Petrogenesis of Ultramafic Lamprophyres from the Terina Complex (Chadobets Upland, Russia): Mineralogy and Melt Inclusion Composition

Abstract

The mineral composition and melt inclusions of ultramafic lamprophyres of the Terina complex were investigated. The rocks identified were aillikites, mela-aillikites, and damtjernites, and they were originally composed of olivine macrocrysts and phenocrysts, as well as phlogopite phenocrysts in carbonate groundmass, containing phlogopite, clinopyroxene and feldspars. Minor and accessory minerals were fluorapatite, ilmenite, rutile, titanite, and sulphides. Secondary minerals identified were quartz, calcite, dolomite, serpentine, chlorite, rutile, barite, synchysite-(Ce), and monazite-(Ce). Phlogopite, calcite, clinopyroxene, Ca-amphibole, fluorapatite, magnetite, and ilmenite occurred as daughter-phases in melt inclusions. The melt inclusions also contained Fe–Ni sulphides, synchysite-(Ce) and, probably, anhydrite. The olivine macrocrysts included orthopyroxene and ilmenite, and the olivine phenocrysts included Cr-spinel and Ti-magnetite inclusions. Crystal-fluid inclusions in fluorapatite from damtjernites contain calcite, clinopyroxene, dolomite, and barite. The data that were obtained confirm that the ultramafic lamprophyres of the Terina complex crystallized from peridotite mantle-derived carbonated melts and they have not undergone significant fractional crystallization. The investigated rocks are considered to be representative of melts that are derived from carbonate-rich mantle beneath the Siberian craton.

Compositional Variations of Spinels from Ultramafic Lamprophyres of the Chadobets Complex (Siberian Craton, Russia)

“…Phlogopite phenocrysts (up to 15-20 vol.%) have a zonal structure; often magnesian cores of #Mg 0.78-0.73 are overgrown with a ferrous rim of #Mg 0.47-0.12. The mineral composition of mela-aillikites differs from the aillikites in the presence of hypidiomorphic elongated clinopyroxene grains: diopside with an aegirine minal (up to 10%) [40]). The groundmass of UMLs contains predominant mineral phases of calcite, dolomite, and phlogopite, as well as disseminated micrograins of spinels, ilmenite, Ti-magnetite, apatite, and rare sulfides (chalcopyrite, pentlandite, etc.…”

“…Macrocrysts are represented by idiomorphic grains of olivine (up to 20%), completely or partially replaced by serpentine and calcite. In olivines, the #Mg varies within grains, individual crystals, and their microcomponent compositions [40]. Phlogopite phenocrysts (up to 15-20 vol.%) have a zonal structure; often magnesian cores of #Mg 0.78-0.73 are overgrown with a ferrous rim of #Mg 0.47-0.12.…”

Polygenic Nature of Olivines from the Ultramafic Lamprophyres of the Terina Complex (Chadobets Upland, Siberian Platform) Based on Trace Element Composition, Crystalline, and Melt Inclusion Data

“…The alkaline silicate rocks of the Terina complex are represented by aillikites, melaaillikites, and damtjernites [23,38], and they can be classified as intrusive coherent intrusive lamprophyres, according to [39], with a varying content of phenocrysts (20-60 modal%). The phenocrysts are mainly olivine (20-40 modal %), less commonly phlogopite, clinopyroxene, or magnetite.…”

“…The mineral composition of the rocks is characterized by fine-grained aggregates of clinopyroxene, biotite, dolomite, calcite, apatite, and in some cases, potassium feldspar, as well as ore minerals, which are present at 15-17 modal% (Cr-spinels, Ti-magnetite, magnetite, perovskite, rarely ilmenite and sulfides). The petrography and mineralogy of the Terina UMLs are described in more detail in another study [38].…”

“…It is impossible to correctly calculate its hematite component due to the small size of ilmenite. For more detail on composition of minerals from melt inclusions, see [38]. Some inclusions contain magnetite (TiO 2 up to 2 wt %) or ilmenite, pentlandite, and pyrrhotite (Figure 7k).…”