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| Titel |
The Subduction of Continental Crust, the Variscan Evolution of the Bohemian Massif, and the Origin of PO Granitoids |
| VerfasserIn |
H. K. Brueckner |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250069464
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| Zusammenfassung |
| Slices of continental crust subducted into the mantle during collisional orogeny may either
undergo metamorphism and exhumation towards the surface as coherent slab-like or domal
high pressure/ultrahigh pressure (HP/UHP) terranes or, if stalled or delayed in the
mantle, melt and return towards the surface as magmas, or undergo a combination of
exhumation and melting. Some exhumed HP/UHP terranes contain synorogenic granitoid
bodies demonstrating melting does occur during exhumation. Therefore, crust that
remains trapped in the mantle will also melt when temperatures reach the appropriate
solidi through adiabatic decompression and/or conductive heating and/or radioactive
decay. Subducted terranes with hydrous phases will undergo hydrate-breakdown
melting and could melt during subduction, when stalled in the mantle or during
exhumation. Terranes lacking hydrous phases probably require melting by adiabatic
decompression as heated crust becomes ductile and rises as diapirs through the mantle
wedge. The generated magmas will intrude through the overlying mantle wedge and
into the overlying continental crust to form late orogenic and post orogenic (PO)
granitoids depending on the time required to reach solidus temperatures. Geochemical
characteristics will depend on P-T conditions, the age/chemistry/mineralogy of
the subducted terrane (especially the presence or lack of hydrous phases), and the
degree of melt interaction (i.e. the traverse length) with the mantle wedge. Melts that
significantly traverse the wedge will acquire the hybrid mantle/crust nature of many
PO granitoids. Melts generated by adiabatic decompression close to or within the
continental crust will retain ancient crustal signatures. The Variscan evolution of the
Bohemian Massif involved two episodes of subduction of continental crust: (1), the
southward (present coordinates) subduction of Saxo-Thuringia beneath Bohemia (aka
Tapla-Barrandia) along an east-west suture at 400-370 Ma followed by, (2), the east-northeast
subduction of Moldanubia beneath Bohemia at 340-320 Ma1. The first subduction was
followed by the intrusion of granitoids, including the Central Bohemian Batholith,
between 370-340 Ma, all occurring south of the Saxo-Thuringian – Bohemia suture
suggesting elements of the subducted Saxo-Thuringian continental crust melted
and intruded the overlying Bohemian Craton. The second subduction event was
accompanied and followed by multiple intrusions of granites (340 Ma synorogenic
granitoids, 340-310 Ma post-orogenic S and high-K granitoids, and 310-290 I-type
granitoids2) all occurring west of the of the Moldanubia-Bohemia suture suggesting
elements of the subducted Moldanubian crust were melted and intruded the overlying
Bohemian/Saxo-Thuringian Craton. Many of the HP/UHP terranes in the Bohemian
Massif occur in domal structures suggesting exhumation and melting occurred
in part by diapiric upwelling3. It is proposed that both the Saxo-Thuringian and
Moldanubian terranes were exhumed by a combination of slab-return and diapiric
upwelling and that elements of both terranes were stranded long enough in the
mantle to heat up and melt to form most or all of the PO granitoids of the Bohemian
Massif. This model involves successive re-distillation of the continental crust and
may play a significant role in the evolution of both the continental crust and upper
mantle.
1Medaris et al., 2005, Lithos 82.
2Finger et al., 1997, Min & Pet 61.
3Stípská et a., 04, J. Met. Geol. 22. |
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