The chemistry of barium anomalies in the Berisal Complex, Simplon Region, Switzerland

Abstract

The petrographic and geochemical features of two zoisite-celsian gneiss outcrops from the Berisal Complex, characterised by a syn-kinematic mineral assemblage that contains celsian and barium white micas and a maximum whole-rock BaO content of 8.36 wt%, are described. The outcrops are enclosed in a larger body of garnet-bearing two-mica augen-gneiss, which has intrusive contacts with the surrounding garnet-biotite-muscovite paragneiss, and also contains small outcrops of two-mica clinozoisite gneiss. The zoisite-celsian gneisses are strongly enriched in all alkaline-earth elements, are depleted in the alkali elements, and have high Zr/Hf and Nb/Ta ratios compared with the surrounding gneisses. The zoisitecelsian, two-mica clinozoisite, and garnet-bearing twomica augen-gneisses have Al/CNK molar ratios >1, and the zoisite-celsian gneisses are also enriched in Zr, Y, and Nb. Chondrite-normalised patterns for the rare earth elements (REE) show light REE enrichment, with a negative Eu anomaly. On the basis of field and geochemical data it is argued that the barium anomalies in the Berisal complex are the result of igneous fractionation of barium into anorthosite-dominated cognate inclusions within a larger volume of calcalkaline peraluminous melt. On the basis of U-Pb zircon ages, a conservative estimate for the age of magma crystallisation is placed at 460 ± 10 Ma, and thus was related to significant late Ordovician granitoid magmatism in Gondwana-derived microcontinents during collision between Avalonia-Cadomia and Gondwana.

Barium silicates of the Berisal Complex, Switzerland: A study in geochronology and rare-gas release systematics

“…Several localities in the lecucocratic gneisses contain elevated concentrations of Ba, and in the case of the zoisitecelsian gneisses, BaO concentrations as high as 8.36 wt% are reported (Hetherington et al, 2007). Barium was introduced into the gneisses prior to, or coeval with, the peak of Alpine metamorphism (Hetherington et al, 2003b;Hetherington et al, 2007) and is now concentrated in synkinematic celsian, barian-muscovite and the Ba-dominant analogue of muscovite, ganterite.…”

“…Age information on the Berisal Complex is scarce (Gebauer, 1999). Previous chronological studies of the area have provided pre-Alpine ages for orthoamphibolites (Stille and Tatsumoto, 1985) and pre-Triassic granites (Allègre et al, 1974;Hetherington et al, 2007), while other studies have concentrated on dating post-peak processes (Frank and Stettler, 1979;Mancktelow, 1992;Seward and Mancktelow, 1994). Exhumation of the Berisal Complex was primarily controlled by the Simplon Fault Zone.…”

“…The zoisite-celsian orthogneiss exhibits a penetrative foliation (F2), is fine grained, with a syn-kinematic mineral assemblage of celsian + zoisite + quartz + margarite + barian-muscovite or ganterite ± anorthite (Hetherington et al, 2007). A sample of fine grained, slightly elongate, and chemically homogenous celsian was separated from a sample of zoisite-celsian orthogneiss for irradiation.…”

“…Exotic barium-bearing minerals are found in a 1 by 2 km east-west trending body of orthogneisses in the Berisal Complex, Simplon Region, Switzerland (Hetherington et al, 2003b(Hetherington et al, , 2007. At some localities in the Berisal Complex concentrations of Ba may be 200 times greater than that reported for average continental crust (700 ppm; Taylor and McLennan, 1981).…”

“…At some localities in the Berisal Complex concentrations of Ba may be 200 times greater than that reported for average continental crust (700 ppm; Taylor and McLennan, 1981). Barium enrichment was proposed to be related to 460 Ma magmatism in a Gondwana derived microcontinent (Hetherington et al, 2007). It is at present concentrated in synkinematic celsian (a Ba feldspar structurally related to anorthite), barianmuscovite, and ganterite (a Ba analogue of dioctahedral muscovite).…”