GEOCHEMISTRY OF THE LYNGDAL HYPERITES (SW NORWAY) - COMPARISON WITH THE MONZONORITES ASSOCIATED WITH THE ROGALAND ANORTHOSITE COMPLEX

The Lyngdal hyperites (LYHY) crop out as two small, post-tectonic intrusions of gabbro-noritic to monzonoritic composition, at the extreme South of Norway. They display subophitic textures with plagioclase laths (An40 to An58), orthopyroxene (En56 to En62), clinopyroxene (Wo47.5En39Fs13.5), ilmenite, magnetite and interstitial myrmekite fringes, K-feldspar, quartz, apatite and biotite. The SiO2 content is very constant (50-53%), but K2O ranges from 1% to 1.6% in differentiated facies. REE patterns are smooth [( La Yb)n = 6]. The hyperites define an Rb Sr isochron of 910 ± 82 Ma (2σ), with an initial ratio of 0.7054. ε{lunate}Nd is slightly positive (0 to +2) and average lead isotopic initial ratios are 17.45 [( 206Pb 204Pb)i] and 15.51 [( 207Pb 204Pb)i]. The LYHY represent magmatic liquids that have not fractionated large amounts of plagioclase. They are quite comparable to the rocks of the monzonoritic series that occur in the nearby Rogaland anorthosite complex. (1) The plagioclases have the same composition; plagioclase phenocrysts are locally present. (2) They display enrichments in TiO2, P2O5 and K2O. (3) They have comparable REE patterns. (4) The isotopic data show that the monzonorites and the LYHY derive from the same isotopic reservoir. The LYHY nevertheless differ from the monzonorites by their high-Al content (Al2O3 = 18.5%) and their lower FeOtot content ( FeOtot FeOtot + MgO = 0.62). The geochemical and isotopic diversity of the monzonoritic magmatism, including the LYHY, implies that each occurrence represents a distinct magma batch generated from a distinct source. The LYHY could tentatively be interpreted as a Fe-poor (Mg-rich) end-member of the monzonoritic magmatism occurring to the East of the anorthosite complex. © 1990.