The temporal geochemical record of Zealandian alkaline intraplate volcanism reveals a significant geochemical evolution from Cretaceous HIMU (high time-integrated μ = ²³⁸U/²⁰⁴Pb Mantle) end member to Cenozoic HIMU-like volcanism with overall lower ²⁰⁷Pb/²⁰⁴Pb and variable ²⁰⁸Pb/²⁰⁴Pb ratios at a given ²⁰⁶Pb/²⁰⁴Pb ratio. In general, this temporal geochemical evolution has been reconstructed by piecing together volcanism taking place at many different localities covering different but limited age ranges. The Chatham Islands, covering an area of ~800km², represent the only known locality within Zealandia where volcanic activity has taken place nearly continuously over ~85 Ma and record the geochemical evolution from HIMU end member to HIMU-like volcanism. Therefore, the Chatham Islands are a key locality for reconstructing and understanding the geochemical evolution of Zealandia. Numerous models, such as an asthenospheric heritage (e.g. mantle plume) or metasomatic overprint of the SCLM by: 1) subduction zone fluids or 2) interaction with Hikurangi Plateau, have been proposed, but the style of geochemical change (abrupt or progressive), as well as the origin of the HIMU end member and HIMU-like sources remain enigmatic, as is the unique longevity of intraplate volcanism over ~85 Ma at a single locality. Geochemical analysis (major and trace elements, radiogenic Sr-Nd-Pb-Hf isotope ratios and mineral composition) of alkaline lavas and enclosed mantle xenoliths from the Chatham Islands, will be used to reconstruct their spatio-temporal geochemical evolution of HIMU to HIMU-like volcanism on the Chatham Islands and associated volcanism on Zealandia over the last ~85 Ma.

Garnet in alkaline igneous rocks is of interest due to its compositional and textural variability that provides insights into magmatic and hydrothermal processes. This study investigates textures and mineral chemistry of garnets from the Tezhsar Alkaline Complex (Armenia) to constrain their petrogenetic origin by determining whether the garnets are a primary magmatic liquidus phase or whether they have a secondary, subsolidus origin. Element mobility during garnet formation is evaluated, focusing on rare earth elements (REE), for which alkaline igneous rocks are a globally important resource and which are a valuable geochemical tracer to understand the evolution of rock-melt-fluid systems.

In the Tezhsar Alkaline Complex, K-rich plutonic and volcanic rocks occur in concentric units, representing the remnants of a palaeocaldera (Sokół et al., 2018). Garnet occurs in euhedral to subhedral clusters in pegmatitic nepheline syenite and more rarely as phenocrysts in syenites. The calcic garnets have a high Ti content (c. 2-4 wt.% TiO₂), which is typical for garnet in alkaline igneous rocks. Garnet in the pegmatitic nepheline syenite is devoid of inclusions and shows only limited chemical variability, interpreted to reflect crystallization from a melt. In the syenite, garnet is rich in mineral inclusions and is interpreted to reflect a metasomatic origin during late/post-magmatic growth. Trace element data is being acquired to constrain the physicochemical conditions of garnet growth and evaluate REE incorporation into garnet.

Reference: Sokół, K. et al., 2018. Lithos 320-321, 172-191.

Burko is a nephelinitic volcano in the Gregory Rift of the East African Rift System (EARS), situated in northern Tanzania. The rocks are olivine-free and phonolitic nephelinites, deposited as tuffs, agglomerates and lavas that frequently contain plutonic inclusions. Based on geochemical data, a carbonatite-metasomatized mantle source has been assumed for these rocks (Mana et al., 2015).

We present a detailed mineralogical and petrological study of the variably evolved Burko rocks (whole-rock Mg numbers <50), which contain either nepheline + Fe-Ti oxide + perovskite + diopsidic pyroxene or nepheline + Fe-Ti garnet + titanite + hedenbergitic pyroxene ± alkali feldspar assemblages. Such an evolution of relatively oxidized and strongly SiO₂-undersaturated alkaline magmas towards peralkaline compositions may be a common process and is probably controlled by T-a_SiO2-f_O2 changes. Based on a comparison with spatially associated nephelinitic volcanos like Sadiman and Ol Doinyo Lengai, we discuss similarities and differences in their genesis.

Mana, S., Furman, T., Turrin, B. D., Feigenson, M. D., and Swisher, C. C., III, 2015, Magmatic activity across the East African North Tanzanian Divergence Zone: Journal of the Geological Society, v. 172, no. 3, p. 368-389.

The Motzfeldt Centre forms part of the Igaliko Complex: one of the major complexes of the Mesoproterozoic Gardar Igneous Province of Southern Greenland. This syenite hosts a Ta-Nb-Rare Earth Element (REE)-Zr alkaline-silicate roof zone, containing several metals classed as “critical” to the economy by the EU.

This study carries out detailed mineralogical, geochemical, and microtextural analysis of REE-enriched syenite variants from Motzfeldt using BSE-SEM imaging, EPMA, WDS mapping, and RAMAN spectroscopy. These data provide evidence that an aggressive, magmatically derived, F-rich fluid (F₁) altered primary magmatic pyrochlore (P₀), replacing it with mineralogically heterogeneous pseudomorphs formed of secondary pyrochlore (P₁) and several alteration phases. Also found also encrusted on the surface of P₁, these intergrown secondary phases are formed of elements lost from P₀ during alteration including Nb, REEs, F, Ca, Zr. In a second hydrothermal event (F₂), these crusts are “scrubbed” from the surface of P₁, dissolving into and enriching that fluid (F₂) in the elements scavenged from those minerals. The escape structures formed by the fluid F₂ as it exits the system are breccia pipes, cemented by a rock mineralogically reminiscent of carbonatite. Previously interpretations were that these structures were entirely magmatic, but our findings indicate these structures are late-stage hydrothermal in origin. This may be a novel mechanism by which carbonatite-type rocks are formed.

These processes display how early intensive hydrothermal events can prime an ore body for further alteration, through conversion of refractory primary phases into secondary phases more vulnerable to hydrothermal attack.

Alkaline igneous rocks are known to host a variety of rare metal deposits, including HFSEs and REEs. The Iivaara alkaline complex (Finland) and the surrounding fenite aureole show Ti-dominated enrichment of HFSEs carried by titanite and apatite. Mineralogy and textural observation indicate a shallow intrusion level, fast cooling, and steep temperature gradients, as well as expulsion of different fluids. So far, the roles of different fluids in rare metal enrichment in such systems have not been fully understood. New results of a fluid inclusion (FI) study including FI petrography, microthermometry, phase identification using Raman spectroscopy, and quantification of elements by LA-ICP-MS reveal the fluid evolution in the Iivaara alkaline complex. Three stages can be distinguished: magmatic fluid, post-magmatic fluid, and late aqueous fluid. Magmatic fluid is characterized by relatively high salinity, with a dominance of methane in the vapor phase. This fluid type is responsible for the fenitization process as well as transporting some important elements and metals like HFSEs and REEs. The post-magmatic fluid is characterized by relatively low salinity with very low to no methane concentration in the vapor phase. This fluid is responsible for recrystallization of some minerals, especially apatite and the formation of cancrinite as a replacement mineral and as a vein. And the last fluid is an aqueous fluid which is very low in salinity, interpreted as meteoric water influx at the very late stage.