Carbonatites and Pseudocarbonatites
Carbonatites and pseudocarbonatites are considered in a comparative aspect. The latter include rocks of a substantially calcite composition, but of different genesis – metamorphites (marbles, calciphyres), metasomatites (including calcareous skarns), hydrothermalites, metamorphosed sedimentary rocks (marls, limestones), which some researchers are trying to convert into carbonatites. Carbonatites, unlike the pseudocarbonatites, are often characterized by localization in specific tectonic structures in shields and platforms (deep faults, rifts), paragenetic relationships with alkaline rocks of various compositions (varying from ultrabasic-alkaline to alkaline-granites), multi-stage formation and intrusive-magmatic genesis, which is confirmed by (1) the presence of primary melt inclusions in their rock-forming minerals, (2) geological and petrographic data, (3) results of experimental studies of petrological systems close to natural ones. The compared groups of rocks are also significantly distinguished in geochemical peculiarities of calcite – by contents of typomorphic trace elements and isotopic composition of strontium, oxygen, and carbon. Among these elements, the leading diagnostic role belongs to strontium, barium and rare earths, which are more concentrated in the mineral of carbonatites than in pseudocarbonatites. Data on the isotopic composition of strontium, oxygen, and carbon indicate that carbonatites and their constituent calcites correspond to products of mantle melting and, in the absence of secondary alterations, do not contain signs of interaction with the material of host rocks, while, on the contrary, isotopic characteristics of pseudocarbonatite calcite, confirm their crustal source.