Impact of hydrothermal alteration on the U-Pb isotopic system of zircons from the Fangcheng syenites in the Qinling orogen,Henan Province,China  

作　　者：BAO Zhiwei WANG Qiang BAI Guodian ZHAO Zhenhua 

机构地区：[1]Key Laboratory for MetaUogenic 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

出　　处：《Chinese Journal Of Geochemistry》2009年第2期163-171,共9页中国地球化学学报

基　　金：supported by National Basic Research Program of China (2006CB403504);National Natural Science Foundation of China (Grant Nos.40672070,40472032)

摘　　要：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 maDisturbance 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 im- pacts 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 angite 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

关 键 词：霞石正长岩 PB同位素 热液锆石 秦岭造山带 同位素体系 防城港 电感耦合等离子体质谱 热液蚀变 

分 类 号：P313.2[天文地球—固体地球物理学] TD985[天文地球—地球物理学]