Sidewall crystallization in the Klokken intrusion: zoned ternary feldspars and coexisting minerals |
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| Authors: | Ian Parsons William L Brown |
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| Institution: | (1) Department of Geology and Mineralogy, Marischal College, University of Aberdeen, AB9 1AS Aberdeen, Scotland;(2) Centre de Recherches Pétrographiques et Géochimiques, B.P. 20, F-54501 Vandoeuvrelès-Nancy Cedex, France |
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| Abstract: | The syenitic layered series in the Klokken intrusion is surrounded by a zone ( 500 m thick) of nearly structureless unlaminated syenite followed outwards by a zone of vertically banded gabbro ( 200 m thick) at the outer rim. The unlaminated syenite is intrusive into the gabbro and develops a thin (2 m) transition zone of syenodiorite at the contact. A traverse across the vertical transition zone and inwards towards the layered series was sampled with a portable drill. Mafic silicates (olivine, clinopyroxene, biotite) show inward evolution in Fe/(Fe+Mg) across the syenodiorite-unlaminated syenite zones. Feldspars change rapidly across the syenodiorite zone from rocks dominated by plagioclase, in some cases together with two alkali feldspars, one a mesoperthite or cryptomesoperthite, the other a cryptoperthite, to rocks in which plagioclase is seen only rarely as cores to cryptomesoperthitic alkali feldspar crystals. Plagioclase is absent from the layered series.Alkali feldspars occurring in pairs have bulk compositions on solvus isotherms in the Or-Ab-An ternary system, estimated at 950° C in a syenogabbro and 910° C in a syenodiorite, at 1 kbar. The more calcic liquids from which they crystallized fractionated on paths that intersected the two- feldspar surface, whereas the more syenitic members crystallized from liquids which terminated crystallization in the one- feldspar field at 900° C. Plagioclases evolve from calcic andesine in syenodiorites, to very rare sodic oligoclase in the most evolved unlaminated syenites. The boundaries between plagioclase cores and alkali feldspar rims, which are usually optically abrupt, involve complex mixed zones on the  m -scale, consistent with arrested reaction between plagioclase primocrysts and crystallizing syenitic liquid. Ternary liquidus-solidus relationships are in qualitative agreement with this interpretation. The syenodiorites are cumulates produced during sidewall crystallization of a trachytic magma against a gabbroic chamberlining. This magma changed little in bulk composition as it evolved, giving rise to the unlaminated syenites by further sidewall crystallization. Water build- up in this liquid probably caused a change in style of chamber filling, giving rise to the layered series by bottom accumulation. Microtextures in the zoned feldspars are described in an accompanying paper.CRPG contribution 729 |
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