LEIDSE GEOLOGISCHE MEDEDELINGEN, Vol. 37, 69-76, 1-6-1968 1968, pp. separately published

Basement rocks of Western as sources for the minerals in the Ría de Arosa

BY

P. Floor

Abstract

The of the in Galicia is outlined with reference the area of the geology hercynian orogen western briefly special to drainage Ría de Arosa. The possible host rocks of translucent heavy minerals found in unconsolidated sediments within and around the ria are tabulated and discussed.

Resumen

del herciniano occidental el área de de la Ría de La geología orógeno en la parte de Galicia, especialmente captación Arosa, traslúcidos sedimentos está resumida. Se discute cuales las rocas primarias de los minerales pesados encontrados en los no consolidados dentro alrededor de la el cuadro 1 las áreas de de los ríos Ulla y ría. Estos son representados en para captación y

Umia el cuadro la directamente circundante la Ría de Arosa. y en 2 para región a

INTRODUCTION Acknowledgements. — The author gratefully acknow- ledges the assistance of Professor A. J. Pannekoek, The Ría de Arosa is situated within the hercynian that in the Professor E. den Tex and Mr. W. S. Koldijk, who orogen occupies a large area western part critically read the manuscript, and of Messrs. C. E. S. of the Iberian Peninsula. It has a slightly curved shape; strikes from W-E in the south NW- Arps, J. J. M. W. Hubregtse, H. Koning, J. A. Saltet average vary over and E. van Scherpenzeel who provided data for Tables SE in the centre to N-S in Galicia, the northwestern 1 an 2. Mr. C. E. S. Arps also assisted in the compila- corner of the peninsula (cf. fig. 1). tion offig. 2.

Western Galicia belongs to arelatively deeply eroded

PRE-HERCYNIAN ROCKS part of the orogen, characterized by abundant regional-metamorphic rocks and granites. Both to the Three groups of pre-hercynian rocks can be distin-

in northern and to the in eastern south, Portugal, east, guished in western Galicia: Galicia and the neighbouring provinces of León and a. Metasediments: mainly pelitic schists and plagio-

Zamora, levels of the are higher orogen exposed. clase-bearing paragneisses (metapelites and meta- In these rocks not areas regional-metamorphic are greywackes respectively) with very subordinate but often could be traced their rare either, they along intercalationsof quartzite and graphite schist; strikes into areas of lower metamorphism, where it has ultrabasic b. Basic and rocks with a metamorphic been establish possible to a stratigraphie succession, grade varying from greenschist facies to high- partly dated by fossils (see Matte, 1967). Strong e.g. pressure granulite facies; deformation in several successive metamor- from phases, c. Granite-gneisses, varying in composition and the abundant emplacement of granites phism, quartzdioritic to per-alkaline granitic, and with rendered the of the recognition metamorphosed blastomylonitic or blastophyllonitic textures; (rare) of these dated rocks equivalents stratigraphically so metarhyolites.

1 far impossible in western Galicia ).

As appears fromthe geological map (fig. 1), metasedi-

ments are in western Galicia. *) Department of Petrology and Mineralogy, Geological present everywhere found in Institute, University of Leiden. Paragneisses are as a major rock-type a narrow belt between Malpica and Tuy, within which and *) In the area S of Orense Ferragne (1966) on lithological which a of the also Infracambrian to Silurian to a along large part granite-gneisses grounds assigns an age Because of the of series ofschists with intercalated quartzites, detrital volcanic crop out. frequent occurrence sediments and metarhyolites. blastomylonitic granite-gneisses and on account of its 70 P. Floor

1. of Galicia. After and data of the Fig. Simplified geological map western Parga-Pondal (1963) mainly unpublished Department of Petrology, University of Leiden. Scale 1 : 1.000.000. Basement rocks of Western Galicia 71

structural features the belt has been named "blasto- the older two-mica granites and country rocks by late-

mylonite graben" by den Tex & Floor (1967). Outside hercynian flattening. the belt have granite-gneisses mostly a blastophylloni-

tic texture. To the W and NW of Lalinanothercomplex The other intrusive granite series is completely

of blastomylonitic granite-gneisses is found. postkinematic and occupies mainly oval-shaped bodies (e.g., the Porrino-Monçào, Caldas de Reyes, Metabasic and Traba These intrusions often and ultrabasic rocks are mainly concen- Pindó, granites). are intrusive trated in a discontinuous arc through Carballo, composed of more than one type. Clearly relations between the could sometimes be and Mellid up to the area of types

