Biotite Equilibria and Fluid Circulation in the Klokken Intrusion

Abstract

Chemical variation in biotite from the KJokken gabbro-syenite intrusion in the Proterozoic Gardar province in South Greenland has been investigated by electron probe and, for F and Li, ion microprobe. Most mica occurs in small amounts as fringes on ilmenomagnetite or fayalite, rarely as an intercumulus or poikilitic phase. The micas range continuously from Phlog70Ann30 in a gabbro, to Phlog4Ann96 in the most evolved (slightly persilicic and peralkaline) syenite. In the syenites Fe-Mg partitioning between biotite and olivine can be described by a single distribution coefficient, K6 = Xfi X%? /X^-Xf°'~3, suggesting that these reactant phases mix ideally at the reaction T. Experimental data for Fe-Mg exchange via aqueous chloride solutions (Schulien, 1980) imply low T (~ 320 °Q. F was absent in the experiments and may significantly affect the exchange equilibrium. K6 in the gabbros is ~ 1, consistent with equilibrium via a fluid depleted in F because of crystallization of large amounts of amphibole. Al, Mn, and Ti vary regularly throughout the series and can be used as markers of cryptic variation in the layered syenites. (Al + Si): 22 O is always in the range 7-7—7-85. A1/(A1 + Si) decreases from ~O31 in gabbros to 0-25 in the most Fe-rich micas. Li is always 300 °C subsolidus even when texturally intercumulus. Stable isotope data are consistent with the separation and retention of a deuteric fluid during the final stage of magmatic crystallization. Klokken was not generally subject to the pervasive, long-range (in both distance and time) dydrothermal interactions demonstrated in