Magmatic crystallization drives zircon Zr isotopic variations in a large granite batholith.

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中文题名岩浆结晶驱动了大型花岗岩岩基中锆石Zr同位素的变化。
作者Zhu, Zhaoxian
作者单位State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
刊名Geochimica et Cosmochimica Acta
2023
342
摘要
Stable zirconium (Zr) isotope of magmatic zircons is a promising new tracer to understand magma differentiation in the continental crust. However, magmatic processes controlling zircon Zr isotopic variations remain poorly constrained. Here we present high-spatial-resolution in-situ methods on magmatic zircons for U-Pb age, trace elements, Hf isotopes, and stable Zr isotopes (δ94/90Zr relative to IPGP-Zr) as well as bulk rock δ94/90Zr for a large granite batholith (SiO 2 = ∼73 wt%) in the Jiaodong Peninsula, eastern North China Craton. Magmatic zircons are classified into high-luminance Type-I and low-luminance Type-II zircons in the cathodoluminescence images. Both types show indistinguishable U-Pb ages and initial Hf isotopic compositions, indicating their same magma source. Yet, they differ in chemical compositions and δ94/90Zr values. Type-I zircons display lower δ94/90Zr values (−0.35‰ to 0.15‰) than Type-II zircons (0.16–0.54‰). The δ94/90Zr values overall increase with decreasing Zr/Hf, Th/U ratios, Ti-in-zircon temperature, and increasing U abundances in zircons, implying that zircon δ94/90Zr values becomes heavier with the enhanced magma differentiation. Such concomitant correlations resulted from closed-system magma crystallization without noticeable segregation of feldspar and zircon, as revealed by high Sr and Ba content, limited Eu anomalies, and mantle-like δ94/90Zr in the bulk rocks (−0.01 0.07‰ and 0.02 0.04‰). Thus, Type-I zircons incorporating light Zr isotopes crystallized at an early stage of magma solidification, while Type-II zircons grew from more evolved residual melts with elevated δ94/90Zr values. Zoned zircon grains with Type-I cores and Type-II rims record the prolonged crystallization history covering both stages with contrasting melt compositions; whereas oscillation zircon grains of Type-I or Type-II, crystallizing at the stable melt composition, display the restricted intragrain variations of δ94/90Zr, Zr/Hf, and Th/U ratios. These results demonstrate that closed-system magmatic crystallization plays a critical role in zircon Zr isotopic variations. We further propose that the variability of δ94/90Zr values and multiple types of Zr isotopic profiles in zircons are predominantly driven by the compositional effect of adjacent melts from which zircons crystallize. The composition-related δ94/90Zr variations provide a fundamental framework for understanding Zr isotopic evolution in the silicic igneous system and can be used for exploring open-system magmatic processes in the mushy pluton bodies, which are important for continental crust evolution.

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