THE RARE-ELEMENT-ENRICHED MONZOGRANITE - PEGMATITE - QUARTZ VEIN SYSTEMS IN THE PREISSAC-LACORNE BATHOLITH, QUEBEC. II. GEOCHEMISTRY AND PETROGENESIS

THOMAS MULJA, ANTHONY E. WILLIAMS-JONES, SCOTT A. WOOD* and MICHEL BOILY**
Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7

* Present address: Department of Geology and Geological Engineering, University of Idaho, Moscow, Idaho 83843, U.S.A.
** Present address: Géon, 10785, rue St-Urbain, Montréal, Québec H3L 2V4.

Abstract

The Archean Preissac-Lacorne batholith in northwestern Quebec includes four felsic plutons (Preissac, Moly Hill, Lamotte, Lacorne), which are zoned from biotite to muscovite monzogranite. The Lamotte and Lacorne plutons are associated spatially with rare-element pegmatites, whereas pegmatites are absent from the Moly Hill pluton and do not contain rare-element minerals in the Preissac pluton. The rare-element pegmatites are zonally distributed from beryl-bearing in the plutons to spodumene-bearing in the country rocks. Molybenite-bearing quartz veins are associated with all four plutons, and in the case of the Lamotte and Lacorne plutons, occur beyond the spodumene pegmatites. Dikes of molybdenite-bearing albitite occur north of the Lacorne pluton. All the plutons are weakly to moderately peraluminous (A/CNK: 1.0-1.3) and exhibit a compositional continuum in major- and trace-element contents from biotite to muscovite monzogranite. This compositional continuum extends to the rare-element pegmatites, indicating that the monzogranites and pegmatites are comagmatic. The chemistry of the pegmatites suggests that they underwent further evolution from beryl-bearing to spodumene-bearing varieties. The monzogranites and pegmatites have 18O(SMOW) values (8.6 ± 0.3‰). The zonation of the plutons, the geochemical trends, and the oxygen isotopic compositions indicate that the various types of monzogranite were mainly the products of fractional crystallization. Trace element (Rb, Ba, Sr) modeling of the Lamotte and Lacorne plutons suggests that the most fractionated monzogranite could have been formed by 80-90% crystallization of the magma that formed the biotite monzogranite. A model is proposed for the evolution of the Lamotte and Lacorne plutons, in which side-wall crystallization produced their observed quasi-concentric zonation, and created volatile-rich residual melts. These melts were subsequently injected sequentially into the overlying parental monzogranite and later the country rocks, producing zonally distributed beryl and spodumene pegmatites, respectively. Fluids exsolved from the most evolved pegmatites back-reacted with earlier-crystallized spodumene-bearing aplite to form albitite, or separated from the melts, filling fractures as molybdenite-bearing quartz veins. The smaller Preissac and Moly Hill plutons, which host molybdenite-bearing quartz veins, did not evolve sufficiently to form rare- element pegmatites. Vapor saturation occurred during late crystallization of the muscovite monzogranite, and culminated in the formation of molybdenite-bearing quartz veins, which filled fractures in the overlying crust of previously solidified magma.

Keywords: rare-element monzogranite, granitic pegmatite, beryl pegmatite, spodumene pegmatite, molybdenite- bearing albitite, quartz veins, geochemistry, fractional crystallization, Preissac-Lacorne batholith, Archean, Québec.