MASS BALANCE OF MAJOR ELEMENTS DURING METASOMATISM OF METAPELITES FROM NORTHERN SPITSBERGEN

Metasomatism leads to changes in chemical composition of rocks or their portions in presence of fluids. During this process different elements might be introduced to the system or removed from it. In this work we studied in detail metasomatic reactions that are recorded in metapelites from northern Svalbard.

In the studied samples zonation of metasomatic changes is observed. The most intense alterations occur near the fractures. Mineral assemblage varies from Grt-Bt-Qtz-Pl in zones that were not affected by fluids to Grt-Chl-Bt-Qtz-Pl in the most altered portions of the rock. In the later biotite is breaking down to chlorite, K-feldspar and titanite. Biotite is characterized by X_Fe­ from 0.55 to 0.6. On the Hay (1954) classification diagram chlorite analyses plot in the brunsvigite field. In order to calculate mass balance, a volume of main rock forming minerals was estimated using point count technique and image analysis. Plagioclase, quartz and biotite are comprising more than 95vol.% of the rock. Amount of garnet oscillates around 2vol.% and chlorite varies from 2.38vol.% to 0vol.%, respectively.

Using mineral volume, their chemistry and whole rock composition, mass balance was calculated using Gresens’s (1967) approach and the isocon method (Grant 1986, 2005). First, we have investigated redistribution of major elements during biotite – chlorite reaction. Based on obtained results the amount of H₂O in the system must have increased by at least 8% in order to form 2.4vol.% of chlorite as observed in the thin sections. Moreover, magnesium and iron must have been introduced to the system, while titanium and potassium released (resulting in formation of titanite and K-feldspar). The second method was based on the whole rock composition. Mass balance revealed that mainly potassium has been removed from the system (decrease of ca. 17%) while magnesium introduced (increase of ca. 13%) during rock – fluid interaction, which is consistent with the previous approach.

Acknowledgements: The study was financed from the statutory research project No 11.11.140.319 and 11.11.140.613, AGH University of Science and Technology.

References: Grant, J.A., 1986. Economic Geology 81, 1976-1982. Grant, J.A., 2005. Physics and Chemistry of the Earth 30, 997-1004. Gresens, R.L., 1967. Chemical Geology 2, 47-55.