Crystal size distribution of feldspar crystals from granitic rocks of Eurajoki (Finland) and the South Mountain Batholith (Nova Scotia, Canada)

Abstract

Crystal size distributions (CSDs) studies of igneous rocks allow the quantification of sizes of mineral and contribute to a better understanding of crystallization conditions. Since most CSD studies have been made on mafic and felsic volcanic rocks and mafic plutonic rocks, one of the goals of this research was to do a large number of CSDs on feldspar crystals from two well-known granitic localities: the Eurajoki stock (Finland) and the South Mountain Batholith (Nova Scotia, Canada). Crystal size data were measured at different scales and on different media (outcrop, rock slabs and thin sections). CSDs were constructed from the data collected at the various scales using the program CSD corrections and plotted as amalgamated CSDs for each sample. CSDs of potassic felspars from Eurajoki samples were curved upwards and the characteristic lengths indicate three populations of crystals sizes. It is proposed that potassic feldspar crystals have been texturally coarsened likely because the Eurajoki melts were emplaced in a hot environment. This minimized undercooling and the temperature was buffered by latent heat of fusion near the liquidus. Thus large crystals grew at the expense of smaller ones. With cooling, nucleation and growth of small crystals resumed, yielding the different crystals sizes populations observed. CSDs of plagioclase and potassic felspars from the South Mountain Batholith were also curved upwards and the characteristic lengths indicate two populations of crystals sizes, except for the muscovite leucogranites from the stage 2 plutons, where CSDs are straight. Textural coarsening formed the larger crystals, megacrysts, in the South Mountain Batholith. Here the constant ascent of new magma into the plutons slowed undercooling and buffered the temperature producing the right conditions for textural coarsening.