Layering in peralkaline magmas, Ilímaussaq Complex, S Greenland

Hunt, Emma J.; Finch, Adrian A.; Donaldson, Colin H.

doi:10.1016/j.lithos.2016.10.023

Cited by 17 publications

(10 citation statements)

References 38 publications

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“…The repeated nature of macro-rhythmic (or cyclic) units within layered intrusions is typically thought to represent individual pulses of magma that have fractionated into an upwardly evolving sequence of cumulate (e.g. Brown 1956;Tepley and Davidson 2003;Holness 2005;O'Driscoll et al 2007;Brandiss et al 2014;Hunt et al 2017;Latypov et al 2017). The macro-rhythmic units in the ELI have traditionally been linked to the settling of successive phases of olivine, plagioclase, and clinopyroxene upon replenishment into the magma chamber (Brown 1956;Emeleus et al 1996).…”

Section: Introductionmentioning

confidence: 99%

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

Hepworth

Kaufmann

Hecht

et al. 2020

Contrib Mineral Petrol

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The Rum Eastern Layered Intrusion (ELI; Scotland) is an open-system layered intrusion constructed of 16 macro-rhythmic units. Each of the macro-rhythmic units consists of a peridotite base and a troctolite (± gabbro) top, previously attributed to the fractional crystallisation of a single magma batch. This classic paradigm has been challenged, however, with evidence presented for the emplacement of peridotite sills in Units 9, 10, and 14, such as cross-cutting relationships, upward-oriented apophyses, and lateral discontinuities. To test whether the other major peridotites within the ELI represent sills, we have carried out new field, petrographic, and mineral chemical analyses of the peridotites in Units 7, 8 and 9. The peridotites display large-and small-scale cross-cutting relationships with the overlying troctolite, indicative of an intrusive relationship. The peridotites also show large-scale coalescence and lateral spatial discontinuities such that the ELI unit divisions become arbitrary. Harrisite layers and Cr-spinel seams found throughout Units 7, 8, and 9 suggest the peridotites were constructed incrementally via repeated injections of picritic magma. Our observations allow for distinct subtypes of peridotite to be defined, separated by intrusive contacts, allowing for their relative chronology to be determined. Older, poikilitic peridotite, rich in clinopyroxene, is truncated by younger, well-layered peridotite, containing abundant harrisite layers. In addition to the new peridotite subtypes defined in this study, we find strong evidence for laterally oriented metasomatism within clinopyroxene-rich wehrlites at the top of the Unit 8 peridotite. The wehrlites and surrounding peridotites record a complex series of metasomatic reactions that transformed thin picrite sills into clinopyroxene-rich wehrlites without any evidence for the sort of vertical melt movement typically posited in layered intrusions. The observations presented in this study from the ELI cannot be reconciled with the classic magma chamber paradigm and are better explained by the emplacement of composite sills into pre-existing feldspathic cumulate (gabbro or troctolite). The evidence for sill emplacement presented here suggests that the layered complex was constructed by a combination of sill emplacement and metasomatism, forming many of the unusual (often clinopyroxene-rich) lithologies that surround the sills. The broad-scale formation of the layered peridotites via incremental sill emplacement, suggested by the occurrence of upward-oriented apophyses, coalescence, and lateral discontinuity, could be applied to much larger ultramafic intrusions, which might have formed by similar mechanisms.

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Section: Introductionmentioning

confidence: 99%

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

Hepworth

Kaufmann

Hecht

et al. 2020

Contrib Mineral Petrol

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“…Much information has, however, emerged over the past decade and two very different models are advocated for the development of igneous layering in Ilimaussaq, including: (1) A classic closed system evolution with layering caused by gravitational accumulation, and e.g., crystal mat formation [16,26,[28][29][30]; and (2) a crystallization model based on repeated replenishment of the magma chamber [31]. The latter model, however, require that the magmas or mushes evolved along evolutionary trends similar to those observed in Ilimaussaq.…”

Section: Consequences For Layered Agpaitic Complexesmentioning

confidence: 99%

“…Such models relocate observed evolution to a not known or specified feeder chamber. The need for such an unspecified feeder chambers for the Ilimaussaq intrusion rests on petrofabric (CDS) studies, sometimes sharp contacts between repeated cyclic layers, and structural characteristics interpreted as the result of turbulent accumulation of new pulses of inclusion-bearing agpaitic mush in the kakortokites [31]. It is also argued that the classic gravitational settling (or flotation) mechanisms cannot explain the petrofabrics and the repetition of 29 tripartite cycles of black, red, and white kakortokites.…”

Section: Consequences For Layered Agpaitic Complexesmentioning

confidence: 99%

Chemical Composition and Petrogenetic Implications of Eudialyte-Group Mineral in the Peralkaline Lovozero Complex, Kola Peninsula, Russia

