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Compositional variations (major and trace elements) of clinopyroxene and Ti-andradite from pyroxenite, ijolite and nepheline syenite, Alnö Island, Sweden |
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| Authors: | Jaana Hode Vuorinen Ulf H lenius Martin J Whitehouse Joakim Mansfeld and Alasdair DL Skelton |
| Institution: | aDepartment of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden bDepartment of Mineralogy, P.O. Box 50007, Swedish Museum of Natural History, S-104 05 Sweden cLaboratory for Isotope Geology, P.O. Box 50007, Swedish Museum of Natural History, S-104 05 Sweden |
| Abstract: | Clinopyroxenes from pyroxenite, ijolite and nepheline syenite from the main intrusion of the Alnö complex define two sub-parallel compositional trends with respect to Na, Ca and FeTOT plotted against alkali-pyroxene fractionation index (Na–Mg). Both trends define a smooth fractionation of increasing Na and FeTOT and decreasing Ca with increasing Na–Mg, but one set of samples contain clinopyroxenes that constantly plot at higher Na and lower FeTOT and Ca (at similar Na–Mg) than the rest of the samples. Clinopyroxenes with higher Ca and FeTOT and lower Na (trend 1) co-exist with substantial amounts of Ti-andradite (up to 70 vol.%), while the sample set defining the more Na-rich trend (trend 2) lack co-existing Ti-andradite. Clinopyroxenes from both trends show fractionated REE patterns with a distinct difference in HREE content, reflecting the content of co-existing Ti-andradite. The rocks of the first Ti-andradite-bearing trend crystallized slightly prior to the rocks of the second trend, probably from a primitive, Ca- and Ti-rich nephelinitic magma. Crystallisation of pyroxenite and melteigite occurred under low aSiO2 and high aCaO and aTiO2 as evidenced by the presence of perovskite and sometimes substantial amounts of magnetite. Subsequent increase in aSiO2 is evidenced in the overgrowth of perovskite by titanite, which in turn is overgrown by Ti-andradite. Nepheline syenitic residuals crystallized under higher aSiO2 and aNa2O and lower aCaO and aTiO2, which reduced Ti-andradite into an accessory phase and produced more Si- and Na-rich clinopyroxenes. Some of these residuals probably also mixed with new primitive magma producing a hybrid magma that crystallised the more Na-rich and Ca- and FeTOT-poor clinopyroxenes of trend 2. The complete lack of Ti-andradite in these rocks indicates different crystallisation conditions and also a different magma composition. |
| Keywords: | Alnö Clinopyroxene Ti-andradite Pyroxenite Ijolite Nepheline syenite |
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