Calcite and dolomite in intrusive carbonatites. I. Textural variations

Abstract

Carbonatites are nominally igneous rocks, whose evolution commonly involves also a variety of postmagmatic processes, including exsolution, subsolidus re-equilibration of igneous mineral assemblages with fluids of different provenance, hydrothermal crystallization, recrystallization and tectonic mobilization. Petrogenetic interpretation of carbonatites and assessment of their mineral potential are impossible without understanding the textural and compositional effects of both magmatic and postmagmatic processes on the principal constituents of these rocks. In the present work, we describe the major (micro)textural characteristics of carbonatitic calcite and dolomite in the context of magma evolution, fluid-rock interaction, or deformation, and provide information on the compositional variation of these minerals and its relation to specific evolutionary processes.

Carbonate-rich dyke in Roztoky Intrusive Complex - an evidence for carbonatite magmatism in the Eger Rift?

“…It rather originated from a continuous process of melt injectioncooling -kinetic fragmentation -annealing by new portions of melt. Similar textures, albeit with larger grain-size contrasts, were found for carbonatites from Kaiserstuhl (Germany) and Kerimasi (Tanzania), and were interpreted in terms of reincorporated autholiths of a disaggregated carbonatite cumulate (Chakhmouradian et al 2016). The intimate calcite-dolomite intergrowths document further evolution through sub-solidus immiscibility re-equilibration (Fig.…”

“…6b). The overall low REE content in comparison with most carbonatites worldwide (Jones et al 2013) probably reflects their low abundance in the carbonatite source; rather low total REE contents of c. 80 ppm were also reported from other intrusive carbonatites (e.g., Chakhmouradian et al 2016). Thorium/uranium, Zr/Hf and Nb/Ta ratios are elevated in the CF over the NCF (Tab.…”

“…1. This facies does not contain any open cracks or joints filled with idiomorphic crystals resulting from hydrothermal precipitation (Chakhmouradian et al 2016), or any heterogenic fragments (alkaline rocks or xenoliths), all of which would indicate explosive, or kinetic brecciating of solidified rock.…”

“…The texture of all three types indicates several phases of magmatic origin. The annealed texture with the absence of wall-rock fragments (Cretaceous sedimentary xenoliths and alkaline igneous rocks), or open cracks with late mineralization suggests that this rock does not represent breccia (e.g., McPhie et al 1993;Chakhmouradian et al 2016). It rather originated from a continuous process of melt injectioncooling -kinetic fragmentation -annealing by new portions of melt.…”

Primary and secondary textures of dolomite in Eppawala carbonatites, Sri Lanka: implications for their petrogenetic history

“…Magnesian calcites can be formed in both carbonatites (Carpenter 1967;Zaitsev and Polezhaeva 1994;Pitawala and Lottermoser 2012;Chakhmouradian et al 2016) and high-grade marbles (Van der Veen 1965;Kretz 1988;Mposkos et al 2006). However, the analyzed magnesian calcites are rich in Sr compared to marbles found in the WC as well as elsewhere in the country (Pitawala et al 2003;Halama et al 2005).…”