Kogarko

Nielsen

2020

Minerals

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Lovozero complex, the world’s largest layered peralkaline intrusive complex hosts gigantic deposits of Zr-, Hf-, Nb-, LREE-, and HREE-rich Eudialyte Group of Mineral (EGM). The petrographic relations of EGM change with time and advancing crystallization up from Phase II (differentiated complex) to Phase III (eudialyte complex). EGM is anhedral interstitial in all of Phase II which indicates that EGM nucleated late relative to the main rock-forming and liquidus minerals of Phase II. Saturation in remaining bulk melt with components needed for nucleation of EGM was reached after the crystallization about 85 vol. % of the intrusion. Early euhedral and idiomorphic EGM of Phase III crystalized in a large convective volume of melt together with other liquidus minerals and was affected by layering processes and formation of EGM ore. Consequently, a prerequisite for the formation of the ore deposit is saturation of the alkaline bulk magma with EGM. It follows that the potential for EGM ores in Lovozero is restricted to the parts of the complex that hosts cumulus EGM. Phase II with only anhedral and interstitial EGM is not promising for this type of ore. Nor is the neighboring Khibiny complex despite a bulk content of 531 ppm of Zr. Khibiny only has interstitial and anhedral EGM. The evolution of the Lovozero magma is recorded in the compositions EGM up through a stratigraphy of 2400 m in Phase II and III of the complex, and distinct in elements like rare earth elements (REE), Sr, Ba, Th, U, Rb, Mn, Fe. The compositional evolution reflects primarily fractional crystallization processes within the magma chamber itself in combination with convective magma flow and layering by precipitation of minerals with different settling velocities. The suggested mechanism for the formation of the EGM deposits is flotation of very small, suspended EGM crystals in the convective magma and concentration below the roof of the magma chamber. Phase III EGM is enriched in total REE (1.3%) and in HREE (Ce/Yt = 8.8) and constitutes a world class deposit of REE in the million tons of Phase III eudialyte lujavrites.

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“…Each of these three sections resembles continuously exposed rock series of several hundreds of meters thickness that allow to relate the various rocks types to each other. The detailed genetic relationships between these three sections have been intensively discussed [9,10,[19][20][21][22].…”

Section: Geology Of the Ilímaussaq Complexmentioning

confidence: 99%

“…Rhythmically layered and coarse-grained rocks (lower layered kakortokites; LLK) are dominant but grade upwards into similar rocks in which rhythmic layering is less well developed (slightly layered kakortokites; SLK). Formation models for this layered sequence range from closed-system formation by crystal settling and mat formation [9,21,23] to open-system models invoking multiple replenishment events and mostly in-situ growth of crystals [22].…”

Section: Floor Sectionmentioning

confidence: 99%

Review on the Compositional Variation of Eudialyte-Group Minerals in the Ilímaussaq Complex (South Greenland)

et al. 2020

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We review the compositional variation of eudialyte-group minerals (EGM) from the Ilímaussaq complex in South Greenland. Investigated samples cover all major rock units and associated pegmatites and aplites. The whole data set (>3000 analyses from >250 samples) exhibits variable XMn (0.1–0.5), REE (0.2–1.7 apfu), Nb (0.1–0.4), and Cl contents (0.4–1.6 apfu). Most EGM compositions are Na-rich (13–15 apfu), while deviations to Na-rich but also to Na-poor compositions occur because of a combination of primary features (peralkalinity, water activity) and secondary alteration. During magma evolution, REE contents in EGM cores generally increase and reach their highest contents in the most evolved rock units of the complex. This points to the moderate compatibility of REE in EGM and a bulk D (cEGM/cmelt) value of <1 during magma differentiation. Chlorine contents in EGM cores continuously decrease, and are lowest at the rims of individual crystals, suggesting a continuous decrease of Cl activity in the magmas by large-scale EGM and sodalite extraction during the orthomagmatic stage and water enrichment during the late-magmatic stage. The overall variations of XMn across stratigraphy are only minor and likely influenced by the co-crystallization of sodic pyroxene and amphibole (c.f. aegirine, arfvedsonite) and local phase proportions. Similarly, Nb and Ti contents are influenced by co-crystallizing aenigmatite, rinkite, and others. Their presence buffers Ti and Nb contents to rather constant and low values, while their absence may cause variable enrichment on a local scale. Very low Sr contents (<0.1 apfu) in magmatic EGM from Ilímaussaq are related to the basaltic nature of the parental magmas of the complex, as large-scale plagioclase fractionation occurred prior to the formation of the Ilímaussaq magmas, effectively removing Sr from the system. This is in line with very strong negative Eu anomalies in EGM from Ilímaussaq. Consistently, Sr contents in EGM from alkaline complexes, for which foiditic parental magmas are assumed, are much higher and, in such cases, negative Eu anomalies are generally absent.

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Layering in peralkaline magmas, Ilímaussaq Complex, S Greenland

Cited by 17 publications

References 38 publications

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

Chemical Composition and Petrogenetic Implications of Eudialyte-Group Mineral in the Peralkaline Lovozero Complex, Kola Peninsula, Russia

Review on the Compositional Variation of Eudialyte-Group Minerals in the Ilímaussaq Complex (South Greenland)

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