The Klokken Gabbro-Syenite Complex, South Greenland: Quantitative Interpretation of Mineral Chemistry
The layered syenite series in the Klokken stock formed by continuous in situ fractionation of a trachytic magma in a chamber lined by gabbro with ≲ 3000 m of cover rocks. The following minerals and reactions are assessed as geothermometers and barometers: two feldspars; hypersolvus ternary feldspars...
| Published in: | Journal of Petrology |
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| Main Author: | |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1981
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| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/22/2/233 https://doi.org/10.1093/petrology/22.2.233 |
| Summary: | The layered syenite series in the Klokken stock formed by continuous in situ fractionation of a trachytic magma in a chamber lined by gabbro with ≲ 3000 m of cover rocks. The following minerals and reactions are assessed as geothermometers and barometers: two feldspars; hypersolvus ternary feldspars; ferrohedenbergite-β-wollastonite; clinopyroxene-olivine Fe-Mg exchange; Fe-Ti oxides; sanidine-magnetite-annite; ferroedenite stability. Estimates of silica activity are obtained from the silica-magnetite-fayalite assemblage. The gabbros ended magmatic crystallization at > 1000–1050°C. The less fractionated members of the syenite range probably crystallized with P H 2 O < P total , at T > 870°C and, P H 2 O ≲ 800 bars. In the more fractionated syenites P H 2 O = P total during intercumulus feldspar growth, and all magmatic phases crystallized within the interval 940–830°C and P H 2 O < 1100 bars. Magmatic f O 2 (bars) was ≲1 log unit below the QFM buffer. a SIO 2 in gabbros was slightly above the albite-nepheline buffer, but rose suddenly to just <1 in the syenites, a jump mirrored by minor elements in pyroxenes and opaque oxides. Biotite grew subsolidus in most rocks, at f O 2 < QFM, except in intermediate rocks when f O 2 > QFM and was defined by the sanidine-magnetite-biotite assemblage. In these rocks P H 2 O of 450 bars at 760°C is obtained using existing experimental data, but application of this data to Fe-rich biotites in the layered series (where biotite is an intercumulus phase) requires P > 10 kb at magmatic temperatures. High TiO 2 or F: OH probably accounts for increased T stability of natural annites at low P . The syenitic liquid fractionated down a low temperature zone in a multicomponent system precipitating alk fsp + ol + cpx + mt and the more fractionated members of the layered series had a negligible crystallization interval, a prerequisite for the development of the unique Klokken-type inversely graded mineral layering. |
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