ИСТИНА

The Yoko-Dovyren peridotite-dunite-troctolite-gabbronorite massif (YDM, up to 3.4 km thick), associated mafic-ultramafic sills, and gabbronoritic dykes represent the intrusive constituent of the Upper Riphean Synnyr-Dovyren volcano-plutonic complex located in Northern Transbaikalia, Russia (Ariskin et al., 2016, 2018). Lesson 1: Comparison of complete cross-sections of the thickest part of the YDM with those from its NE and SW terminations reveal key differences in its lateral architecture. The marginal domains are largely dominated by melanotroctolite to troctolite and olivine gabbro, with relatively thin zones of plagiodunite, whereas the core mainly comprises a thick dunite zone. These differences, highlighted with petrochemical reconstructions and mineral chemistry indicate that the central part of the intrusion was formed by a high temperature ‘Fo88-magma’, whereas its peripheral zones and ultramafic apophyses crystallised from the lower temperature ‘Fo86-magma’. Lesson 2: These results suggest a diversity of olivine-laden to multiply-saturated magmas, which formed the YDM. The COMAGMAT-5 calculations, indicate that the temperatures of the emplacing olivine-laden parental magmas in the central and peripheral parts of the intrusion ranged across 100oC, approximately from 1290oC (∼11 wt% MgO, olivine Fo88) to 1190oC (∼8 wt% MgO, olivine Fo86). Lesson 3: Based on the present thermodynamic modeling, the high-MgO magma in the centre was S-undersaturated, whereas its derivatives became S-saturated at the temperature of 1240°C or below. This is consistent with geological observations that most sulphide-rich Cu-Ni ores were discovered in the sills of plagioperidotite underneath the YDM, as well as at the periphery of the intrusion. Lesson 4: Data on concentrations of precious metals and chalcogens in rocks and sulfides throughout a completed cross-section of the YDM allow one to predict evolution of an original immiscible sulfide liquid, so that all main types of the YDM sulfide mineralizations can be interpreted by combining modeling of the sulfide immiscibility in olivine cumulates with calculations of a crystallization history of the modeled sulfide liquids. Lesson 5: The C-shaped distribution of TiO2, K2O, P2O5, and incompatible trace elements along cross-sections of the YDM reflects the depleted geochemistry of most Dovyren rocks. Accounting for a marked misbalance between estimates of the average weighted compositions of the intrusion and proxies of the parental magmas, these data suggest that roughly 50-70% of complementary basaltic melts had to be expelled from the YDM during its consolidation. Lesson 6: A complex history of the evolution of the open magmatic system includes: (1) the formation of a thick sequence of Ol-rich cumulate piles, (2) followed by their compaction and in situ crystallisation, (3) which give rise to upward migration and infiltration of the intercumulus melts. These processes proceeded in parallel to those in the upper part of the Dovyren chamber, which could evolve as a magma-staging system, through which large amounts of olivine gabbronorite magmas have passed, leaving behind a complementary melt-depleted succession of troctolite to gabbroic cumulates. Lesson 7: We suggest that at a final stage of evolution the magma chamber became closed, thus favoring in situ magma fractionation within the upper portion of the residual heterogeneous reservoir. This is consistent with the most evolved mineral compositions observed in olivine-free gabbronorite and quartz-pigeonite gabbro from the uppermost YDM. Lesson 8: Reconstructions of the time-dependent evolution of εNd(t) for the YDM rocks suggest for the Dovyren magmas an anomalous mantle protolith formed at the Meso-Neoarchean boundary at ∼2.8 Gyr. It remained isolated from magmatic events for ~2 Gyr, and then reactivated at 728 Ma. REFERENCES Ariskin, A.A., Kislov, E.V., Danyushevsky, L.V. et al. (2016) Cu-Ni-PGE fertility of the Yoko-Dovyren layered massif (Northern Transbaikalia, Russia): thermodynamic modeling of sulfide compositions in low mineralized dunite based on quantitative sulfide mineralogy. Mineralium Deposita 51, 993-1011. Ariskin, A., Danyushevsky, L., Nikolaev, G. et al. (2018) The Dovyren Intrusive Complex (Southern Siberia, Russia): Insights into dynamics of an open magma chamber with implications for parental magma origin, composition, and Cu-Ni-PGE fertility. Lithos 302-303, 242-262.