Chiavenna, Bergell Alps)

Research Collection Doctoral Thesis Petrological and chemical investigation of a metamorphosed oceanic crust-mantle section (Chiavenna, Bergell Alps) Author(s): Talerico, Caterina Publication Date: 2000 Permanent Link: https://doi.org/10.3929/ethz-a-004138034 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 Diss. ETH No. 13934 Petrological and chemical investigation of a metamorphosed oceanic CRUST-MANTLE SECTION (CHIAVENNA, BERGELLALPS) A dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZÜRICH for the degree of Doctor of Natural Science presented by Caterina Talerico Laurea in Scienze Geologiche - Università delta Calabria - Italia born September 11th 1967 citizen of Italy and Zurich (ZH) accepted on the recommendation of Prof. V. Trommsdorff, examiner Dr. P. Ulmer, co-examiner Prof. L. Morten, co-examiner 2000 Contents Abstract I Riassunto Ill 1 Introduction 1 1.1 Context Central Alps 1 1.2 Aims 1 2 Geological overview 3 2.1 Geotectonic position of the Chiavenna unit 3 2.1.1 Former studies on the tectono-metamorphic evolution of the Chiavenna unit 4 2.1.2 General discussion of the ophiolitic nature of the chiavenna unit 5 2.2 Field observations and petrography 7 2.2.1 Ultramafic body and preserved pre-alpine features 7 2.2.2 Amphibolites, calc-silicate and metamorphosed rodingites boudins 9 2.2.3 Metacarbonates: Calcite marbles and ophicarbonatic veins and pockets 14 3 Metamorphism 17 3.1 Problem Statement 17 3.2 Metamorphism in ultramafic rocks 17 3.2.1 Temperature conditions 17 3.2.2 Metamorphic evolution 20 3.2.3 Mineral chemistry 23 3.2.4 Microtextural features 26 3.2.5 Interpretation 27 3.3 Metamorphic evolution of the mafic rocks 28 3.3.1 Mineral assemblages 28 3.3.2 Metamorphic related Compositional Changes in amphibole and plagioclase 29 3.3.2.1 Amphibole breakdown reactions 30 3.3.2.2 Amphibole compositional changes 31 3.3.2.3 Plagioclase compositional changes 33 3.3.3 Summary 34 3.4 Metamorphosed calcareous rocks 35 3.4.1 Phase stability calculations 36 3.4.1.1 Implications 38 3.5 Estimated pressure conditions in the Chiavenna unit 38 3.5.1 Ultramafic and carbonate rocks 38 3.5.2 Tonalitic system 39 3.6 Summary and Interpretation 40 4 Dating of amphiboles from metabasic rocks 43 4.1 Introduction 43 4.1 WAr method 43 4.2 Sample description 44 4.3 39Ar-40Ar data presentation 46 4.3.1 Sample SM71 47 4.3.2 Sample SM44 49 4.3.3 Sample SM60 50 4.3.4 Sample SM81 52 4.3.5 Sample SM28 53 4.3.6 Sample BM4 55 4.4 Discussion and Interpretation of the results on a regional scale 57 4.4.1 Interpretation of the 39Ar/40Ar data 57 4.4.2 Overview of the existing isotope data on the surrounding areas of the Chiavenna unit 58 4.5 Conclusions 59 5 Model of Thermal cooling 61 5.1 Introduction 61 5.2 Mathematical Model 61 5.3 Results 63 5.4 Discussion 65 5.4.1 Was the Gruf Unit responsible for the thermal overprinting event in the chiavenna rocks? 65 5.4.2 Alternative heat-sources 66 5.4.2.1 Masino-Bregaglia related intrusions 66 5.4.2.2 Deep seated bodies 67 5.4.3 Geological implications of gruf unit enhanced contact metamorphism 67 5.5 Conclusions 68 6 Geochemistry 69 6.1 Introduction 69 6.1.1 Aims of geochemical analysis 69 6.2 Bulk rock chemistry of metaultramafic Rocks 69 6 2 1 Major element composition 69 6 2 2 Transition elements 72 6 2 3 Re- and trace element composition 75 6 3 Trace element distribution in ultramafic mineral phases of metamorphic parageneses a laser ablation icp-ms study 79 6 3 1 Introduction 79 6 3 2 Samples description 79 6 3 3 Results 80 6 3 4 Significance of metamorphic amphibole for mantle processes 82 6 4 Summary and discussion on the Geochemistry of the Chiavenna metapendotites 83 6 5 Metabasic rocks 88 6 5 1 Major and Minor element-chemistry of amphibohtes 89 6 5 2 Magmatic classification 92 6 5 3 Trace and rare earth elements in amphibohtes 93 6 5 4 Rodingites 97 6 5 5 Tectono-magmatic interpretation 97 6 6 Metacarbonates and Metaophicarbonatic rocks 99 6 6 1 Compositional Features and Discussion of the metacarbonates data 99 6 7 Conclusions 102 7 Dumortiente 105 71 The Gruf complex 106 7 2 Crystal structure 107 7 3 Petrogenetic system and stability field 107 7 3 1 Chemical analysis 109 7 3 1 1 Discussion 110 8 Conclusions 113 8 1 Alpine metamorphic evolution 113 8 1 1 Recorded metamorphic events 113 8 1 2 Time constraints of the alpine evolution 115 8 1 3 Interpretation and thermal model 115 8 1 4 General considerations on the tertiary metamorphic evolution of the Chiavenna unit 116 8 2 Bulk-rock chemical characterisation 117 A Appendix 119 A 1 Analytical Methods 119 A.1.1 X-Ray Fluorescence (XRF) 119 A.1.2 Inductively Coupled Plasma Mass Spectrometry (ICP-MS) 119 A.1.3 ICP-MS Laser Ablation 120 A.1.4 Mineral separation and Ar/Ar Dating 120 A. 1.5 Electron Microprobe (EMP) 