Secondary Ca–Al silicates in plutonic rocks: implications for their cooling history |
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Authors: | Regina Freiberger Lutz Hecht Michel Cuney Giulio Morteani |
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Affiliation: | Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universit?t Freiburg, Albertstr. 23b, 79104 Freiburg, Germany, Lehrstuhl für Angewandte Mineralogie und Geochemie, Technische Universit?t München, Lichtenbergstr. 4, 85747 Garching, Germany, UMR G2R 7566–CREGU, BP 23, 54501 Vand?uvre-lès-Nancy Cedex, France,
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Abstract: | Secondary Ca-Al silicates are used to constrain the P-T-x conditions of the very early post-magmatic stage of the intermediate to basic Hercynian plutonic complexes of Charroux-Civray (NW Massif Central, France) and Fichtelgebirge (NE Bavaria, Germany). The secondary Ca-Al silicates hydrogarnet, prehnite, pumpellyite, epidote and laumontite form lenses within unaltered or only slightly chloritized biotite. Hydrogarnet as the first occurring Ca-Al silicate phase crystallizes at temperatures above 340 °C. The common paragenesis prehnite + pumpellyite post-dates hydrogarnet and indicates rather narrow ranges of temperature (200-280 °C) and pressure (2-3 kbar). Laumontite is formed at the end of Ca-Al silicate crystallization (180-260 °C, 1-3 kbar), mostly in small fractures in association with prehnite and adularia. The observed crystallization sequence of the Ca-Al silicates and their stabilities define a retrograde alteration path for the plutonic rocks. The Ca-Al silicate assemblage results from an early pervasive alteration of the plutonic rocks by low XCO2 fluids during post-magmatic cooling. Subsolidus cooling starts at about 4 kbar at solidus temperatures as indicated by magmatic epidote stability, hornblende barometry and fluid inclusion data, and continues under slightly decreasing pressure (uplift) down to 2-3 kbar at 200-280 °C (prehnite-pumpellyite paragenesis). This shows that Ca-Al silicate assemblages may be a unique tool to constrain the P-T conditions of the subsolidus cooling of intermediate to basic plutonic bodies. |
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