The Strange Lake peralkaline complex, Québec-Labrador : the hypersolvus-subsolvus granite transitiion and feldspar mineralogy

The four main units of the epizonal Strange Lake peralkaline complex (Quebec-Labrador) document a transition from early hypersolvus granite, to transsolvus granite, to geochemically more evolved subsolvus granite and, finally, to highly K-enriched pegmatites. The felsic and mafic minerals constrain the temperatures of crystallization of the different units. The hypersolvus granite has a solidus above 650$ sp circ$C, and the pegmatite has a solidus below ${ sim}590 sp circ$C at 0.7 kbar. The textural transition and reduced solidi of the units are explained by an increase in (Na+K)/Al and fluorine in the evolved granitic melt, which allows crystallization to lower temperatures, and thus the nucleation of two separate primary feldspars. The composition and structural state of the feldspars of the pluton indicate temperatures of equilibration of ${ sim300} sp circ$ to 100$ sp circ$C. The increasing K/Na value of the bulk feldspar is explained by: (1) the shift of the haplogranite minimum toward the Qtz-Or sideline due to the addition of excess alkalis, and (2) the loss of Na by degassing. The high degree of order in the K-feldspar is attributed to the presence of an alkaline fluid phase. The geochemical evolution of the alkali feldspars is consistent with the fractionation of a single batch of evolved granitic magma. Degassing of the magma led to an important increase in $ int( rm O sb2),$ and to the escape of the peralkaline fluid into the wallrocks. Mineralization in the complex is attributed to the late re-distribution of ore constituents via a contaminated peralkaline fluid, which gained Ca, Sr and Mg by interaction with the country rock