Albitization of Granitic Rocks: The Mechanism of Replacement of Oligoclase by Albite

Abstract

large-scale metasomatic albitization in the albitite terranes of the Bamble sector of southeastern Norway has affected both mafic and granitic lithologies. In partially metasomatized tonalite, the albitization fronts advance normal to fractures and can be recognized in the field by a distinct reddening of the rock in which original plagioclase crystals are replaced by albite. To determine the mechanism of albitization within single crystals of Ca-bearing plagioclase (oligoclase: an 21-23 ), intragranular replacement interfaces have been studied by electron-probe micro-analysis, scanning (SEM) and transmission electron microscopy (TEM). The albite replacement-product (An 2-5 ) has micrometric pores observable by SEM, as well as nanopores imaged by TEM. The albite contains fine-grained white mica commonly associated with the pores, as well as precipitates of hematite. the interface between oligoclase and albite is sharp, with corresponding compositional gradients across no more than tens of nanometers, and the crystallographic orientations of oligoclase and albite are coincident within less than a degree. these are all characteristic features of an interface-coupled dissolution-reprecipitation mechanism of replacement which, in combination with fracturing, is responsible for the fluid infiltration and the mineral-replacement process.

Sculpting of Rocks by Reactive Fluids

“…During albitisation, when more Ca-rich plagioclase is replaced by the almost pure end member albite, the interface between parent and product is very complex and the product phase is invariably porous on a submicrometre scale. This has been observed both for experimentally produced albite (Hövelmann et al, 2010;Norberg et al, 2011) and for natural samples (Engvik et al, 2008). Relatively large porosities (several percent) can arise during replacement (Fig.…”

Microtexture and compositional variation of alkali feldspars from the Kakkonda granitic pluton, northeast Japan: Implications to the formation processes of granitic texture

“…This difference in the compositions between the C13 and C11 plagioclase suggests that the albitization in the C11 feldspars occurred hydrothermally furthermore after the main magmatic reactions. Albitization in granitic feldspars is a common phenomenon of their hydrothermal reactions Lee and Parsons, 1997;Hashimoto et al, 2005a;Putnis et al, 2007;Engvik et al, 2008;Parsons and Lee, 2009;Hovelmann et al, 2010;Norberg et al, 2011). Therefore, the formation process of the present microperthite discussed above probably occurred at the late magmatic or hydrothermal stage during the cooling.…”

“…Thus, perthitic textures of alkali feldspars become a very important tool to clarify the cooling histo-ries of host plutons during magmatic to hydrothermal stages (Brown and Parsons, 1994;Parsons and Lee, 2009). Moreover, it has been pointed out that feldspar textures in granites should record metasomatic replacement reactions (Putnis, 2002;Putnis et al, 2007;Hovelmann et al, 2010;Norberg et al, 2011), and that the textures might be related to large-scale, crustal fluid flows of multiple-stage (Engvik et al, 2008;Plumper and Putnis, 2009;Słaby et al, 2012).…”

Dynamic Metasomatism: Stable Isotopes, Fluid Evolution, and Deformation of Albitite and Scapolite Metagabbro (Bamble Lithotectonic Domain, South Norway)

“…Albite is the dominant mineral in foliated felsites, in chlorite schist, in carbonate-rich albitite, and in large-scale albitite bodies [23,50]. Albitisation has been studied in detail in the Ringsjø-Ødegården Verk area [20,35]. Both mafic (gabbro, scapolite metagabbro, and amphibolite) and granitoid protolith are transformed to albitite along veining, where the central vein consists of nearly pure albite (Figures 3(a)-3(b)).…”

“…While the scapolitisation process with respect to mineral reactions is relatively well understood in the Kragerø region, albitisation is a more complex process and less constrained. Extensive albitisation is seen along veins, as brecciation, as formation of foliated albititic felsites and chlorite schists, as carbonate-rich albitite, and as large-scale albitite bodies [23,35].…”