An unusual mantle source region for nephelinites from the Oslo Rift, Norway

Abstract

Abstract-Wehave analyzed the trace-element and isotopic composition of a series of lavas and their constituent pyroxenes and calcite from near the town of Skien in the Permo-Carboniferous Oslo Rift. The series decreases in alkalinity upsection: the lowermost layaS are n~heli~t~, followed by basanites and finally alkali basalts. This sequence is accompanied by decreasing light ram earth element (LREE), Th, Nb and U concentrations. Nd isotopic change is strongly correlated with the petrologic and chemical evolution. The nephelinites are characterized by cNd (300 Ma) of 1.0 to 1.6, the basanites 1.9 to 2.2 and the alkali basalts 2.4 to 3.9. cSr (300 Ma) ranges from -10 to -15 and shows little change from one group to the next. Calcite segregations in the basaltic rocks show 6'-'C = -2.825, 6'*0 = 7.7 to 9.6%, tsr = -6.5 to -9.9 and Q.+, = 0 to +2, values which are compatible with formation from magmatic CO, exsolved from a mantle-derived silicate melt.The Sr isotope values are unradiogenic relative to Nd for the nephelinites, causing these samples to fall to the lefthand side of most rn~tl~~y~ rocks on a eNd vs. 6sr diagram. They are similar to rocks from "high p" (HIMU) sources and the recently proposed "LoNd array" f HART et al., 1986). The alkali basal&, with their higher cNd, lie close to the mantle array for 300 Ma ago and resemble tholeiitic magmatism in the Oslo Rift. We interpret these trends as representing evolution in which the earliest magmas were derived from a source with HIMU-LoNd characteristics, and successive magmas gradually acquired the more dominant mantle isotopic composition. Model age calculations permit either 1) that the nephelinite source was originally depleted then subsequently re-enriched in LREE or 2) that the nephelinite source was primitive with respect to Nd but had undergone Rb depletion. The nearby Fen carbonatite complex is about 250 Ma older than Skien and its em is slightly more positive. A direct ~~tion~ip between Fen and Skien would favor the first hypothesis.

The Permo-Carboniferous Oslo Rift through six stages and 65 million years

“…31, No. 1 melts (Anthony et al, 1989). The main mantle source for the basaltic magmatism was a prevalent depleted mantle.…”

“…It may represent the composition of the base of the local lithospheric mantle, and the asthenosphere beneath, which partly melted in response to localized thinning of the lithosphere due to the extension. Anthony et al (1989) also suggested an alternative scenario involving a mantle plume, with depleted characteristics, actively up-welling beneath the lithosphere. The most primitive lavas appear to involve low degree partial melting of one or more sublithospheric mantle sources.…”

“…Thin volcaniclastic sediments and pyroclastic products are found between some of the lava flows. The Skien basalts, and most likely also the Brunlanes basalts, came from an enriched mantle source, potentially as a result of low degree partial melting within the garnet-bearing part of the mantle (Anthony et al, 1989). Thus, the earliest and southermost basalts in the Oslo Rift indicate a low degree of partial melting in an asthenospheric mantle source, subsequently modified by lithospheric mantle and crustal components (Neumann et al, 2004).…”

Isotope Geochemistry and Chemical Evolution of the Mantle and Crust

“…A range of studies were made of comparable phenomena in other regions of the world: Red Sea Rift [ Hegner and Pallister , 1989], Ethiopian Rift [ Hart et al , 1989], Cameroon Line [ Halliday et al , 1988], Rhine Graben‐related volcanism [ Wörner et al , 1987; Harmon et al , 1987], the Oslo Rift [ Neumann et al , 1988; Anthony et al , 1989], the White Mountain plutons of the northeastern U.S. [ Foland et al , 1988] and the Deccan basalts [ Mahoney , 1988; Lightfoot and Hawkesworth , 1988]. Magma chamber phenomena, including assimilation, was studied at the Skaergaard Intrusion by Stewart and DePaolo [1990].…”

Recent Developments in Igneous Petrology

“…[1989] presented a three component model that includes subducted crust and sediment, respectively. Baker [1987], Macdonald [1987] and Woolley and Jones [1987] described magmatism in the East African Rift, while Anthony et al . [1989] presented Sr and Nd isotopic data on nephelinites, basanites and alkali olivine basalts from the Oslo Rift.…”