Cabo structure Ortegal, where a complex of high-grade meta- observed. The is generally porphyritic to rocks is found the the morphic (Vogel, 1967). In neigh- coarse-grained inequigranular depending on

bourhood of this arc of basic rocks granite-gneisses are relative size of the minerals between the alkalifeldspar

4 locally abundant as well. Small but numerous lenses phenocrysts, which rarely measure more than cm.

of Biotite is contain dark amphibolite are common in the "blastomylonite never absent; some types a

Elsewhere metabasic rocks There in others muscovitc. graben". are rare. green amphibole addition, are textural, mineralogical and compositional indi- cations that basic rocks have An intrusive of is some a sedimentary gabbro complex hercynian age origin (Floor, 1966). situated N of Santiago de Compostela. West of Mellid found. some small gabbro intrusions were

determinations madeof Isotopic age were some granite-

gneisses collected in the southern part of the "blasto- Lamprophyres and dolerite dykes are the youngest W is small. mylonite graben". They gave whole-rock ages of intrusive rocks in Galicia. Their number about 500 determina- m.y. (Priemet al., 1966). Age of of tions of granite-gneisses from other parts of western For a summary the results recent geological Galicia carried in Galicia the reader is referred are being out at present. investigations western to the proceedings of the "Primera reunión sobre de Galicia de geología y norte Portugal" (1965, HERCYNIAN METAMORPHISM AND INTRUSIVES published in 1966). the During hercynian orogeny all rocks mentioned above deformed and of were metamorphosed, some SUPPLY OF MINERALS TO THE SEDIMENTS them for the first time (granite-gneisses, some basic

and ultrabasic rocks and others for Since the of the is a metasediments), purpose present paper to provide the sedi- second time (metasediments, many basic and regional geological basis for the discussion of the ultrabasic den of it is that this rocks; see Tex, 1966). ments the Ría de Arosa, thought of purpose is served best by the separate description the of the Ulla and Umia Rivers and Metamorphism was locally of such a grade that the drainage areas

rocks were subjected to partial melting with formation the area immediately surrounding the Ría de Arosa. of localization of migmatites, anatexitic granites and, at deeper The emphasis will be laidon the possible

levels, also of homogenized granites capable of in- source rocks of minerals found in the unconsolidated

trusion. The members of this two-mica granite series sediments. leucocratic and characterized the of are by presence

and Umia Rivers. — The brotite, muscovite and plagioclase with anorthite The drainage areas of the Ulla below The rocks have drained the Ulla and Umia Rivers is indicated contents generally 15 per cent. area by either the linear or planar oriented or unorientedtextures on the geological map, fig. 1. It can be seen that and of Ulla River and its tributaries erode about the display many shapes outcrop (from elongate via greatest irregular to about circular) thereby demonstrating variety of rocks to be found in western Galicia:

their members of all and of two syn- to postkinematic character. pre-hercynian groups hercynian granite series distinguished in the preceding In addition the two-mica series section. do The to granite two clearly Only postkinematic granites not occur. intrusive series found. The oldest is and of the is granite are one eastern most complex part area being of intrusive into migmatic rocks but was itself intruded studied by the Department of Petrology, University two-mica The main Leiden. The of its rocks therefore by homogeneous granite. type is a compositions are megacrystal-bearing granodiorite; hornblende and well known.

