Oxygen isotopes in the Cretaceous-Paleogene granites of Primorye, and some problems of their genesis
Oxygen isotope composition has been first determined in the Cretaceous-Paleogene granites of the East Sikhote- Alin volcanic belt and in the Upper Cretaceous granitoids of the Tatibinskaya series (Central Sikhote-Alin). By 18O value, three groups of granitoids have been recognized: I - with 18O = +5.5 to +6.5‰; II - with 18O = +7.6 to +10.2‰; and III - with 18O = <+4.5‰. The rocks of group I with 18O values analogous to those of oceanic basalts could originate at the expense of partial melting of basaltic crust. The rocks of group II, represented by granites of the Tatibinskaya series, have higher values of 18O suggesting the participation of sedimentary material in the generation of initial melts that formed these intrusions. The rocks of group III developed either through melting of the earlierlived rocks with low 18O content, or through the later exchange of melts with low-isotope fluids or meteoric waters under subsolidus conditions. Relatively low values of isotopic characteristics ( 18O and 87Sr/86Sr) of Primorye granitoids testify to the fact that granitoid melts were generated at the expense of rocks with a short-term crust history and can be the result of (1) melting of sedimentary rocks containing abundant young volcanic material accumulated in the trench along the continental margin (granites of Tatibinskaya series), and (2) melting of mixture of deep-sea sediments and basalts of the oceanic bottom and upper mantle in the lithosphere plate submerged under the continent in the subduction zone (granites of the East Sikhote-Alin volcanic belt).