Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden

Abstract

Petrographic examinations and electron microprobe analyses of Proterozoic granitic rocks, SE Sweden aimed to characterize and unravel the mechanisms and conditions of plagioclase alterations. These alterations include saussuritization, albitization and replacement of plagioclase by K-feldspar. The hydrothermal alterations, which are inferred to have occurred at ca. 250-4008C, resulted in concomitant formation of Alrich titanite, epidote, calcite, pumpellyite, prehnite and iron oxides. Replacement of plagioclase by K-feldspar occurs in red-stained zones, which have developed close to thin fractures owing to the precipitation of tiny Fe-oxide pigment particles within the altered plagioclase, whereas saussuritized plagioclase has less systematic spatial relationships to these fractures. Albitization of plagioclase occurred in rocks that are poor in biotite compared to rocks that suffered extensive saussuritization. The chemical and textural characterization of various types of plagioclase alterations allows elucidation of the granitic hydrothermal systems. Features of feldspar alteration in the granitic rocks are similar to those encountered in feldspathic sandstones and should hence be considered in studies on diagenetic changes of siliciclastic successions during basin evolution.

Surprisingly young Rb/Sr ages from the Simav extensional detachment fault zone, northern Menderes Massif, Turkey

“…There is a link between saussuritization of plagioclase and chloritization of biotite at a temperatures of ca. 400 • C (e.g., Eggleton and Banfield, 1985;Turpault et al, 1992;Lee and Parsons, 1997;Boulvais et al, 2007in Morad et al, 2010 and this is recognized by abundances of the two alteration features in the same rock (Eliasson, 1993in Morad et al, 2010. The saussuritization of plagioclase is therefore most extensive hydrothermal alteration type in granitic rocks that are rich in chloritized biotite whereas albitization of plagioclase occurs in rocks that are poor in biotite; hydrothermal alteration occurs at ca.…”

Evaluation of Deep Geothermal Exploration Drillings in the Crystalline Basement of the Fennoscandian Shield Border Zone in South Sweden.

“…A possible explanation is that these are low temperature alteration of feldspars, a process that is common in granitoids (cf. Plümper et al 2009;Morad et al 2010).…”

Paragenesis of secondary Ca–Al silicates during hydrothermal alteration of Proterozoic granitic rocks (SE Sweden)

“…This mineral forms an important component of saussuritized plagioclase, that is, together with epidote and albite (Morad et al, 2010). In some cases, however, saussuritized plagioclase does not contain epidote, but is merely albitized and replaced by sericite.…”