| Author||Don Isidro Parga-Pondal|
|Title||Nota explicativa del Mapa geológico de la parte N.O. de la provincia de la Coruña|
|Journal||Leidse Geologische Mededelingen|
|Abstract||Explanation of the geological map (Northwestern part) of the province of La Coruña, Galicia, N.W. Spain Main rock groups We distinguish five main groups of rocks which probably differ in age. Whether this age difference applies only to the time of their intrusion or metamorphism or also to their sedimentary origin remains obscure.|
These five groups are from top to bottom in the time scale: — (1) Younger rocks, mostly not tectonized, post tectonic. To this group belong: (a) a red sandstone patch west of Malpica on the coast, unconformable on the underlying schists, age unknown; (b) the Traba granite pluton and some other plutonic rocks south of it; (c) rhyolitic or syenitic dykes south of the Traba granite; (d) a swarm of WNW striking basic dykes, mostly dolerites which are probably older than the Traba granite; (2) A group of intrusive rocks which are partly tectonized and partly not, and often have a porphyritic texture. (a) A group which we call the trondjemetic differentiation series, in the north mostly diorites, in the south gradually losing their content of dark minerals. The rocks often contain very large calcic felspar crystals. (b) A group called the Coruña granite, mostly biotite granite but with pegmatites containing muscovite. (3) A group of basic rocks, covering a large territory forming an arc with liameter of ± 60 km and consisting of gabbros, pyroxenites, serpentines and amphibolites. The coarse gabbros in the centre of its western branch are not tectonized but the omphibolites on its outer margin are often strongly tectonized. (4) A group of rocks containing migmatites, white granites, gneisses, mica schists and even less metamorphic rocks, which we call the Lage group. The muscovite granite of Lage is certainly a syntectonic granite, and is associated with migmatites and micaschists on the one hand and with much less disturbed granites on the other hand. (5) A group of rocks, showing locally a very high degree of metamorphism, which we call the “ancient complex”. It contains hornblende gneisses, amphibolites eclogites, muscovite gneisses, granite gneisses and micaschists, and occupies a long NS trending band. It differs from the Lage complex by the frequent occurence of concordant amphibolites.
The relation of these groups of rocks is very doubtful in many cases but we believe that the youngest rocks are Paleozoic and the oldest Pre-Cambrian. The doleritic dykes, striking WNW are often regarded as Mesozoic or even Tertiary (Torre de Assunçao, 1950, 1951) it follows that Traba granite might also be Tertiary and could perhaps be compared to the Cintra granite of Portugal (Torre de Assunçao and Brak-Lamy, 1952) The Lage complex could be compared perhaps with the pre-Ordovician schists which have been called by Teixeira (1954, 1955) the “ante-Ordovician schistograywackes” and are perhaps Pre-Cambrian.
Probably our “ancient complex” represents then an even older Pre-Cambrian orogenic cycle.
On the other hand the analyses of the Rb/Sr relation (Hoja de Tuy, 1953) indicate that the pegmatites of the Lage granite are either Caledonian or Hercynian in age, as they imply ages vary between a 270 and 350 million years. The analyses are not isotopic however and one perhaps ought not attach too much value to their result.
According to the field relation of the Coruña granites and the Lage migmatites or schists there can be little doubt that the Coruña granite is younger. The Coruña granite should then be Hercynian because a Caledonian orogeny is almost unknown in the Iberian Peninsula (Carrington da Costa, 1952).
The intrusion of gabbroïc rocks is still more difficult to date. On the one hand it has partly been tectonized on its margin whereas the rocks of the centre are perfectly fresh, but on the other hand they are younger than the schists in which they intruded and the Lage orogeny itself. Provisionally we regard their intrusion as late-Hercynian.
In general the structures of this western region of Galicia shows a dominant NS trend, bent in an arc convex towards the west. This convexity has been increased by a set of younger faults striking WNW. The schistosity of the rocks is generally parallel to the trend of their boundaries but exact measurements are mostly lacking. Discordant with the prevailing structures are the abovementioned faults, the doleritic dykes which accompany them and the intrusions of younger granites of the Traba group.
Petrology 1. The Traba granite and associated rocks. — This group of rocks occurs only in the eastern coastal section of the province of La Coruña. They form either great batholiths like the Traba and the Pindo masses, or small outcrops closely related to the big batholiths as near Mugía, Leis and Caneliñas.
The reddish granite generally contains biotits and Na-K felspar, and has sometimes a porphyritic texture. It never shows any preferred orientation of its minerals.
