Iron Isotope Composition of Adakitic Rocks: The Shangcheng Pluton, Western Dabie Orogen, Central China

Chenglai Deng,Ming Li,Wu Li,Changqing Hu

doi:10.3390/min11121356

Chenglai Deng, Ming Li + Show 2 more

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https://doi.org/10.3390/min11121356

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Abstract

There has been little research on the metal isotopic composition of adakitic rock. The main objective of our investigation was to obtain more knowledge on the iron isotopic composition of adakitic rocks and provide new evidence for the genesis of Shangcheng pluton from an iron isotope perspective. The Dabie orogen is divided into eastern and western areas by the Shangcheng-Macheng fault, and the Shangcheng pluton is located in the western Dabie orogen area. The iron isotopic composition of these rocks ranges from 0.08‰ to 0.20‰ (2SD, n = 3). The δ56Fe values of two rocks from the SGD (Sigudun) unit are relatively low (0.11 ± 0.03‰ and 0.08 ± 0.04‰), while the δ56Fe values of the other samples are basically consistent (0.18–0.2‰). Evidence from elemental geochemical characteristics and petrogenesis defines the Shangcheng pluton as adakitic rocks. Our investigation on the elemental and isotopic compositions hints that the enrichment of heavy iron isotopes cannot be explained by weathering/alteration and fluid exsolution. Fractional crystallization of magnetite may account for the enrichment of light iron isotopes in two rocks from the SGD unit, while the fractional iron isotope trend in the other five samples can be explained by Δ56Fecrystal-melt = ~0.035‰. Two investigated rocks from SGD units may have been derived from the partial melting of amphibolite, while the other five samples may have been derived from the partial melting of eclogite containing 10–15% garnet.

Highlights

Recent theoretical developments have revealed that the fractionation of iron isotopes occurs during magmatic processes
Fractional crystallization, and fluid exsolution can all be responsible for the enrichment of heavy iron isotopes [1,2,3,4,5,6,7,8,9,10,11,12,13]
After combining the mineralogy and elemental composition characteristics (Figure 5a,b), we propose that the Shangcheng pluton mainly includes two kinds of rocks: biotite monzonite and quartz monzonite

Summary

Introduction

Recent theoretical developments have revealed that the fractionation of iron isotopes occurs during magmatic processes. Fractional crystallization, and fluid exsolution can all be responsible for the enrichment of heavy iron isotopes [1,2,3,4,5,6,7,8,9,10,11,12,13]. Factors besides magmatic processes can cause significant fractionation of iron isotopes. Redox conditions account for the fractionation of iron isotopes. Li et al [15] reported that the δ56Fe content of the Archaean BIF deposit is 1.5–2.6‰, due to a low degree of Fe (II) oxidation along with very low oxidation rates, resulting in the enrichment of heavy iron isotopes. Fe3+ released from primary minerals will form secondary minerals under an oxidative environment, preventing a loss of Fe and a change in the iron isotopic composition [16,17]

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Conclusion