Alkali-calcic and alkaline post-orogenic (PO) granite magmatism: petrologic constraints and geodynamic settings

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State: Public
Version: Final published version
Serval ID
serval:BIB_A8CEB4E6AACF
Type
Article: article from journal or magazin.
Collection
Publications
Institution
Title
Alkali-calcic and alkaline post-orogenic (PO) granite magmatism: petrologic constraints and geodynamic settings
Journal
Lithos
Author(s)
Bonin B.L., Azzouni-Sekkal A., Bussy F., Ferrag S.
ISSN-L
0024-4937
Publication state
Published
Issued date
1998
Peer-reviewed
Oui
Volume
45
Pages
45-70
Language
english
Abstract
The end of an orogenic Wilson cycle corresponds to amalgamation of
terranes into a Pangaea and is marked by widespread magmatism dominated
by granitoids. The post-collision event starts with magmatic processes
still influenced by subducted crustal materials. The dominantly
calc-alkaline suites show a shift from normal to high-K to very high-K
associations. Source regions are composed of depleted and later enriched
orogenic subcontinental lithospheric mantle, affected by dehydration
melting and generating more and more K- and LILE-rich magmas. In the
vicinity of intra-crustal magma chambers, anatexis by incongruent
melting of hydrous minerals may generate peraluminous granitoids bearing
mafic enclaves. The post-collision event ends with emplacement of
bimodal post-orogenic (PO) suites along transcurrent fault zones. Two
suites are defined, (i) the alkali-calcic
monzonite-monzogranite-syenogranite-alkali feldspar granite association
characterised by [biotite + plagioclase] fractionation and moderate
[LILE + HFSE] enrichments and (ii) the alkaline
monzonite-syenite-alkali feldspar granite association characterised by
[amphibole + alkali feldspar] fractionation and displaying two
evolutionary trends, one peralkaline with sodic mafic mineralogy and
higher enrichments in HFSE than in LILE, and the other aluminous
biotite-bearing marked by HFSE depletion relative to LILE due to
accessory mineral precipitation. Alkali-calcic and alkaline suites
differ essentially in the amounts of water present within intra-crustal
magma chambers, promoting crystallisation of various mineral
assemblages. The ultimate enriched and not depleted mantle source is
identical for the two PO suites. The more primitive LILE and HFSE-rich
source rapidly replaces the older orogenic mantle source during
lithosphere delamination and becomes progressively the thermal boundary
layer of the new lithosphere. Present rock compositions are a mixture of
major mantle contribution and various crustal components carried by
F-rich aqueous fluids circulating within convective cells created around
magma chambers. In favourable areas, PO suites pre-date a new orogenic
Wilson cycle. (C) 1998 Elsevier Science B.V. All rights reserved.
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01/10/2012 20:07
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