Composition and volume changes accompanying soda metasomatic alterations, Västervik Area, SE Sweden

Abstract

ZusammenfassungMit Hilfe yon Gleichungen, die von GRESENS (1967) entwickelt wurden, werden die Volumenentwicklung fiir Natrium-Metasomatite und ihrer unver~inderten Nebengesteine diskutiert. Ein mSglicher Volumenfaktor wurde abgeleitet und das Verhalten der Elemente beschrieben. Die Erhaltung des Volumens der Nebengesteine charakterisiert eine erste Phase, wobei diese in ein pseudomorphes Quarz-Plagioklasgestein umgewandelt wurden. W/ihrend dieser Phase waren Si und A1 immobil. Die Auslaugung von K, Fe, Mg, Ti sowie der meisten Spurenelemente wurde durch die Zufuhr yon Na und Ca sowie von Sr, Th, Nb und Ta ausgeglichen.In einer zweiten Phase vcurden die pseudomorphen Quarz-Plagioklasgesteine unter einer gleichzeitigen Ausscheidung yon Quarz und Ti-Oxid rekristallisiert. Diese Phase ist durch eine 8Oprozentige Volumenverminderung charakterisiert. Hier waren Na, Ca, A1 und Sr immobil, die meisten Spurenelemente wurden dagegen ausgelaugt. AbstractApplying a set of equations derived by GRESENS (1967), composition-volume relationship are discussed for pairs of soda metasomatites and unaltered enclosing country rocks. A probable volume factor is inferred and the behaviour of elements interpreted. The metasomatic alteration was basically a process of leaching in which two stages can be recognized differing from each other in the prevailing volume factors and in the behaviour of elements.A first stage, during which country rocks were altered into pseudomorphous quartzplagioclase rocks, was characterized by preservation of the volume of the parent rocks; Si and A1 were immobile, and the leaching of K, Fe, Mg, Ti and most of the trace elements was balanced by introduction of Na and Ca, accompanied by Sr, Th, Nb and Ta.A second stage, during which pseudomorphous quartz-plagioclase rocks were recrystallized under simultaneous expulsion of quartz and Fe-Ti-oxides, was characterized by a volume decrease of some 800/0; Na, Ca, A1 and Sr were immobile and most of the trace elements were leached. R~sum~Sur la base d'une s~rie d'~quations donn~es par GRESENS (1967), l'auteur diseute les relations composition-volume pour des couples m~tasomatites sodiques et roches intactes les renfermant. I1 en d6duit un facteur vohim~trique probable et interpr~te le comportement des ~16ments. La m~tasomatose est fondamentalement un processus de disSOlution dans lequel on peut reconnattre deux stades diff6rant l'un de l'autre par la prevalence du volume et par le comportement des ~l~ments.Le premier stade, au cours duquel les roches sont alt~r~es en roches pseudomorphos~es quartzo-plagioclasiques, est caraet~ris~ par la conservation du volume de la roche originelle; Si et A1 sont immobiles, et la dissolution de K, Fe, Mg, Tiet de la plupart des 616-

Sr–Nd isotope and geochemical characterisation of the Paleoproterozoic Västervik formation (Baltic Shield, SE-Sweden): a southerly exposure of Svecofennian metasiliciclastic sediments

“…In addition, unaltered plagioclase was detected in some of these samples that indicate later growth during or after the metamorphic overprint. Present accessory phases include the following: sillimanite, cordierite, andalusite and minor occurrences of garnet indicative for HT/LP metamorphism as already stated by Elbers and Hoeve (1971), Kresten (1971a, b), Hoeve (1978) and Gavelin (1984). A sample of the so-called Black Sand (Gavelin 1984), presumably representing a heavy-mineral alluvial deposit and a metapelitic sample showing a garnetrich peak metamorphic assemblage, was also selected.…”

“…Hence, it seems that ancient removal of common Sr in these rocks by fluids and possibly secondary input of Rb is responsible for the observed pattern, thus leading to exceptionally high present-day Sr isotope ratios by radiogenic ingrowth with time. Some 5-10 km north-east of the study area quartz-plagioclase rocks of metasomatic origin are exposed, the so-called soda metasomatites of Elbers and Hoeve (1971) and Hoeve (1978). These authors inferred removal of K 2 O and introduction of CaO into the metasediments during metasomatism possibly connected to the HT/LP overprint of the Västervik area.…”

“…Published Rb-Sr isotope data for Västervik metasediments and Loftahammar granites of Å berg (1978) fit nicely into that correlation resulting in a combined R 2 of 0.997. As argued earlier, the disturbance of the Rb-Sr isotope system is probably caused by the HT/LP overprint of the Västervik area leading to the formation of 'soda metasomatites' in the north-eastern part (Elbers and Hoeve 1971;Hoeve 1978) and 'K-metasomatites' in the southern part (Rheosomes of Kresten 1971a, b) of the study area. Beunk and Page (2001) observed that the LLDZ, which is located within the area where the soda metasomatites are exposed, functioned as a pathway for the fluids that was suggested to cause the metasomatic overprint of the area and in addition postulated the breakdown of biotite during the metasomatic reactions.…”

“…This high-grade metamorphic event presumably led to the formation of migmatites, diatexites and in situ production of voluminous S-type granitic melts (Kresten 1971a, b;Gavelin 1984). Probably related to the metamorphism, the Västervik area suffered a Na-Ca metasomatic overprint leading to the formation of plagioclase-rich rock types (the so-called soda metasomatites of Elbers and Hoeve (1971) and Hoeve (1978)) in close vicinity of the Loftahammar granites. In the southern part of the study area (Misterhult archipelago), a presumably metasomatic rock type called 'Rheosome' is described by Kresten (1971a, b).…”

Granitization - fact or fiction?

“…Often, metasomatic processes seem to have affected rather small areas but large-scale remobilization of older granites has been described (Kresten 1974(Kresten , 1986. In addition, large areas, mainly within the Loftahammar massif, have been affected by soda metasomatism (Hoeve 1974(Hoeve , 1978.…”