Early Precambrian Crustal Evolution in the Irkut Block (Sharyzhalgai Uplift, Southwestern Siberian Craton): Synthesis of U–Pb, Lu–Hf and Sm–Nd Isotope Data

Abstract

Abstract —The paper presents a synthesis of zircon U–Pb and Lu–Hf and whole-rock Sm–Nd isotope data from main early Precambrian (3.40 to 1.85 Ga) metamorphic and magmatic units of the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). The Archean complexes consist of relict Paleoarchean (3.4 Ga) melanocratic granulites and predominant Neoarchean mafic and felsic granulites (2.70–2.66 Ga), paragneisses (≤2.75 Ga), and gneissic granites (2.54 Ga). The Paleoproterozoic complexes include paragneisses (1.95–1.85 Ga), granitoids and charnockites (1.86–1.84 Ga), as well as mafic intrusions and dikes (1.86 Ga). Few detrital zircons with Hf model ages of ≥3.6 Ga mark the Eoarchean onset of crustal growth in the Irkut block. Isotopic data record two major stages of crustal growth in early Precambrian evolution of the Irkut block: Paleoarchean (3.6–3.4 Ga) and Neoarchean (~2.7–2.66 Ga). The Paleoarchean crustal growth was most likely associated with plume magmatism fed from depleted and primitive mantle sources. The spatial distribution of Paleoarchean crust is traceable in isotopic signatures of magmatic and detrital zircons from most of Mesoarchean to Paleoproterozoic units. The Neoarchean crustal growth from a depleted mantle source was due to subduction magmatism. Moderate crustal growth occurred in the Paleoproterozoic from 2.30 to 1.85 Ga. At the turn of 1.86–1.85 Ga, mafic magmas and products of their fractionation formed from both depleted and enriched sources under postcollisional extension; the latter sources were the subcontinental lithospheric mantle formed during Neoarchean subduction. Three major stages of crustal recycling have been established: Mesoarchean (~3.0 Ga), Neoarchean (~2.55 Ga), and Paleoproterozoic (1.86–1.85 Ga), which are characterized by near-coeval intracrustal melting and metamorphism. The recycling during the ~2.55 Ga and 1.86–1.85 Ga events apparently occurred in a collisional setting. The 2.7 Ga subduction-related felsic magmas also formed through the recycling of the Paleo-Mesoarchean crust. The hypothesized scenario for the geological evolution of the Irkut block is the dominant vertical growth and crustal recycling for about two billion years. Available isotope data record similar major crustal growth in the Paleoarchean and growth combined with recycling during the Neoarchean and Paleoproterozoic events in both the southwestern and northern and central parts of the Siberian craton. The Irkut block in the southwest differed in a long and continuous recycling during the Mesoarchean and pronounced Neoarchean crustal growth.