Structural petrology of an area near Santiago de Compostela (NW Spain)

The area around Santiago de Compostela has been subjected to petrological and structural investigations. The rocks present in the mapped area have been divided into two complexes (the Ordenes Complex and the Complex of Santiago de Compostela) on the basis of their petrography, structure and grade of metamorphism. An additional group consists of the intrusive rocks still recognizable as such. The intermediate-grade rocks of the Ordenes Complex mainly comprise retrograded mafic granulite-facies rocks, garnet-amphibolites, amphibolites, metagabbros, peridotite and kyanite-staurolite-garnet-gneisses or schists. Part of these rocks underwent amphibolite-facies metamorphism during a probably pre-Hercynian orogeny and in places granulite-facies conditions were attained. Isoclinal to tight folding (F1) accompanied the first metamorphic phase; the axial planes are subhorizontally to gently inclined and the fold axes plunge N or NNW. A period of thrusting, accompanied by mylonitization and resulting in an imbricate structure, was followed by recrystallization under amphibolite-facies conditions, forming blastomylonites in certain zones. The emplacement of the metagabbros probably preceded this recrystallization. The related F2-structures were found in thrust zones, having approximate E-W axes and in general gently inclined axial planes. F3 was tentatively placed after F2 and is a postcrystalline deformation, characterized by the formation of a steeply inclined E-W striking crenulation cleavage and subhorizontally plunging axes. All these events presumably occurred during a pre-Hercynian orogeny, but this will not be certain until conglomerates marking an angular unconformity are found or until radiometric data become available. The pre-Hercynian orogeny was followed by a period in which intrusions of dioritic and granitic rocks took place. The latter are tentatively correlated with similar rocks in S Galicia which were radiometrically dated as Cambro-Ordovician. The Complex of Santiago de Compostela contains low-grade schists, for example albiteporphyroblast-bearing schists and migmatized metasediments. During the first Hercynian phase, these rocks underwent predominantly greenschist-facies conditions. The rocks of the Ordenes Complex have been locally retrograded to epidote-bearing amphibolites during this phase. Metamorphism continued giving rise to the growth of albite-porphyroblasts and culminating in partial fusion of the metasediments. This period terminated with intrusions of two-mica granites, which resulted in contactmetamorphic aureoles. Subsequently the rocks were retrograded during the greenschist-facies metamorphism (the basic rocks in places into greenstones). The structural main phase (F4) of the Hercynian orogeny resulted in the formation of folds with steeply inclined axial planes and axes plunging gently to the N. F5 coincided with the growth of the albite-blasts. This folding produced the same N-S striking axial planes but the B-axis plunges steeply to the E. A well-developed crenulation cleavage (F6) occurs throughout the area. The associated lineation (‘Runzelungen’) on the schistosity planes plunges gently to the N. It is argued that during F6 the basic rocks were upthrusted to about their present position. Late-Hercynian phyllonitization and fault-movements were accompanied by the previously mentioned greenschist-facies retrogradation which presumably started at the time that F6 was active. The emplacement of pegmatites and dolerites also took place at the end of the orogeny. Microscopical fabric analyses of metabasites and albiteporphyroblast-bearing schists corroborate the field observations. The amphibole fabrics show that at least three generations of hornblende, each of them formed under different metamorphic and structural conditions, may be postulated. The mathematical distribution model, proposed by Bingham, is shown to fit the observed distributions (an elongated or circular maximum which may lie in a great-circle girdle) quite well.