Comparison of petrology and isotope geochemistry of the Bulka peridotite–gabbro massif and granitoids of the Kyzykchadra complex (West Sayan)

Abstract

The study provides new petrologic and isotope geochemical data for rocks of the 465 ± 5 Ma Bulka massif (Borodina et al., 2011). The primary amphibole from granitoid stocks cutting across the layered series of the massif yielded an Ar-Ar age of 415.9 ± 3.7 Ma. The rocks of the Bulka massif have 143 Nd/ 144 Nd ratio of 0.513243 and εNd (Т) values of +12.00. The granitoids have 143 Nd/ 144 Nd ratios between 0.512919 and 0.512961 and εNd (Т) values between +8.03 and +9.25. The Nd isotope composition indicates that the rocks of the Bulka massif and granitoids were derived from a depleted mantle source. Depletion of the rocks of the massif in LILE, LREE, and HFSE over LILE is inherited from the mantle source, which has geochemical signatures of N-MORB and subduction-related components. Granitoids are metaluminous I-type granites, which were probably generated either by differentiation of intermediate to mafic mantle-derived magmas or by melting of metabasites. The rocks of the granitoid stocks are characterized by enrichment in LILE and LREE and depletion in HFSE over LILE, which suggests derivation from arc-related parental magmas. BORODINA et al. microscope (Japan) at the Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences (Novosibirsk). Mineral phases were identified using energy-dispersive spectroscopy.Isotopic measurements were performed on a grabbronorite from the upper part of the layered series (sample BL-94b) (Fig. 2a) and granitoids stocks (samples BL-111a, 111b) (Fig. 2b) at the Geological Institute of the Kola Scientific Center, Russian Academy of Sciences (Apatity, analyst Serov P.A.). The Sm and Nd concentrations and Nd isotopic compositions were analyzed using а multi-collector Finnigan-MAT 262 (RPQ) solid-phase mass spectrometer in static mode using Re and Ta double-filament assemblies. The analytical uncertainty on the Nd isotopic composition for the La Jolla standard = 0.511857 ± 7 (n = 7) was 0.003% (2σ). Errors for the 147 Sm/ 144 Nd ratio were 0.2% (2σ) (average of seven runs of the BCR standard). The total procedural blanks were 0.3 ng for Nd and 0.06 ng fo Sm. Isotope ratios were normalized to 146 Nd/ 144 Nd = 0.7219 and corrected relative to a 143 Nd/ 144 Nd value of 0.511857 for La Jolla standard.