Geochemical characteristic of the gabbro Kharanur and Kho;bun-Khairkhan ophiolite massif (East Sayan)

Abstract

The ophiolite massifs Kharanur and Kholbyn-Khairkhan are considered to be a single overthrust, consisting of two tectonic sheet, separated by a fault. The ophiolite gabbros, in these two sheets, have different geochemical characteristics. Gabbro Kharanur massif  have low content Ti and content of Nb, Zr  are look like with the content in  N-MORB. On the spider diagram, gabbro have positive anomalies in Ba, Th, Sr and negative anomalies in Ta, Nb, Ti. The REE distribution is similar to these in upper continental crust and island-arc rock. On the discrimination diagrams, the gabbro of the Kharanur massif lie in the fields of island-arc rocks. The value (Ta/Yb) vary 0.14-0.19, Sr/Y  more 25, (La/Nb)PM is three, it may indicate the participation of the subduction component in the melts (Pearce et al., 1995).Gabbro of the Kholbyn-Khairkhan massif have a flatter REE distribution spectrum than gabbro of the Kharanur massif, more consistent with the REE distribution in E-MOR, BAB (Drill et al., 1997) basalts. They have positive anomalies in Ba, Sr and negative anomalie in Th. There are not negative anomalies in Nb, Ta, Ti. On the discrimination diagrams, the gabbro of the Kholbyn-Khairkhan massif lie in the fields MOR – OIB basalts.The isotope data of the overmantioned gabbroi are the Sm/Nd ratios: 0.14 – 0.19, 143Nd/144Nd 0.511730 – 0.512715, Ɛ Nd (0) (-10) – (+1.5) and the epsilon Nd changes from negative to positive values with increasing rock alkalinity; Ɛ Nd (1020 m.y.) (-5) ÷ (+2.5). The rocks have a strong negative correlation (La/Sm)PM – (Nb/La)PM, which suggests the assimilation by crust material of the magmatic source. The like characteristics is observed in the mafic rocks of the Ulan-Sardag massif (Kiseleva et al., 2022). The magmatic source of gabbro from the Kholbyn-Khairkhan massif is supposed to be assimilated by the crustal material of the Archean Gargan TTG complex.The Kholbyn-Khairkhan gabbro was formed from an enriched (terrigenous material from the TTG complex) mantle source. The calculated percentage of the crustal component is 9–25%. The analyses of the whole-rock major, trace, and rare-earth element compositions were carried out at the Analytical Center for Multi-Elemental and Isotope research (VS Sobolev Institute of Geology and Mineralogy, Novosibirsk, Russia). Work was done on state assignment to IGM SB RAS. This research was funded by the Ministry of Science and Higher Education of the Russian Federation and the Russian Foundation for Basic Research, grants No. 19-05-00764a. ReferencesDril S.I., Kuzmin M.I., Tsipykova S.S., Zonenshain L.P. (1997) // Marine Geology. 1997.  V. 142.  P. 57-83.Kiseleva O.,  Serov P., Airiyants E., Travin A., Belyanin D., Nharara B., Zhmodik S. // Minerals. 2022. 12. 92.  doi.org/10.3390/min12010092 Pearce J.A., Peate D.W. // Annual Review of Erath and Planetary Science. 1995. V.23. P.251 – 285.