Mineralogy of the Labrieville anorthosite, Quebec

The Labrieville anorthosite massif is an example of crystallization differentiation in a plutonic environment. The lithologic sequence is: anorthosite (earliest), gabbroic anorthosite, iron-titanium oxide-rich gabbro, and syenite (latest). The earliest liquid consisted of at least 84 percent feldspar constituents and had water fugacity of less than 500 bars. At the oxide-rich gabbro stage, biotite indicates that the lowest crystallization temperature exceeded 880 degrees C and the highest water fugacity was less than 2,000 bars. A massive hemo-ilmenite deposit formed early as a result of immiscibility of silicate and iron-titanium oxide liquids, which separated at about 1100 degrees C or higher, as shown by fractionation of oxygen isotopes. Mineralogic data are given for feldspars, iron-titanium oxides, pyroxenes, apatites and biotites.