ALBERT

Description: Within the strongly migmatized axial zone of the Hesperian massif in western Galicia a graben-like structure has been distinguished, characterized essentially by the presence of non-migmatic rocks that comprise orthogneisses with blastomylonitic textures, leucocratic gneisses, plagioclase-blastbearing paragneisses, pelitic schists, and numerous amphibolitic layers and lenses. In the southern and central part of the graben and at the borders in the north the majority of the amphibolites are metamorphosed mafic dike swarms that intruded in the Early Palaeozoic after the emplacement of biotite granites but before the intrusion of subalkaline and peralkaline granites. Few amphibolites are of sedimentary origin. The other amphibolitic rocks in the north are of inferred Proterozoic age and have a different appearance. They consist of retrograde eclogite facies mafites and garnet- and epidote-amphibolites that are typically associated with leucocratic gneisses and younger subalkaline orthogneisses. It is inferred that the northern part of the graben mainly represents a lower basement segment that underwent Precambrian and Early Palaeozoic catazonal metamorphism and subsequent retrogradation, while the central and southern parts represent higher basement levels of mesozonal metamorphic grade.

Description: A study of the beaches on the NW shores of the Ría de Arosa was made with respect to shape and structure of the beaches, grain-size distributions, heavy mineral associations, and relations of the heavy minerals to the bedrock of the hinterland. Most of these numerous small beaches are situated in inlets determined by the occurrence of relatively weak bedrock (mostly schists). Grain-size generally fall in fractions 〉 50 µ, and some beaches even contain important amounts 〉 2000 µ. Many grain-size distributions are bimodal owing to the mixing of different beach layers by wave action or, in a few cases, to mixing with fluvial sands. The heavy mineral compositions of the fluvial and beach deposits approximately reflect the mineral compositions of the bedrock, especially in the deeper inlets of the Ría, whereas most of the beaches on the more exposed parts have a more varied heavy mineral composition. In some cases the minerals indicate a weak long-shore transport of the sands.

Description: The present area covers a part of the axial zone of the Hesperian Massif. Rocks of metamorphic and intrusive origin occur in three structural units: the central zone (which in fact forms part of what is called the “blastomylonitic graben”), the migmatic complex and the rocks in the S and SW. During the Hercynian orogeny these units were placed in juxtaposition. With respect to their age and composition the rocks can be subdivided into pre-Hercynian metasediments, metabasic rocks and orthogneisses, and Hercynian migmatites, palingenic granites and intrusive granodiorites, granites, pegmatites, aplites, quartz veins, granite porphyries, lamprophyres and olivine dolerites. Based on penological evidence, the geological history can be reconstructed as follows: Pre-Hercynian metasediments, bearing traces of a probably Precambrian deformation and metamorphism (resorbed staurolites and garnets enclosed in Hercynian metamorphic minerals) have intruded along fundamental N-S trending faults in the Upper Ordovician (460-430 x 106 years ago) by means of megacrystal-bearing two-mica granites, biotite granites and peralkaline granites. The intrusion of a basic dyke-swarm took place between the emplacements of the latter two types of granite series. During the Hercynian orogeny the pre-Hercynian rocks were at first penetratively deformed; the metasediments were (re)- folded and the granites were phyllonitized into coarse-grained augengneisses or mylonitized into granite mylonites (central zone). This Hercynian main phase deformation was followed by a period of Abukuma-type plutonometamorphism. A widespread metablastic recrystallization of the non-migmatic rocks of the central zone is characteristic. The metamorphic mineral assemblages in the non-migmatic rocks of the central zone are indicative of the cordierite-amphibolite facies and the mineral assemblages of the metasedimentary rocks in the SW and W indicate greenschist facies conditions, while the inclusions in the palingenic two-mica granite suffered superimposed hornblende-hornfels facies conditions. At deeper levels the Hercynian metamorphism culminated in anatexis. During a short period of tension and normal faulting the non-migmatic rocks of the central zone were brought into juxtaposition with the migmatites. More or less synchronously the active fundamental faults, separating the central zone from the western migmatites facilitated the rise of large amounts of granodioritic magma; immediately afterwards the ultimate products of anatexis, in as much as they were capable of intrusion, penetrated the migmatic complex and the granodiorites in the form of successive phases of two-mica granites. This period of plutonic activity was interrupted by the second and third Hercynian deformations that were only locally of a penetrative nature. The majority of the Hercynian granites and granodiorites were weakly foliated during the second deformation while the third deformation locally phyllonitized and mylonitized the rocks in the N, SW and W. The Hercynian orogenic cycle was concluded with the intrusion of a post-kinematic two-mica granite, two biotite granites (280 ± 11 X 106 years ago), some granite porphyries and lamprophyres. The few olivine dolerite dykes are probably younger. Accessory zircon occurring in metasediments, orthogneisses, migmatites and granites has been investigated for crystal habit, growth phenomena, inclusions, resorption, fluorescence and other properties. The various rock-types contain zircon populations with a more or less typical habit: i.e. a combination of the forms {101}, {100} and /or {110} and sometimes {301} and {201} for the zircons of the blastomylonitic orthogneisses, the post-kinematic granites and granite porphyries, while a combination of {101}, {211} and {100} and/or {110} is characteristic for the pre-Hercynian coarse-grained augengneisses and the Hercynian megacrystal-bearing granodiorites and two-mica granites. The zircons in the rocks belonging to the megacrystal-bearing granodiorite series clearly show differences in evolution. The zircons in the migmatic rocks indicate that conditions for anatexis favoured the regeneration of zircon. The fluorescence colours of the zircon crystals are yellow, orange and brown; some zircon populations also contain emerald green fluorescent crystals. A zonal variation in fluorescence colours is also present.

