Abstract

Halogen abundances in apatite, biotite, amphibole, titanite, and other halogen-rich minerals (sodalite and eudialyte) from ultramafic pyroxenites and glimmerites, shonkinites, and gabbroic, monzonitic, and syenitic rocks of the alkaline to peralkaline Tamazeght complex (Morocco) are presented. Apatite, biotite, and amphibole show the highest F contents in the most evolved rocks, which also contain interstitial fluorite (restricted in syenites). The Cl contents of apatite, biotite, and amphibole are highest in the intermediate rocks. Sodalite (Cl-rich) is equally common in the evolved rocks, although Cl contents of apatite, biotite, and amphibole are low. We suggest that the early magmatic crystallization of sodalite causes a strong depletion of Cl in the remaining melt, resulting in the subsequent crystallization of Cl-poor apatite, biotite, and amphibole. Late-magmatic eudialyte in some of the rocks may indicate a late-stage re-enrichment of Cl.

The Br contents of biotite, amphibole, eudialyte, and sodalite reveal that Cl-rich minerals generally contain high Br. The Cl/Br ratios of individual minerals are, however, highly variable. This suggests that besides melt composition, other factors, such as crystal-melt partitioning and fluid mobility, may influence Cl/Br ratios in igneous minerals, resulting in potentially strong fractionation of Cl from Br in minerals.

Estimation of F and Cl contents in alkaline melts suggests that primitive ultramafic melts are poorer in Cl than evolved melts, whereas their F contents are relatively similar. The low apparent Cl contents of the syenitic melts, calculated based on apatite compositions, may reflect the halogen abundance during late-magmatic conditions, as the presence of early magmatic sodalite indicates Cl-rich syenitic melt compositions.

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