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| Titel |
Amphibole zonation as a tool for tracing metamorphic histories: examples from Lavrion and Penteli metamorphic core complexes |
| VerfasserIn |
Ioannis Baziotis, Alexander Proyer, Evripides Mposkos, Antonios Marsellos, George Leontakianakos |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250086892
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| Publikation (Nr.) |
 EGU/EGU2014-835.pdf |
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| Zusammenfassung |
| The amphiboles represent the predominant minerals in high pressure/low temperature
metamafic rocks. Amphibole composition (e.g. cations Si, Al, Na) depends on the bulk
composition, the solid solution‘s gaps and the P-T conditions during metamorphism. The
AlIV , NaA and Ti increase with temperature, whereas AlV I and NaB with pressure
(Triboulet, 1992). Increase of Si+4 and decrease of AlV I and (NaA+NaB) in clinoamphibole
is concurrent with pressure decrease. We focus on Na- and Ca-amphiboles from greenschists
and blueschists from Lavrion (upper tectonic unit-UTU) and Penteli (lower tectonic
unit-LTU) metamorphic complexes (Baziotis et al. 2009; Baziotis & Mposkos, 2011) in order
to unravel the relation of zonation with: (1) the physicochemical conditions and (2) the P-T
stages during metamorphic evolution.
Amphiboles in Lavrion metabasites are the dominant rock constituents:
(ferro)-glaucophane in the blueschists and actinolite in the greenschists. Blue-amphiboles
occur in various textural modes: parallel to the foliation, replacing omphacite, rimmed
by Ca-amphibole, as inclusions in albite and epidote or in calcitic veinlets. The
blue-amphibole composition varies from glaucophane to ferro-glaucophane with
the NaB ranging from 1.617 to 1.979 a.p.f.u., whereas the green-amphiboles are
actinolites. In the blueschists, actinolite has higher R3+ at a given NaB compared to the
greenschists, attributed to Fe+3-rich bulk composition of the blueschists. Three types
of zoning were observed in these amphiboles: glaucophane rimming actinolite,
glaucophane with increasing AlIV , AlV I and NaB values towards the rim, and actinolite
mantling glaucophane. The zoning patterns in the blue-amphiboles are characterized by
increasing Tschermak and glaucophane components towards the rim, consistent
with increasing temperature and pressure during prograde metamorphism. Two
different modes of actinolite represent different stages of the metamorphic history.
The actinolite inclusions formed during prograde evolution, whereas the actinolite
overgrowths on glaucophane took place at post-peak conditions, during greenschist-facies
overprinting. The actinolite overgrowths indicate retrogression at slightly higher
temperature for a given pressure compared to the prograde path; such overgrowths
started with the assemblage actinolite–glaucophane–epidote–albite, immediately at
the post-peak pressure conditions with minor fluid ingress. In the greenschists,
however, the extensive actinolite overgrowths on glaucophane took place in the typical
greenschist-facies actinolite–chlorite–epidote–albite stability field, triggered by internal
dehydration.
The metamorphic history of Penteli amphiboles is depicted by a prograde path with AlV I/
NaB increase and AlIV decrease; this suggests the formation of glaucophane around
actinolite during ongoing subduction. Glaucophane replacement by actinolitic hornblende
and core-to-rim decrease of AlV I both suggest temperature increase during decompression. A
complex amphibole, with Fe-rich hornblende or actinolitic hornblende at the core, an
intermediate Na–amphibole and outer rim of actinolitic or actinolitic hornblende, suggest
with pressure-temperature decrease. A patchy glaucophane grown at the outer rim, suggests
another cycle of pressure increase, perhaps during renewed subduction. This might be
interpreted as a temporary recapturing of that rock fragment by the downgoing
slab.
References
Baziotis, I. et al. 2009. Eur. J. Mineral. 21, 133–148.
Baziotis, I. & Mposkos, E. 2011. Lithos, 126, 161-173.
Triboulet, C. 1992. J. Met. Geol., 10, 545-556. |
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