Description: Three cut sphenes, originating from the Tissamaharama area in Sri Lanka, are described. Their properties are compared with those of gem quality sphenes from Capelinha, Brazil. There is hardly any difference between the chemical data of the major elements and physical properties of the specimens of these two sources. Even the inclusions are very similar. It is the first time that fine gem quality sphene has been reported from Sri Lanka.

Description: The new computer-based registration system, a project of the National Museum of Geology and Mineralogy in the Netherlands, will considerably increase the accessibility of the Museum collection. This greater access is realized by computerisation of the data in great detail, so that an almost unlimited number of operations to select and sort the data is possible. A flexible design of annotating information, mainly in plain words, saves considerable time. The fast mechanical data processing permits the efficient preparation of catalogues which contain selected information about the geological collection; as an additional benefit labels may be produced at low costs. The RGM system disposes of the burden of classical registration books and card-indexes, without any essential quality being lost. It combines a minimum of man-hours with an optimal flexibility in storage and retrieval of data, and an acceptable employment of computer time and equipment. The system may also serve for the registration of collections of other institutions, in geology as well as zoology and botany.

Description: Within the strongly migmatized axial zone of the Hesperian massif in western Galicia a graben-like structure has been distinguished, characterized essentially by the presence of non-migmatic rocks that comprise orthogneisses with blastomylonitic textures, leucocratic gneisses, plagioclase-blastbearing paragneisses, pelitic schists, and numerous amphibolitic layers and lenses.\nIn the southern and central part of the graben and at the borders in the north the majority of the amphibolites are metamorphosed mafic dike swarms that intruded in the Early Palaeozoic after the emplacement of biotite granites but before the intrusion of subalkaline and peralkaline granites. Few amphibolites are of sedimentary origin.\nThe other amphibolitic rocks in the north are of inferred Proterozoic age and have a different appearance. They consist of retrograde eclogite facies mafites and garnet- and epidote-amphibolites that are typically associated with leucocratic gneisses and younger subalkaline orthogneisses.\nIt is inferred that the northern part of the graben mainly represents a lower basement segment that underwent Precambrian and Early Palaeozoic catazonal metamorphism and subsequent retrogradation, while the central and southern parts represent higher basement levels of mesozonal metamorphic grade.

Description: A study of the beaches on the NW shores of the R\xc3\xada de Arosa was made with respect to shape and structure of the beaches, grain-size distributions, heavy mineral associations, and relations of the heavy minerals to the bedrock of the hinterland.\nMost of these numerous small beaches are situated in inlets determined by the occurrence of relatively weak bedrock (mostly schists).\nGrain-size generally fall in fractions 〉 50 \xc2\xb5, and some beaches even contain important amounts 〉 2000 \xc2\xb5. Many grain-size distributions are bimodal owing to the mixing of different beach layers by wave action or, in a few cases, to mixing with fluvial sands.\nThe heavy mineral compositions of the fluvial and beach deposits approximately reflect the mineral compositions of the bedrock, especially in the deeper inlets of the R\xc3\xada, whereas most of the beaches on the more exposed parts have a more varied heavy mineral composition.\nIn some cases the minerals indicate a weak long-shore transport of the sands.

