Aflevering 3, Pp. Eclogites, Granulites, Intensively Greenschists
Total Page：16
File Type：pdf, Size：1020Kb
LEIDSE GEOLOGISCHE MEDEDELINGEN, Deel 49, Aflevering 3, pp. 487-497, 1-12-1975 The Palaeozoic strata near Moeche in Galicia, NW Spain BY C.G. van der Meer Mohr Abstract Coral fragments, crinoidal debris and foraminifers found in limestone beds near Moeche (Galicia) are evidence of a Devonian-Silurian Coruña Hoja age for the strata in the greenschist facies belt around Cabo Ortegal, hitherto considered on the Spanish 1:200,000 map-La 1 to currents into ancient floor is (IGME), be Precambrian in age. Their deposition by turbidity troughs on an ocean being suggested. INTRODUCTION metamorphosed sediments considered to be of Ordovician the to Silurian age on Mapa Geologico de Espana The northwestern mentioned extremity of Spain, near El Ferrol, Cabo above. Ortegal and Ortigueira is an area of igneous and metamorphic rocks which forms part of the Galician-Castilian zone (Lotze, 1945) in western Spain. STRATIGRAPHY Towards the East the exposed strata become progressively in in the in the under younger age and the Cantabrian Mountains The stratigraphic succession area Palaeozoic succession is finally buried under a thick cover consideration is very difficult to be established in the field of Mesozoic sediments. due to the lack of palaeontological data and marker beds, complicated folding of the strata and the metamorphism of Since 1957 petrological fieldwork has been carried out the sediments. The most pertinent palaeontological and around Cabo Ortegal by students and staffof the petrologic stratigraphical data bearing on an area close to our region departmentofthe Geological Institutein Leiden.This work His contains have beenpublished by Matte(1968). work an resulted e. g. in theses by Vogel (1967), Maaskant (1970), excellent synopsis of the stratigraphy of the Palaeozoic and Engels (1972) in a geological map by Vogel, Engels and succession in northwestern Spain. From his data, Ho Len Fat. Also the Mapa Geologico de Espana, correlations can be established between Silurian and older sheet 1, the Instituto Minero 1:200,000, by Geologico y de strata in the Cantabrian Mountains and their equivalents Espana (IGME) is most important with regard to the the flank of the near El Picon, on northern Barquero present paper. anticline (Fig. 2). This is important since the strata in the greenschist facies belt around Moeche are almost on strike GENERAL GEOLOGICAL SETTING with the Palaeozoic succession near El Picon. The area around Cabo Ortegal can be divided into a The stratigraphic succession described by Matte from the NE-SW trending complex of eclogites, granulites, northern flank of the Barquero anticline is presented in amphibolites and paragneisses with abundant catazonal Figure 3. The most important aspects of this succession are mafic inclusions considered to be of Precambrian age. in stratigraphical order: These intruded are by ultramafic rocks and metagabbros. the Olio de volcano-detrital Faults separate this complex from intensively folded 1. Sapo formation, considered Matte be metasediments and greenschists to the Southeast, the by to of Precambrian age (although elsewhere it South and the Southwest. Within this belt of greenschist might be Cambrian in age); facies rocks are isolated outcrops of serpentinite. Near Moeche (Pereiro) and Insua, halfa kilometer SE of Pena Grande, fossiliferous limestone beds can be found associated * within serpentinite (Fig. 1, Photo 1). Eastward Geological Institute, Department of Stratigraphy and Garenmarkt The of the greenschist facies belt lies an area of very slightly Palaeontology, lb, Leiden, Netherlands 488 Complex. Ortegal Cabo the of East the to strata Palaeozoic the of map sketch Geological 1. Fig. 489 Fig. 2. Stratigraphic cross-section of the Lower Palaeozoic strata between Cabo de Peñas and the anticline of Barquero close to the Cabo Ortegal Complex. Data after Radig (1962) and Matte 1968 (modified). 2. a thick sequence of Ordovician strata ofpredominantly light green to grey coloured, finely laminatedschists with believed abundant and lenses. in the shales with a pronouncedinterval of quartzites to quartzveins They occur be equivalent to the Culebra quartzites (Arenig.); greenschist facies belt as well as on top of the serpentinite of metavolcanites show 3. a sequence of shales, intercalated with limestone lenses, body near Moeche. Several the a greenish quartzites, rhyolitic tuffs and, near the top, relict variolitic texture (Photos 2 and 3). The cavities have limestones containing crinoids and siliceous shales with been generally filled with quartz and feldspar. Monograptus halli Barr. and Rastrites linnaei Barr. THE STRATA NEAR MOECHE From Matte's cross-section through the Barquero the of and anticline, Spanish geological