Ultra-alkaline magmatism with or without rifting

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doi: 10.1016/0040-1951(87)90079-5
Authors:Le Bas, M. J.
Author Affiliations:Primary:
Univ. Leicester, Dep. Geol., Leicester, United Kingdom
Other:
Moscow State Univ., USSR
Volume Title:Continental rifts; principal and regional characteristics
Volume Authors:Ramberg, Ivar B., editor; Milanovsky, E. E.; Qvale, G.
Source:Tectonophysics, 143(1-3), p.75-84; Symposium on Rifts of the continents and oceans, a part of the 27th international congress, Moscow, USSR, Aug. 4-14, 1984, edited by Ivar B. Ramberg, E. E. Milanovsky and G. Qvale. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0040-1951
Publication Date:1987
Note:In English
Summary:Examination of the relationship between rifting and types of alkaline magmatism reveals features at present overlooked by current theories of plate tectonics. For example, in East Africa the majority of ultra-alkaline igneous rocks (carbonatites, melilitites, nephelinites, ijolites) lie west of the site of the Gregory Rift Valley and pre-date it by tens of millions of years. The mildly alkaline volcanic rocks (basalts, basanites, tephrites, trachytes) occur closely along the site of the Rift Valley, but began erupting approx 10 m.y. before rifting commenced. Rift fracturing on a big scale in East Africa has occurred only in the last 6 m.y., whilst magmatism there began approx 50 m.y. ago. Clearly in East Africa at least, it cannot be said that ultra-alkaline magmatism is controlled by the rifting in either space or time, except for a small number of examples in the last few m.y. Similar relations hold in other parts of the world. It is concluded that rifting is not a necessary accompanying feature of ultra-alkaline magmatism in the continents, nor does it occur in oceanic regions of ultra-alkaline magmatism. Uplift by mantle epeirogenesis, however, is a significant feature. (Author's abstract) [A.W.H.]
Subjects:Alkalic composition; Continental crust; Crust; Genesis; Igneous rocks; Plate tectonics; Rift zones; Rifting; Igneous activity
Abstract Numbers:88M/4492
Record ID:1988004197
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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