A proposed model for the formation of reversely zoned plutons based on a study of the Lacorne Complex, Superior Province, Quebec

Abstract

The Lacorne Complex is a calc-alkaline mass of Archean age located in the Abitibi region of Quebec. The complex can be subdivided into three zones. The inner zone varies in composition from hornblende > biotite gabbro breccia to monzonite, with all rocks containing ≤ 5 % modal quartz. The outer zone rocks contain more than 5% modal quartz and range in composition from biotite > hornblende quartz diorite to granodiorite. The third zone, consisting of muscovite–biotite–garnet leucomonzogranite, is exposed only in the northwestern portion of the complex. The transition between each zone is relatively abrupt but continuous. Based on 400 modal estimates of stained slabs and 107 whole-rock and trace-element analyses, it is demonstrated that both the inner zone and the outer zone rocks are reversely zoned: mafic cores are surrounded by more felsic borders. Various methods that could explain the observed geochemical trends are considered. The data indicate that crystal–liquid equilibria were maintained during crystallization and that mechanisms such as magma mixing can be rejected. A two-stage model involving sidewall crystallization and upwelling of fractionated magma accumulating near the roof of the magma chamber is proposed to explain the reverse zoning.