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

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.