Petrology of ijolite series rocks from the Prairie Lake (Canada) and Fen (Norway) alkaline rock-carbonatite complexes

Savard, Johnathan; Mitchell, Roger H.

doi:10.1016/j.lithos.2021.106188

Cited by 10 publications

(2 citation statements)

References 39 publications

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“…Recent hypotheses for the emplacement of magmas in small volume sub-volcanic environments have proposed that large continuously fractionating magma chambers do not exist (Emeleus & Troll, 2014;Floess et al 2019;Hepworth et al 2020;Savard & Mitchell, 2021). Following these concepts, the mineralogical variations of the Type 1-3 lavas can be explained by fractional crystallization of many individual batches of magmas derived from a common source coupled with magma mixing as a result of intrusion of magma into crystal-melt mushes and/or entrainment of crystals derived from disaggregation of pre-existing fully crystallized melts; i.e.…”

Section: Discussionmentioning

confidence: 99%

Petrology and Sr–Nd isotope geochemistry of Mosonik: a polygenetic phonolitic nephelinite–phonolite volcano located in the North Tanzanian Divergence of the East African Rift

Mitchell

Dawson

2022

Geol. Mag.

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Mosonik, a 3.25 Ma extensively dissected stratovolcano located in the North Tanzanian Divergence of the East African Rift, consists predominantly of phonolite and three types of phonolitic nephelinite distinguished by the presence or absence of amphibole or garnet antecrysts and differing populations of complexly zoned antecrystal and phenocrystal pyroxenes. The antecryst–phenocryst assemblage is typical of hybrid lavas derived by magma mixing. Compositional data are given for all major minerals. Owing to the high modal proportions (30–60 vol. %) of antecrysts and phenocrysts of pyroxene and nepheline plus the hybrid character of the lavas, bulk-rock compositions do not represent those of the parental liquids. Thus, assimilation–fractional crystallization modelling of the bulk-rock major- and trace-element abundances is inappropriate and an unevolved parental magma cannot as yet be defined. Sr–Nd isotopic data for Mosonik and other Older Extrusive Series rocks suggest derivation by partial melting of ancient metasomatized lithospheric mantle with mixing of Sr and Nd from two sources coupled with minor lower crustal contamination, melting being induced by the plume currently impinging on the Tanzanian craton, and representing the initial interaction of the plume with the cratonic lithosphere. In contrast, the Younger Extrusives, as exemplified by Oldoinyo Lengai nephelinite–carbonatite volcanism, could be derived from this ancient metasomatized lithospheric mantle plus a recent plume-derived asthenospheric component and no contamination by crustal material. The isotopically and genetically distinct Natron–Engaruka melilitites are considered to represent direct adiabatic melting of the Tanzanian plume without lithospheric contributions. Carbonatites and melilite-bearing nephelinites also occur at Mosonik but are not considered in this study as they are only a very minor volumetric component of the volcano.

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Section: Discussionmentioning

confidence: 99%

Petrology and Sr–Nd isotope geochemistry of Mosonik: a polygenetic phonolitic nephelinite–phonolite volcano located in the North Tanzanian Divergence of the East African Rift

Mitchell

Dawson

2022

Geol. Mag.

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“…However, recent research (Emeleus and Troll 2014; Hepworth et al 2020) has questioned this hypothesis, and it is now proposed that the layered suite originated from repeated small sill-like intrusions into previously intruded material and that a magma chamber did not exist. With respect to carbonatite complexes, Savard and Mitchell (2021) have concluded that there is no geological evidence for the presence of a large-scale magma chamber at the Prairie Lake complex, and that ijolites and other rocks are formed by the crystallization of repeated injections of small batches of ijolite-forming magma into consolidated or partially-crystallized previously emplaced batches of magma of similar, but not identical, composition. Intrusion of magma into previously consolidated rocks can result in subsolidus recrystallization of the latter to form annealed ijolites, whereas intrusion into partially-consolidated material can result in filter pressing and mobilization of residual fractions such as carbonatite-forming liquids.…”

Section: Critique Of the Geological Evidence Offered By Advocates Of ...mentioning

confidence: 99%

The genesis of calcite and dolomite carbonatite-forming magma by liquid immiscibility: a critical appraisal

Gittins,

Mitchell

2023

Geol. Mag.

