Secondary Ca-Al silicates in plutonic rocks: implications for their cooling history

Secondary Ca-Al silicates are used to constrain the P-T-x conditions of the very early postmagmatic stage of the intermediate to basic Hercynian pluto nic complexes of Charroux-Civray (NW Massif Central, France) and Fichtelgeb irge (NE Bavaria, Germany). The secondary Ca-Al silicates hydrogarnet, preh nite, pumpellyite, epidote and laumontite form lenses within unaltered or o nly slightly chloritized biotite. Hydrogarnet as the first occurring Ca-Al silicate phase crystallizes at temperatures above 340 degreesC. The common paragenesis prehnite + pumpellyite post-dates hydrogarnet and indicates rat her narrow ranges of temperature (200-280 degreesC) and pressure (2-3 kbar) . Laumontite is formed at the end of Ca-Al silicate crystallization (180-26 0 degreesC, 1-3 kbar), mostly in small fractures in association with prehni te and adularia. The observed crystallization sequence of the Ca-Al silicat es and their stabilities define a retrograde alteration path for the pluton ic rocks. The Ca-Al silicate assemblage results from an early pervasive alt eration of the plutonic rocks by low X-CO2 fluids during post-magmatic cool ing. Subsolidus cooling starts at about 4 kbar at solidus temperatures as i ndicated by magmatic epidote stability, hornblende barometry and fluid incl usion data, and continues under slightly decreasing pressure (uplift) down to 2-3 kbar at 200-280 degreesC (prehnite-pumpellyite paragenesis). This sh ows 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 b odies.