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Early Variscan P-T evolution of an eclogite body and adjacent orthogneiss from the northern Malpica-Tuy shear-zone in NW Spain

Botao Li and Hans-Joachim Massonne
Early Variscan P-T evolution of an eclogite body and adjacent orthogneiss from the northern Malpica-Tuy shear-zone in NW Spain
European Journal of Mineralogy (July 2016) 28 (6): 1131-1154

Abstract

An eclogite and its surrounding gneiss from the Malpica-Tuy zone, northwestern Spain, were studied. In addition to garnet and omphacite, the eclogite contains barroisitic amphibole, quartz, phengite with Si contents between 3.27 and 3.45 per formula unit (pfu), rutile, and accessory talc and (clino)zoisite-epidote. Garnet exhibits a chemical zonation with Alm (sub 59) Prp (sub 13) Grs (sub 24) Sps (sub 4) , Alm (sub 57) Prp (sub 13) Grs (sub 27) Sps (sub 3) , Alm (sub 56.6) Prp (sub 12.5) Grs (sub 30.2) Sps (sub 0.7) , and Alm (sub 54) Prp (sub 20) Grs (sub 25) Sps (sub 1) as inner core, outer core, inner rim, and outermost rim compositions, respectively. The gneiss is a former granodiorite now mainly composed of quartz, plagioclase, K-feldspar, biotite, garnet, (clino)zoisite-epidote, and phengite with Si contents between 3.27 and 3.38 pfu. Garnet shows inner core, outer core, and rim compositions of Alm (sub 26.9) Prp (sub 0.2) Grs (sub 70) Sps (sub 2.9) , Alm (sub 33.8) Prp (sub 0.7) Grs (sub 64) Sps (sub 1.5) , and Alm (sub 42.6) Prp (sub 1.4) Grs (sub 54) Sps (sub 2) , respectively. A series of P-T pseudosections, calculated with PERPLE_X for the bulk-rock compositions of the studied eclogite and gneiss, were contoured by isopleths of various parameters, including molar fractions of garnet components. Using mainly the variable garnet and phengite compositions and these isopleths, P-T paths were derived. After an isothermal burial path the eclogite reached peak pressures of 22.5 kbar at 540 degrees C. The subsequent exhumation path passed through P-T conditions of 21 kbar and 575 degrees C and 13.5 kbar and 625 degrees C. The latter data represent the peak P-T conditions of the gneiss. These P-T data suggest that the protolith of the eclogite, a basic rock of an island-arc setting, underwent high-pressure metamorphism in a subduction zone. During its exhumation it came in contact with the buried metagranodiorite, representing the tip of a descending continental plate, at depths of 45-50 km probably 350-355 Ma ago.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 28
Serial Issue: 6
Title: Early Variscan P-T evolution of an eclogite body and adjacent orthogneiss from the northern Malpica-Tuy shear-zone in NW Spain
Affiliation: Universitaet Stuttgart, Institut fuer Mineralogie und Kristallchemie, Stuttgart, Germany
Pages: 1131-1154
Published: 20160717
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 87
Accession Number: 2016-085309
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, geol. sketch map
N43°18'40" - N43°18'40", W08°48'07" - W08°48'07"
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201641
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