Hydrogen and oxygen isotope geochemistry of A-type granites in the continental margins of eastern China

Hydrogen and oxygen isotope studies of both whole-rock and mineral separate s were carried out for five representative A-type granites in eastern China . From North to South, the whole-rock deltaD and delta O-18 values vary fro m - 145 to -82 parts per thousand and from -1.2 to 5.0 parts per thousand f ur Nianzishan pluton, from - 135 to -110 parts per thousand and from 1.6 to 6.7 parts per thousand for Shanhaiguan pluton, and from - 135 to -111 part s per thousand and from 1.5 to 9.1 parts per thousand for Laoshan massif, r espectively. The highest whole-rock deltaD and delta O-18 values are observ ed in Suzhou pluton and range from -81 to - 59 parts per thousand and from 3.5 to 9.2 parts per thousand, respectively. Homogeneous deltaD and delta O -18 values are found in Kuiqi massif, ranging from - 113 to -99 parts per t housand and from 5.6 to 8.5 parts per thousand, respectively. The results s how that the large variabilities in the hydrogen and oxygen isotope ratios ale mainly related to the diverse processes of magma evolution, such as mag ma degassing, fractional crystallization as well as post-magmatic alteratio n by local meteoric water under subsolidus conditions. The initial deltaD a nd delta O-18 values of the A-type granites in eastern China are constraine d to be about -50 +/- 54 parts per thousand and 7.5 +/- 1.0 parts per thous and, respectively. This is consistent with mantle-like protoliths for grani toid rocks and thus precludes the possibility that the A-type granites is d erived from the anatexis of the O-18-rich upper crust. However, recycling o f crustal materials by earlier subduction cannot he excluded in the process es of generating the A-type granitic magmas. The variation in the D-depleti on of the granites with latitude implies that the paleogeographic localitie s of the A-type granites have not changed significantly since the Mesozoic. (C) 2000 Elsevier Science B.V. Ail rights reserved.