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| Titel |
Interplay between water infiltration, metamorphic reactions and strain localization during subduction of gabbro from the basement rocks of the Lofoten anorthosite complex |
| VerfasserIn |
P. Nasipuri, H. Stünitz, L. Menegon, A. Berger |
| 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 |
250064301
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| Zusammenfassung |
| Eclogitization of lower crustal rocks occurs during subduction processes. During the
prograde segment of subduction cycle and subsequent exhumation, ranges of mineral
assemblages are produced under different P-T conditions. In this contribution, we describe
the role of water and formation of syn-kinematic mineral assemblages in the ductile
shear zone within leucogabbro and in the retrograde equivalent (characterized by
alternate bands of mafic and felsic layers) that are separated by a distance of 100-150
meter.
In the undeformed gabbro, Plagioclase (Pl0), olivine (Ol0) and orthopyroxene (Opx0)
form the igneous texture. The onset of metamorphism (M1, 700 0C, 1GPa) during subduction
processes is indicated by growth of metamorphic Opx1 and Grt1 corona around Pl0 and Ol0.
Cm- wide alternating bands of a mixture of Pl-amphibole (Amph) ± Spl and GrtI(M1) – Opx
–Omph – Grt II (M2A, 650-750 0C, > 1.8 GPa) phase mixtures characterize shear zones.
Omph overgrow deformed Opx grains (D1). Clinopyroxene (Cpx) -Pl symplectite
(M2B, 600-650 0C, 1 GPa) occur at the outer rim the Omph. Breakdown of Grt
I-IIto Amph ± Pl (An rich) ± Spl with increased water activity form during M3
metamorphism.
In the retrograde eclogite, the mafic layer is composed of Cpx, sodic Pl and Qtz (grain
size 50 μm). Coarse-grained calcic Pl (grain size 50-75 μm) forms the felsic layer. Grt
porphyrolcast (eq. Grt I-II) with inclusions of Pl and Qtz occur within the mafic layer. A thin
layer of amphibole and clinozoisite layer overgrows at the contact between the mafic and
felsic layer (6000C, 0.6-0.8 GPa).
In the ductile shear zone, crystallographic data of recrystallized Opx0 (grain size
20-30 μm) and the relict Opx0, chemical similarity and the grain size reduction
indicate that Opx deformed by at the onset of subduction. The deformed Opx grains
show sinistral sense of shear. Although, the fine-grained Pl is expected to show
random CPO characteristics of diffusion creep, the strong CPO of Pl is indicative of
inheritance of crystallographic preferred orientation from D1 deformation. The
CPO of amphibole suggests that amphibole re-orients by rigid body rotation during
deformation, forming aligned (100) planes with opposite shear sense with the Opx0 defined
fabric.
In the retrograde variety, the CPO data of Cpx from the mixed phase layers is consistent
with the (010) [001] dominant slip system. However, the CPO data of Cpx is interpreted as
oriented growth fabric during diffusion creep. Crystallographic data of recrystallized
plagioclase in the mono-mineralic felsic layers indicate (010) [100] as dominant slip system
during dislocation creep.
This study indicates that the deformation of the Opx0in the ductile shear zone
occurredat the amphibolite-granulite facies condition with the influx of water. After the
static growth of Omph, increase in the water activity promotes the growth of Amph
and the deformation after the post eclogite stage (M3). However, in the retrograde
eclogite, the coarser grain size of the minerals (Cpx, Pl) is achieved probably due to
water-saturated condition and promotes diffusion creep accommodated deformation. |
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