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Liquid immiscibility has become the preferred mode of genesis for the carbonatite rocks, which commonly, but not exclusively, accompany silicate rocks in alkaline-rock complexes. This concept has been universally based on the presumption that nephelinitic and phonolitic magmas can evolve to a stage where two conjugate immiscible liquids separate. It is assumed that these two liquids separate quickly, or even instantaneously, into discrete bodies of magma capable of being intruded or extruded with subsequent independent crystallization. Supporting evidence generally given is: alleged consanguinity as discrete occurrence of the two rock types; similarity of radiogenic isotope ratios; trace element contents similar to those predicted from experimentally derived partition coefficients. We do not accept that a general case for liquid immiscibility has been demonstrated; although we do accept that silicate and carbonate liquids are inherently immiscible, we maintain that they are not conjugate in a petrogenetic context. We have reviewed and critically examined the experimental data purporting to establish liquid immiscibility and find that when applied to natural rocks, they are based on inappropriate experimental designs, which are not relevant to the genesis of calcite or dolomite carbonatites, although they might have some relevance to Oldoinyo Lengai nyerereite–gregoryite lavas. The design of these experiments guarantees immiscibility and ensures that the carbonate liquids formed will be calcitic or sodium-rich. We dispute the validity of comparing the trace element contents of natural rocks, which in many instances do not represent liquid compositions, to experimentally determine partition coefficients. We consider that experimental design inadequacies, principally assuming but not proving, that the liquids involved are conjugate, indicate that these coefficients are merely an expression of the preference of certain elements for particular liquids, regardless of how the liquids formed. Proof of consanguinity in alkaline complexes requires more accurate age determinations on the relevant rock types than has generally been the case, and in most complexes, consanguinity can be discounted. We dispute the contention that melt inclusions represent parental melts, although they might elucidate the character of magmas undergoing fractional crystallization from magmatic to carbothermal stages. Radiogenic isotope data are shown to be too widely variable to support a case for liquid immiscibility. We address the contention that calcite cannot crystallize from a dolomitic liquid formed by direct mantle melting, and must therefore have crystallized from a calcite carbonate liquid generated by liquid immiscibility, and demonstrate that it is an unsupported hypothesis as calcite can readily crystallize from dolomitic liquids. We observe that, because immiscible dolomite liquids have never been produced experimentally, the liquid immiscibility proposition could at best be applied only to calcite carbonatites, thus leaving unexplained the large number of dolomite carbonatites and those of either type, which are not accompanied by alkaline silicate rocks. The assumed bimodality of alkaline-rock carbonatite complexes is considered to be fallacious and no actual geological or petrographic evidence for immiscibility processes is evident in these complexes. Several examples of alkaline rock carbonatite complexes for which immiscibility has been proposed are evaluated critically and shown to fail in attempts to establish them as exemplifying immiscibility. We conclude that no actual geological or experimental data exist to establish liquid immiscibility being involved in the genesis of calcite or dolomite carbonatite-forming magmas.

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PL57 garnet as a new natural reference material for in situ U–Pb isotope analysis and its perspective for geological applications

Mitchell

et al. 2022

Contrib Mineral Petrol

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Petrology of ijolite series rocks from the Prairie Lake (Canada) and Fen (Norway) alkaline rock-carbonatite complexes

Cited by 10 publications

References 39 publications

Petrology and Sr–Nd isotope geochemistry of Mosonik: a polygenetic phonolitic nephelinite–phonolite volcano located in the North Tanzanian Divergence of the East African Rift

Petrology and Sr–Nd isotope geochemistry of Mosonik: a polygenetic phonolitic nephelinite–phonolite volcano located in the North Tanzanian Divergence of the East African Rift

The genesis of calcite and dolomite carbonatite-forming magma by liquid immiscibility: a critical appraisal

PL57 garnet as a new natural reference material for in situ U–Pb isotope analysis and its perspective for geological applications

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