biotite-bearing granodiorite and quartzdiorite with-

found within The of the Umia River is underlain out alkalifeldspar megacrysts are locally drainage area by a number of metasediments the megacrystal-bearing type. Rocks of this series seem much smaller rock-types: to be mainly restricted to the loci of fundamental (mainly micaschists), some granite-gneisses, migma- NNW-SSE of the two-mica and striking faults (e.g., the fault zone separating tites and granites granite series

eastern Galicia from western Galicia and the fault members of the postkinematic biotite granite series the the Caldas de zone delimiting "blastomylonite graben" to (the Reyes intrusion). Petrographically, the in detail. west). They have often been deformed together with area has not yet been mapped 72 P. Floor

green) garnit tourmaline (blue)

bluish brown-green) also: zircon (mainly (mainly Rivers pegmatites/aplites monazite anatase tourmaline tourmaline zircon zircon titanite rutile zoisite rutile monazite zircon Umia in

and amphibole

Ulla green the of Santiago titanite anatase zoisite zircon zoisite pistacite clinozoisite rutile titanite anatase anatase titanite brownish areas granites) of granites) E drainage (near (mainly (near amphibole amphibole spinel the Compostela)

1 in andalusite sillimanite andalusite kyanite sillimanite pistacite clinozoisite pistacite clinozoisite kyanite pistacite sillimanite monazite monazite enstatite zircon de rutile zircon garnet garnet green zircon zircon zircon brown brown TABLE minerals turbid) amphibole amphibole heavy amphibole amphibole amphibole (mainly amphibole (mainly amphibole (cummingtonite) brown clinopyroxene brown green green clinopyroxene thene translucent garnet staurolite chloriioid garnet staurolite actinolite green colourless light garnet light enstatite colourless dark garnet titanite sillimanite tourmaline brown) dark pistacite hypers titanaugite

their

and granites

s amphibolites and and tinites blastomylonitic rock-types Paragneisses Greenschists Amphibolitf Blastophyllonitic Migmatites, homogeneous Micaschists Garnet Granulites Peridotites serpen and types anatexitic Principal

series rocks rocks series granite dykes Granite-gneisses Postkinematic Metasediments Metabasic Ultrabasic Megacrystal granodiorite series Two-mica granite biotite Gabbro Dolerite

1 ' Î4. S0Pt z< Z boí PiS 1/3. ü0 oí < ><Üci s Basement rocks of Western Galicia 73

2. of the Ría de Arosa After von Raumer Pondal Fig. Geological map area. (1963), Parga- (1963), unpublished reports of the Department of Petrology, University of Leiden, by C.E.S. Arps (1), J. B. M. ten Bosch (2) revised by C. E. S. Arps, and and observations of Scale 250.000. W. Vogel (3) N. Rengers (4), personal the author. 1 : 74 P. Floor

The most important rocks, with the translucent heavy The megacrystal-bearing two-mica granite situated

minerals they could supply to the Ulla and Umia north of the Ría de Abanqueiro is considered as the listed Table member of the two-mica series. Its river systems, are in 1. youngest granite

age relation with the Caldas de Reyes intrusion crop-

ping out both north, within and south of the Ría de — The and The country around the Ría de Arosa. coast Arosa could not be established by observations in the hillsides drained by small rivers flowing directly into

field. Both complexes are clearly postkinematic. the Ría de Arosa are constituted of about the same

the of the Umia rock-types as drainage area River, the but their distribution is better known through The Caldas de Reyes intrusion, though depicted as a of and the the is in fact investigations von Raumer (1963) Depart- homogeneous mass on geological maps, intrusive into ment of Petrology, University of Leiden (fig. 2). composed of several types of granite, of other. The has been and Translucent heavy minerals found in thin sections each complex not yet mapped varia- rocks from this area are listed in Table 2. investigated petrologically. Qualitatively, the

tions can be studied excellently in the many quarries of present everywhere in the complex, but mapping Rocks belonging to the "blastomylonite graben" are will be difficult lack of natural the northern shore of the Ría de types owing to out- present only on

2 The variations are mainly structural. Arosa ). They comprise plagioclase-bearing para- crops. two-mica few thin bands of gneisses, schists, a quartz- and biotite is the ite and graphite schist, granite-gneiss some Coarse-grained inequigranular granite

amphibolite lenses. Hercynian megacrystal grano- most common type. The number ofpotassium feldspar of intruded is and the of the diorite and several types two-mica granite crystals too great grain ground mass into this Calcsilicate rocks found call this Irre- complex. are as rare too coarse to type coarsely porphyritic.

