The Anarak, Jandaq and Posht-e-Badam metamorphic complexes in central Iran: New geological data, relationships and tectonic implications


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The Anarak, Jandaq and Posht-e-Badam metamorphic complexes in central Iran: New geological data, relationships and tectonic implications
Bagheri S., Stampfli G.M.
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The Anarak, Jandaq and Posht-e-Badam metamorphic complexes occupy the NW
part of the Central-East Iranian Microcontinent and are juxtaposed with
the Great Kavir block and Sanandaj-Sirjan zone. Our recent findings
redefine the origin of these complexes, so far attributed to the
Precambrian-Early Paleozoic orogenic episodes, and now directly related
to the tectonic evolution of the Paleo-Tethys Ocean. This tectonic
evolution was initiated by Late Ordovician-Early Devonian rifting events
and terminated in the Triassic by the Eocimmerian collision event due to
the docking of the Cimmerian blocks with the Asiatic Turan block.
The ``Variscan accretionary complex'' is a new name we proposed for
the most widely distributed metamorphic rocks connected to the Anarak
and Jandaq complexes. This accretionary complex exposed from SW of
Jandaq to the Anarak and Kabudan areas is a thick and fine grain
siliciclastic sequence accompanied by marginal-sea ophiolitic remnants,
including gabbro-basalts with a supra-subduction-geochemical signature.
New Ar-40/Ar-39 ages are obtained as 333-320 Ma for the metamorphism of
this sequence under greenschist to amphibolite facies. Moreover, the
limy intercalations in the volcano-sedimentary part of this complex in
Godar-e-Siah yielded Upper Devonian-Tournaisian conodonts. The
northeastern part of this complex in the Jandaq area was intruded by 215
+/- 15 Ma arc to collisional granite and pegmatites dated by ID-TIMS and
its metamorphic rocks are characterized by Some Ar-40/Ar-39 radiometric
ages of 163-156 Ma.
The ``Variscan'' accretionary complex was northwardly accreted to the
Airekan granitic terrane dated at 549 +/- 15 Ma. Later, from the Late
Carboniferous to Triassic, huge amounts of oceanic material were
accreted to its southern side and penetrated by several seamounts such
as the Anarak and Kabudan. This new period of accretion is supported by
the 280-230 Ma Ar-40/Ar-39 ages for the Anarak mild high-pressure
metamorphic rocks and a 262 Ma U-Pb age for the trondhjemite-rhyolite
association of that area. The Triassic Bayazeh flysch filled the
foreland basin during the final closure of the Paleo-Tethys Ocean and
was partly deposited and/or thrusted onto the Cimmerian Yazd block.
The Paleo-Tethys magmatic arc products have been well-preserved in the
Late Devonian-Carboniferous Godar-e-Siah intra-arc deposits and the
Triassic Nakhlak fore-arc succession. On the passive margin of the
Cimmerian block, in the Yazd region, the nearly continuous Upper
Paleozoic platform-type deposition was totally interrupted during the
Middle to Late Triassic. Local erosion, down to Lower Paleozoic levels,
may be related to flexural bulge erosion. The platform was finally
unconformably covered by Liassic continental molassic deposits of the
One of the extensional periods related to Neo-Tethyan back-arc rifting
in Late Cretaceous time finally separated parts of the Eocimmerian
collisional domain from the Eurasian Turan domain. The opening and
closing of this new ocean, characterized by the Nain and Sabzevar
ophiolitic melanges, finally transported the Anarak-Jandaq composite
terrane to Central Iran, accompanied by large scale rotation of the
Central-East Iranian Microcontinent (CEIM). Due to many similarities
between the Posht-e-Badam metamorphic complex and the Anarak-Jandaq
composite terrane, the former could be part of the latter, if it was
transported further south during Tertiary time. (C) 2007 Elsevier B.V.
All rights reserved.
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