Mineralogy, geochemistry and age of metacarbonate-silicate rocks of the Ilmenogorsky complex

Abstract

Research subject. This paper presents the results of a series of mineralogical, petrological, geochemical and isotope­chronological studies carried out to investigate metacarbonate­silicate rocks in the area of Ishkul Lake, the Ilmenogorsky complex.Materials and methods. The microprobe analysis of the composition of minerals was performed using a REMMA­202M scanning microscope equipped with a microanalyzer. The content of major, trace and rare­earth elements (REE) was determined using a mass spectrometer ICP­MS. The U­Pb age of zircons was obtained by a microprobe SHRIMP II. The content of REE in zircons was determined by an ion probe CAMECA IMS­4F.Results. The studied metacarbonate­silicate rocks are shown to be represented by spinel­forsterite­calcite, diopside­scapolite­calcite and clinopyroxenite varieties with a di verse range of minerals, including diopside, calcite, forsterite, spinel, scapolite, anorthite, enstatite, alumoenstatite, augite, fassaite, tschermakite, pargasite, hornblend, tremolite, barium­containing feldspar, celsian, phlogopite, graphite, titanite, fluorapatite, picroilmenite, pyrrhotite, pentlandite, sphalerite, violarite, gersdorffite, maucherite. In terms of petrochemical properties, the metacarbonate­silicate rocks under study are characterized by significant variations in the content of SiO2, CaO, MgO at a Ca/Mg ratio of 1.4–8.2, as well as by increased Ni and Cr content, low ratios of Sr/Ba, Th/U, Zr/Hf and Nb/Ta. The small ΣREE amounts of 6–25 ppm (rarely up to 70–72 ppm) correspond to sedimentary formations with a significant amount of ultrabasic material.Conclusions. The specific features of the composition of olivine, spinel, ilmenite, as well as the titanium content in the early generation zircon indicate the formation (transformation) of spinel­forsterite­calcite rocks at t = 830–850°C. According to the established specifics of REE distribution and the Th/U ratio, the early generation zircons refer to the granulite type zircons, while the late generation zircons correspond to those of transformed syenites­miaskites and various metasomatites. The formation (transformation) of the rocks is found to correspond to the following age stages: PR1 (1720–1780 Ma) ­ “granulite” metamorphism; D1–C (345–399 Ma) – metasomatic transformations caused by the formation of alkaline rocks associated with rifting processes; P1 (282 Ma) – tectonic­metasomatic transformations caused by shear processes.

Keywords

• metacarbonate-silicate rocks
• mineralogy
• geochemistry
• zircon
• age
• ilmenogorsky complex