Compositional zoning in hydrothermal aegirine from fenites in the Proterozoic Gardar Province, South Greenland
Ranløv, Jens; Dymek, Robert F.
European Journal of Mineralogy Volume 3 Number 5 (1991), p. 837 - 854
published: Oct 2, 1991
manuscript accepted: Apr 8, 1991
manuscript received: Jul 27, 1990
ArtNo. ESP147050305007, Price: 29.00 €
Abstract Compositionally zoned Ti-bearing aegirine is a common constituent in fenites and hydrothermal veins and vugs associated with nepheline syenites from the mid-Proterozoic Gardar Province of South Greenland. A typical assemblage in these mineralizations includes quartz, albite, microcline, aegirine, calcite, stilpnomelane, apatite, monazite and Nb-Fe-Ti-Mn oxides, with a wide range of unusual REE-Zn-Be-Ba-Mn-silicates as accessory phases. Individual aegirine crystals display evidence of sequential growth, concentric- and sector-zoning that represent large variations in wt. % TiO2 (0.1 - 3.0), Na2O (11.6 - 12.8), CaO (0.0 - 2.0), FeOt (25.4 - 30.9) and Zr02 (0.0 - 0.74). Concentrations of Al2O3 (<1.34 but typically less than 0.7 wt. %) and MgO (<0.76 but typically less than 0.4 wt. %) are both very low. In terms of pyroxene end-members, these compositions correspond to from 70-95 mole % NaFe3+[Si2O6] (Aegirine), <1-6 mole % NaAl[Si2O2] (Jadeite), <1-8 mole % Ca (Mg,Fe2+)[Si2O6] (Enstatite-Ferrosillite), and from < mole % of an unnamed end-member having the composition Na(Mg,Fe2+)0.5(Ti,Zr)0.5[Si2O6]. As such, the principal chemical variation in the analyzed crystals involves the substitution: VI(Fe2+,Mg) + VI(Ti4+ = 2VI(Fe3+) in the M1-position. Textural relationships, together with the chemical inhomogeneities of the pyroxenes, suggest rapid nonequilibrium crystallization from a hydrothermal fluid of changing composition and/or temperature.