Trace and Minor Element Chemistry of Alkali Feldspars in the Klokken Layered Syenite Series |
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| Authors: | MASON R A; PARSONS I; LONG J V P |
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| Institution: | 1Department of Earth Sciences, University of Cambridge, Bullard Laboratories Madingley Rise, Madingley Road, Cambridge CB3 OEZ, England
2Department of Geology and Mineralogy, Marischal College, University of Aberdeen Aberdeen AB9 IAS, Scotland |
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| Abstract: | Trace and minor element concentrations in alkali feldspars fromthe layered syenite series in the Klokken gabbro-syenite complex,South Greenland have been measured using an ion microprobe.The technique has high precision, avoids problems of mineralseparation, and has allowed investigation of zoning and theeffects of deuteric alteration. Li, Be, B, Cs and Pb occur at< 1 p.p.m. levels, but Ba, Sr, Rb, Mg, Fe, Ti and P are presentin the 1–7000 p.p.m. range. Provided unaltered strain-controlledcrypto- or microperthitic feldspar is sampled and deutericallycoarsened material is avoided, all of the latter elements exceptMg show systematic variation. This confirms the evidence forin situ fractionation of a single syenitic magma-pulse affordedby major element variation in coexisting mafic minerals, particularlypyroxenes and olivines. The distinction between granular andlaminated syenites in the series is supported by the trace elements,which confirms the view that the granular syenites are a condensedroof-chill series with inverted cryptic variation, while thelaminated syenites are a bottom accumulated sequence. Using mainly experimentally determined partition coefficientsfrom the literature and a simple Rayleigh fractionation modelbased on the variation in Ba, Sr and Rb (elements whose concentrationin the melt is largely controlled by feldspar) we calculategeologically reasonable minimum thicknesses for the entire layeredseries of 1370–3260 m. Ti variation in the feldspars wascontrolled by Fe-Ti oxide fractionation, and Fe contents alsowere controlled by mafic phases. Controls of Mg, and particularlythe irregular P levels, are not clear. Behaviour of Ca, whichis present at < 6000 p.p.m. levels in the feldspars, is qualitativelyexplained in terms of the ternary feldspar phase diagram. |
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