Research Collection Doctoral Thesis Petrological observations in the Maggia hydro-electric tunnel between Lake Maggiore and Centovalli Author(s): Venkayya, Emani Publication Date: 1956 Permanent Link: https://doi.org/10.3929/ethz-a-000099633 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Prom. No. 2509 Petrological observations in the Maggia hydro-electric tunnel between Lake Maggiore and Centovalli THESIS PRESENTED TO THE SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH FOR THE DEGREE OF DOCTOR OF NATURAL SCIENCES BY Emani Venkayya M.Sc. Citizen of India Accepted on the recommendation of Prof. Dr. C. Burri and Prof. Dr. F. de Quervain ZURICH 1956 DISSERTATIONSDRUCKEREI LEEMANN AG Reprint from "Schweiz. Mineralogische und Petrographisohe Mitteilungen" Vol. 36, 1956 This thesis is dedicated to the Board of the Volkart Foundation of Winterthur and to my Professors in the Mineralogical and Petrological Institute of the Swiss Federal Institute of Technology for their goodwill, un¬ failing friendliness and promotion of Indo-Swiss cultural relations. Contents Abstract 71 Zusammmenfassung 71 Chapter I Introduction 72 Explanation of the map and profiles 72 Methods of optical determinations 74 Previous literature 77 Short geological and petrographical summaries of the different geological zones described in the thesis 78 Acknowledgements 79 Chapter II Kinzigite zone 80 Section 1. Petrography of the kinzigite rocks 82 Chemical composition of the kinzigite rocks 94 Discussion and summary of observations 96 Section 2. Basic rocks (amphibolites) of the kinzigite zone 97 Discussion and summary of observations 109 Relationship between the amphibolite bands of the kinzigite zone and those of the Ivrea zone 110 Chapter III Ivrea zone 112 Section 1. Basic rocks of the Ivrea zone 112 Section 2. Pegmatites 136 70 Emani Venkayya Section 3. Inclusions in the basic rocks 137 Section 4. Cataclasis of the basic rocks 155 Discussion 157 Chapter IV Canavese zone 161 Section 1. Petrography of the country rocks 162 Section 2. Basic rocks (amphibolites) of the Canavese zone 172 Contact between the and Canavese the . gneisses amphibolites . 178 Comparison between the basic rocks of the Ivrea zone and those of the Canavese zone 179 Discussion 179 Chapter V Arcegno zone - 181 Section 1. Petrography of the country rocks 181 Section 2. Maia diorite-amphibolite . 188 Section 3. Serpentinised peridotite and associated amphibolites 191 Section 4. Other types of amphibolites not related to those of sections 2 and 3 197 Discussion 204 Chapter VI Locarno zone 206 Section 1. Country rocks (mica-gneisses) 206 Section 2. Basic rocks (amphibolites) of the Locarno zone 207 Discussion 212 Chapter VII Petrochemistry of the different basic rocks of the five zones 213 Appendix 219 References 224 Maggia hydro-electric tunnel between Lake Maggiore and Centovalli 71 Abstract A description is given in the present paper of the rocks contained in the profile through the Pennine and upper East-Alpine Root-Zone as exposed by the hydro-electric tunnel of the Maggia Electricity Works. The profile comprises the kinzigite zone and the basic zone of Ivrea (zona diorito-kinzigitica of the Italian authors), the Canavese Zone and the zones of Arcegno and Locarno. The kinzigite zone contains kata-metamorphic pelites to marly sediments of at least Palaeozoic age, into which the basic rocks of the Ivrea zone later intruded. These intrusives vary in the investigated area from hypersthene-bearing diallage-gabbro to diorite- amphibolite and occasionally contain inclusions of kinzigites. The age of the intrusion cannot be directly determined, but the general course of the zone, con¬ formable in respect to the West-Alpine arc, points to a connection with the Alpine orogenesis. The Canavese, Arcegno, and Locarno zones originally contained pelitic, marly and calcareous sediments, out of which staurolite-bearing two-mica gneisses, epidote-bearing para-amphibolites and banks of calk-silicate felses were produced by metamorphism. In the Canavese zone compact black limestones also occur. The age of these rocks is presumably Mesozoic. Subsequently to the development of the present grade of metamorphism basic intrusions took place. In the Arcegno zone ultrabasic magmas also intruded which now have the character of amphi- bolites and Berpentinised peridotites. In late to post-Alpine times an intensive pegmatitic injection took place which in the Locarno zone gave rise to extensive phenomena of migmatisation. With the exception of those in the Locarno zone all basic and ultrabasic rocks in the investigated area are considered to be of magmatic origin and genetically inter-related. The members found in