Primary textural-structural features of carbonatites and their metamorphic evolution

Abstract

The study of form is the study of its transform on." -Goethe.The genesis of the endogenic carbonatites can be established primarily through a study of the carbonatites themselves, both as rocks and as aggregates of minerals. In doing this, however, one must first prove, on the basis of the factual material, that the textural-structural pattern being studied is primary -that it arose simultaneously with the rock's formation as an aggregate of minerals, and that the conditions of its genesis are recorded in it. -An example of the identification of primary and secondary structures and textures of the rocks is presented below.I distinguish the following stages in the textural and structural evolution of the carbonatites.Formation of the carbonatites -the moment of the appearance of the carbonatites as aggregates of minerals, as geologic bodies, and as rocks. From the physicochemical standpoint, the genesis of the carbonatites may be determined either by the filling of cavities by a moving or standing stagnant solution, or by the injection of a solution-melt into fractured structures, or again by the pseUdomorphotis replacement (metasomatisrn) of 'any substrate. This is the very moment of appearance of the crystals of the minerals that make up the carbonatites, and corresponds to the concept of the rock's genesis. The structural-textural features of the rock that arise in this stage are accordingly considered to be primary.Early metamorphism of the carbonatites (autometarnorphism) may begin immediately after the stage of their genesis, without interruption, and may be caused by magmatogenic heating (activation) of the rock or the presence of intergranular (pore) syngenetic solutions. The main processes characterizing this stage are recrystallization (granulation or, by contrast, increase in grain size) and autometasomatic reactions (for example, serpentinization of the forsterites, martitization of the magnetite, amphibolization of the pyroxenes and the like). This is most probably the stage of destruction of the "protocarbonates," accompanied by the formation of decay structures within the grains (for example, structures of the decomposition of dolomite in the early magnesial calcite) (VanclerVeen, 1965). In this stage the recrystallization of the rock's carbonate mass may so change the form of the boundaries between grains and the sizes of the grains themselves, that even the textural and structural patterns formed in the stage of genesis are also substantially altered. Subsequent superimposed metamorphism, including regional metamorphism, of the carbonatites is due mainly to external factors such as tectogenesis of the surrounding host rocks and the internal stresses arising as a result -regional metamorphism and epigenic superimpositions (the thermal fields of the intrusives, hot springs). The main forms of the alteration of carbonatites in this stage consist in the continuing process of recrystallization (often under "dry" conditions), metamorphic differentiation of the carbonatites with the spatial separation of...

The kinetics of solid-state isotope-exchange reactions for clumped isotopes: A study of inorganic calcites and apatites from natural and experimental samples

“…A relevant question is whether or not the measured ⌬ 47 values reflect the kinetics of isotope-exchange reactions or recrystallization during cooling or later metamorphism/diagenesis. Recrystallization is of concern because carbonate minerals in carbonatites commonly recrystallize, often rapidly, after crystallization (Zhabin, 1971). Such recrystallization is documented to have occurred in samples from Siilinjarvi (Tichomirowa and others, 2006) and has likely occurred in all samples.…”

“…First, apatites are thought to be more resistant to recrystallization in carbonatites than carbonate minerals (Zhabin, 1971), yet we find apatites yield consistently lower apparent equilibrium temperatures (40 -60°C lower) than co-genetic carbonates (table 1 and fig. 3).…”

“…Such recrystallization is documented to have occurred in samples from Siilinjarvi (Tichomirowa and others, 2006) and has likely occurred in all samples. However, much of this recrystallization occurs at elevated temperatures close to the time of original crystallization (Zhabin, 1971). Thus, it is possible that this recrystallization occurs above the nominal apparent equilibrium blocking temperature of the carbonate and allows for internal isotope-exchange reactions to continue during further cooling.…”

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

“…Flow-aligned calcite prisms are common in many calcite kimberlites, melilitites and carbonatites. Typical examples are: the Jos, Nikos and International kimberlites, Canada (Mitchell, 1997b); Kaiserstuhl lapilli tuffs, Germany (Keller, 1989);calcite carbonatite dyke rocks in the Guli and Maimecha-Kotui complexes, Siberia (Zhabin and Cherepivskaya, 1965; Zhabin, 1971); carbonatitic lavas at Catanda, Angola (Campeny et al , 2015); Saltpetre Kop melilitites, South Africa (de Wit, 1975); and hypabyssal dykes at the Prairie Lake complex (this work). In all of these examples the textures are best considered as examples of rheological concentration of primary liquidus calcite as there are no petrographic indications that they might be pseudomorphs after some pre-existing phase.…”

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

“…In addition, they have typically low Fe and high Sr contents, comparable to those in matrix dolomite grains. For that reason, type-5 dolomite could be a result of postrecrystallization processes of the matrix dolomite grains (Grigorev 1961;Zhabin 1971). Type-2 calcite, which has low Mg but high Sr contents, may have formed simultaneously from earlier calcites.…”