Trace and Minor Element Chemistry of Alkali Feldspars in the Klokken Layered Syenite Series
Trace and minor element concentrations in alkali feldspars from the 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 mineral separation, and has allowed investigation of zoni...
| Published in: | Journal of Petrology |
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| Main Authors: | , , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1985
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| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/26/4/952 https://doi.org/10.1093/petrology/26.4.952 |
| Summary: | Trace and minor element concentrations in alkali feldspars from the 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 mineral separation, and has allowed investigation of zoning and the effects 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 present in the 1–7000 p.p.m. range. Provided unaltered strain-controlled crypto- or microperthitic feldspar is sampled and deuterically coarsened material is avoided, all of the latter elements except Mg show systematic variation. This confirms the evidence for in situ fractionation of a single syenitic magma-pulse afforded by major element variation in coexisting mafic minerals, particularly pyroxenes and olivines. The distinction between granular and laminated syenites in the series is supported by the trace elements, which confirms the view that the granular syenites are a condensed roof-chill series with inverted cryptic variation, while the laminated syenites are a bottom accumulated sequence. Using mainly experimentally determined partition coefficients from the literature and a simple Rayleigh fractionation model based on the variation in Ba, Sr and Rb (elements whose concentration in the melt is largely controlled by feldspar) we calculate geologically reasonable minimum thicknesses for the entire layered series of 1370–3260 m. Ti variation in the feldspars was controlled by Fe-Ti oxide fractionation, and Fe contents also were controlled by mafic phases. Controls of Mg, and particularly the irregular P levels, are not clear. Behaviour of Ca, which is present at < 6000 p.p.m. levels in the feldspars, is qualitatively explained in terms of the ternary feldspar phase diagram. |
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