First Evidence of MesoArchean Continental Crust?

Evolution history of the Neoproterozoic eclogite-bearing complexof the Muya dome (Central Asian Orogenic Belt): constraints from zircon U-Pb age, Hf and whole-rock Nd isotopes

Authors:

Shatsky et al

Abstract:

U-Pb dating and Hf-isotope analysis of zircons and whole-rock Nd-isotope analyses were carried out on country rocks of the eclogite-gneiss complex of the North Muya dome in the Anamakit-Muya zone of the Baikal Muya accretionary fold belt. Zircons from garnet-biotite gneisses (Qtz + Kfsp + Pl + Bt + Grt) and garnet-biotite-muscovite schist (Pl + Kfsp + Bt + Mu + Grt + Qtz) were dated using the LA-ICP-MS technique. Based on U-Pb isotope data and CL images zircon grains were divided into three groups: detrital, magmatic and metamorphic zircons. Metamorphic zircons display no zoning or the cloudy zoning. The grains morphology together with the well-developed oscillatory zoning clearly identifies the igneous origin of magmatic zircons. The metamorphic zircons (ages 576-680 Ma) have Th/U ratios varying from 0.271 to 0.004, whereas the ratio in magmatic zircons ranges from 0.779 to 0.11. Magmatic zircons from granite-gneisses of the North Muya dome exhibit a relatively narrow spread in the crystallization age with the major peak at ca 764 Ma. Younger ages are interpreted as due to the partial resetting of U-Pb system during the subsequent metamorphic evolution. Detrital zircons from two-mica schist sample Mu-93-10 give ages of 1.88-2.66 Ga. The oldest detrital zircon from this sample plots near concordia and has a Pb207/206Pb age of 3.2 Ga. Zircons from this sample are characterized by the widest scatter of ɛHf(t) values (from +13.9 to -15.3) and View the MathML sourceTDMC model ages (0.82-3.86 Ga). Zircons from other samples have a much narrower ranges of ɛHf(t) (+11.6 to -0.7) and View the MathML sourceTDMC (0.85-1.52 Ga). The involvement of older crustal material is also evident from the whole-rock Nd isotopic compositions. The gneisses and schists exhibit a range of Nd isotopic compositions with ɛNd(t) values ranging from -3.5 to +3.6 and tNd(DM) from 1.64 to 1.09 Ga. The integration of the Hf-isotope data with the age spectra provides with the first evidence for the existence of Mesoarchean crust in the Baykal-Muya sector of the Central Asian Orogenic Belt.