Ilímaussaq ‘en miniature’: closed-system fractionation in an agpaitic dyke rock from the Gardar Province, South Greenland (contribution to the mineralogy of Ilímaussaq no. 117)

Abstract

Based on petrography, mineral chemistry, and petrology, the physico-chemical evolution of an agpaitic dyke was found to be very similar to that of the neighbouring Ilṍmaussaq complex. Various mineral assemblages were used to reconstruct the crystallization conditions of the dyke rock for different stages during cooling. The early magmatic phenocryst assemblage is alkali feldspar + nepheline + augite + olivine + magnetite and indicates liquidus temperatures of~850ºC, silica activities of~0.5, and oxygen fugacities of FMQ -1.5 to -3. The groundmass assemblage albite + microcline + nepheline + sodalite + arfvedsonite + aegirine + aenigmatite + astrophyllite indicates lower temperatures of between 600 and 450ºC, at silica activities of 0.25, and oxygen fugacities around the FMQ buffer. Amphibole composition strongly responds to fluorite saturation and proves crystallization occurred in a system closed to oxygen. Late-stage hydrothermal conditions are indicated by the conversion of nepheline and sodalite to analcime and the growth of aegirine on arfvedsonite. These late-stage reactions are constrained to temperatures of <300ºC, water activities of between 0.5 and unity, and oxygen fugacities above MH. The dyke has to be regarded as a small equivalent of the larger Ilṍmaussaq complex, in which identical differentiation processes proceeded at a scale very different in terms of magma volume and cooling times.

Hydrothermal Alteration of Eudialyte-Hosted Critical Metal Deposits: Fluid Source and Implications for Deposit Grade

“…Conversely, the positive εNd(t) value ( Figure 7b) and off-isochron position of the zircon-bearing pseudomorphs (Figure 8) suggest isotopic disturbance occurred and are consistent with an external origin for the alteration fluid. Although we do not have additional Nd isotopic data for the hybrid kakortokite, positive εNd(t) values (>1) have not previously been recorded for any of the Ilímaussaq intrusive units [51] and have only been reported for overlying basalts of the Eriksfjord Formation [57] and a single micro-kakortokite dyke [58]. We thus infer that the positive εNd(t) value of the zircon-type pseudomorphs reflects modification of the Nd isotope system through external fluid interaction.…”

Astrophyllite-group minerals from the Ilímaussaq complex, South Greenland (contribution to the mineralogy of Ilímaussaq no. 123)

“…group minerals crystallized at Ilímaussaq, it is informative to compare the conditions under which astrophyllite and niobophyllite crystallized in the pegmatites with those estimated for astrophyllite precipitation in an agpaitic dyke which intruded basement granites close to Ilímaussaq (Larsen and Steenfelt, 1974) and which has been taken by Marks and Markl (2003) to represent an 'en miniature' analogue for the complex. The groundmass assemblage albite + microcline + nepheline + sodalite + arfvedsonite + aegirine + aenigmatite + astrophyllite indicated to Marks and Markl (2003) temperatures of between 600 and 450ºC and f O 2 around the FMQ buffer.…”

Rare Earth and Rare Metal Elements Mobility and Mineralization During Magmatic and Fluid Evolution in Alkaline Granite System: Evidence from Fluid and Melt Inclusions in Baerzhe Granite, China

“…Previous studies have indicated that these elements are enriched in highly fractionated and volatile‐rich residual magmas (Salvi & Williams‐Jones, ; Schmitt et al ., ). In addition, the late stage hydrothermal activities may have a great effect on the movement and deposition of these elements (Salvi et al ., ; Marks & Markl, ). Melt inclusions, which preserve portions of primary melt, are the ideal samples to indicate the mobility and mineralization of ore forming elements (Kovalenko et al ., ).…”