On protolith-, metamorphic overprint, microstructure and rheology of mineral assemblages in orogenic peridotites of the central Scandinavian Caledonides
2013
The Scandinavian Caledonides (SC) are a deeply eroded Alpine-type orogenic belt formed by closure of the
Iapetus ocean and collision between Baltica and Laurentia (500-380 Ma). The SC consists of a stack of Nappe
Complexes (from bottom to top called Lower, Middle, Upper and Uppermost Allochthons) thrusted to the east over
the Baltic Shield (Brueckner and Van Roermund, 2004; Gee et al., 2008). Fossil lithospheric mantle fragments,
called orogenic peridotites, have been found within the (upper part of) middle, upper and uppermost Allochthons,
as well as in the reworked basement gneisses (a.o Western Gneiss Complex (WGC)) along the Norwegian west
coast. They occur as isolated lenses that contain diverse mineral parageneses and/or bulk rock compositions.
Crustal incorporation of orogenic peridotite is classically interpreted to be the result of plate collisional processes
related to orogeny (Brueckner and Medaris, 2000).
The WGC and parts of the upper part of the Middle Allochthon (a.o. Seve Nappe Complex (SNC) in N Jamtland/S
Vasterbotten, central Sweden), are well known for the occurrence of high (HP) and ultrahigh pressure (UHP)
metamorphic terranes (of Caledonian age). The (U)HPM evidence clearly demonstrates the deep metamorphic
origin of these rocks interpreted to be caused by continental subduction and/or collision. Other metamorphic rocks
(of Caledonian age) exposed in allochthonous nappes are solely characterised by greenschist-, amphibolite- and/or
MP granulite “facies” mineral assemblages that can be interpreted, in the absence of retrogression, to have formed
in less deeply subducted (and/or metamorphic) environments. This duality in metamorphic “facies” allows for
a discrimination (at least theoretically) between “deep” versus “shallow” rooted nappes (in central parts of the
Scandinavian Caledonides).
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