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FERROMAGNESIAN MINERALS /ROM THE VOLCANIC SUITE OF TENERIFE A thesis submitted for the degree of Doctor of Philosophy of the University of London by Peter Wright Scott October 1970. Dept. of Geology, Imperial College, London, S.W.7. ABSTRACT The Tenerife rocks have been classified as ankaramites, alkali basalts, trachybasalts, trachyandesites, trachytes and phonolites on the basis of their chemical and petrographic characteristics. Wet chemical and microprobe analyses and some optical data for olivines, pyroxenes, micas, garnets and sphene are presented; and, cell parameters for olivines, pyroxenes, garnet and sphene are also given. Olivine is restricted in its occurrence to the more basic rocks and is always forsterite rich (approx. Fo 80), although it may become more fayalitic in the groundmasses. A salitic clinopyroxene is present throughout the suite but aegirine is developed in the groundmass of some phonolites and in a nepheline-syenite boulder. The salites show only limited enrichment in hedenbergite with magmatic differentiation whilst the aegirines are enriched in diopside relative to hedenbergite. This is thought to be due to the limited amounts 2+ of Fe present in the "parental" magmas after the removal of varying proportions of olivine and kaersutite. Seven types of zoning have been recognised in the salites: normal, reversed, irregular, large scale oscillatory, minute scale oscillatory, hourglass, and a special "zoning" caused by the presence of green cores. The origin of most types of zoning can be explained by changes in conditions during crystallisation, but it appears unlikely that hourglass crystals have formed as suggested by Strong (1969). A kaersutite of uniform composition occurs in most rock types although it becomes barkevikitic in the salic differentiates. It is thought to have formed as a stable cumulus phase at depth in magmas of intermediate composition but to have become unstable on being brought to the surface. The presence of magnesio- arfedsonite and aenigmatite in the groundmass of some phonolites indicates crystallisation under low oxygen fugacities. A titaniferous, relatively magnesium-rich, biotite, and melanite and sphene are found in the phonolites as minor phenocrystal phases. 3 - CONTENTS Page No. Acknowledgements. 5 SECTION 1. INTRODUCTION 1. Situation and physiography of Tenerife. 6 2. Previous Geological work on Tenerife. 7 3. Stratigraphy of the volcanic series. 7 4. Geological relationship to the other Islands and Africa. 11 5. Extent of the present work. 11 SECTION 2. CLASSIFICATION AND PETROGRAPHY OF THE TENERIFE ROCKS. 1. Basis of classification. 14 2. Comments on chemical analyses of the volcanic rocks. 16 3. Petrography of the rock types. 17 SECTION 3. MINERALOGY 1. Olivine. 43 2. Pyroxene. 49 Intrbduction. 49 A. Crystallography. 49 B. General characteristics of pyroxenes from the individual rock types. 50 C. Variation in optical properties. 56 D. Chemical and Microprobe analyses. 57 E. X-Ray Diffraction Studies. 68 F. Zoning in the pyroxenes. 71 3. Amphibole. 93 A. Kaersutite. 93 B. Alkali amphiboles. 98 - 4 - Page No. 4. Biotite. 101 5. Garnet. 104 6. Sphene. 107 SECTIOi•I CONCLUSIONS. 172 Appendix 1. Analytical and X--Ray techniques: 177 Appendix 2. Specimen localties. 182 Bibliography. 187 Enclosure. Simplified geological map of Tenerife.. Figures and tables are located at the end of each Section. 5 Acknowledgements. I wish to thank Dr. G.D. Barley for the constant help, encouragement and supervision throughout the period of research, and P. 13uddaby and M.T. Frost for help and advice with microprobe, x-ray and computing techniques. Thanks are due to many post-graduate students and members of staff at Imperial College for helpful discussion and advice with experimental techniques, especially Dr. J. Nolan who allowed me to compare the pyroxene x-ray diffraction traces with those from his synthetic pyroxenes, and Dr. W.I. Ridley, who provided some separated pyroxene fractions and allowed access to his thin sections of rocks from the Las Canadas area. I am grateful to Mrs S. Bird for typing the thesis. My wife traced most of the diagrams, helped with the preparation of the photomicrographs and checked the final manuscript but, above all, she is to be thanked for encouragement and patience during the experimental and latter stages of preparation of the thesis. Receipt of an N.E.R.C. Research Studentship is acknowledged. -6-- SECTION 1 •INTRODUCTION 1. ------------Situation and physiography of Tenerife The Canary Islands comprise seven major, and a few smaller islands situated off the west coast of Africa (fig. 1.). Tenerife is the largest of these islands, with an area of 2058 sq.Kms., and a maximum height above sea level of 3718 metres (Pico Teide). In shape, Tenerife is roughly triangular, but is elongated in the north--east into the Anaga peninsula (fig. 2). This peninsula, and the north-western part of the