Zircon chemistry and magma mixing, SE China: In-situ analysis of Hf isotopes, Tonglu and Pingtan igneous complexes

Field relations and whole-rock geochemistry indicate that magma mixing has been important in the genesis of the late Mesozoic I-type igneous complexes at Pingtan and Tonglu in SE China. Morphological and trace-element studies of zircon populations in rocks from each of these complexes have defined several distinct growth stages [Mineral. Mag. (2001)]. In-situ LAM-MC-ICPMS microanalysis shows large variations in 176Hf/177Hf (up to 15 εHf units) between zircons of different growth stages within a single rock, and between zones within single zircon grains (up to 9 εHf units). These variations suggest that each of the observed magmas in both complexes developed through hybridisation of ≥2 magmas with different sources. Although this mixing has produced similar Sr and Nd isotopic compositions in the different rock types of each complex, the zircons have functioned as “tape recorders” and have preserved details of the assembly of the different magmas. In the Tonglu complex the most primitive magma is a mafic monzonite (preserved as enclaves), whose isotopic composition suggests derivation from the lower crust; rhyodacites, rhyolites and quartz diorites reflect the mixing of the monzonite with ≥2 more felsic magmas, derived from older crustal materials. In the Pingtan complex, zircons in a quartz diorite enclave suggest mixing between a crustal magma and a more primitive mantle-derived component. Zircons from granites and granodiorite enclaves indicate mixing between the quartz diorite and more felsic melts with lower 176Hf/177Hf. Major changes in 176Hf/177Hf correlate with discontinuous changes in the trace-element composition and morphology of the zircons, in particular the development of sector zoning that suggests rapid disequilibrium crystallisation. We suggest that the magma mixing recorded by the changes in 176Hf/177Hf occurred during transport in magma conduits. The in-situ analysis of Hf-isotopic stratigraphy in zircons is a new and powerful tool for the detailed study of magma generation processes.