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

Abstract

Hydrogen and oxygen isotope studies of both whole-rock and mineral separates were carried out for five representative A-type granites in eastern China. From North to South, the whole-rock δD and δ 18O values vary from −145 to −82‰ and from −1.2 to 5.0‰ for Nianzishan pluton, from −135 to −110‰ and from 1.6 to 6.7‰ for Shanhaiguan pluton, and from −145 to −111‰ and from 1.5 to 9.1‰ for Laoshan massif, respectively. The highest whole-rock δD and δ 18O values are observed in Suzhou pluton and range from −81 to −59‰ and from 3.5 to 9.2‰, respectively. Homogeneous δD and δ 18O values are found in Kuiqi massif, ranging from −113 to −99‰ and from 5.6 to 8.5‰, respectively. The results show that the large variabilities in the hydrogen and oxygen isotope ratios are mainly related to the diverse processes of magma evolution, such as magma degassing, fractional crystallization as well as post-magmatic alteration by local meteoric water under subsolidus conditions. The initial δD and δ 18O values of the A-type granites in eastern China are constrained to be about −50±5‰ and 7.5±1.0‰, respectively. This is consistent with mantle-like protoliths for granitoid rocks and thus precludes the possibility that the A-type granites is derived from the anatexis of the 18O-rich upper crust. However, recycling of crustal materials by earlier subduction cannot be excluded in the processes of generating the A-type granitic magmas. The variation in the D-depletion of the granites with latitude implies that the paleogeographic localities of the A-type granites have not changed significantly since the Mesozoic.