Provenance of late palaeozoic sediments in the southern Patagonian Andes [Elektronische Ressource] : age estimates, sources, and depositional setting / vorgelegt von Carita Augustsson

Informations

• Publié par: westfalische_wilhelms-universitat_munster
• Publié le: 01 janvier 2004
• Nombre de lectures: 22
• Poids de l'ouvrage: 3 Mo

Extrait

Geologie
Inaugural-Dissertation
zur Erlangung des Doktorgrades
der Naturwissenschaften im Fachbereich Geowissenschaften
der Mathematisch-Naturwissenschaftlichen Fakultät
der Westfälischen Wilhelms-Universität Münster
vorgelegt von
Carita Augustsson
aus Norra Sandby (Schweden)
- 2003 -Augustsson, C., 2003: Provenance of Late Palaeozoic sediments in the southern Patagonian
Andes: age estimates, sources, and depositional setting, Dissertation, Westfälische Wilhelms-
Universität Münster, Germany, 94 pp.
Correspondence:
Carita Augustsson
Geologisch-Paläontologisches Institut und Museum
Westfälische Wilhelms-Universität
Corrensstraße 24
48 149 Münster
Germany
E-mail: [e-mail]
Tel. [phone]
Fax: [phone]
Dekan: Hans Kerp
Erster Gutachter: Heinrich Bahlburg
Zweiter Gutachter: Klaus Mezger
Tag der mündlichen Prüfung:
Tag der Promotion:
2Table of contents
Abstract...................................................................5
1. Introduction .............................................................7
1.1. Geological setting .................................................7
1.2. Aims ..........................................................12
1.3. Age ...........................................................13
2. S am ple descriptions ...................................................... 16
3. P etrography............................................................. 17
3.1. Light framework mineral content ....................................17
3.1.1. Light framework mineral content: methods .....................17
3.1.2. Light framineral content: results and implications .........17
3.2. Cathodoluminescence of quartz .....................................17
3.2.1. Cathodoluminescence of quartz: methods .......................21
3.2.2. Cathodoluminescence of quartz: results ........................22
3.2.3. Cathodoluminescence of quartz: new data evaluation method .......22
3.2.4. Cathodoluminescence of quartz: implications ....................27
3.3. Heavy minerals ..................................................28
3.3.1. Heavy minerals: methods ...................................29
3.3.2. Heavy minerals: content ....................................29
3.3.3. Heavy minerals: chemical composition of tourmaline .............33
3.4. Petrography: discussion ...........................................37
4. W hole-rock chem istry .................................................... 39
4.1. Whole-rock major and trace element chemistry .........................39
4.1.1. Whole-rock major and trace element chemistry: methods ..........39
4.1.2. Waistry: results and chemical
weathering .........................................39
4.1.3. Whole-rock major and trace element chemistry: implications .......45
4.2. Sm-Nd and Rb-Sr isotopes50
4.2.1. Sm-Nd and Rb-Sr isotopes: methods ...........................50
4.2.2. Sm-Nd isotopes: results and implications .......................51
4.2.3. Rb-Sr isotopes: results and im ........................52
4.3. Whole-rock chemistry: discussion ...................................54
5. Z ircon U -P b ages and H f isotopes ........................................... 57
5.1. U-Pb ages ......................................................57
5.1.1. U-Pb ages: methods ........................................57
5.1.2. U-Pb ages: results .........................................58
5.1.3. U-Pb ages: implications .....................................68
5.2. Hf isotopes .....................................................70
5.2.1. Hf isotopes: methods .......................................71
35.2.2. Hf isotopes: results and implications .......................... 72
5.3. Zircon U-Pb and Hf isotope signatures: discussion ...................... 74
5.3.1. The Early to Late Carboniferous .............................. 74
5.3.2. The Early Permian ........................................ 80
6. F inal rem arks ........................................................... 81
6.1. Conclusions .................................................... 82
Acknowledgements ........................................................ 84
References ............................................................... 84
4Provenance of Late Palaeozoic sediments in the southern Patagonian Andes
Abstract
In the southern Patagonian Andes of Chile and Argentina the Late Palaeozoic basement of the