xenoliths in megacrystal granodiorite. gularly shaped concentrations of alkalifeldspars

Outside the "blastomylonite graben" micaschists with very subordinate quantities of interstitial quartz,

predominate. Coarse-grained augengneisses (blasto- plagioclase and biotite are seen in some outcrops. and far dark is found in addition phyllonitic granite-gneisses) run parallel to not Locally, green amphibole from the boundaries of the "graben". Calcsilicate rocks to biotite. It is not known whether this is a variety SW of biotite are found intercalatedin micaschists in the extreme coarse-grained granite or a separate type. of the W and part area. of the "graben", schists augen- intruded the Barbanza two-mica gneisses are by Other members of the intrusion are medium to fine-

3 granite ). grained and can be distinguished by their content The structural between the inliers continuity larger habit of biotite and the or absence of and presence and schists surrounding the granite demonstrates that Their some alkalifeldspar megacrysts. composition the former should be considered roof as pendants. is granodioritic.

Andalusite is very common in the schist, garnet often absent, whereas of staurolite only a few were grains Medium-grained equigranular two-mica granite has found in schist xenoliths in the Barbanza granite. The been found as a small mass N of Cambados and as has weak subvertical NW-SE foliation. granite a xenoliths in of biotite The rock many types granite.

must therefore be relatively early in the intrusive East ofthe two-mica be older than the "graben" granites are abundantly sequence of the complex or even

as well. Their rocks often have a whole of member of the two-mica present country mig- complex (xenoliths a matic with the aspect contrasting generally non- granite series).

migmatic gneisses and schists to the west. Translucent mineralsin the rocks heavy migmatic are incorporated Late in the magmatic history of the complex, an with those of Table the two-mica granite series on 2. autometasomatic phase caused the alteration of the

granite with the formation of strongly pleochroic

Before the intrusion of at the of and some relatively young granites green epidote, probably cost biotite, the deformed concentrated in veins. This alteration be rocks in the area were in places by an partly can

ENE-WSW seen to start joints and veins and to work flattening compression, causing e.g. a along gra- strongly phyllonitic structure in the megacrystal dually into the granite. Where this phenomenon has shore granodiorites and some two-mica granites, and a new acted most completely (mainly on the northern de Santa de Ribeira cataclastic texture in the blastomylonitic granite- of the Ria Arosa near Eugenia

gneisses. and Puebla del Carmiñal), the granite has a con-

spicuous brick-red colour.

2 South of the ria ) they are not very characteristically Pre-existing rocks, belonging to groups that crop out and out in the extreme SW of the drainage developed crop north and south of the intrusion, are found as xeno- of the Umia area River. in series of liths, roofpendants or septa, e.g., a quarries harbour of Villa- 3 the western extremity of the ) After Sierra de Barbanza, the ridge on the peninsula along de Arosa. north ofthe Ría de Arosa. garcia Basement rocks of Western Galicia 75

tes: selvages /apli (blue) granite

in pegmatites altered rutile titanite garnet tourmaline epidote tourmaline-enriched in Arosa pleochroic

de in autometasomatically pistacite clinozoisite titanite zircon zircon zircon monazite zircon anatase green Ría zircon the brown-green

surrounding granites) granites) green, veins) (brown) area (near (near (bluish the and (rare) amphibole in dykes (brown-green) 2 sillimanite zircon sillimanite tourmaline garnet monazite pistacite clinozoisite tourmaline sillimanite pistacite titanite garnet tourmaline monazite zircon brown minerals of TABLE heavy brown- „blasto-

turbid) of locally) W amphibole translucent (mainly graben" (rare) (mainly (only hornblende hornblende (only (rare) their green tourmaline andalusite clinopyroxene cummingtonite mylonite sillimanite andalusite tacite titanaugite and garnet green) garnet green zircon zircon green garnet garnet zircon dark pis rock-types and Principal Blastophyllo- augen- granite- homogeneous Paragneisses Micaschists Calcsilicate rocks Quartzites Graphite schists nitic gneisses Blastomylo- nitic gneisses migmatites. anatexitic granites

granite dykes Granite-gneisses wo-mica Postkinematic Metasediments Amphibolites Megacrystal granodiorite series granite series biotite series Dolerite 1

C/5 0Pi Z< zIX üoí W ai M ¡s¿ U 0 ¡g< Z C¡5 WS 76 P. Floor

d'Orense, C. R. Acad. Se. Paris, 832— CONCLUSIONS Espagne). 262, p.

