Abstract
A-type granites are usually derived from partial melting of the lower crust and/or the upper mantle within extensional settings, but their exact sources for specific plutons are required to unravel. A systematic laser fluorination analysis of oxygen isotopes was carried out for five well-documented late Mesozoic A-type granites in eastern China. Zircon from 41 samples was separated and 109 aliquots of fresh zircon were analyzed along with whole-rock powder and quartz from the same samples. Hydrothermally altered whole-rock samples show scattered δ 18O values of − 2.3 to 7.4‰, with a total variation up to 9.7‰. In contrast, zircon δ 18O values vary only from 3.1 to 5.4‰, with a range of 2.3‰. Most available zircon δ 18O values of A-type granites are lower than normal mantle zircon values of 5.3 ± 0.6‰ (2SD), and high-temperature equilibrium fractionations are well preserved between quartz and zircon in most cases. The above lines of evidence suggest that the low δ 18O zircons in A-type granites were crystallized from low δ 18O magmas. Because low δ 18O magmas cannot be derived entirely from the mantle, reworking and/or recycling of crustal igneous rocks are involved for their petrogenesis. The low δ 18O zircons in this study thus offer evidence for a dominantly crustal origin of the late Mesozoic A-type granites in eastern China.
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