Basement Rocks of Western Galicia As Sources for the Minerals in the Ría De Arosa
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LEIDSE GEOLOGISCHE MEDEDELINGEN, Vol. 37, 69-76, 1-6-1968 1968, pp. separately published Basement rocks of Western Galicia 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 son 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 Santiago de Compostela 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