Petrogenesis of Massif Anorthosites

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Massif anorthosites are plutonic igneous rocks consisting almost entirely of plagioclase feldspar. Recurring problems in the study of these enigmatic rocks include the mechanisms that lead to such unusual concentrations of plagioclase on the scale of batholiths, the nature and source of their parental magmas, their relationship to associated iron-titanium oxide ores, their relationship to associated granitic igneous rocks of the admellite-charnockite-mangerite suite, and the crystallization - differentiation processes that link some or all of these rocks together as parts of single or repeated episodes of magmatism. The emplacement of massif anorthosites appears restricted in space and time to relatively narrow belts of mid-Proterozoic age. Accordingly, the formation of massif anorthosites and their associated rocks represents an important stage in the growth and development of the continental crust spanning only a short interval of Earth history. We will investigate the origin of massif anorthosite and associated rock types through an integrated field, petrological and geochemical study on the St. Urbain and Labrieville anorthosites, Quebec. Our study of St. Urbain is a continuation of work begun a number of years ago, whereas the study of Labrieville represents a new initiative. Each massif contains a range of rock types pertinent to a critical evaluation of the anorthosite problem within plutons that are small enough to be investigated in great detail. Our work will include the analysis of mineral compositions by electron microprobe methods, and the analysis of whole-rocks for major and selected trace elements by a combination of X-ray fluorescence (XRF) and instrumental neutron activation (INA) methods. We will establish a comprehensive petrological and geochemical data base that will allow us to develop quantitative models for the chemical assembly of the anorthositic rocks, to evaluate processes responsible for their magmatic differentiation, and to provide well-constrained estimates of their parental magma compositions.