Mafic silicates from the nepheline syenites of the Motzfeldt centre
ABSTRACT. At least five successive intrusions of nephe-line-bearing magmas fractionated in situ and developed repeatedly overlapping, yet distinct chemical trends in the mafic silicates. Representative el ctron probe analyses are given for olivine, clinopyroxene, amphibole, biotite, and aenigmatite...
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| Format: | Text |
| Language: | English |
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1984
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.617.2650 http://www.minersoc.org/pages/Archive-MM/Volume_48/48-346-1.pdf |
| Summary: | ABSTRACT. At least five successive intrusions of nephe-line-bearing magmas fractionated in situ and developed repeatedly overlapping, yet distinct chemical trends in the mafic silicates. Representative el ctron probe analyses are given for olivine, clinopyroxene, amphibole, biotite, and aenigmatite and their variations discussed. Increasing Na with fractionation produced aegirine-rich clinopyroxene and caused Na/K in amphibole to increase from 2.4 to 7.0. Si~/AI z increased in both amphibole and in the less abundant mica. Mn-enrichment is common to all of the mafic silicates, including the olivines and it is suggested that the parent magmas were initially Mn-rich. Mineralogically the Motzfeldt centre is evolved and similar to the other centres in the Igaliko complex. However, the Motzfeldt centre contains the most frac-tionated rocks (lujavrites) and is in this respect similar to the peralkaline Ilimaussaq intrusion. The compositional ranges and trends of the mafic silicates in Motzfeldt hus provide links between the uncommon peralkaline plutons and the more abundant and typical nepheline syenites. THE PROCESSES which lead to the emplacement of large plutonic 'batholiths ' of alkaline and per-alkaline magma into shallow levels of the crust are poorly understood. Good exposures throughout the Gardar province in southern Greenland (for general description see Upton, 1974; Emeleus and Upton, 1976) give a clear indication of prolonged Proterozoic alkaline magmatism ( ~ 1350-1150Ma) associated with tectonic tilting, which produced many plutonic centres. The dominant message which comes from the studies of these centres is one of multiple overlapping intrusions. Vertical sec-tions to a height of nearly 2 km are typical, no bases |
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