Fluid evolution in the nepheline syenites of the Ditrău Alkaline Massif, Transylvania, Romania

Abstract

The Ditrău Alkaline Massif (Romania) is located in 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 represent the youngest intrusion of the complex. Petrographic observations and fluid inclusion studies were performed on nepheline syenites in order to examine the evolution and the effect of the magmatic fluids on the alteration of nepheline to secondary minerals as sodalite, cancrinite and analcime. Fluid inclusion studies in nepheline, aegirine, albite and cancrinite provide evidence for the role of highly saline fluids in incongruent transformation reactions by which sodalite, cancrinite and analcime crystallize mostly on the expense of nepheline. The fluids, in most cases, can be modeled by the H2O-NaCl system with various NaCl contents; however inclusions with more complex fluid (containing also K, Ca, CO3, etc. besides H2O and NaCl) composition are abundant. Raman spectroscopic studies of daughter minerals in inclusions demonstrate the presence of alkali-carbonatic fluids in some of the earliest inclusions of nepheline, aegirine and albite. The alteration process is supported by the presence of H2O-NaCl 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 high salinity, carbonate rich solution that evolved to decreased salinity with time. The following observations support this:

• paragenesis of mineral phases and their fluid inclusions: the early phases have high salinity inclusions and the late phases have low-salinity inclusions

• the partitioning of chlorine depends on the pressure of the system: at about 2.0 kbars, the fluids coexisting with the melt have a high initial salinity and the salinity decreases with time; inclusions in nepheline show the lowest trapping pressure at ~2.5 kbars, hence the system has a high initial salinity and decreases with time

• aH2O increases with time, resulting in the formation of H2O-bearing phases in a late stage of the crystallization of nepheline syenites.