Alkali activity during charnockitization of biotite-hornblende gneisses from the Limpopo Belt (South Africa): petrologic and experimental study

 

Moiseeva E.I., Safonov O.G.

 

Lomonosov Moscow State University, Moscow, Russia; Institute of Experimental Mineralogy RAS, Chernogolovka, Russia

myzikant88@mail.ru

 

The Central Zone (CZ) of the Precambrian Limpopo Granulite Belt (LGB) experienced two regional high-grade events: D2/M2 at 2.67-2.53 Ga and D3/M3 at 2.02 Ga (Boshoff et al., 2006; Perchuk et al., 2008). Unlike the earlier event manifested by formation of large granitic bodies, the metamorphism D3/M3 was associated with highly localized granitization and charnockitization along shear zones. Thermobarometry of the charnockitized Bt-Hbl gneisses (Sand River gneisses) indicates that the local charnockitization took place at 5.5-6.2 kbar within the temperature interval 800-650^ОС synchronously with the isobaric heating and subsequent decompression cooling stages of the high-grade metamorphism in CZ (Boshoff et al., 2006; Perchuk et al., 2008) as a result of influx of deep-seated fluids along the shear zones. These parameters are close to the conditions of the “arrested” charnockitization pervasively observed in the Gondwanian metamorphic complexes of Southern India, Sri Lanka, and Antarctica (e.g. Perchuk et al., 2000; Ravindra-Kumar, 2004).

Charnockitization results in change of a rock bulk composition toward the granitic composition. Reaction textures in the rocks and fluid inclusions indicate that fluid-mineral reactions during charnockitization were predominantly controlled by high alkali activity (specifically, potassium activity) and low water activity in a brine fluid coexisted with a CO₂ fluid. Textural evidence for perfect mobility of alkalis during charnockitization is extensive K-feldspar veining accompanying the formation of orthopyroxene and/or clinopyroxene, as well as alkali-exchange equilibria such as Pl_>Ab + K₂O (in fluid) = Pl_<Ab + Kfs + Na₂O (in fluid) and Opx_>Al + K₂O (in fluid) + Qtz = Opx_<Al + Kfs (Perchuk, Gerya, 1993) which were conjugate with the amphibole and biotite breakdown. Activity of the chloride-rich brines is reflected in composition of amphibole and biotite, which become more Cl-enriched in the charnockites.

 Analyses of the mineral assemblages in terms of μ_K2O and μ_Н2O show an inverse relationship between water and potassium activities in the fluid during charnockitization (see Figure), as it would be expected if these parameters were controlled by salt concetration in the H₂O-chloride brine (e.g. Aranovich, Newton, 1997). As potassium activity increases and water activity decreases, the following sequences of mineral assemblages occur (see Figure): Bt+Hbl+Pl (initial gneiss) → Bt+Opx+Pl (transition enderbite GAB-8) → Opx+Kfs+Pl (charno-enderbite vein GAB-6), and also Bt+Hbl+Pl → Bt+Cpx+Kfs → Opx+Cpx+Kfs. The two pyroxene-K-feldspar assemblage reflects a maximal potassium and minimal water activities. Distribution of these assemblages in charnockitized zones reflects local variations of water and alkali activity of the fluid. It can be caused both by immiscibility phenomena in the H₂O-CO₂-salt fluid as well as by drop in temperature by 50-100^OC during the process at constant pressure. The above sequences seem to reflect two “waves” of the gneiss interaction with the H₂O-CO₂-salt fluids. The first one (lower trend in figure) corresponds to the main charnockitization stage at 750-800^OC finalized in partial melting with formation of secant charnockitic veins with “monzonitic” texture. Fluid inclusion data show a notable increase of the fluid salinity from the transition enderbite to the charno-enderbite vein. During cooling down to 650-700^OC melt crystallized exsolving residual concentrated brine and CO₂ fluid, which provoked further progression of the charnockitization under higher alkali activity (upper trend in figure).

The above model was verified by an experiment on interaction of the Sand River biotite-hornblende gneiss (LIM-173/1) with the H₂O-CO₂-KCl fluid at 750^ОС and 6.2 kbar using a piston-cylinder apparatus (NaCl-pyrex-graphite cell). Phase assemblages produced in the experiment are very similar to those observed in the natural samples, while their distribution within the run sample depends on proximity to the fluid and thermal gradient in the gneiss sample. Similar to natural rocks, formation of clinopyroxene after biotite accompanied by intensive K-feldspar veining in plagioclase corresponds to the highest alkali activity and lower temperature in comparison to those for the Opx+Kfs and Opx+Cpx+Kfs assemblages. Partial melting is observed in the run products in accordance with the natural rocks, as well.

Present petrologic and experimental data thoroughly support a model for charnockitization driven by alkali activity (Perchuk, Gerya, 1993) applied to the arrested charnockitization of gneisses (Perchuk et al., 2000).

 

Figure. Diagram μ_K2O – μ_Н2O for the charnockitic assemblages in the system CaO-MgO-Al₂O₃; FeO, SiO₂, TiO₂ in excess; K₂O, H₂O, Na₂O are perfectly mobile components. Arrows show a progress of charnockitization evaluated from the paragenetic analyses of different types of rocks. GAB-7 (1) and LIM-173/1 are initial Bt+Hbl gneiss; GAB-7 (2) – Cpx+Kfs assemblage locally developed in the gneiss; GAB-8 – transition zone, enderbite (Opx+Bt+Pl); GAB-7 (3) – Opx+Cpx+Kfs; GAB-6 (2) – charno-enderbite vein (Opx+Pl+Kfs).

 

The study is supported by the Russian Foundation for Basic Research (08-05-00354, 09-05-00991, 10-05-00040), Russian President Grant (MD-380.2010.5), and Russian Science Support Foundation.

 

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