Age, provenance and Precambrian evolution of the Anabar shield from U-Pb and Lu-Hf isotope data on detrital zircons, and the history of the northern and central Siberian craton
Résumé
The Anabar shield in northern Siberia is one of the world’s least studied Precambrian areas, and provides a ‘window’ into the crustal basement of the central and northern Siberian craton. We report U-Pb and Hf isotope data for detrital zircons sampled in a profile across its major structural units. They define a U-Pb age range from 1.8 to 3.4 Ga with three main periods: 1.8–2.0 Ga, 2.4–2.8 Ga and 3.0–3.4 Ga. The oldest zircons yield super-chondritic εHf(t) implying that the parental magmas of their source rocks were juvenile, i.e. formed from depleted mantle (DM). Thus, the crustal basement of the Anabar shield, and probably the whole central and northern Siberian craton, started to form in the mid-Paleoarchean, and included no recycled crust. Zircons with 2.5–2.7 Ga ages define two εHf(t) intervals. One is super-chondritic (+2 to +7) implying juvenile sources, the other is sub-chondritic (−3 to −12) indicative of recycled crust, probably formed at 3.2–3.4 Ga, in magma sources. Nearly all 1.8–2.0 Ga zircons have sub-chondritic εHf(t) (−2 to −29) implying derivation from sources dominated by recycled crust formed at ∼2.6 Ga and ∼3.4 Ga and little or no juvenile addition. These events accompanied amalgamation of the entire craton by welding of Archean domains.
The Bekelekh unit of the Daldyn series has the highest proportion of ∼2.6 Ga zircons and may be the oldest ‘nucleus’ of the Anabar shield, whereas the Kilegur unit of the same series is essentially Proterozoic (1.95 Ga). The largest amount of 3.1–3.4 Ga zircons, as well as common 2.6–2.7 Ga zircons, occur in the Ambardakh unit of the Upper Anabar series. Our data suggest alternation of areas with dominant ages of 1.95 Ga and ∼2.6 Ga, with the younger zircons coming from granites and granulites, and the older ones from gneisses. They show no evidence for significant ages differences for the Anabar and Olenek provinces. The final amalgamation of the entire Siberian craton by welding of Archean blocks, may have taken place at around 1954 ± 6 Ma.