the Locarno zone are of a peculiar nature as is especially apparent from their high K-content. Zusammenfassung Die vorliegende Arbeit beschreibt die Gesteine des Profils durch die penni- nisch-oberostalpine Wurzelzone, wie es zwischen Centovalli und Lago Maggiore (Tessin) durch den Druckstollen der Maggia-Elektrizitatswerke aufgeschlossen wurde. Es umfasst die Kinzigitzone und die basische Zone von Ivrea (Zona diorito- kinzigitica der italienischen Autoren), die Zone des Canavese und diejenigen von Arcegno und Locarno. Die Kinzigitzone umfasst katametamorphe pelitische bis mergelige Sedimente von mindestens palaozoischem Alter, in welche spater die basischen Gesteine der Zone von Ivrea eingedrungen sind. Diese variieren im Un- tersuchungsgebiet von Hypersthen-Diallaggabbro bis Dioritamphibolit und ent- halten gelegentliche Einschlusse von Eonzigiten. Das Alter der Intrusion lasst sich nicht direkt festlegen; der gesamte Verlauf der Zone, konform dem Westalpen- bogen, macht jedoch eine Verkniipfung mit der alpinen Orogenese nicht unwahr- Die Zonen scheinlich. des Canavese, von Arcegno und Locarno enthielten ursprung- lich pelitische, mergelige und kalkige Sedimente, aus welchen spater durch Meta¬ morphose staurolithfuhrende Zweiglimmergneise, epidothaltige Paraamphibolite und Lagen von Kalksilikatfelsen entstanden. In der Canavesezone finden sich auch dichte schwarze Kalke. Das Alter der Gesteine ist vermutlich mesozoisch. Nachdem sie den heutigen Grad der Metamorphose erreicht hatten, intrudierten basische, 72 Emani Venkayya in der Zone von Arcegno auch ultrabasische Magmen, welche heute als Amphibo- lite und serpentinisierte Peridotite vorliegen. In spat- bis postalpiner Zeit erfolgte eine starke pegmatitische Injektion, welche in der Zone von Locarno ausgedehnte Migmatitisierung zur Folge hatte. Mit Ausnahme der Zone von Locarno werden alle basischen und ultrabasischen Gesteine des untersuchten Gebietes als magmati- schen Ursprungs und als miteinander genetisch verwandt betrachtet. Eine be- sondere Stellung nehmen diejenigen der Zone von Locarno ein, wie sich besonders auch aus ihrem hohen K-Gehalt ergibt. CHAPTER I Introduction In order to meet the increasing demands for power for industrial purposes the Maggia Hydro-electrical Company Ltd. of Locarno bored a tunnel of several kilometers length during the years 1950—1954. It traverses several rock formations of the root zones of the upper east- Alpine and pennine nappes of the Alps in the canton of Tessin west of Locarno and W. N. W. of the Lago Maggiore. This afforded an excellent opportunity to study the different rock formations in their almost un- weathered condition in vertical profiles along the walls and roof of the tunnel. Interesting data was thus provided to supplement our present knowledge of the geological relationship of the formations (of the "root zones") to one another and the petrogenetical conditions of their evolu¬ tion to their present state. As the tunnel is several kilometers in length and as it would be beyond the possibility of a detailed study by a single researcher within a short period of time, the late Prof. Dr. P. Niggli assigned to the author the first part of the tunnel from its opening N. E. of Brissago as far as Palagnedra in the Centovalli (a length of 7.4 km) for detailed geological and petrological studies. The present thesis, there¬ fore, contains the investigations carried out by the author in the Minera- logical and Petrological Institute on specimens collected from the first part of the tunnel during the years 1951—1954. Explanation of the map and profiles (Plates I—III) The map and the accompanying profiles appended to this thesis represent a simplified version of the geological and petrological map of P. Walter (1950), who investigated this area of the root zone of the pennine nappes up to the Swiss border with meticulous thoroughness. The map gives a general idea of the disposition of the principal zones and Maggia hydro-electric tunnel between Lake Maggiore and Centovalli 73 their possible stratigraphical boundaries against one another. The forma¬ tions met with in the above zones (kinzigite, Ivrea, Canavese, Arcegno, and Locarno zones) have a general E.-W. strike with minor fluctuations in E.-N. E. to W.-S. W. or E.-S. E. to W.-N. W. direction. Such fluctua¬ tions do not veer to any large extent from the general E.-W. direction of strike. All the formations have steep northerly dips in general ranging from 70° to almost 90°. The direction of the hydro-electrical tunnel is indicated in the above map by a thick black line ABCD. The tunnel cuts through the first four geological formations (kinzigite, Ivrea, Cana¬ vese, and Arcegno zones) in