Source nature and magma evolution of I-type granites from the North Qinling orogen, China, revealed by zircon morphology and grain-scale variations in Hf O isotope composition

Abstract

The North Qinling orogen, central China, formed above an ancient ocean-continent subduction zone and contains widespread early-Paleozoic I-type granites . Previous studies have shown that the mantle wedge beneath the north Qinling unit was significantly metasomatized and isotopically enriched relative to the lower crust in the early-Paleozoic. Therefore, material contributions from the mantle wedge and the lower continental crust can be readily distinguished in this geological setting. In this study, we show that the early-Paleozoic Zaoyuan I-type granite was emplaced at c. 417 Ma and zircon antecrysts are preserved. Our geochemical and isotopic results demonstrate that there is no evidence of a mantle source contribution in the Zaoyuan granite. Instead, it is argued that the granites formed solely by low-temperature fluid-fluxed melting of a mixture of lower and upper crustal material, in a continental arc setting. Cathodoluminescence images of magmatic zircons from the Zaoyuan granite reveal core to rim domains. Zircons from both domains are isotopically uniform and have consistent morphology. Increasing Th and U contents and δ 18 O zir and decreasing ε Hf (t) values from core to rim domains are explained by contamination of the melt during crystallization. Two granite samples have different Hf isotope compositions indicating some source heterogeneity during the formation process. Our results provide evidence for partial melting of a mixed crustal source and subsequent minor assimilation with upper crustal metasediments during crystallization. We further conclude that the suggested magma formation mode might be more common process for the generation of I-type granites in order to acquire their isotope mixing characteristics. • The Zaoyuan granite was of I-type affinity and emplaced at 417 ± 3.3 Ma. • They were formed by low-temperature fluid-fluxed melting of a mixture of different crustal sources in continental arc setting. • Zircon morphology and grain-scale Hf O isotope variations indicate contamination of upper crustal material.