Fragments of deeper parts of the hanging wall mantle preserved as orogenic peridotites in the central belt of the Seve Nappe Complex, Sweden

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• 作者：Frediano Clos ; ^{clos.frediano@gmail.com" class="auth_mail} ; Mattia Gilio ^{mattia.gilio@gmail.com" class="auth_mail} ; Herman L.M. van Roermund ^{H.L.M.vanRoermund@uu.nl" class="auth_mail}
• 关键词：Cr# ; 100  ; × ;   ; (Cr)/(Cr  ; +  ; Al) ; FGP ; Friningen garnet peridotite ; GBM ; grain boundary migration ; GSI ; grain size insensitive ; GSS ; grain size sensitive ; KSP ; Kittelfjä ; ll spinel peridotite ; Mg# ; 100  ; × ;   ; (Mg)/(Mg  ; +  ; Fe) ; RSP ; Ruotats spinel peridotite ; SCLM ; sub-continental lithospheric mantle ; SOLM ; sub-oceanic lithospheric mantle ; SNC ; Seve Nappe Complex ; UHP ; ultra high pressure ; WGR ; Western Gneiss Region
• 刊名：Lithos
• 出版年：April, 2014
• 年：2014
• 卷：192-195
• 期：Complete
• 页码：8-20
• 全文大小：2551 K

文摘

Formation conditions of olivine microstructures are investigated in the Kittelfj盲ll spinel peridotite (KSP), a fragment of lithospheric mantle which occurs as an isolated body within high grade metamorphic crustal rocks of the Seve Nappe Complex (SNC), southern V盲sterbotten, central Sweden. The KSP is an orogenic peridotite containing a well developed penetrative compositional layering, defined by highly depleted dunite with olivine Mg# (100 脳 Mg/Mg + Fe) of 92.0-93.5 and harzburgite with lower Mg# (91.0-92.5). Dunite is characterized by three contrasting olivine microstructures formed in response to different tectonometamorphic events: Coarse-grained, highly strained olivine porphyroclasts (M1) up to 20 cm long are surrounded by dynamically recrystallized olivine grains (M2) defining a characteristic olivine 鈥渇oam鈥?microstructure (grain size: 200-2000 渭m). An olivine 鈥渕ortar鈥?(M3) microstructure (10-50 渭m) forms a penetrative fabric element only in strongly localized, cm-to-m sized shear zones that crosscut earlier structures/foliations. Olivine fabric analysis in synergy, with mineralogical and chemical analyses, reveals that the KSP body represents old, possibly Archean, sub-continental lithospheric mantle that was crustally emplaced into the Caledonian tectonic edifice from the hanging wall mantle during exhumation of the subducted Seve Nappe Complex (J盲mtlandian orogeny ~ 454 Ma). Olivine porphyroclasts (M1) grew at high temperature during dominant isobaric cooling after extensive polybaric melt extraction (> 40%) and subsequent refertilization. The onset of the early Caledonian deformation is interpreted to be related to the crustal emplacement of the KSP during eduction of the SNC. This phase is characterized by the development of the olivine M2 foam microstructure, formed at 650-830 掳C/1-2 GPa by dislocation creep processes producing an E-type CPO's by the operation of the [100](001) and subordinate [001](100) slip systems with operating flow stress levels around 8-48 MPa. In contrast the M3 olivine 鈥渕ortar鈥?microstructure formed at 550-600 掳C/0.45-0.6 GPa and represents deformation after the subducted slab had returned to shallow crustal levels. It is proposed here that the presence of a penetrative olivine M2 鈥渇oam鈥?microstructure can be used as an easy tool in the field to discriminate between mantle wedge (i.e. sub-continental affinity), ophiolite (i.e. sub-oceanic affinity), and/or hyper-extensional peridotite in the Scandinavian Caledonides. The latter two peridotite subtypes may have similar M2 microstructures, but exclusively restricted to the structural base of the bodies. Alternatively in basal parts of ophiolites, M3 microstructures directly overprint coarser grained proto-granular olivine microstructures.