The age and petrologic and geochemical conditions of formation of the Kogtakh gabbro-monzonite complex in the Kuznetsk Alatau

Abstract

We present new data on the U-Pb age of zircons, geochemistry of trace elements, and isotope (Nd, Sr, O) composition of rocks of the Kogtakh gabbro-monzonite complex on the eastern slope of the Kuznetsk Alatau Ridge. The established age of zircon in the rocks of the main intrusive phases (500-480 Ma) is taken as the time of formation of the Kogtakh complex in the Late Cambrian-Early Ordovician, during the accretion-collision stage of evolution of the Central Asian Orogenic Belt. The distribution and ratios of LILE and HFSE in the rocks suggest that the intrusions proceeded under interaction of the PREMA + EM/OIB mantle plume and the suprasubductional lithospheric IAB mantle. The mantle-crust interaction led to the heterogeneous isotopic composition of neodymium in the magma source (ɛNd(T) ~ 3.5-5.4). The mixing of different mantle and continental-crust materials resulted in an increase in isotope parameters: (87Sr/86Sr)T ~ 0.7039-0.7052, ɛSr(T) ~ 0-19, and δ18O ~ 6.5-8.8‰ (SMOW). The REE ratio in the least differentiated gabbroids indicates different depths of probable magma chambers and the formation of their primary (Tb/YbPM > 1.8) mafic melts at different degrees of melting of the model garnet peridotite.