121 A.2 Sample Location 122 A.3 Analyses 123 A.3.1 XRF-Analyses 123 A.3.2 Laser ablation ICP-MS analyses 129 A.3.3 Icp-ms analyses 131 A.3.4 Argon analyses 135 A.3.5 Electron microprobe analyses 138 References 147 Ringraziamenti Curriculum vitae / Abstract Aim of this study is to constrain the metamorphic evolution of the Chiavenna unit (Central Alps) and to define its genetic derivation. The Chiavenna unit, exposed between the Middle Pennine Tambo nappe to the north and the North Pennine Adula-Gruf nappe to the south, consists of metamorphosed ultramafic, mafic and calcareous rocks. The entire unit underwent an intense Alpine deformation that imprinted the rocks with a typical E-W schistosity (S2). The metamorphosed ultramafics consist of deformed peridotitic rocks. Former mantle structures are rarely preserved and are limited to a weakly developed mantle layering formed by dunite and smaller pyroxenite bands crosscut by a N-S magnetite schistosity (S1) discordant to the main E-W schistosity (S2). The ultramafics contain metamorphosed boudins of rodingites surrounded by black-wall as well as various fractures and pockets filled with carbonatic material. Banded and massive amphibolites technically underlay the ultramafics. The banded amphibolites exhibit nematoblastic textures with a developed metamorphic banding. The massive amphibolites, without a preferred orientation of the mineral phases display textures typical of doleritic rocks. None of the amphibolites show typical gabbro textures. Schistose calcite marbles containing amphibolite-breccias and Cr-rich nodules directly underlay the amphibolites. During the Tertiary Alpine metamorphism the Chiavenna unit underwent a progressive metamorphic evolution at different temperature conditions varying from less than 500°C (diopside-out reaction) in the northern part of the unit to almost 700°C (enstatite-in reaction) in its southern part. Petrological and microtextural analyses indicate the occurrence of two different metamorphic events characterised by different deformation styles, temperature conditions and space distribution. A first dynamic metamorphic event is distinguished by: 1) A well developed antigorite or chlorite schistosity (S2) where large olivine blasts and magnetite bands are elongated parallel to this schistosity. 2) The presence of tremolite and olivine aggregates replacing rotated former diopside, preserving a relic magnetite schistosity (S1). In the amphibolites and in the calcite marbles, the synkinematic event develops foliated textures (S2) formed by the elongation of low-grade greenschist faciès metamorphic phases such as actinolite, low edenite-Mg- hornblende, epidote and low Ca-plagioclase and calcite, quartz as well as phlogopite, respectively. The successive near static thermal event overprinted the dynamic event with mineral assemblages of progressively increasing metamorphic grade. The major features of this event are: 1) The occurrence of metamorphic reactions indicating increasing temperature along a prograde path. The inferred temperature increase is of approximately 200°C, rising from less than 500°C to almost 700°C. The mapped isograds are subparallel to the contact between the Chiavenna and the Gruf units. 2) The development of coarse¬ grained granoblastic textures partially replacing the schistose microfabrics. 3) The occurrence of exchange reactions which involve solid solution phases such as hornblende and plagioclase. During the temperature increase the amphiboles of the mafic system modified their compositions towards progressively higher edenite, AIIV, AIVI and Ti contents, while plagioclase increased its anorthite component until clinopyroxene appeared. The compositional variations of these phases occurred without inducing significant changes in the rock-microfabric. The changes are linked to increasing temperature conditions and correlate with the distance from the Gruf unit. Independent pressure estimates obtained from the high-grade metamorphic calcite marbles as well as from tonalitic dikes crosscutting the amphibolites yield values of approximately 4 kbar. The calcite marbles contain a five-mineral phase assemblage whose composition permits to determine pressure and temperature conditions (4.3 kbar and approximately 670°C) for a fluid composition of 0.17 Xco2. The amphibole composition of the tonalité dikes has an average total aluminium content (AIT) of 1.47 (a.p.f.u.). The substitution of this AIT value in a barometric equation calibrated for amphiboles of calc-alcaline intrusive rocks yields pressures of approximately 4 ± 0.6 kbar. This value refers to conditions achieved during the high temperature conditions and represents the pressure acting during the near static thermal event.

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