Andes is mainly composed of metasediments. SHRIMP U-Pb ages of detrital zircons from
turbiditic rocks of the Cochrane unit, which belongs to the Eastern Andean Metamorphic
Complex in Chile, constrain the maximum depositional age to Devonian to Early
Carboniferous for this unit. SHRIMP U-Pb ages further restrict deposition of the Bahía de la
Lancha Formation in Argentina to late Early Carboniferous times. The main sediment sources
of the Cochrane unit and the Bahía de la Lancha Formation were generally dominated by
felsic rocks, as revealed by the whole-rock chemistry (Th/Sc = 0.6-1.9). The sedimentary
detritus is dominated by recycled metasedimentary and metamorphic material, as revealed by
cathodoluminescence of quartz, chemistry of tourmaline and zoning of zircon. The source
material was of continental origin and had been recycled within the crust before becoming
part of the studied turbidite deposits as indicated by whole-rock Sr and Nd isotope signatures
87 86and U-Pb ages of detrital zircons ( Sr/ Sr(T = 250 Ma) = 0.709-0.718; (T = 320-350 Ma)Nd
= -8 to -2; Nd T * = 1170-1540 Ma; ca. 90 % of the zircons have U-Pb ages of < 1500 Ma,DM
with a dominance in the interval 350-700 Ma). As indicated by the U-Pb ages and Hf isotope
signatures of single zircons, the original main source areas for the detrital grains encompassed
present extra-Andean Patagonia, as well as the Argentinean Sierra de la Ventana slightly
north of Patagonia. Furthermore, southern Africa, East Antarctica and the Falkland Plateau,
which formed an area in the interior of Gondwana, were probably important original source
areas for the sediments. The part of the Eastern Andean Metamorphic Complex that crops out
on the shoreline of Fiordo Peel (ca. 50°45'S) in the southern Chilean archipelago has a Late
Carboniferous - Early Permian maximum depositional age, as deduced from the SHRIMP U-
Pb dating. Magmatic source material dominates the sediments, as revealed by the abundance
of zircons with magmatic zoning. The primary sources were mainly felsic and had a
87 86continental setting (Th/Sc = 0.8-1.1; Sr/ Sr(T = 250 Ma) = 0.705-0.710; (T=280) = -5 toNd
-4; Nd T * = 1270-1320 Ma; > 40 % of the zircons have U-Pb ages between 290 and 330DM
Ma). The sediments most likely had short transportation paths. With the data above, this study
supports an onset of subduction at the Late Palaeozoic Pacific margin of Gondwana (present
coordinates) in Late Carboniferous times.
Andes, Argentina, cathodoluminescence, Chile, detrital zircons, geochemistry, Gondwana,
heavy minerals, Hf, Palaeozoic, Patagonia, provenance, quartz, Rb-Sr, SHRIMP, Sm-Nd,
tourmaline, turbidites, U-Pb.
5Carita Augustsson
Kurzfassung
In den Südpatagonischen Anden in Chile und Argentinien besteht das Spätpaläozoische
Grundgebirge der Anden hauptsächlich aus Metasedimenten. SHRIMP-U-Pb-Datierungen
von detritischen Zirkonen aus Turbiditablagerungen der „Cochrane unit”, welche dem
chilenischen „Eastern Andean Metamorphic Complex” angehört, weisen auf ein maximales
devonisch-frühkarbonisches Ablagerungsalter hin. Die Bahía de la Lancha Formation wird in
das späte Frühkarbon gestellt. Im Allgemeinen sind die Quellgesteine der Sedimente felsisch,
wie an der Gesamtgesteinschemie (Th/Sc = 0,6-1,9) gezeigt werden kann. Katodolumineszenz
von Quarzkörnern, Turmalin-Chemismus und die Zonierungen der Zirkone deuten darauf hin,
dass der Detritus dieser geologischen Einheiten hauptsächlich aus wiederaufbereitetem und
recyceltem, metasedimentärem und metamorphem Material bestand. Das Quellmaterial war
kontinental und wurde in der Kruste vor der Ablagerung der untersuchten Sedimente recycelt.
Dies deutet sich in Sr- und Nd-Isotopie der Gesamtgesteinsanalysen ebenso an wie in den U-
87 86Pb Altern detritischer Zirkone ( Sr/ Sr(T = 250 Ma) = 0,709-0,718; (T = 320-350 Ma) =Nd
-8 to -2; Nd T * = 1170-1540 Ma; ca. 90 % der Zirkone haben U-Pb-Alter jünger als 1500DM
Ma mit einer Dominanz im Intervall 350-700 Ma). U-Pb-Datierungen und Hf-Isotopie von
Einzelzirkonen lässt vermuten, dass die ursprünglichen Hauptquellen in Patagonien östlich
der Anden sowie in der Sierra de la Ventana in Argentinien etwas nördlich Patagoniens zu
finden sind. Auch der südliche Teil Afrikas, die Ostantarktis und das Falklandplateau, welche
ein zusammenhängendes Gebiet innerhalb Gondwanas bildeten, waren wahrscheinlich
wichtige ursprüngliche Liefergebiete. Der aufgeschlossene Teil des „Eastern Andean
Metamorphic Complex” an der Küste von Fiordo Peel (ca. 50°45'S) im Archipel Südchiles
hat ein spätkarbonisch-frühpermisches maximales Ablagerungsalter. Die Zirkonzonierung
zeigt, dass in den Sedimenten überwiegend magmatisches Material vorliegt. Die prim