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Fluid inclusion in giant pegmatitic microcline crystals, Alto da Cabeca Parelhas, Rio Grande do Norte, Brazil

The Boqueirao pegmatite body of Alto da Cabesa, Parelhas, Rio Grande do Norte, Brazil, contains giant microcline crystals which are classified as pure potassium feldspars. Their unit cell parameters are: a=8.584(3)A, b=12.964(4)A, c=7.215(3)A,  =90.62(4)°,  =115.97(4) ,  =87.76(4), V = 721.3(4)A3 . Most probably they crystallized as orthoclase and subsequently ordered their crystal structure during the history of pegmatite formation. Fluid inclusion (FI) studies of microcline, including laser Raman spectroscopy revealed three different inclusion types: 1. One phase FIs, lacking any phase transition below 400°C, Raman spectroscopy suggests the presence of quenched melt inclusions ; 2. Aqueous FIs, Te = -31°C (-25.2 to -32.7°C), Tm Ice = -5°C (-1.8 to -6.2°C), Th V/L= +190°C (+170 to +220°C) ; 3.Aqueous- carbonic FIs, Tm CO2 = -65°C (-60.5 to -67.8°C), laser Raman spectroscopy determined 0.1 to 2.0 mole% N2. Th CO2 L/V and V/L = +29°C, critical phenomena were not observed, Tm clath = +9, O°C (+7.5 to +9.5°C), Th tot V/L = 200°C (+165 to +230°C). FIs in quartz differ only in Th CO2 = +19.9 °C. The crystallization path is approximated by using the intersection of isochores of coexisting aqueous and aqueous-carbonic FIs (Bowers and Helgeson, 1983 ; Potter and Brown, 1977), The presence of spodumene and quenched glass inclusions implies a melt pressure above 1500 bars and a gradual transition of silicic melt to hydrothermal solutions. Crystallization of tourmaline (schorl and elbaite). and consequent consumption of boron, produced acidic, aqueous fluids which caused ubiquitous kaolinitization and enhanced crystallization of microcline, quartz and other alluminosilicate phases introducing almost quenching effect on the borosilicate melt which separated into silicic melt and a aqueous fluids.

pegmatite; microcline; fluid inclusion

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