized during this stage include baratovite KLi3Ca7Ti2[Si6O18](2)F-2, mi serite K-1.3(Ca,REE)(6)[Si-8(O,OH)(22)] (OH,F)(2) and turkesranite (K,Pb)(0 .5) (Ca,Na,REE,Pb)(2)(Th,U,REE)[(Si,Al)(8)O-20] (formulae given on the basi s of compositional data). Sr-bearing calcite postdates the albitization ass emblage, and may be a manifestation of a distinct metasomatic event (carbon atization). Conspicuous strain-induced features exhibited by the clinopyrox ene, quartz, baratovite, miserite, microcline and calcite indicate that the rock underwent deformation during the late stages of crystallization. Defo rmation-induced fracturing of the rock facilitated circulation of a relativ ely low-temperature fluid enriched in B, F, Ti and incompatible elements. T he fluid precipitated datolite, fluorite, quartz, and a rich diversity of r are minerals containing REE, Pa, Zr, Sn, Hf and Nb. The REE are concentrate d predominantly in the borosilicates tadzhikite Ca3.2-3.8LREE1.0-1.8Y0.1-0. 9(Ti,Fe,Al)(1.0)[B4Si4O20.3 (OH)(1.7)](OH)(2) and stillwellite- (Ce) LREE[B SiO5]. The new analytical data confirm the existence of two distinct minera l species, "tadzhikite(Y)" and "tadzhikite-(Ce)". The high-field-strength e lements and Ti are concentrated in titanite (less than or equal to 8.1 wt.% SnO2. 2.3 wt.% ZrO2 and 1.7 wt.% Nb2O5), bazirite (less than or equal to 2 .9 wt.% SnO2, 2.1 wt.% HfO2) and zircon (less than or equal to 3.2 wt.% HfO 2). The B-F-enriched fluid may have been derived from alkaline pegmatites t hat cut the metasomatic suite and contain a similar assemblage of rare mine rals.