island, the Teno peninsula, are formed of deeply dissected volcanic mountains, and, where they meet the sea, frequently form cliffs a few hundreds of metres high. To the west of Anaga lies the plain of La Laguna, which further towards the west, rises to form the ridge of Cumbre de Pedro Gil. The sides of this ridge initially fall sharply, and then more gently to the sea forming the Oratava and Guimar valleys. In the centre of the island is a wide semi-circular depression (2000 - 2500 metres above sea level) known as Las Canadas, which is bounded by a steep wall (Portillo and Tauce escarpmets) to the east, south and south-west. People have been known to compare this rugged desert area with the surface of the Moon. Rising out of Las Canadas to the north is the huge twin-peaked volcano of Pico Teide and Pico Viejo. To the north of this volcano the land falls fairly steeply to the sea, and to the west, this central part of the island is joined to the Teno peninsula by the Valle de Santiago. The top of the wall bounding Las Canadas falls uniformly to the sea at the south. Water is scarce on the island but has great erosive power, cutting deep valleys, called "Barrancos", through the rock formations: -7 2. Previous Geological work on Tenerife. There is an abundance of geological literature concerning Tenerife dating back to the early 18th Century. The majority is in 1,7nguages other than English. Much of the early litera- ture deals with field relations of the volcanic formations, with obvious emphasis on the impresive Pico Teide and Las Canadas, sometimes followed by short petrographic descriptions of the rock-types. 11 considerable variation is found in rock nomen- clature. Occasionally chemical analyses of rocks are presented, but only rarely have the minerals, in particular ferromagnesian minerals, been studied in any detail. Very little data is given concerning the properties of the minerals. Kunitz (1936) states analyses of two pyroxenes from Tenerife and these appear to be the first modern analyses of any mineral from the island. Petrogra-chic descriptions, together with rock analyses, and some optical data on the minerals, are given by Jeremine (1933), Smulikowski (1937), and Smulikowski et al.(1946). Hausen (1956) gives a detailed review of literature concerning the geology of Tenerife, and quotes roc1,- analyses and optical properties of minerals from earlier sources, along with new data. The reader is referred to Hausen (1956) for a review of knowledge to that date. Recent work, with the exception of Bravo (1955) and Ridley (1970) adds little to the knowledge of the mineralogy. A general account and most recent map of the geology is contained in Fuster et al. (1968). Ridley (1968) is concerned with the Las Canadas area. Geophysical data is contained in Macfarlane and Ridley (1968) and Bosshard (1969) and some ages of the rocks are given by Abdel-Monem et al. (1968). Plutonic nodules, found within the volcanic rocks, ore described by Borley et al. (1969). 3. Strati.gFaDhy- of the volcanic series. With the exception of a few recent sedimentary deposits, the rocks are entirely of volcanic origin. _8_ A brief stratigraphical description of the volcanic formations, from oldest to youngest is given below. It is based on divisions made by Fuster et al. (1968), Hausen (1956), and Ridley (1968) (Las Canadas area), and is augmented by the authorb own knowledge. Petrographical, mineralogical and geochemical characteristics described later are related to the formations as described here. Ancient Series: These are the oldest rocks exposed on the island and are found on the Anaga and Teno peninsulas. The rocks comprising Ladera de Guimar, and a few isolated areas to the south are also grouped within this series. The latter areas either stand out as "Islands" (e.g. Roque del Conde) or are exposed by erosion in the floors of barrancos (e.g. Barranco del Rio). This series corresponds to an early shield building stage with abundant eruptions from a fissure complex traversing Tenerife, and embracing the island of Gomera as well. Abdel- Monem et al. (1968) date the lavas from 1-2 million years (Ladora'de Guimar) to 16 million years (northern side of Anaga). If these dates are correct, then the very similar rocks comprising this series. cover a time-span considerably greater than all tlie otlier formations together. Flows are mainly basaltic or ankaramitic, with subordinate pyroclasts, and they are cut by a network of mainly basic dykes. Phonolitic intrusions into the Ancient 5eries: Also intruded into the Ancient ,Series are some thick phonolitic dykes, sheets, and plugs. Hausen (1956) calls these "Phonolitic and trachytic differentiates of the same series, but they are distinct, in their occurrence, from any other formation. Fuster et al. (1968) considers these are feeding the more alkaline Canadas rocks which have since been eroded. Xenoliths are common in these intrusions.

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