Origin of A-type granites in East China: evidence from Hf-O-Li isotopes
thesisposted on 28.03.2022, 02:53 by Yuya Gao
A-type granites are a distinct group of granites which have great metallogenic significance and are the only granitoid type found on other terrestrial planets. Several petrogenetic models have been proposed to account for the mineralogical, geochemical and unique O-isotopic characteristics of A-type granites in eastern China. This study elucidates the origin of Cretaceous A-type granites in eastern China using a non-traditional geochemical tracer, lithium isotopic composition, in addition to conventional geochemical data. This required improvement of in-situ lithium isotopic analytical methods, laid the groundwork for understanding Li isotope behavior in granites and in zircons, and developed Hf-O-Li isotope fingerprints as a useful geochemical tracer for future studies of granite petrogenesis. Six A-type granite plutons (Nianzishan, Houshihushan, Laoshan, Suzhou, Taohuadao, and Putuoshan) along eastern China were examined systematically for their geochronology, petrography, whole-rock geochemistry, whole-rock Li isotope compositions, and in situ zircon Hf-O-Li isotopes. The different stability of the U-Pb isotopic and oxygen-isotope systems in zircon have been re-evaluated. A screening criterion (Ddpa) has been developed to quantify the degree of radiation damage by taking both time and [U], [Th] into consideration. This makes it possible to obtain more reliable U-Pb and O isotope data from highly evolved high-U granites. The correlation between Ddpa, U-Pb ages and δ¹⁸O values strongly suggests that anomalously high U-Pbages and depleted oxygen isotope compositions are caused by different mechanisms related to different Ddpa thresholds. The screened isotopic results suggest that the Suzhou, Laoshan, and Houshihushan plutons formed in a single magmatic period and are not really "low-δ¹⁸O" granites as previously assumed. Whole-rock and in situ Li isotopes are applied to constrain the source region of the granites. Though extreme isotopic variations of Li have been observed during in situ SIMS (Secondary Ion Mass Spectrometry) analysis of zircon, Li isotopes in zircon arepotentially useful for studying the origin of its host rock if the zircon grains are large enough. Detailed ion imaging and in situ profiles indicate that the large variations of [Li]and extreme fractionation of δ⁷Li (~20‰) in zircons were only observed at the rim of the zircon and were generated by diffusion during cooling from igneous temperatures. Homogeneous δ⁷Li in the cores of large zircon grains most likely carry reliable information about the isotopic signatures of their source magmas. Geochemical evidence from Hf-O-Li isotopes indicates that the source region of Cretaceous A-type granites in eastern China has a more complex history than previously thought. This source region could be generated from partial melting of a juvenile upper crustal source (Nianzishan), re-melting of ancient igneous protoliths, triggered by input of depleted mantle-derived magmas (Suzhou, Putuoshan), a hybrid origin from metasomatized mantle and lower crust (Laoshan, Taohuadao), or dehydrated mantle source mixed with some component of lower crustal-derived magmas (Houshihushan).Their production was most likely triggered by the upwelling of the asthenosphere related to the delamination of a flat-subducting oceanic slab and the thinning of the subcontinental lithosphere.