HR: 0800h
AN: V31A-1417    [Abstracts]
TI: Fluid Evolution in the Nepheline Syenites of the Ditrau Alkaline Massif, Romania
AU: * Fall, A
EM: afall@vt.edu
AF: Virginia Polytechnic Institute and State University, Dep. of Geosciences, Fluids Research Lab, 4044 Derring Hall, Blacksburg, VA 24061 United States
AU: Bodnar, R J
AF: Virginia Polytechnic Institute and State University, Dep. of Geosciences, Fluids Research Lab, 4044 Derring Hall, Blacksburg, VA 24061 United States
AU: Szabo, C
AF: Eotvos University Budapest, Dep. of Petrology and Geochemistry, Lithosfhere Fluid Research Lab, 1/C Pazmany Peter setany, Budapest, 1117 Hungary
AB: The Ditrau Alkaline Massif (Romania) is situated in the central part of the Eastern Carpathians, as an intrusion in the Bukovina nappe system of the Mesozoic crystalline zone. Nepheline syenites are the most abundant rocks occurring in the central and eastern part of the Massif, and representing the youngest intrusion of the complex. The nepheline syenite is composed of perthitic feldspars, nepheline, biotite, amphibole, pyroxene and titanite as primary minerals, and sodalite, cancrinite, calcite, analcite as secondary minerals formed at the expense of nepheline. Petrographic observations and fluid inclusion studies were performed on nepheline syenites in order to examine the effect of residual magmatic fluids on the alteration of nepheline to secondary minerals listed above. The alteration of nepheline to secondary minerals is obvious from textural relationships and comparison of the compositions of the minerals. Fluid inclusion studies provide evidence for the role of highly saline fluids in the incongruent transformation reactions (nepheline to sodalite and/or cancrinite and/or analcite). The fluids, in most cases, can be modeled by the H$_{2}$O-NaCl system with various NaCl contents; however inclusions with more complex fluid (containing also K, Ca, CO$_{3}$, etc. besides H$_{2}$O and NaCl) composition are abundant in the nepheline. The alteration process is supported by the presence of fluid inclusions in cancrinite, showing lower salinity compared to those in nepheline. During the crystallization period of the nepheline syenites the rock was in equilibrium with a salty solution whose salinity increased with time, mostly by the loss of H$_{2}$O to produce H$_{2}$O-bearing minerals like amphiboles and micas. One possible interpretation of the fluid inclusions and textural observations is that nepheline alteration to sodalite, cancrinite and analcite was associated with increasing salinity of the fluids with time.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting