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Record: oai:ARNO:505958

AuthorJ.P. Engels
TitleThe catazonal poly-metamorphic rocks of Cabo Ortegal (NW Spain), a structural and petrofabric study
JournalLeidse Geologische Mededelingen
AbstractThe petrological study of the southern part of the Cabo Ortegal area is a complement of Vogel’s (1967) investigation of the northern half. The present investigations include a structural as well as a petrofabric study. The rocks belong to an eugeosynclinal sequence which during the Precambrian underwent prograde metamorphism from the staurolite-almandine-muscovite and the kyanite-almandine-muscovite subfacies of the almandine-amphibolite facies through the clinopyroxene-pyralmandine (± hornblende) granulite facies into the eclogite facies (M1). Several of these zones, bounded by isogrades, which are sometimes tectonic in nature, have been mapped. From the fact that the banded gneisses are only metatexitic and from the jadeite content of omphacite, the P/T conditions for the eclogite facies are estimated as: T\u2248700°-750°C, P\u224811-13 Kb, and water vapour pressures: very low.
In the gneisses isoclinal folding (F1) accompanied the metamorphism. The axial planes are subhorizontal and the fold axes plunge just west of north. The fabric analyses of eclogite (point-maximum for [010]) and basic granulite (point-maximum for [001]) show that the same stressfield which produced the F1-folds in the paragneisses, influenced the preferred orientation of clinopyroxene.
A second Precambrian metamorphic phase retrograded the rocks into the hornblende granulite facies (M2). Before the onset of this phase pronounced cataclasis caused the formation of thick mylonitic horizons, followed by E-W trending drag folding (F2). The fabric diagram for clinopyroxene does not show a preferred orientation. During this deformation phase the M1 metamorphic zoning was turned upside down by a combined process of folding and thrusting. In the paragneisses the hornblende granulite metamorphism is marked by a second generation of kyanite. Gabbros, intruded along thrust planes, were partly metamorphosed into garnet-coronites. During this second metamorphic phase isoclinal folds (F3) with subhorizontal axial planes and N-S axial directions were formed. The fabrics of these folds show a marked orientation of the c-axes of (metastable) diopside and brown-green hornblende parallel to the fold axis direction.
A third metamorphic phase caused further retrogradation of the rocks into the amphibolite facies (M3). The characteristic amphibole of this phase is a blue-green hornblende. The former metabasites were metamorphosed into (garnet-)amphibolites. Intruded gabbros were transformed along their margins into ‘flaser’ amphibolites. Folds with vertical axial planes and N-S axial directions reflect the synchronous F4-deformation. The large syn- and antiform structures are products of this phase. The fabric of the hornblende in the amphibolites is determined by the stress field of F4. Older hornblende orientations were destroyed. Whether a Hercynian age should be attributed to the amphibolite facies is not certain; if so, F4 is the first Hercynian deformation phase.
After the overthrusting of the complex over its low-grade country rocks, a phase of chevron folding (F5) was active locally. On the thrust plane small folds of the second Hercynian folding phase can be discerned. A third Hercynian folding phase can be seen in the Paleozoic rocks but not in the Cabo Ortegal Complex proper.
Local greenschist retrogradation (M4) and the emplacement of dolerite dykes are late Hercynian. The tectonic history ends with a phase of normal block faulting which caused the E-W faults.
Document typearticle
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