Journal article
Details of the gabbro-to-eclogite transition determined from microtextures and calculated chemical potential relationships

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Publication Details
Author list: Schorn S, Diener J
Publisher: Wiley
Publication year: 2017
Journal: Journal of Metamorphic Geology
Volume number: 35
Start page: 55
End page: 75
Total number of pages: 21
ISSN: 0263-4929
eISSN: 1525-1314

Permian-aged metagabbros from the eclogite type-locality in the eastern
European Alps were partially to completely transformed to eclogite
during Eoalpine intracontinental subduction. Microtextures developed
along a preserved fluid infiltration and reaction front in the gabbros
record the incipient gabbro-to-eclogite transition, allowing the details
of the eclogitization process to be investigated. Original,
anorthite-rich igneous plagioclase is pervasively replaced by
fine-grained intergrowths of clinozoisite, kyanite and Na-rich
plagioclase. Where plagioclase was in contact with igneous
orthopyroxene, 100–200 μm thick bimineralic coronae of
symplectic kyanite and diopsidic clinopyroxene form along the edges of
the grains. The rims of igneous orthopyroxene develop a complementary
bimineralic corona of diopsidic clinopyroxene and garnet. Igneous
clinopyroxene does not show any breakdown textures; however, jadeite
content gradually increases towards the rims. In addition, exsolution
lamellae inherited from the igneous clinopyroxene become progressively
more jadeitic as eclogitization proceeds. Given that the igneous
plagioclase is pervasively replaced by clinozoisite, kyanite and Na-rich
plagioclase, whereas kyanite–diopside symplectites are confined to
narrow rim zones, we suggest that the development of these textures was
controlled by the (im)mobility of different elements on different length
scales. The presence of hydrous minerals in the core of anhydrous
plagioclase indicates that H2O diffusivity occurred on a
mm-scale. By contrast, the size of the anhydrous diopside–kyanite and
diopside–garnet symplectites indicate that Fe–Mg–Ca–Na diffusivity was
limited to a 10s of μm scale. Chemical potential relations
calculated in the idealized NCASH chemical system show that the
clinozoisite–kyanite–albite intergrowths formed due to an increase of μH2O
to plagioclase, whereas all other elements remained effectively
immobile on the scale of this texture. Fluid conditions indicated by
this texture span from virtually dry conditions (math formula0.15) to H2O-saturation,
and therefore does not imply that the rocks were ever fluid-saturated.
Calculations in the CMAS and NCFMAS systems show that the
gabbro-to-eclogite transition is characterized by the growth of garnet,
diopsidic clinopyroxene and kyanite due to diffusion of Ca (+ Na) and Mg
(+ Fe) along a μCaO (+ Na2O)–μMgO (+ FeO)
chemical potential gradient developed between orthopyroxene and
plagioclase compositional domains. The anhydrous nature of the textures
indicate that the gabbro-to-eclogite transition is not driven by
hydration; however, increased μH2O acts as a
catalyst that increases diffusivity of all elements and rates of
dissolution–precipitation, allowing the overstepped metamorphic
reactions to occur. Our results show that crustal eclogite formation
requires low H2O content, confirming that true eclogites are dry rocks.

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Last updated on 2018-12-03 at 10:14