Impact of hydrothermal alteration on the U-Pb isotopic system of zircons from the Fangcheng syenites in the Qinling orogen,Henan Province,China |
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| Authors: | Zhiwei Bao Qiang Wang Guodian Bai Zhenhua Zhao |
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| Institution: | (1) Key Laboratory for Metallogenic Dynamics, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China;(2) Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China;(3) Henan Institute of Geological Survey, Zhenzhou, 450007, China |
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| Abstract: | Disturbance of the zircon U-Pb isotopic system has been investigated extensively, but mostly in lab, in the last decades.
Here, we reported a field-based study on intensive sericitization, K-feldsparthization and the impacts of mylonitization on
zircons from the Fangcheng syenites. The Fangcheng syenites occur in the eastern part of the Qinling orogen and consist mainly
of aegirine-augite syenite, aegirine nepheline syenite, biotite syenite and hornblende nepheline syenite. Zircons from the
slightly sericitized aegirine augite syenite are colorless, transparent crystals and exhibit well-developed oscillatory and
sector zoning on the cathodoluminescence (CL) images which are typical of magmatic zircons from alkaline rocks. Zircon U-Pb
determinations by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) showed that the syenite was formed
in Neoproterozoic time, the weighted average of 206Pb/238U ages is 844.3±1.6 Ma (MSWD=0.86). In contrast, the hydrothermally altered zircons (hydrothermal zircon) from the intensively
sericitized, K-feldsparthized, and weakly mylonitized aegirine augite syenite are conglomerates, yellowish to brown in color,
generally translucent and internally textureless. The CL and backscatter electron (BSE) images of hydrothermal zircons exhibit
fractured, textureless or mosaic textures, and occasionally show “sponge texture” with the veinlets and inclusions of K-feldspar;
however, relicts of magmatic oscillatory zoning can still be discerned locally in individual grains. LA-ICPMS analyses of
the hydrothermal zircons demonstrated that the zircons are chemically inhomogeneous, with enhanced and widely varied Pb, U,
and Th contents. The U and Th contents of the hydrothermal zircons are estimated to be 32×10−6−1550×10−6 and 188×10−6−4059×10−6, respectively, with Th/U ratios within the range of 0.7–44.9. 206Pb/238U apparent ages of the hydrothermal zircons are negatively correlated with the contents of U, and radiogenic and common Pb.
As the U and Th concentrations of the magmatic zircons are rather low, the α-decay doses (3.65×1014−2.04×1015 α-decay events/mg) are much lower than those at the first percolation point (3.5× 1015 a-decay events/mg), thus, Pb mobility resultant from diffusion could be safely neglected. Disturbance of the U-Pb isotopic
system of zircons is most likely to be attributed to the intensive sericitization and K-feldsparthization coupled with mylonitization,
the hydrothermal fluids reacted with zircons along the rims and fractures of the distorted zircon crystals, giving rise to
the chemically inhomogeneous hydrothermal zircons through a series of complicated mechanisms such as dissolution-reprecipitation.
The significance of the U-Pb age (the lower intercept age on the discordia U-Pb plot) of hydrothermal zircons is uncertain.
We argued that interpretations of the hydrothermal zircon data as the age of hydrothermal events or hydrothermal ore-forming
processes are questionable and hence cautions must be taken. |
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| Keywords: | zircon hydrothermal alteration mylonitization U-Pb system alkali-syenite Fangcheng |
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