834. 1. The disposition of rock-types in the drainage 1966. Floor, P., Petrology of an aegirine-riebeckite gneiss- areas of the rivers discharging into the Ría de Arosa bearing part of the Hesperian massif: the Galiñeiro and is such, that within and along this ria the variety surrounding areas, , . Leidse Geol. Med., 36, of translucent minerals in the unconsoli- heavy 1—203. p. dated sediments can be expected to be the greatest Matte, Ph., 1967. La schistosité primaire dans l'arc hercynien ofall western Galicia. de Galice; variation de son pendage parallèlement et

perpendiculairement aux structures et rôle des phases de 2. This variety is due mainly to the erosion of rocks déformation ultérieures. In: Etages tectoniques, Ed. la the Ulla River and its tributaries. 243 —251. by Baconnière, Neuchâtel, p.

Parga-Pondal, I., 1963. Mapa petrográfico estructural de Notable is the small of 3. amount garnet, staurolite, Galicia. Inst. Geol. Min. Esp. clinopyroxene, amphibole and the absence of I. & Martin 1952. Estudio de kyanite, zoisite, chloritoid, spinel, orthopyroxene in Parga-Pondal, Cardozo, M., la corindón andalucita de diaspora con y Goyán (prov. rocks of the area immediately surrounding the de Notas Inst. Geol. Min. Pontevedra). y comun., Ría de Arosa. This is not due to incomplete regional 163—168. Esp., 26, p. knowledge, because the greater part of the area H. N. A., N. A. I. has been petrographically investigated. Priem, Boelrijk, M., Verschure, R.H., Hebeda, E. H. & Floor, P., 1966. Isotopic evidence for

Upper-Cambrian or Lower-Ordovici?n granite emplace- 4. Corundum and brookite were never observed in in the north-western Geol. ment Vigo area, Spain. en thin sections. is known to Corundum, however, 36—40. Mijnb., 45, p. occur in andalusite-quartz nodules in micaschists von Raumer, J., 1963. Zur Tektonik und Genese des nord- near the Portuguese frontier SW of Túy (Parga- west-spanischen Kernkristallins bei Noya (La Coruña). Pondal & Martin Cardozo, 1952), of which the Geotekt. 1—63. Forschungen, 17, p. schists in the W of the Ría de part Arosa area den Tex, E., 1966. Aperçu pétrologique et structural de la may very well be the continuation. Brookite is Galice cristalline. Leidse Geol. Med., 36, p. 211—222. mineral formed probably a during alteration and den Tex, E. & Floor, P., 1967. A blastomylonitic and poly- weathering of titaniferous minerals and therefore metamorphic „graben" in western Galicia (NW Spain). not observed by hard-rock petrographers. In: Etages tectoniques, Ed. la Baconnière, Neuchâtel,

p. 169—178.

D. 1967. of and REFERENCES Vogel, E., Petrology an eclogite- pyrigarnite-

bearing polymetamorphic rock complex at Cabo Ortegal,

Contribuciones a la reunión sobre de NW Leidse Geol. primera geología Spain. Med., 40, p. 121—213.

Galicia de 1966. y norte Portugal (1965), Leidse Geol. Warnaars, F. W., 1967. Petrography ofa peridotite-, amphi- Med., p. 205—320. bolite- and gabbro-bearing polyorogenic terrain NW of Ferragne, A., 1966. Sur l'existence d'une série volcano- Santiago de Compostela (Spain). Thesis, University of

Nord de Celanova détritique silurienne au (Province Leiden.