Multiscale evidence of silicate-carbonate liquid immiscibility at the Crevier alkaline intrusion, Grenville Province (Québec)
Résumé
The Crevier alkaline intrusion (CAI, QC) is located in the Proterozoic Grenville Province and is composed of a suite of undersaturated alkaline rocks from ijolite to nepheline syenite and carbonatites. We present (i) field relationships; (ii) comagmatic carbonatite, carbonate-free nepheline syenite and carbonate-rich syenite; (iii) textures of interstitial carbonates in nepheline syenite, silicate rims around carbonates in carbonatite, primary carbonates at the edges of nepheline and feldspar or as inclusion in albite in the carbonate-rich syenite, an orbicular facies with spheres of carbonate-bearing nepheline syenite in a carbonatitic matrix; (iv) coupled to LA-ICP-MS U-Pb geochronology on zircon from nepheline syenite and apatite from carbonatite, that argue in favor of a coeval emplacement of the silicate rocks and the carbonatites, and of their parental linkage from liquid immiscibility. The (i) petrography, (ii) textures and (iii) trace elements contents of pyrochlore provide evidence for the coexistence of these two magmatic differentiation trends. In the early carbonate-rich syenite, pyrochlore defines a Nb-Ta differentiation trend, whereas they follow a more classical Nb-Ti trend in the younger nepheline syenite. The later crystallization of Nb-rich pyrochlore with more constant Nb/Ta ratio and a correlative classic Nb-Ti trend in the nepheline syenite suggests that the Nb and Ta fractionation has stopped in response to the end of the immiscibility between the two liquids. In addition, the (i) textures, (ii) trace elements, (iii) Ti-in-zircon thermometry, and (iv) oxygen isotope compositions of zircon grains from the CAI, allows to constrain the temperature range of ca. 1000-815°C at which the immiscibility process occurred. Accordingly, the deposit to microscopic multiscale evidence provided by petrographic textures, whole rock, accessory mineral geochemical signatures and isotopic compositions of the various lithologies demonstrate the petrogenesis of the CAI through a silicate-carbonate liquid immiscibility from a parental silicate liquid of ijolite composition.