Oxygen isotope evidence for two-stage water-rock interactions of the Nianzishan A-type granite in NE China

The oxygen isotope ratios of whole-rock, common rock-forming minerals and zircon from Mesozoic A-type granitic pluton at Nianzishan in northeastern China were analyzed by the conventional BrF5 method and the laser-probe technique, respectively, Both whole-rock and rock-forming minerals show large delta O-18 variations up to 5.5 parts per thousand with significant oxygen isotope disequilibrium between zircon and the other minerals, whereas the delta O-18 values of zircon are tightly clustered between 3.12 parts per thousand and 4.19 parts per thousand and thus lower than the normal-mantle delta O-18 values. These results indicate that the Nianzishan A-type granite experienced two-stage water-rock interactions subsequentially, The remarkably low zircon delta O-18 values are genetically due to seawater exchange with granite protolith in the first stage, and the oxygen isotope disequilibrium fractionations between zircon and rock-forming minerals are caused by meteoric-hydrothermal alteration in the second stage. It is inferred that the O-18-depleted A type granitic magma was derived from partial melting of subducted lower oceanic crust which was isotopically exchanged with seawater at high temperatures, In the process of granite emplacement into the upper crust, meteoric-hydrothermal circulation was triggered to overprint crystallizing granite under subsolidus conditions.