The U-Pb and Lu-Hf isotope composition of zircon xenocrysts from the V. Grib kimberlite pipe (Arkhangelsk region, NW Russia): a key to reconstruct the history of continental crustal growth beneath the northern East European platform

Abstract

<p>Crystalline continental crust of the northern East European platform, particularly within the Arkhangelsk diamondiferous province (ADP), is covered by sedimentary rocks, and no direct samples are available. Scarce data on the age and evolution of continental crust of the province is based upon the studies of lower-crustal xenoliths (Koreshkova et al., 2014; Shchukina et al., 2018). In order to obtain more information on the age and evolution of continental crust of the ADP we study a set of 135 zircon xenocrysts selected from heavy minerals fraction of the V. Grib kimberlite. Zircons are between 100 and 250 micron in size and mostly rounded with some remnants of crystallographic shapes. CL images show a variety of types of zoning, dark or oscillatory zoned cores often have hexagonal shapes surrounded by bright rims. Other zircons have blocked irregular structure and some grains are homogeneous.  Zircons show a wide range of <sup>207</sup>/<sup>206</sup> Pb ages from 3.12 to 1.12 Ga with the two major peaks on the relative probability plot. The first corresponds to the Archean age of 2.5-2.6 Ga and the second to Paleoproterozoic of 1.9 Ga. If we consider core age only the two major peaks remain but Proterozoic peaks are less sharp and the proportion of Archean to Proterozoic ages is increased. Over half of the grains analyzed have discordant ages mostly within 10 % and plots below the Concordia line indicating Pb loss. Zircon also have a wide variation in their Hf isotope composition. The range of ɛHf (t) values calculated with corresponding U-Pb ages are vary from 4.5 to – 36. The vast majority of zircons have negative ɛHf (t) values that suggest re-setting U-Pb ages of older zircons by later tectono-thermal events. The calculated Hf T<sub>DM</sub> model ages are mostly Archean being of 2.9-3.4 Ga with the oldest of 3.8 Ga.  From the synthesis of zircons U-Pb and Lu-Hf isotope data the several steps of formation and evolution of continental crust of ADP can be proposed. The first fragment of continental crust of ADP was formed in the Eoarchean time app at 3.6-3.8 Ga as evidenced from the Hf T<sub>DM</sub> model ages of two zircons. The major event of continental crust growth occurred at 3.4-3.1 Ga. However, all of this crust has been reworked later by collision with the other Archean terrane at app 2.8-2.7 Ga. At that time, some magmatic activation in the area was occur as three of this age zircons have juvenile Hf isotope composition with positive ɛHf (t) values. The next tectono-magmatic episode at 1.8-2.0 Ga significantly affected all the crust in the area, both Archean and newly formed Paleoproterozoic. This event corresponds to final amalgamation and stabilization of Kola-Karelian craton. After that, continental crust of ADP remains stable with some magmatic activation in Meso-Proterozoic time. The rocks of this age not experienced metamorphism as their zircons U-Pb ages are concordant and keep positive ɛHf (t) values. </p> <p>This work was supported by the Russian Science Foundation, grant No. 20-77-10018</p>