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

1989; Elsevier BV; Volume: 53; Issue: 5 Linguagem: Inglês

10.1016/0016-7037(89)90211-1

1872-9533

Elizabeth Y. Anthony, Tom V. Segalstad, E.‐R. Neumann,

Geology and Paleoclimatology Research

We have 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 lavas are nephelinites, followed by basanites and finally alkali basalts. This sequence is accompanied by decreasing light rare 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 ϵNd(300 Ma) of 1.0 to 1.6, the basanites 1.9 to 2.2 and the alkali basalts 2.4 to 3.9. ϵsr(300 Ma) ranges from −10 to −15 and shows little change from one group to the next. Calcite segregations in the basaltic rocks show δ13C = −2.82‰, δ18O = 7.7 to 9.6‰, ϵsr = −6.5 to −9.9 and ϵNd = 0 to +2, values which are compatible with formation from magmatic CO2 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 mantle-derived rocks on a ϵNdvs.ϵSr diagram. They are similar to rocks from "high μ" (HIMU) sources and the recently proposed "LoNd array" (Hartet al., 1986). The alkali basalts, with their higher ϵNd, 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 ϵNd is slightly more positive. A direct relationship between Fen and Skien would favor the first hypothesis.