Apatite in the Ilímaussaq alkaline complex: Occurrence, zonation and compositional variation

Abstract

Apatite from the augite syenite and the roof sequence of layered nepheline syenites (pulaskite, foyaite, sodalite foyaite and naujaite) of the Ilímaussaq alkaline complex has been analysed by electron microprobe for Ca, P, Na, Si, selected rare earth elements (REE), F and Cl. In the augite syenite, chilled facies and middle zone, euhedral apatite crystals are relatively abundant as inclusions in other liquidus phases. The REE content is ca. 1 wt.% REE 2O 3. The same level is found in interstitial apatite in the coarse-grained innermost facies. REE-enriched intercumulus apatite overgrowths are common and can have up to ca. 18 wt.% REE 2O 3. Apatite is a common phase in the pulaskite and foyaite, whereas it is rare in the sodalite foyaite and naujaite. Based on compositional variation and zoning, it is possible to distinguish four generations of apatite in the layered roof sequence: 1) primary apatite belonging to the high-temperature mineral assemblage. This apatite occurs in the pulaskite, foyaite and sodalite foyaite as oscillatory-zoned crystals, whereas only remnants are found in the naujaite. The REE enrichment is dominated by the Ca 2+ + P 5+ = REE 3+ + Si 4+ substitution and maximum REE 2O 3 exceeds 20 wt.%. 2) irregularly zoned apatite occurs as rim zones on the primary apatite in the foyaite and as the dominant type of apatite in the sodalite foyaite and naujaite. Most irregularly zoned grains are altered primary apatite. During the dissolution and recrystallisation process the composition gradually changed towards a lower REE content. 3) overgrowths of homogeneous apatite on the irregularly zoned apatite in the naujaite. 4) Na- and REE-rich apatites occur in the foyaite, sodalite foyaite and naujaite either as overgrowths on earlier generations of apatite or as individual crystals. Calculated chondrite normalised patterns show that the later generations of apatite are relatively enriched in LREE and depleted in HREE. The most pronounced evolution is found in Na- and REE-rich apatite. Apatite/melt partition coefficients for La, Ce, Nd, Sm, Dy, Y and Yb are calculated for the augite syenite and nepheline syenites of the roof sequence. The coefficients show the characteristic convex shape with a maximum around Nd and Sm. Using the calculated partition coefficients and the chemical composition of the Na- and REE-rich apatite, the REE content of a late aqpaitic melt in the roof sequence is estimated. The shape of the chondrite normalised pattern is similar to that of a late arfvedsonite lujavrite dyke and steeper than the patterns of the primary magma.