Oxygen isotope composition of heterochronous zircons from gneiss of the Chupa sequence (Khitostrov, Upper Pulonga Lake), Belomorian Province
Krylov D. P., Skublov S. G., Li Xian-Hua, Li Qiu-Li, Levashova E. V.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 6. P. 18-27
Isotope composition of oxygen has been analyzed in heterochronous zircon populations from the Chupa gneiss at the Pulonga Lake area with the purpose to reveal the possible influence of fluids which caused the extreme 18O depletion in the enclosed high-aluminous silica-undersaturated (corundum-bearing) rocks. Previous studies of Chupa gneiss did not indicate an 18O decrease in these rocks, and any 18O/16O shifts in accessory minerals. There are distinguished among zircon populations the ancient detrital cores (the estimated U-Pb age exceeding 3 Ga), internal rims (ca. 2.8 Ga) of magmatic origin, metamorphic outer rims (ca. 2.6 Ga) and envelopes (1.8–1.9 Ga). The isotope ratios of oxygen in cores, internal and outer rims of zircon (δ18O 5.67 ± 0.53, 7.48 ± 0.26 and 5.70 ± 0.44‰, respectively) are determined by the magmatic and/or metamorphic crystallization from the 18O-undepleted melts or by the interaction between zircon and 18O-undepleted fluids. Low δ18O values (–22.04 ± 0.41‰), near identical to those in corundum-bearing rocks, are typical only for the latest zircon envelopes. Thus, there may be suggested an infiltration of fluids, responsible for the δ18O decrease in corundum-bearing rocks, inside the host gneiss during the regressive alteration.
Keywords: zircon, negative δ18O values, Chupa sequence, Belomorian Province