Dear Santanu and all,

Neither amphibolitized eclogite (that normally has post-eclogite
albite/plagioclase) nor amphibolitized blueschist (that would retain
albite or produce plagioclase under amphibiolite facies conditions)
occur in the oceanic mélanges indicated below. There, tectonic blocks of
"amphibolite" (lacking plagioclase) occur in a serpentinite matrix
interpreted as formed in a subduction channel. Calculated pressures are
similar (15 kbar), though temperature is higher (700-750 ºC). In these
rocks both relatively high pressure and hydrous partial melting of MORB
"amphibolites" formed rocks lacking plagioclase which bear a similar
mineral assemblage as that indicated in the query. The tectonic scenario
is onset of subduction of a young hot oceanic lithosphere. A paragraph
in the first paper listed below is devoted to the raised problem of
nomenclature of these peculiar metabasite.

2008. GARCÍA-CASCO, A., LÁZARO, C., ROJAS-AGRAMONTE, Y., KRÖNER, A.,
TORRES ROLDÁN, R. L., NUÑEZ, K., MILLÁN, G., NEUBAURER, F., QUINTERO, I.
Partial melting and counterclockwise P-T path of subducted oceanic crust
(Sierra del Convento, E Cuba). Journal of Petrology 49, 129-161.
doi:10.1093/petrology/egm074

2009. LÁZARO, C., GARCÍA-CASCO, A., NEUBAURER, F., ROJAS-AGRAMONTE, Y.,
KRÖNER, A., ITURRALDE-VINENT, M.A. Fifty-five-million-year history of
oceanic subduction and exhumation in the northern edge of the Caribbean
plate (Sierra del Convento mélange, Cuba). Journal of Metamorphic
Geology, 27, 19-40. DOI 10.1111/j.1525-1314.2008.00800.x

2010. BLANCO-QUINTERO, I.F., GARCÍA-CASCO, A., ROJAS-AGRAMONTE, Y.,
RODRÍGUEZ-VEGA, A., LÁZARO, C., ITURRALDE-VINENT, M. A. Metamorphic
evolution of subducted hot oceanic crust (La Corea mélange, Cuba).
American Journal of Science, 310, 889–915. DOI: 10.2475/11.2010.01

If you are curious and want to know more about this rare case of
partial melting of a hot slab:

2007. GARCÍA-CASCO, A. Magmatic paragonite in trondhjemites from the
Sierra del Convento mélange, Cuba. American Mineralogist 92, 1232-1237.
DOI 10.2138/am.2007.2598

2008. LÁZARO, C., GARCÍA-CASCO, A. Geochemical and Sr-Nd isotope
signatures of pristine slab melts and their residues (Sierra del
Convento mélange, eastern Cuba). Chemical Geology. Chemical Geology,
255, 120-133.DOI 10.1016/j.chemgeo.2008.06.017

2011. BLANCO-QUINTERO, I. F., GERYA, T. V., GARCÍA-CASCO, A., CASTRO,
A. Subduction of young oceanic plates: a numerical study with
application to aborted thermal-chemical plumes. Geochem. Geophys.
Geosyst, 12, Q10012. DOI 10.1029/2011GC003717.

2011. Blanco-Quintero, I.F., Rojas-Agramonte, Y., García-Casco, A.,
Kröner, A., Mertz, D.F., Lázaro, C., Blanco-Moreno, J., Renne, P.R.
Timing of subduction and exhumation in a subduction channel: Evidence
from slab melts from La Corea mélange (eastern Cuba). Lithos, 127,
86–100. DOI 10.1016/j.lithos.2011.08.009

2011. BLANCO-QUINTERO, I. PROENZA J.A., GARCÍA-CASCO A., TAULER, E.,
GALÍ, S. Serpentinites and serpentinites within a fossil subdution
channel: La Corea melange, eastern Cuba. Geologica Acta, 9, 3-4,
389-405, DOI 10.1344/105.000001662,

2011. Lázaro, C., Blanco-Quintero, I.F., Marchesi, C., Bosch, D.,
Rojas-Agramonte, Y., García-Casco A. The imprint of subducion fluids on
subducted MORB-derived melts (Sierra del Convento mélange, Cuba).
Lithos, 126, 341–354. DOI 10.1016/j.lithos.2011.07.011

2011. BLANCO-QUINTERO, I.F.; LÁZARO, C.; GARCÍA-CASCO, A.; PROENZA, J.;
ROJAS-AGRAMONTE, Y. Barium-rich fluids and melts in the subduction
environment (La Corea and Sierra del Convento mélanges, eastern Cuba).
Contributions to Mineralogy and Petrology, 162, 395–413. DOI
10.1007/s00410-010-0603-2

2011. BLANCO-QUINTERO, I.F., GARCÍA-CASCO, A., GERYA, T. Tectonic
blocks in serpentinite melange (eastern Cuba) reveal large-scale
convective flow of the subduction channel. Geology, 39, 79-82. DOI
10.1130/G31494.1

Wish you happy Christmas

Antonio
--
Antonio Garcia-Casco
  Professor of Petrology and Geochemistry
(http://www.ugr.es/~agcasco/personal/)
  Department of Mineralogy and Petrology, University of Granada, Spain
(http://www.ugr.es/~minpet/)
  Andalusian Institute of Earth Sciences, IACT, UGR-CSIC, Spain
(http://www.iact.ugr-csic.es/web_eng/index.html)
  Tel: + 34 958 246613



El 2014-12-17 16:52, Jarek Majka escribió:
> Santanu,
>
> It seems to me like a former blueschist, which has undergone some
> heating. Have you searched carefully for a proper Na-amphibole
> relicts
> in your hornblende?
>
> Best,
> Jarek
>
>
>
>> Dear Colleagues,
>>
>> I am seeking your suggestions on the nomenclature of a metamorphic
>> rock with the assemblage
>> garnet+hornblende+epidote+phengite+rutile+quartz. This assemblage
>> which became stable during peak metamorphism yielded P-T estimate of ~
>> 15-16 kbar,~ 590-610 °C. If this assemblage is evaluated with
>> metamorphic rocks that are related to metamorphism during subduction
>> initiation, the closest rock name that comes to my mind is
>> garnetiferous amphibolite. However, in the strictest sense, the rock,
>> in the absence of plagioclase cannot be an amphibolite, if we use
>> amphibolite in facies sense. At the same time, the estimated P-T range
>> lies in the eclogite facies, also supported in part by complete
>> elimination of plagioclase. However, with omphacitic clinopyroxene
>> being absent, the rock is also not an eclogite in the strictest sense.
>> One additional information in this regard is that Na in the protolith
>> is completely partitioned in Hbl, which shows substantial presence of
>> Na in the M4 site, apart from its general occurrence in the A site.
>>
>> Your comments and observations are welcome.
>>
>> With regards,
>>
>> Santanu
>> -------------------------------------------------------
>> Dr. Santanu Kumar Bhowmik
>> Professor
>> Department of Geology & Geophysics
>> Indian Institute of Technology
>> Kharagpur-721 302
>> India
>> Mobile:+919434041791
>> Telephone:+91-3222-283390(O), 283391, 279766(R)