| Author||L. van der Plas|
|Title||Petrology of the Northern Adula Region, Switzerland (with particular reference to the Glaucophane-Bearing Rocks)|
|Journal||Leidse Geologische Mededelingen|
|Abstract||Geological and petrographical investigations were carried out in the northern part of the so-called Adula Nappe, one of the deepest Pennine nappes. The area under consideration lies in the SE of Switzerland, near Vals, S of Ilanz. This area is situated north of the Lepontinic gneiss-region, the deepest part of the Alpine orogen.|
The rocks of the mapped area fall into four groups, viz gneisses; micaschists; amphibolites and allied rocks; and Mesozoic rocks, either of sedimentary or of igneous origin. The first three of these groups presumably represent metamorphic Hercynian or older rocks.
Structurally three unites were distinguished, viz the Valserschuppen, the Fanellalappen and the Zervreilerlappen. The investigations clearly showed the important role of thrusting, isoclinal folding being of minor importance.
The fissure-filling and rock-making minerals, a. o., chloritoid, chloromelanite, ferrian phengite, garnet, crossite, glaucophane and ferrohastingsite, are described in chapter III. They are listed on p. 451.
A petrographical description of the region is to be found in chapter IV. The four different groups of rocks are treated separately, with a summary at the end of each section.
The phengite-gneisses have a blastic structure pointing to a total recrystallization of the original granitoid material. Generally the feldspars do not show alteration. The large orthoclase porphyroclasts in some samples are assumed to represent relics of an older mineral assemblage.
The mica-schists show the influence of several different phases of metamorphism as witnessed by, e. g., the chloritization of garnets, the replacement of chloritoid by transverse muscovite, the occurrence of several successive generations of amphibole and the occurrence of biotite fringes around the muscovites.
The amphibolites and allied rocks vary in mineralogical composition. The latter comprise, a. o., sodium-pyroxene garnet rocks with occasional glaucophane and rocks rich in albite with subordinate quartz, amphibole and/or garnet. The amphibolites and allied rocks are mainly found between mica-schists and phengite-gneisses. The occurrence of zonal amphiboles and of very narrow veins filled with a different metamorphic mineral assemblage, points to a polyphasic metamorphic history. The mineral assemblage of such a narrow vein is assumed to have originated within a relatively short time and to represent a stable association of approximately contemporaneous minerals.
The Mesozoic rocks mainly comprise metamorphic carbonate-bearing sediments and metamorphic ophiolites. These rocks inform us about the age of a large number of metamorphic minerals, since these minerals can only have been produced during metamorphism of Alpine age. A list of such minerals is given on p. 521. The occurrence of zonal amphiboles, the replacement of chloritoid by garnet and that of chloritoid and garnet by chlorite, point to polyphasic metamorphism.
Chapter V treats the chemical composition of the various groups of rocks. If the chemical composition of the rock-making minerals is approximately known, the chemical composition of a sample can be calculated from the mode. In this way a number of chemical compositions of samples was added to those obtained by chemical analysis.
A theoretical discussion of point counter analysis is given first. The results are summarized on p. 530—531. In order to avoid correlation between the measurements it seems advisable to choose as the distance between the points, the diameter of the largest grains that occur in appreciable quantities, or any larger distance. The theory that the grain size is irrelevant in modal analysis, is disproved.
In chapter V 26 chemical analyses and 35 calculated analyses of rocks have been used to study the characteristic differences between the various groups by means of statistical methods. Differences in the chemical composition of both groups of rocks strongly suggest that the amphibolites did not derive from ophiolites. In view of their low potassium content it is highly improbable that the amphibolites represent a basic front of the phengitegneisses at their boundaries with the mica-schists. Hence it is the author’s opinion that the amphibolites, the mica-schists and the phengite-gneisses have no genetical relation whatsoever.
In chapter VI the results of the mineralogical, petrographical and chemical investigations are combined in order to arrive at a synthesis. The rocks of the northern Adula region are shown to have been influenced by three successive phases of metamorphism of Alpine age. Some rocks even show traces of pre-Alpine metamorphism. The first Alpine metamorphic phase produced, a. o., glaucophane, crossite, sodium-pyroxene, garnet, epidote and chloritoid. The second phase is characterized by the production of, a. o., blue-green amphibole, ferrohastingsite, garnet, albite, epidote and biotite. The third phase produced, a. o., actinolite, chlorite, green biotite, epidote, zoisite and albite. The existence of these three Alpine phases was proved by making use of the following phenomena: (a) the occurrence of armoured relics; (b) the difference between the mineral assemblages of the host-rock and of narrow veins originated during the metamorphism; (c) the zonal habit of amphiboles; (d) the frequency of occurrence of a number of mineral associations in a group of about 300 samples, a result of a quantitative investigation with the aid of an International Business Machines equipment.
The amphibolites and allied rocks seem to be metamorphosed mafic igneous rocks of Hercynian or older age. Scanty evidence suggests that the phengite-gneisses are the products of metamorphism of Hercynian igneous rocks. The fact that the amphibolites are nearly always found between micaschists and phengite-gneisses might be explained by assuming a teetonical cause for this association.
In chapter VII some general aspects of the results obtained in the Adula region are discussed, as well as the bearing of these results on the geology and petrology of the southern part of the Swiss Alps. The tentative conclusion is reached that the Alpine glaucophane was produced simultaneously with the chloritoid and stilpnomelane of E. Niggli’s different zones, whereas the blue-green amphibole may be contemporaneous with the kyanite and the coarse-flaky brown biotite of Alpine age. Consequently the typical minerals of Niggli’s zones may be of different age. The distribution pattern of glaucophane in the Penninides and their immediate surroundings shows a conspicuous gap south of the Gotthard Massif, more or less coinciding with the area of brown biotite of Alpine age. This phenomenon may be connected with the occurrence of post-Palaeozoic granitoid rocks in the regions of Bellinzona and Tessin.
Maps showing the distribution of glaucophane and lawsonite in Europe and in the rest of the world are added, as well as a bibliography giving localities of these minerals.
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