The Ilímaussaq Complex, South Greenland, is a layered intrusion dominated by silica-undersaturated, highly peralkaline and agpaitic syenites that crystallized in the sequence Roof Series (from the roof downwards), Floor Series (from the floor upwards), and Intermediate Series (sandwiched between the Roof and Floor Series). In the Ilímaussaq, minerals belonging to the solid solution between rinkite-(Ce) [(Ca3Ce)Na(NaCa)Ti(Si2O7)2(OF)F2] and nacareniobsite-(Ce) [Ca3(Ce)Na3Nb(Si2O7)2(OF)F2] are present as intercumulus grains over the entire stratigraphy. We combine new chemical data measured by electron microprobe for rinkite-(Ce)–nacareniobsite-(Ce) solid solution minerals, clinopyroxene, and amphibole with detailed petrography of the studied samples.

One pegmatitic lithology in the Roof Series (naujaite pegmatite) yields strongly zoned rinkite-(Ce)–nacareniobsite-(Ce) solid solution minerals and clinopyroxene. We argue that the naujaite pegmatites crystallized from fluid-rich magma that became isolated from the main magma body and that the fractionation trends observed in the naujaite pegmatites are equivalent to those observed in the bulk intrusion. A detailed petrographic study of the naujaite pegmatites allows us to discuss the relative stability of rinkite-(Ce) and other Ti-minerals in the Ilímaussaq and the parameters controlling the rin# [calculated as (Ti+Zr)/(Ti+Zr+Nb)]. In the naujaite pegmatites, crystallization without eudialyte-group minerals occurs from relatively low-Zr melts and creates zonation patterns with high-rin# cores (rin# = 0.6–0.7) and low-rin# overgrowths zones (rin# = 0.2–0.3). In contrast, crystallization of early eudialyte-group minerals from Zr-rich melts leads to intermediate rin# in the core (rin# = 0.3–0.6) relative to later overgrowth zones (rin# = 0.7–0.9 and 0.2–0.4, respectively). Mass balance considerations and chemical trends in co-crystallizing clinopyroxene indicate that the rin# is highly influenced by the Na/Ca and Nb/Ti ratios in the system.

Combining our new data with literature data shows that rin# decreases with fractionation in the Ilímaussaq magma chamber from high values in the Roof Series (rin# = 0.66–0.81 in border pegmatite, rin# = 0.88–0.90 in pulaskite pegmatite, rin# = 0.56–0.63 in non-pegmatitic naujaite) over intermediate values (rin# = 0.63–0.75) in the Floor Series, to low values (rin# = 0.18–0.35) in the Intermediate Series. These systematic changes are consistent with the overall fractionation trend in the Ilímaussaq and suggest that studies of rinkite-(Ce)–nacareniobsite-(Ce) solid solution mineral compositions may supplement petrologic studies of peralkaline rocks based on eudialyte-group minerals, especially in F-rich environments where rinkite-(Ce)–nacareniobsite-(Ce) solid solution minerals crystallize instead of eudialyte-group minerals.

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