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 z ircon from Mesozoic A-type granitic pluton at Nianzishan in northeastern Ch ina were analyzed by the conventional BrF5 method and the laser-probe techn ique, respectively, Both whole-rock and rock-forming minerals show large de lta O-18 variations up to 5.5 parts per thousand with significant oxygen is otope disequilibrium between zircon and the other minerals, whereas the del ta O-18 values of zircon are tightly clustered between 3.12 parts per thous and 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 exp erienced 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 mete oric-hydrothermal alteration in the second stage. It is inferred that the O -18-depleted A type granitic magma was derived from partial melting of subd ucted lower oceanic crust which was isotopically exchanged with seawater at high temperatures, In the process of granite emplacement into the upper cr ust, meteoric-hydrothermal circulation was triggered to overprint crystalli zing granite under subsolidus conditions.