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Monzonorites from Rogaland (Southwest Norway): a series of rocks coeval but not comagmatic with massif-type anorthosites

par Duchesne, Jean-Clair;Wilmart, Edith;Demaiffe, Daniel ;Hertogen, Jan
Référence Precambrian research, 45, 1-3, page (111-128)
Publication Publié, 1989-11

Article révisé par les pairs

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Résumé :

Monzonorites, members of the anorthositic suite of rocks, are usually considered as residual after the formation of massif-type anorthosites. In Rogaland (S.W. Norway) they occur as large dykes, intrusions and chilled margins to differentiated massifs, emplaced during and soon after the main anorthosite massifs, in granulite facies conditions. Ti, P and Fe are enriched in monzonorites and steadily decrease towards quartz mangerites, the FeO/FeO+MgO ratios varying slightly during alkali enrichment (Bowen trend). Trace element spidergrams show deep troughs in Rb, Th, Nb-Ta, Sr, Zr-Hf and Ti. The REE slightly decrease in the evolution with (La/Yb)N ratios about 9 and neutral to positive Eu anomalies. Several occurrences, however, show highly contrasted trace element features. Sr isotope ratios (ISr) show a wide interval of variation (0.704-0.710) between the various dykes and intrusions without any correlation with the elements indicative of crustal contamination. In the Tellnes main dyke, evolution towards acidic rocks occurs without contamination and variation in ISr. Fractional crystallization with subtraction of apatite-bearing noritic cumulates can account for the major and trace element evolution from monzonorite to quartz mangerite, but is unable to explain the large differences between occurrences. It is concluded that monzonorites cannot be comagmatic (though coeval) with massif-type anorthosites. They result from the crystallization of distinct magma batches, possibly formed through partial melting of basic to intermediate rocks in the lower crust. © 1989.