Chemical Geology ( IF 3.9 ) Pub Date : 2022-08-24 , DOI: 10.1016/j.chemgeo.2022.121072 Peng Gao, Yan Wang, Chris Yakymchuk, Hai-Ou Gu, Guo-Chao Sun, Changqing Yin, Jian Zhang, Jiahui Qian
Zircon Hf isotope compositions provide valuable constraints on granite petrogenesis and the evolution of continental crust. However, the causes of variations in zircon Hf isotope ratios during magma evolution are poorly understood. Here we compiled zircon UPb ages and LuHf isotope compositions of Permian-Triassic granites in South China to demonstrate that Hf isotope variability is common in the early stages of granitic magma evolution, but Hf isotope ratios are homogenized during the final stages of granite crystallization. Late-grown zircons have relatively high concentrations of U and have εHf(t) values approximating the median values of multiple analyses for the low-U zircon in the same samples. Hafnium isotope variations measured in low-U zircon could be related to incremental assembly of multiple isotopically distinct magma batches derived from partial melting of heterogeneous crustal rocks; these discrete magma batches may have remained isolated from each other during segregation and extraction in the source and during ascent through the crust. After emplacement, these magma batches may have experienced variable degrees of mixing resulting in homogenization of Hf isotopes in late crystallized zircon. Coupled with whole-rock geochemical data of the Permian-Triassic granites, we argue that the Hf isotope homogenization is related to crystallization of zircon from late-stage magmas or precipitation of hydrothermal zircon from exsolved hydrothermal fluids during the final stages of magma solidification. Hafnium isotope homogenization in late-stage melt/fluid may have been driven by an increased concentration of fluorine and the efficient diffusion of hafnium-fluoride complexes through silicic melt. These results provide a deeper understanding of the mechanisms that contribute to Hf isotope heterogeneity in zircon during granite crystallization.
中文翻译:
花岗岩结晶后期锆石Hf同位素均质化
锆石 Hf 同位素组成对花岗岩成因和大陆地壳演化提供了有价值的约束。然而,人们对岩浆演化过程中锆石 Hf 同位素比值变化的原因知之甚少。本文通过对华南地区二叠系-三叠系花岗岩锆石U Pb年龄和Lu Hf同位素组成进行统计,证明Hf同位素变化在花岗岩浆演化早期较为普遍,但在花岗岩结晶后期Hf同位素比值均一化。 . 晚生锆石具有相对较高的 U 浓度和 ε Hf(t) 值近似于同一样品中低 U 锆石的多次分析的中值。在低 U 锆石中测量的铪同位素变化可能与源自非均质地壳岩石部分熔融的多个同位素不同岩浆批次的增量组装有关;这些离散的岩浆批次可能在源头的分离和提取过程中以及在通过地壳上升的过程中保持相互隔离。就位后,这些岩浆批次可能经历了不同程度的混合,导致晚期结晶锆石中 Hf 同位素的均质化。结合二叠系-三叠系花岗岩的全岩地球化学数据,我们认为,Hf同位素均质化与晚期岩浆中锆石的结晶或岩浆凝固最后阶段从溶出的热液中热液锆石的沉淀有关。后期熔体/流体中的铪同位素均质化可能是由增加的氟浓度和氟化铪配合物通过硅熔体的有效扩散驱动的。这些结果提供了对花岗岩结晶过程中导致锆石中 Hf 同位素异质性的机制的更深入理解。