Abstract
The Labrieville anorthosite massif is an example of crystallization differentiation in a plutonic environment. As in other St. Urbain-type anorthosites (characterized in particular by andesine anti-perthite and hemo-ilmenite), the lithologic sequence is: anorthosite (earliest), gabbroic anorthosite, iron-titanium oxide-rich gabbro and syenite (latest). In this sequence the following trends are observed: antiperthites become enriched in BaO/K2O by a factor of 2, magnetites become impoverished in V2O3/Fe2O3 by a factor of 0.5, hemo-ilmenites become enriched in MnO/FeO by a factor of 4.3, pyroxenes decrease in Mg/Mg+Fe by a factor of 0.5, and apatites become enriched in Cl/F, Ce/Y and Eu/Yb. These changes are associated with a progressive decrease in the Fe2O3 content of the (hemo-) ilmenites from 34 per cent in anorthosite to 6 per cent in oxide-rich gabbro and syenite, indicating that the residual liquids became highly reducing relative to temperature: 1) oxygen fugacity cooling curves for isocompositional ferrian ilmenites, 2) water of constant bulk composition (or constant H2/H2O ratio) and 3) isocompositional titaniferous magnetites.
The Fe2O3 content of hemo-ilmenite in a massive hemo-ilmenite deposit (about 25 per cent) indicates that the deposit formed at a relatively early stage of the differentiation. The deposit contains appreciable corundum, whereas pockets of quartz, perthite and pyrite occur in the anorthosite, requiring physical isolation of the deposit from the anorthosite and subsequent crystallization differentiation of both, which implies that separation occurred by means of silicate liquid—iron-titanium oxide liquid immiscibility.
The 2.0 per mil deduced primary fractionation of oxygen isotopes between ilmenite and plagioclase indicates that separation of iron-titanium oxides from silicates occurred at a temperature near or above 1100° C. The biotite compositions limit the lowest crystallization temperature to a value greater than 880° C. and the highest water fugacity to a value less than 2000 bars at the late oxide-rich gabbro stage.
Analysis of the changes in the compositions of the minerals in the Labrieville massif in terms of published data on the Skaergaard intrusion (Wager and Mitchell, 1951) and the Rayleigh distillation equation reveals that the Labrieville feldspars are more than 75 per cent perfectly fractionated and that residual liquids became enriched in iron-titanium oxide and pyroxene components relative to feldspar components. According to these arguments, the earliest liquid evident from the Labrieville data consisted of at least 84 per cent feldspar constituents and was characterized by a water fugacity of less than 500 bars.
