Role of Alkalies and Ca2+ Mobility During Formation of Patchy Charnockites

Leonid L. Perchuk Department of Geology, Moscow State University, Vorobievy Gory, Moscow, 119899 Russia

Oleg G. Safonov Department of Geology, Moscow State University, Vorobievy Gory, Moscow, 119899 Russia

Taras V. Gerya Department of Geology, Moscow State University, Vorobievy Gory, Moscow, 119899 Russia

For the first time the direct evidence for alkali mobility, such as the K-feldspar microvein network reaction textures and systematic zonations of contacting plagioclase, biotite and orthopyroxene with coronitic K-feldspar rims in arrested charnockite formation from Sri Lanka (Udadiganga quarry at Kurunegala village) are found. Well known for zoned charnockite-gneiss metamorphic complexes (Perchuk & Gerya, 1993; Perchuk et al., 1994), such data were unknown for the rocks from the incipient charnockite formations so far. The analytic data suggest that the alkali feldspar in microveins is resulted from high activity of perfect mobile of potassium and sodium in fluid due to the following reaction:

1/2Opx + 3Qtz + nK+ · (1-n)Na+ = (1-n)Ab nOr+ 1/2Opx.

Al rich in fluid Al poor

The reaction textures developed and regular changes in mineral compositions are the direct evidence for the leading role of metasomatic transformation of biotite gneiss into charnockite at the front of incipient charnockitization. The core of the patch consists of eutectic hyprsthene granite (charnockite). Along with these data, narrow, 10-15 microns, the Qtz+Pl23-19 reaction rim of graphic texture occurs between the K-feldspar microveins and matrix plagioclase in the transition zone of a patch. Such rims are resulted from an increase in sodium and calcium activity of the metasomatic fluid due to the following reaction

(1+x)Kfs + xCa+2 + (1-x)Na+ = xAn·(1-x)Ab+ 4xQtz + (x+1)K+.

in fluid in fluid in fluid

Calculated bulk composition of the leucocratic material from the Qtz-Pl23-19 zone and frontal metasomatic Kfs80, is identical to that of charnockite from the core of patch. The Qtz+Pl intergrowths observed also in microveins developed along creaks and grain boundaries between different minerals from all three zones. So, the 70 microns Qtz+Pl17-19 microvein fills a creak in large Kfs matrix grain of charnockite from the core of patch. Similar microveins evidently formed at the late stage of charnockitization from sodium rich fluids. Presumably at high PT-parameters this homogeneous CO2-aqueous fluid was of high salinity (SiO2, Al2O3, CaO, NaCl, KCl). At low thermodynamic conditions the fluid could split for immisible silica rich + chloride brine mixture. The relics of such fluid formed Qtz-Pl microveins, while the brine was preserved as inclusions. Such brines were observed by Touret (1995) in many granulite facies terranes.


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Touret, J.L.R., Bol. Sociedad Espanola Mineralogia 18/1, 250-252 (1995).