Abstract

The ilmenite norite of the Tellnes Fe-Ti deposit in the Aana-Sira massif-type anorthosite is connected to a main dike which varies in composition from monzonorite to quartz mangerite. Fine-grained satellite dikes also occur in the vicinity of the main dike.A number of rock samples have been analyzed for 36 major and trace elements. An Rb-Sr whole-rock isochron (12 samples) gives an age of 930 Ma. Initial Sr isotope compositions of the ilmenite noritc, the main dike, and the satellite dikes are 0.7050, 0.7069, and 0.7100, respectively, thus precluding a comagmatic origin without invoking contamination.The evolution of the main dike can be accounted for by a closed-system fractional crystallization of a monzonoritic magma (that is, without progressive contamination) which gave rise to a series of liquids prior to intrusion. Mass balance calculation permits a two-stage process in which noritic cumulates were subtracted. Trace element behavior is also consistent with such a model.The satellite dikes represent undifferentiated monzonorites, with trace elements and isotopic ratios implying a source material quite different from that of the main dike monzonorite.The orebody is made up of ilmenite norite and small amounts of more plagioclase-rich rocks, which plot on linear trends in variation diagrams. It is interpreted as a noritic cumulate which was enriched in dense, oxide-rich magma before being emplaced as a crystal mush. The parental magma was noritic in composition, unsaturated in apatite, and probably derived from the crystallization of the enclosing massif anorthosite. It is not precluded either that it could have given rise to the main dike monzonoritic liquid after being contaminated by lower crustal material. Nevertheless, as an alternative, it is not possible to reject two distinct parental magmas for the orebody and the main dike. They would have evolved in two separated magma chambers, before the tapping of these chambers and simultaneous injection of their contents in the dike.The main dike illustrates the process by which a quartz mangerite can derive from a monzonoritic magma, and gives support to a comagmatic origin of charnockitic and monzonoritic rocks.

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