ИСТИНА

The Udzha structure is located between the Anabar shield and Olenek uplift un the northern part of the Siberian Craton and oriented along a north-south trending paleo-rift. Inside the Udzha paleo-rift, Mesoproterozoic volcano-sedimentary rocks are exposed, which are cross-cut by mafic intrusions. Precambrian successions are covered by Phanerozoic sediments along an unconformity. The pre-Late Neoproterozoic sequence is divided into two units: (i) Limey-shales with tuff interbeds of the Ulahan-Kurung and Ungoakhtah formation, (ii) carbonate-dominated Khapchanyr formation and terrigenous Udzha formation, which has an overall thickness of ~1500 m. We provide new trace elements geochemistry, U-Pb dating and paleomagnetic data on mafic dykes on Udzha complex. The chemical composition of the Udzha dyke complex corresponds to moderately alkaline basalts (SiO2 = 46–52%; K2O + Na2O = 1.7–4.4%). On the trace elements and paleomagnetic data, all intrusions are separated on two magmatic pulses. In general, there is a steady increase in the normalized ratios from the least to the most incompatible elements. PM-normalized trace elements patterns of the first pulse define a large compositional range, and in general pronounced enrichment in LREE, a wide range in Ta and Nb, and negative Ti. The second pulse demonstrates flat shape REE pattern, fewer variations of Ta and Nb depletion and weak Ti depletion. Both pulses pronounced enrichment of Pb and depletion of Sr. Nd (eNd values ~ 0) and Sr (eSr values from 10 to 60) isotopy signature shows from moderate to significant input of crust material in the source. U-Pb dating was carried out on the regionally largest dike from the second pulse with a coarse grain texture, plagioclase (65 vol.%), clinopyroxene (20 vol. %), quartz (~ 10 vol. %) and hornblende (5 vol. %) mineral composition. Apatite was employed for U-Pb dating, which was performed at the University of Tasmania (Hobart, Australia) by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) using an Agilent 7900 quadrupole mass-spectrometer connected to a 193 nm Coherent Ar-F excimer laser and Resonetics S155 ablation cell. Analyzed apatite grains form pronounced regression line on the Tera-Wasserburg diagram, which intersects the concordia at 1386+-30 Ma which is indistinguishable from the age of the Chieress dyke (Anabar Shield, 1384+-2 Ma). The obtained age yields new constraints on the timing of sedimentation in the area. The magmatic event occurred in two phases, that supported by paleomagnetic data. The proposed geodynamic model for the based on Nd-Sr isotopy and trace elements is the mafic intrusions were caused by passive rifting extension in which MORB type melts became enriched by wet mantle plume with sub-continental geochemical signs. The research was supported by the Russian Science Foundation grant (16-17-10068). The interpretation of geochemical data was supported by funding from the Russian Science Foundation (Project No. 19-77-10048)