Mineralogy of volcanic calciocarbonatites from the Trig Point Hill debris flow, Kerimasi volcano, Tanzania: implications for the altered natrocarbonatite hypothesis

Abstract

A major debris flow, the Trig Point Hill flow, originating from Kerimasi volcano (Tanzania) contains numerous blocks of extrusive/pyroclastic carbonatites similar to those exposed at the rim of the currently inactive crater. The blocks of calcite carbonatite consist of: (1) large clasts of corroded and altered coarse grained calcite; (2) primary prismatic inclusion bearing phenocrystal calcite; and (3) a matrix consisting primarily of fine-grained prismatic calcite. The large clasts are inclusion free and exhibit a ‘corduroy-like’ texture resulting from solution along cleavage planes. The resulting voids are filled by brown Fe–Mn hydroxides/oxides and secondary calcite. The prismatic or lath-shaped phenocrystal calcite is not altered and contains melt inclusions consisting principally of primary Na–Ca carbonates which contain earlier-formed crystals of monticellite, periclase, apatite, Mn–Mg-magnetite, Mn–Fe-sphalerite and Nb-perovskite. Individual Na–Ca carbonate inclusions are of uniform composition, and the overall range of all inclusions analysed (wt.%) is from 28.7 to 35.9 CaO; 16.7–23.6 Na2O; 0.5–2.8 K2O, with minor SO3 (1.1–2.2) and SrO (0.34–1.0). The Na–Ca carbonate compositions are similar to that of shortite, although this phase is not present. The Na–Ca carbonates are considered to be primary deuteric phases and not secondary minerals formed after nyerereite. Monticellite shows limited compositional variation and contains 2–4 wt.% MnO and 12 wt.% FeO and is Mn-poor relative to monticellite in Oldoinyo Lengai natrocarbonatite. Periclase is Fe-bearing with up to 13 wt.% FeO. Spinels are Cr-free, Mn-poor and belong to the magnetite–magnesioferrite series in contrast to Mn-rich spinels of the magnetite–jacobsite series occurring in Oldoinyo Lengai natrocarbonatite. The matrix in which the ‘corduroy’ clasts and phenocrystal calcite are set consists of closely packed small prisms of calcite lacking melt inclusions, with interstitial fine-grained apatite, baryte, strontianite and minor fluorite. Pore spaces are filled with secondary Mn–Fe hydroxides/oxides, anhydrite and gypsum. The hypothesis that flow-aligned calcite in volcanic calciocarbonatites from Kerimasi, Tinderet, Homa and Catanda is altered nyerereite is discussed and it is considered that these calcite are either primary phases or altered melilite. The nyerereite alteration hypothesis is discussed with respect to the volumetric and compositional aspects of pseudomorphism by dissolution–precipitation replacement mechanisms. This study concludes that none of the volcanic calciocarbonatites containing flow-aligned calcite phenocrysts are altered natrocarbonatite.

Origin of alkali-rich volcanic and alkali-poor intrusive carbonatites from a common parental magma

“…As our data imply that: (1) alkaline phases must have abundantly crystallized during at least the intercumulus stage of formation and (2) present granular and anhedral calcite is not bona fide primary magmatic, but rather formed during near-solidus or subsolidus recrystallization 27 , 46 , the replacement of the alkaline minerals by calcite (and minor dolomite) provides a feasible explanation for the studied cases. High plasticity and extensive recrystallization of calcite at depths did not allow for development of significant porosity of the present rocks, which has been claimed to be inevitable during modification of extrusive carbonatites at atmospheric pressure 61 . These post-magmatic transformations could also be responsible for chemical re-equilibration of the minerals and, thus overprint any chemical evidence for “primary” and “secondary” generations (Fig.…”

“…The reality of such process has been proved in examples of the modern Oldoinyo Lengai lavas 57 , 58 and widely applied to other extrusive and intrusive carbonatites 13 , 19 , 59 , 60 . However, another hypothesis contends that in most carbonatites, calcite is a primary and dominant magmatic phase, whereas alkaline minerals are rare, particularly in intrusive carbonatites of cumulative origin 25 , 61 . As our data imply that: (1) alkaline phases must have abundantly crystallized during at least the intercumulus stage of formation and (2) present granular and anhedral calcite is not bona fide primary magmatic, but rather formed during near-solidus or subsolidus recrystallization 27 , 46 , the replacement of the alkaline minerals by calcite (and minor dolomite) provides a feasible explanation for the studied cases.…”

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

“…5c and 5d of Guzmics et al (2011) cannot represent any realistic parental melt. Interestingly, similar assemblages of minerals that occur as inclusions in calcite in volcanic carbonatites at Kerimasi cannot represent ‘melt inclusions’ (Mitchell & Dawson, 2021). For an as yet unexplained reason, unambiguous melt inclusions containing quenched silicate glass co-existing with quenched Na–Ca carbonate are found only in nepheline from Oldoinyo Lengai combeite wollastonite nephelinite (Mitchell, 2009) and Kerimasi afrikandite (Guzmics et al 2015).…”

Mineralogy and petrology of alkaline rocks and carbonatites: Celebrating the life and work of Gregory Yu. Ivanyuk (1966–2019)

“…They present an improved methodology for the radiometric age determination of perovskite using a non-matrix-matched standard and reach an intriguing conclusion that the Guli complex may have taken longer to form than previously recognised. The paper by Mitchell and Dawson (2021) discusses the applicability of the alteration model, originally proposed for gregorite- and nyerereite-bearing natrocarbonatites from the active Oldoinyo Lengai volcano, to carbonatites at the extinct Kerimasi and Tinderet volcanoes in East Africa. These authors suggest that calcite phenocrysts in these lavas cannot be pseudomorphs after nyereryite or gregoryite.…”