Description: The present area covers a part of the axial zone of the Hesperian Massif. Rocks of metamorphic and intrusive origin occur in three structural units: the central zone (which in fact forms part of what is called the \xe2\x80\x9cblastomylonitic graben\xe2\x80\x9d), the migmatic complex and the rocks in the S and SW. During the Hercynian orogeny these units were placed in juxtaposition. With respect to their age and composition the rocks can be subdivided into pre-Hercynian metasediments, metabasic rocks and orthogneisses, and Hercynian migmatites, palingenic granites and intrusive granodiorites, granites, pegmatites, aplites, quartz veins, granite porphyries, lamprophyres and olivine dolerites.\nBased on penological evidence, the geological history can be reconstructed as follows: Pre-Hercynian metasediments, bearing traces of a probably Precambrian deformation and metamorphism (resorbed staurolites and garnets enclosed in Hercynian metamorphic minerals) have intruded along fundamental N-S trending faults in the Upper Ordovician (460-430 x 106 years ago) by means of megacrystal-bearing two-mica granites, biotite granites and peralkaline granites. The intrusion of a basic dyke-swarm took place between the emplacements of the latter two types of granite series. During the Hercynian orogeny the pre-Hercynian rocks were at first penetratively deformed; the metasediments were (re)- folded and the granites were phyllonitized into coarse-grained augengneisses or mylonitized into granite mylonites (central zone). This Hercynian main phase deformation was followed by a period of Abukuma-type plutonometamorphism. A widespread metablastic recrystallization of the non-migmatic rocks of the central zone is characteristic. The metamorphic mineral assemblages in the non-migmatic rocks of the central zone are indicative of the cordierite-amphibolite facies and the mineral assemblages of the metasedimentary rocks in the SW and W indicate greenschist facies conditions, while the inclusions in the palingenic two-mica granite suffered superimposed hornblende-hornfels facies conditions. At deeper levels the Hercynian metamorphism culminated in anatexis. During a short period of tension and normal faulting the non-migmatic rocks of the central zone were brought into juxtaposition with the migmatites. More or less synchronously the active fundamental faults, separating the central zone from the western migmatites facilitated the rise of large amounts of granodioritic magma; immediately afterwards the ultimate products of anatexis, in as much as they were capable of intrusion, penetrated the migmatic complex and the granodiorites in the form of successive phases of two-mica granites. This period of plutonic activity was interrupted by the second and third Hercynian deformations that were only locally of a penetrative nature. The majority of the Hercynian granites and granodiorites were weakly foliated during the second deformation while the third deformation locally phyllonitized and mylonitized the rocks in the N, SW and W.\nThe Hercynian orogenic cycle was concluded with the intrusion of a post-kinematic two-mica granite, two biotite granites (280 \xc2\xb1 11 X 106 years ago), some granite porphyries and lamprophyres. The few olivine dolerite dykes are probably younger.\nAccessory zircon occurring in metasediments, orthogneisses, migmatites and granites has been investigated for crystal habit, growth phenomena, inclusions, resorption, fluorescence and other properties.\nThe various rock-types contain zircon populations with a more or less typical habit: i.e. a combination of the forms {101}, {100} and /or {110} and sometimes {301} and {201} for the zircons of the blastomylonitic orthogneisses, the post-kinematic granites and granite porphyries, while a combination of {101}, {211} and {100} and/or {110} is characteristic for the pre-Hercynian coarse-grained augengneisses and the Hercynian megacrystal-bearing granodiorites and two-mica granites. The zircons in the rocks belonging to the megacrystal-bearing granodiorite series clearly show differences in evolution. The zircons in the migmatic rocks indicate that conditions for anatexis favoured the regeneration of zircon. The fluorescence colours of the zircon crystals are yellow, orange and brown; some zircon populations also contain emerald green fluorescent crystals. A zonal variation in fluorescence colours is also present.

Description: In Galicia occur several polymetamorphic complexes that contain mafic and ultramafic rocks. Mafic volcanics and gabbros are situated in or near the complexes. An episode of metamorphism and granitization encompassed the generation of the volcanics and gabbros. Gravity surveys revealed the existence of deep roots below polymetamorphic complexes. Geochronological investigations yielded Palaeozoic ages from about 500 Ma for the generation of ultramafic rocks and calcalkaline granite series, till ages of around 300 Ma for postkinematic alkaline and calcalkaline granites. A model involving mantle-plume diapirism and rejuvenation of the lower crust is proposed for the Early Palaeozoic evolution of the continental lithosphere of Galicia.\nAn excursion route is given, leading along 22 exposures of mostly mafic and ultramafic rocks, to be visited in 4 1/2 days.