Petrogenesis of Jurassic fractionated I-type granites in Southeast China: constraints from whole-rock geochemical and zircon U-Pb and Hf-O isotopes

Whole-rock geochemical and Sr-Nd-Hf isotopic data and zircon in situ U-Pb ages and Hf-O isotopes are reported for twelve granitic intrusions from inland to coastal areas of the Cathaysia Block, Southeast (SE) China, in order to constrain their magma sources and petrogenesis. They are mainly composed of biotite granites with minor granodiorites. Zircon U-Pb dating by LA-ICPMS and SIMS methods gave emplacement ages of 165-154Ma for these rocks, which are metaluminous to weakly peraluminous (A/CNK=0.95-1.10), with variable SiO₂ contents and high K₂O contents. They are all depleted in high field strength elements (HFSEs) and enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and show negative Sr, Ba and Eu anomalies in the spidergrams. Petrographic and geochemical features suggest the granodiorites that contain hornblende to be typical of I-type, while the biotite granites without hornblende are fractionated I-type granites. Their variable major and trace element concentrations are due to fractional crystallization of feldspar, biotite, hornblende, and accessory minerals, including apatite, allanite and monazite. Moreover, our data indicate significant variations in isotopic compositions of these granitoids from inland to coastal areas. The inland granites, from the Lapu, Fogang, Xinfengjiang and Baishigang plutons, have relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7105-0.7155) and low ε_Nd(t) (-11.2 to -7.3) and ε_Hf(t) (-10.9 to -6.3) values for whole rocks, and variable δ¹⁸O values of 7.69-9.25‰ and ε_Hf(t) values of -13.5 to -6.7 for zircons. In contrast, the coastal granites, from the Longwo, Wushikeng, Lianhuashan, Shigushan, Chiliao, Hulutian, Mantoushan and Fenghuang plutons, show relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7056-0.7110) and high ε_Nd(t) (-7.6 to -6.2) and ε_Hf(t) (-6.5 to -2.8) values for whole-rocks, together with ε_Hf(t) values of -7.2 to 3.9 and δ¹⁸O values of 6.69-8.38‰ for zircons. The spatial isotopic variations suggest that these Jurassic granitoids were generated by different-percentage mixing of mantle- and ancient (Paleoproterozoic) crust-derived magmas, with subsequently minor crustal assimilation. Our results highlight that Jurassic fractionated I-type granites occur widely from inland to coastal areas of SE China, arguing against the previous view that Jurassic granites occur only in inland while coastal granites are mainly Cretaceous. The widespread occurrence of Jurassic granitoids in SE China is attributed to ascending of mantle-derived magmas, which provide heat for partial melting of lower crustal materials, in an extensional setting.