Isotope Geology of Izrandites (Plagioclase–Olivine Clinopyroxenites) of the Alexandrovsky Complex (Southern Urals)

Research object. The izrandites of the Alexandrovsky metamorphic complex (Southern Urals) are massive, melanocratic plagioclase–olivine clinopyroxenites, characterized by high density (3.27–3.33 g/cm³), specific paragenesis (65–70% clinopyroxene, 10–16% olivine, 5–8% plagioclase, 3–8% amphibole), and Proterozoic–Archean ages.

Research subject.  Isotopic and geochemical patterns of the ¹⁴⁷Sm–¹⁴³Nd isotopic systematics obtained by ID–TIMS for izrandites and monomineralic fractions of plagioclase and clinopyroxene from the Alexandrovsky metamorphic complex.

Objectives. To determine the timeframes of metamorphic events related to the evolution of the complex; to reconstruct the isotopic evolution of the source material based on calculations of ε_Nd(T) and Nd model ages (T_DM); to compare the ¹⁴⁷Sm–¹⁴³Nd signature with the results of previously applied K–Ar, Rb–Sr, and U–Pb methods; to verify two petrogenetic hypotheses: (a) orthocumulate genesis resulting from fractional crystallization in a layered intrusion; (b) mantle-restite origin with subsequent metasomatic and metamorphic overprinting.

Materials and methods. The research was conducted by integrating modern analytical methods (ID–TIMS) to a representative series of geological samples. Data interpretation was performed using current geochronological models, taking into account previously published petrological, mineralogical, and isotopic data for the region.

Results. Ages (in million years) of two metamorphic events were established: 2014 ± 130 (whole-rock isochron for four samples, MSWD = 1.8) interpreted as the time of protolith rehomogenization; 1127 ± 84 (Pl+Wr+Cpx isochron, MSWD = 1.7, ε_Nd = +7.5), an event corresponding to amphibolite-facies metamorphism. The calculated ε_Nd(T) values demonstrate an anomalously high degree of isotopic depletion. While ε_Nd(1127) = +7.5 is practically identical to ε_DM = +7.6 for the mineral isochron of 1127 Ma, the rocks are characterized by ε_Nd(T) significantly exceeding the model value for ε_DM(2014)=+5.7. T_DM ages ranging from 1788 to 2308 Ma indicate that the material basis of izrandites separated from the mantle reservoir during the Archean–early Paleoproterozoic era.

Conclusions. Using ¹⁴⁷Sm– ¹⁴³Nd  ID-TIMS isochron dating, two metamorphic events were distinguished: a Paleoproterozoic event (~2.01 Ga), corresponding to a high-temperature stage, and a Mesoproterozoic event (~1.13 Ga), corresponding to amphibolite-facies metamorphism. The affiliation of the izrandite protolith to an ancient depleted mantle reservoir was confirmed. The high ε_Nd(1127) value of +7.5, the anomalously high ε_Nd(2014) values (>> +5.7), and T_DM ages in the range of 2.3–1.8 Ga indicate that the initial material was formed in the Archean–Early Paleoproterozoic as a result of multiple episodes of melting and melt extraction, which confirms the presence of a relict restitic mantle. A fundamental petrogenetic paradox between mineralogical and textural features indicative of a magmatogenic-cumulate genesis and a ¹⁴⁷Sm–¹⁴³Nd signature characteristic of mantle restites was identified and conceptually resolved. The most plausible model suggests that the izrandites represent fragments of highly depleted Archean mantle that underwent autochthonous partial melting during the Paleoproterozoic (~2.01 Ga). Subsequent in situ crystallization of the generated melt led to the development of the observed cumulate textures.

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