Anders Lindh
Professor emeritus
Strong, localised country-rock contamination and partial homogenisation of a mafic magma: An example from west central Sweden
Author
Summary, in English
A dolerite sill cutting slightly older basalt in west-central Sweden shows a strong chemical variation (54%<SiO2<73%) within a restricted area (<100x100 m(2)). The linear correlation among almost all elements is extremely high; in addition, epsilon(NdT) is strongly correlated with the SiO2 content. Least-square hyperbolic-ratio and three-element ratio modelling (common denominator) suggests that most of the chemical variation is explained by mixing and/or micro-mingling. In all, we test 407 hyperbolas, of which 402 are fitted. The five ratio pairs, which could not be fitted to a hyperbola using a least-square fitting procedure, have the ratio Th/Eu in common. Testing the goodness of fit is problematic for hyperbolic distributions; for comparing purposes we 4 sum the distances to chords approximating the hyperbola. Mobile and immobile elements behave similarly, suggesting that no A elements are lost or added from outside the system. The data suggests that already the most mafic of the analysed rocks is a mixture of the 'normal' dolerite and a siliceous crustal rock. A mafic magma intruded into the base of the crust, where it fractionated resulting in a decreased Mg number. The magma was then contaminated with country rocks in an intermediate magma chamber due to country rock melting; during mixing/mingling almost no fractionation took place. The contaminated rock suggests the presence of a fluid phase. This was probably a prerequisite for country-rock melting. Enrichment in some incompatible elements suggests that besides major mixing/mingling a thermochemical separation process has affected the most felsic rock enriching it in light rare earths and Zr.
Department/s
- Lithosphere and Biosphere Science
Publishing year
2006
Language
English
Pages
212-228
Publication/Series
Lithos
Volume
86
Issue
3-4
Document type
Journal article
Publisher
Elsevier
Topic
- Geology
Keywords
- major elements
- contamination
- trace elements
- mixing
- mingling
- ratio correlation
- hyperbolic modelling
- Sm-Nd isotopes
- dolerite
Status
Published
ISBN/ISSN/Other
- ISSN: 0024-4937