Its direct thermal metamorphic zone is restricted to some tens of meters, but its influence is felt in a much larger region. Everywhere on the sheets 67 and 92 and the western half of the sheets 68 and 93 one finds numerous small stocks and dykes. These dykes consist either of hornblende syenites, fine grained dacites and quartz porphyries. A semicircular dyke system of these rocks suggests a circular zone of subsidence.
Another dyke system, which also traverses the fundamental structure of the Galician system, has an approximately E—W trend. These dykes consist either of basic rocks (lamprophyres, diabase porphyries or dolerites), or of light coloured acid, aphanitic rocks. Their age is certainly younger than the Lage granite, which they traverse, and older than the Traba granite which in its turn appears to cut off the dykes.
The Traba granite mass contains zones full of thin mineralized quartz veins containing cassiterite, wolframite, molybdenite and monazite. Some large quartz dykes traverse the granite from north to south. 2. The non orientated, homogeneous and porphyritic, late tectonic granites. — This group contains all those granitic masses which appear as rounded hills, which in Galicia are called “penedos”, or occupy large flat surfaces.
In general they form large batholiths with well defined boundaries and cause thermal metamorphism in the adjacent rocks, but others are clearly granitizised masses. At the contacts of the latter masses one sees a gradual absorption of the rocks of the “ancient complex” accompanied by a complete reorganization of the elements. The marginal zones coutain numerous xenoliths, distributed without regular orientation, as for instance south of Mugia. When this process of absorption continues the rock becomes homogeneous and a palingenetic granite, sometimes of porphyritic texture like that of la Ruña or Monte Pedrouso, or of homogeneous grain like that of Muros, is the result.
The different types are: (a) The biotite-granodiorite of Bayo, (b) The biotite-granite of La Coruña, (c) Porphyritic muscovite granite of La Ruña, (d) Homogeneous muscovite granite of Muros. a. The biotite-granodiorites of Bayo, or rocks of the trondjemitic differentiation series. — These granodiorites form elongated masses concordant with the trend of the “ancient complex” or Lage group. Apparently they have assimilated large tracts of the surrounding rocks. The most basic types contain much pyroxene and hornblende, all of them contain biotite and plagioclase, and in the most acid types the plagioclase predominates.
The Bayo mass is some 50 km long and has a width varying between 1 and 5 km. The masses of Santa Comba-Negreira are also elongated in a N—S direction. b. Biotite granite of La Coruña. — In eastern Galicia there are several batholithic granite masses which resemble in many respects those of the Bayo type but cannot be included in the same group because their mineralogy and emplacement is different. They form large plutons which are not concordant with the general trend and find their greatest development in the Cambrian and Ordovician of western Galicia, for instance the Lugo granite described by Barrois in 1881.
In our region the Coruña granite belongs to this group, further east we find the batholith east of Betanzos and Curtis and the large batholith of Vivero-Mondoñedo. c. The porphyric muscovite granite of la Ruña. — The Ruña mountain, 640 m altitude, gives typical exposures of these muscovite-biotite granites. The granite consists of large idiomorphic Na-K felspars up to 7 cm with quartz, muscovite and biotite, it does not show any preferred orientation beyond a faint parallel arrangement of the phenocrysts probably due to the intrusion.
The mass is clearly discordant with its surrounding rocks and contains large blocks of the augen gneiss of Lage. We suppose that it constitutes a granitization product of the Lage granite, a palingenic granite in situ. Several similar masses occur in west Galicia for instance, in the Pontevedra province and south of Vigo, and also near Friol near Lugo in eastern Galicia. d. The homogeneous muscovite granite of Muros. — This type of granite is very frequent in western Galicia. The name is derived from the occurrence near the district of that name north-west of the ría de Noya. We consider it for the present as closely related to the Ruña type, more homogeneous, but of the same origin. 3. The basic rocks belonging to the “Lopolith”. — The map shows that these rocks form a discontinuous arcuate outcrop some 100 km long in the N—S direction and some 60 km wide. They dip everywhere inwards and are covered by the Ordenes schists, so that the shape of the mass resembles a dish.
