Petrology and isotope geology of the Hunnedalen monzonoritic dyke swarm, SW Norway; a possible late expression of Egersund Anorthosite magmatism

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Authors:Maijer, Cornelis; Verschure, Robert Henri
Author Affiliations:Primary:
Universiteit Utrecht, Instituut voor Aardwetenschappen, Utrecht, Netherlands
Vrije Universiteit, Netherlands
Volume Title:Bulletin - Norges Geologiske Undersokelse
Source:Bulletin - Norges Geologiske Undersokelse, Vol.434, p.83-107. Publisher: Universitetsforlaget, Trondheim - Oslo, Norway. ISSN: 0332-5768
Publication Date:1998
Note:In English. 93 refs.; illus., incl. 6 tables, geol. sketch map
Summary:The widest and northernmost dyke of the post-tectonic Late Proterozoic Hunnedalen Monzonoritic Dyke Swarm has been studied petrographically, chemically (major and trace elements) and isotopically (Rb-Sr; Sm-Nd; K-Ar) together with its country rocks. Samples from the core of the dyke appear fresh or nearly fresh and have a monzonoritic composition. Samples from within 1 m of the contacts appear severely altered, petrographically, chemically and isotopically, most probably during a very low-grade regional phase of metamorphism, M4, related to Early Palaeozoic burial and to Caledonian orogenesis. The unaltered central part the dyke is slightly inhomogeneous with some concentration of earlier formed minerals in the centre of the dyke (higher plagioclase content, higher Mg-ratio) due to flow differentiation. Trace element discrimination diagrams indicate a continental, within-plate setting. In altered samples the change in chemistry involved variable shifts in the contents of Rb and Sr, as well as in Sr-isotope ratios. Fresh samples produce concordant mineral-whole isochrons with both the Rb-Sr (834±9 Ma; Sr(i) 0.70442±0.00004; MSWD 0.594) and Sm-Nd (835±47 Ma; Nd(i) 0.51166±0.00004; MSWD 1.57) methods, most probably representing the age of intrusion. Whole-rock samples give Rb-Sr and Sm-Nd ages with large uncertainties, due to restricted spread of data points. K-Ar whole-rock ages of fresh samples exhibit some spread, but the average (715±30 Ma) is considerably lower than the probable intrusion age. On the other hand, biotite K-Ar ages are higher (average 893 Ma). This apparent discrepancy is explained by the incorporation of excess Ar during emplacement, which still is present in minerals (e.g. biotite) with closure temperatures above the temperature of the M4 phase of metamorphism, but lost from other minerals (e.g. feldspars) with a closure temperature below the M4-temperature. A remarkable similarity exists between the Hunnedalen monzonorites (c. 835 Ma) and other monzonorites in Rogaland-Vest Agder, especially the chilled margins of the Hidra monzonorite and of the Bjerkreim-Sokndal layered intrusion (c. 930 Ma). This similarity in both mineralogy and chemistry (major and trace elements, element ratios, REE patterns, spidergrams, Eu/Eu* anomalies and Sr(i) values) supports a genetic relationship.
Subsections:Igneous petrology
Subjects:Absolute age; Alkali metals; Alkaline earth metals; Caledonian Orogeny; Dates; Dikes; Geochemistry; Intrusions; Isotope ratios; Isotopes; K/Ar; Lower Paleozoic; Magmas; Magmatism; Major elements; Metals; Metamorphism; Neoproterozoic; Paleozoic; Post-tectonic processes; Precambrian; Proterozoic; Radioactive isotopes; Rb-87/Sr-86; Rb/Sr; Rubidium; Sampling; Sm/Nd; Spectra; Stable isotopes; Strontium; Trace elements; Upper Precambrian; X-ray fluorescence spectra; Europe; Norway; Scandinavia; Western Europe; Egersund Anorthosite; Hunnedalen dike swarm; Southwestern Norway
Coordinates:N575500 N590000 E0080500 E0050000
Abstract Numbers:02M/978
Record ID:1999045215
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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