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Magmatic Differentiation and Mixing Processes Among Mafic to Evolved Cand. Scient. Thesis in Geosciences Magmatic differentiation and mixing processes among mafic to evolved lavas and syenites in the Diego Hernández Formation, Tenerife, Canary Islands: Evidence from the geochemistry of clinopyroxenes and amphiboles Thomas Denstad Magmatic differentiation and mixing among mafic to evolved lavas and syenites in the Diego Hernández Formation, Tenerife, Canary Islands: Evidence from the geochemistry of clinopyroxenes and amphiboles Thomas Denstad Cand. Scient. Thesis in Geosciences Discipline: Petrology and Geochemistry Department of Geosciences Faculty of Mathematics and Natural Sciences UNIVERSITY OF OSLO May 2007 © Thomas Denstad, 2007 Tutor: Else-Ragnhild Neumann (PGP) This work is published digitally through DUO – Digitale Utgivelser ved UiO http://www.duo.uio.no It is also catalogued in BIBSYS ( http://www.bibsys.no/english ) All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission . Table of Contents 1 INTRODUCTION................................................................................................. 6 2 GEOLOGICAL SETTING.................................................................................... 8 2.1 CANARY ISLANDS ..................................................................................................................... 8 2.2 TENERIFE ................................................................................................................................. 8 2.3 DIEGO HERNÁNDEZ FORMATION .............................................................................................. 10 2.4 STRATIGRAPHY OF THE DIEGO HERNÁNDEZ FORMATION .......................................................... 12 3 ANALYTICAL METHODS.................................................................................. 14 4 PETROGRAPHY................................................................................................ 16 5 MINERAL GEOCHEMISTRY ............................................................................. 21 5.1 CLINOPYROXENES .................................................................................................................. 21 5.1.1 Major Elements ................................................................................................................ 21 5.1.2 Trace Elements................................................................................................................ 26 5.2 AMPHIBOLES .......................................................................................................................... 32 5.2.1 Major Elements ................................................................................................................ 32 5.2.2 Trace Elements................................................................................................................ 34 6 WHOLE ROCK ANALYSIS ............................................................................... 36 7 TEMPERATURE AND PRESSURE ESTIMATES ............................................. 40 8 DISCUSSION ..................................................................................................... 42 8.1 FRACTIONAL CRYSTALLISATION ............................................................................................... 42 8.1.1 Whole Rock Chemical Variations .................................................................................... 43 8.1.2 Clinopyroxene .................................................................................................................. 44 8.1.3 Amphibole ........................................................................................................................ 48 8.2 MAGMA MIXING ...................................................................................................................... 50 8.3 MIXING AND FRACTIONAL CRYSTALLISATION ............................................................................ 56 8.3.1 Melt Modelling.................................................................................................................. 59 8.4 COMPOSITIONAL DIFFERENCES BETWEEN LAVAS AND SYENITES .............................................. 67 9 SUMMARY AND CONCLUSIONS..................................................................... 71 APPENDIX I PETROGRAPHY.............................................................................. 74 APPENDIX II GEOCHEMICAL DATA.................................................................... 83 TABLE A2.1. CLINOPYROXENE MAJOR ELEMENT: LAVAS. ........................83 TABLE A2.2. CLINOYROXENE MAJOR ELEMENT DATA: SYENITES...........90 TABLE A2.3. AMPHIBOLE MAJOR ELEMENTS: LAVAS...............................101 TABLE A2.4. AMPHIBOLE MAJOR ELEMENT DATA: SYENITES. ...............103 TABLE A2.5. CLINOPYROXENE TRACE ELEMENT DATA: LAVAS.............112 TABLE A2.6. CLINOPYROXENE TRACE ELEMENT DATA: SYENITES.......118 TABLE A2.7. AMPHIBOLE TRACE ELEMENT DATA ....................................132 TABLE A2.8. WHOLE ROCK MAJOR AND TRACE ELEMENT DATA...........139 ACKNOWLEDGEMENTS ....................................................................................... 145 REFERENCES ........................................................................................................ 146 Introduction 1 Introduction Petrology involves the description, identification, classification, the interpretation of data and the generation of theories on the origin of rocks (Philpotts, 1990). When applied on igneous systems this means the interpretation of rocks formed from a molten material, found either as extrusive bodies on the earths surface, or as intrusive bodies found within the earth. One of the primary goals of igneous petrology is the development of geochemical models of the differentiation processes that produce the observed variety of igneous rocks. In this context it is fundamental to understand the chemistry and distribution of elements between different phases found in the rocks (i.e. different minerals, trapped liquids, mineral-melt partitioning). This thesis will, in addition to whole rock data, centre around the geochemistry of two important rock forming minerals, commonly associated with the products of primary and differentiated alkaline ocean island basalts: Clinopyroxenes and Amphiboles . While whole rock data give only the average result of the processes to which magmas have been subjected, individual minerals faithfully record information about the changing physical and chemical compositions in the magmas from which they grow (Neumann et al., 1999). Pyroxenes and amphiboles are both termed inosilicates and crystallise as single or double chained silicates, respectively. Pyroxenes are the most important group of rock-forming ferromagnesian silicates, and occur as stable phases in almost every type of igneous rock. Clinopyroxenes can be considered broadly in terms of two major subgroups based on their cation occupancy in the general formula [(M2)(M1)(T) 2O6]: Calcic pyroxenes in which Ca occupies more than two-thirds of the M2 position, and sodium pyroxenes where the M2 site is largely occupied by Na. Since there exists no miscibility gap between clinopyroxenes, coupled substitutions within the crystallographic lattice create a haven for a number of differently charged cations. Amphiboles occur in a wide range of P-T environments and are common constituents of both metamorphic and igneous rocks. Among igneous rocks they are found in all the major groups ranging from ultrabasic to acid and alkaline types, but 6 Introduction are particularly common constituents of the intermediate members of the calc-alkali series, where they can put important constrains on the crystallisation process. Amphiboles occur characteristically in the plutonic rocks and, in general, are relatively unimportant minerals of the volcanic rocks. A rising number of recent papers have shown that ocean island magmas may undergo complex processes such as fractional crystallisation at different depths, crystal accumulation, magma mixing, and contamination by oceanic sediments, melts generated in the mantle lithosphere, or through assimilation of hydrothermally altered basement (Neumann et al., 1999). Substantial literature on the mineralogy and petrology of nepheline syenites and related rocks has accumulated, with particular emphasis on compositional trends and relations among mafic phases such as aegirine-augite and alkali amphiboles. To a great extent all these studies deal with intrusive rock bodies presently exposed at the surface, such as the plutons of the Precambrian (Gardar) and Tertiary Provinces in Greenland and elswere. However, the levels of erosion in these areas are usually such that undisputable co-magmatic volcanics are absent. In contrast, the occurrence of nepheline syenite blocks in the late Quarternary phonolitics of the Diego Hernández Formation (0,37 – 0,175 Ma), a product of the latest caldera forming event in the Las Cañadas caldera, Tenerife, provides an excellent opportunity to compare syenite mineralogy with that of the co-magmatic eruptive products. In this thesis, through petrographical observations and geochemical modelling, I will try to shed light on the evolution of primitive to highly evolved magmatic products, both intrusive and extrusive, produced during the magmatic cycle of the Diego Hernández Formation, and study the effects of shallow level processes working on open system magma chambers beneath oceanic islands. 7 Geological
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