The petrography of the rocks is very variable, in general we can recognize: 1. Basic diorites with andesine, pyroxene and hornblende. These we find intercalated between the schists of Barrañán (Carballo sheet). 2. An extensive outcrop north of Carballo of ilmenite-rich gabbro rich in alternating with amphibolites. 3. A large mass of fresh looking olivine-gabbro, also with amphibolites, which extends from Mte Castelo to Carballo in an area of some 200 km2. This mass has on its western margin a band of pyroxenites. 4. A large mass of amphibolites east of Santiago de Compostela which contains important mineralizations of pyrrhotite and cupriferous pyrite. 5. A series of outcrops of peridotites, pyroxenites and serpentines on the southern border of the río Ulla, near Bandeira, Las Cruces and Berredo. This outcrop of basic rocks narrows north of the río Ulla and continues to the east of Mellid where it broadens again on the hills of Corno do Boy and reaches the Rías near Sobrado. North of Sobrado it broadens again and the ultra-basic rock reaches Teijeiro. After an interruption of some km the serpentines reappear near Irijoa east of Betanzos and in a small outcrop north of Puentedeume. 6. Finally we find a major outcrop of the basic rocks in the extreme north of the province from Moeche to the Cape of Ortegal, occupying the hills of the Sierra de la Capelada. The cupriferous pyrite mines of Cerdido are situated on their eastern border.
We do not know yet the age of these intrusions, which might be older than we suppose now. Neither do we know much about the rocks or their structural circumstances. 4. The migmatitic granite of Lage. — This gneissic granite with two micas occupies a large area in eastern Galicia. The most typical rocks are exposed between the isles of Sisargas and Lage (Schulz, 1835). The texture of the rock is very variable (Expl. sheet Lage no. 43, Tuy no. 261, Oya no. 260), and can perhaps be regarded as an antexitic granite. In the gneissgranite we find parallel zones of migmatized schists and micaschists. Their orientation is roughly N15°E.
Along the western margin of the outcrop of the polymetamorphic “ancient complex” these gneisses get an augen structure by the development of large felspatic “eyes” up to 10 cm long, surrounded by biotite perhaps indicating a kind of mylonitization. These gneisses seem to possess two planar structures at an angle of 15° to 20°, one due to the mica orientation, the other to the felspar eyes. The Lage gneisses differ from the “ancient complex” gneisses by the absence of parallel basic bands. 5. The “ancient complex”. — A narrow zone of highly metamorphic rocks extends from Malpica (sheet 44) in the north to the ria de Arosa (sheet 152) in the south. This zone of 80 km length and roughly 6 km wide is slightly convex to the west. The most typical rock is a glandular biotite-felspar gneiss, but we find also gneissic mica schists and other varieties. In the centre, between Baiñas and Mazaricos the gneisses contain riebeckite. The whole complex contains numerous parallel narrow zones or dykes of very much tectonized basic rocks, amphibolites, pyroxenites and eclogites.
The fact that these rocks show a higher grade of metamorphism and often are polymetamorphic as compared to the Lage group induses us to believe them to be older.
Mineralogically these rocks are characterized by the instability of their micas, biotite and muscovite, and hornblendes. The first group is often found as much deformed relics. Only in the perhaps younger riebeckite gneiss intrusions the hornblende is more stable and uniformly developed.
Near Malpica biotite gneisses with some muscovite predominate, near Puenteceso and Zas biotite-hornblende gneiss and near Baiñas and Mazaricos riebeckite gneiss. Near Noya the biotite gneisses deappear. The basic rocks have their greatest development between Zas and Mazaricos.
Inside the complex we can suspect many faults bringing zones of different grades of metamorphism in contact.
It seems quite probable that similar zones of highly metamorphic character exist also elsewhere in Galicia. We suppose for instance that the riebeckite granite an dgranite-gneiss east of the Monte del Carrio and those of Silleda in Central Galicia belong to the same group. Perhaps the Ordenes schists of a much lower metamorphic grade above the basic rocks described before, belong to the same group.
Conclusion Perhaps the complicated skeleton which we have presented here as an explanation of our map, and which is the result of numerous excursions in Galicia during recent years can be summarized in the following table: Age of orogeny Mock groups Deformation; genesis Petrographical type Alpine Traba none intrusion Traba granite Bardullas syenite Rhyolitcs Hercynian Muros weak granitization Bayo diorite Coruna granite Runa granite Maros granite t Lopolith .' Basic rocks Huronian — 800 m.y. Lage intense migmatization Granite-gneiss of Lage Augen granite of Cabrai Archean — 1200 m.y. Malpica (ancient complex) very intense migmatization Penedo granite Borneiro gneiss Baiñas gneiss Metamorphic basic rocks The correlation of the rock-groups with known orogenic periods is of course very doubtful. The reader must realize that we give this outline only in order to stimulate further research.
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