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, Kd = XF XBiotMs/XBiotFe ∼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 (∼32Q°C). F was absent in the experiments and may significantly affect the exchange equilibrium. Kd 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 ∼0.31 in gabbros to 0.25 in the most Fe-rich micas. Li is always < 260 ppm w. In the syenite series, F shows a near-linear inverse relationship with Fe/(Fe + Mg) which passes close to OF at Ann100 with ∼l.4 wt% F(0–7 F to 44 positive charges) at Ann44. Biotites in the gabbro unit (which forms an outer sheath to the intrusion) have relatively less F, probably because it was consumed by coexisting amphibole. δI8O is similar for both gabbros and syenites, and it is unlikely that an envelope fluid was involved in the reactions. G reaches a maximum of 0.3 wt. % in biotite except for that in one syenite sample with 0–7 wt. %. Calculation of relative F-OH fugacities from the reaction OH-phlogopite + F-annite = F-phlogopite + OH-annite, as calibrated by Munoz (1984), appears to suggest that each horizon in the layered series was in equilibrium with a slightly different fluid. In view of the intimate interleaving of these lithologies, this is improbable. The equilibrium constant of the exchange reaction, obtained from the experimental data, seems not to be appropriate to the Klokken assemblage, or to other examples of regular F-Fe avoidance. Explanations may include short-range Fe-Mg ordering in the natural examples or the effect of additional components in the fluid. F contents are high in comparison with biotites from calc-alkaline complexes; high magmatic F may account for the igneous layering common in the Gardar. Temperatures calculated from reactions involving fayalite and magnetite show that most biotites grew subsolidus. The F-poor annites grew > 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 calcalkaline and theleiitic intrusions, although more extensive fluid flow is indicated for the more permeable laminated syenites. The biotites preserve chemical variation indicating local equilibrium with other mafic phases, and halogens provide a useful marker of subsolidus fluid flow.