map El Coruna, Serpentinite bodies occur around Moeche, Pena Grande mapping done by Hoogervorst (internal report, 1973) it can and near Ladrido. They consist of greyish-green rock be deduced that Silurian the rhyolitic tuffs are probably contains composed ofa mass of serpentine which no longer analogous with a distinct quartzite zone that can be any remnants ofolivine or pyroxene. Only near Ladrido did followed It many kilometers inland (Fig. 1). changes its Ho Len Fat (internal report, 1968) find a in which the original SSW strike just about SE of Moeche. After that this quarry serpentinite still contained some remnants of pyroxenite. quartzite zone continues in a northwestern direction In quarries near Moeche (Photo 1) and Pena Grande around the Cabo Ortegal complex. It finally strikes limestone beds occur interlayered with serpentinites. northwards to Punta Corbeira at the Atlantic Coast (see Repetition of this sequence is probably due to recumbent IGME-map). folding. These limestones of which several samples were The stratalying tothe West and to the North ofthis quarzite collected in the Moeche quarry and Pefia Grande are to the facies zone up greenschist belt couldbe Silurianinage lightgreen, slightly fossiliferous, well stratified carbonate according to Matte's data. They are marked as phyllites on rocks. the geological et al. map by Vogel (1967). In thin section they turned out to consist of a mass of microsparite with about 5-10% calcite 'porphyroblasts' The greenschists are from the phyllites probably separated which in many cases show the outlinesofcrinoid columnals by faults. Faults also border this facies belt towards the of these crinoid (Photo 4). Many fragments show up as northeast forming its contact with the amphibolites and the ghost-structures inside the porphyroblasts. We have to do gneisses. here with crinoid fragments which often became The chlorite-schists and greenschists are mainly surrounded by a rim of syntaxial calcite cement and in that metavolcanites. are According to Hoogervorst (1973) they fashion withstood the severe conditions, including 490 and laminar Tabulatathese beds cannot even be older than Middle Ordovician. However, since we know from the work by Matte (1968) and Nissen (1959) that the Silurian strata near El Picon contain crinoidal limestones, an identical age for the Moeche and Pena Grande limestone beds seems to be the most likely possibility. There of also is, course, a possibility that the limestones might even be younger in age; perhaps they might date from Devonian times. However the nearest Devonian strata known to-day are at a considerabledistance from Moeche. Yet it be ruled that Devonian cannot out strata might once have been deposited far more to the West than they are and in anotherfacies presently found, than they are present in the Cantabrian Mountains. Corals are abundantly present in the Devonianstrata in the CantabrianMountains but do not occur in the underlying pre-Devonian, marine deposits. The emplacementofthe fossiliferous limestonesis another The problem. average percentage of crinoid fragments in these limestones is in no way comparable to the amounts found in shallow water carbonates known from so many other Neither places. did we get the impression that the corals the foraminifers or were originally present in great numbers. It must be however admitted that shearing and recrystallization could have caused the destruction of the skeletal structure of many bioclasts, particularly those of corals or Bryozoa. Turbidity currents, sweeping material from a nearby platform, could have caused the of bioclastic material into transportation deeper parts of a sea. There such detritus could have become deposited in calcareous muds overlying lavaflowsof a basic, and mantle slices of an ultrabasic composition. CONCLUSIONS The preceding observations and deductions lead to the conclusion that the greenschist facies belt is of Silurian-Devonianage and that theirstratamight have been deposited in deep troughs. A Caledonianfoldingfor the Gafician-Castilian zone has Fig. 3. Stratigraphic succession of Lower Palaeozoic strata near been et His idea isbased on the Barquero anticline, after Matte (1968). suggestedby Wagner al. (1974). a pre-Upper Wenlockian uncornformity reported by Martinez-Garcia(1972) from NW-Zamora.However, inthe concrete vicinity of Cabo Ortegal we do not find any metamorphism, to which they were submitted afterwards. unconformable evidence of a Caledonian orogeny. The Apart from crinoid fragments some foraminifers contact, in northern Spain, between Ordovician ■ and (Glomospira?)i (Photo 5) were observed in the thin sections Silurian strata, can be explained by uplifts of a foreland and as well as fragments of tabulatecorals (Photos 6 and 7). The block faulting. latter were found in a limestone