糜棱岩化-绢云母化对东秦岭方城正长岩中锆石U-Pb体系的影响

Mylonitization and hydrothermal alteration impacts on U-Pb system of zircons:A case study of Fangcheng syenites in eastern Qinling Orogen

方城碱性正长岩岩体产于秦岭造山带东段北秦岭构造域,侵位于新元古界栾川群黑云母、绢云母石英片岩中,主要由霓辉正长岩、霓霞正长岩、黑云母正长岩和角闪霞石正长岩组成。具有轻微绢云母化的霓辉正长岩中的锆石为无色透明的短柱状晶体,其阴极发光图像显示清晰的振荡韵律环带,锆石的LA-ICPMS分析表明岩体形成于新元古代,206Pb/238U加权平均年龄为(844.3±1.6)Ma(MSWD=0.86)。而靠近断裂带的糜棱岩化和强烈绢云母化霓辉正长岩中的锆石则为淡黄-黄褐色半透明颗粒,其阴极发光和背散射电子图像显示其裂隙非常发育,有时为钾长石穿插呈海绵状,仅个别颗粒的局部区域仍可见残留的岩浆锆石的环带结构。LA-ICPMS分析结果表明,交代蚀变所形成的热液锆石的Pb、U、Th含量显著增高,而且成分极不均一,含量变化范围很大,U、Th含量分别在32～1550μg/g和188～4059μg/g之间。锆石Th/U比值具有很大的变化范围,Th/U=0.7～44.9。热液锆石206Pb/238U表面年龄与其U、放射成因Pb及普通Pb含量成反比。由于岩浆锆石U、Th含量较低,其α衰变放射性剂量较小(Dα=3.65×1014～2.04×1015/mg),因此扩散作用所引起的放射成因铅的活动性可忽略不计,所以糜棱岩化、绢云母化霓辉正长岩中锆石U-Pb体系的扰动应是糜棱岩化对锆石晶体结构的损伤和强烈热液交代的结果。由于热液锆石的形成经历了溶解-再沉积等复杂的过程,其U-Pb年龄(不一致线下交点年龄)的地质意义并不明确。

Zircon, a ubiquitous accessory mineral in a wide variety of igneous, metamorphic and sedimentary rocks, is one of the most widely used minerals for determining the age, origin and thermal history of rocks by U-Th-Pb geochronology. It is considered as the most useful mineral for U-Pb geochronology, because, in addition to an extremely high ratio of U to initial common Pb, it is mechanically and chemically durable, and has one of the highest known thermal closure temperatures. However, as the employment of new, state of art equipments and methodologies, which enable more precise and in situ analysis, many studies demonstrated that disturbance of zircon U-Pb system, particularly significant Pb loss is common in zircon in low temperature environments. Here, we report a field study of mylonitization and intensive sericitization impacts on zircons from the Fangcheng syenites. The Fangcheng syenites occur in the northern Qinling structural domain of eastern 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 the cathodoluminescence （CL） images exhibit well developed oscillatory and sector zoning typical of magmatic zircons from alkaline rocks. LA-ICPMS zircon U-Pb analyses show that the syenite was formed in Neoproterozoic, weight average of ^206 Pb/^238 U age is （844.3 ± 1.6） Ma （MSWD = 0.86） . In contrast, the hydrothermal inflicted zircons （hydrothermal zircon） from the mylonitized and intensively sericitized aegirine-augite syenite are conglomerate, yellowish to brown, generally translucent and internally textureless. The CL and backscatter electron （BSE） images of hydrothermal zircons exhibit fractured, textureless or mosaic texture, and occasionally show ＂sponge texture＂ with the veinlets and inclusions of K-feldspar, however, relicts of magmatic oscillatory zoning can still be observed locally in individual grains. LA-ICPMS analyses of the hydrothermal zircons demonstrate 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 32 - 1550 μg/g and 188 - 4059 μg/g, respectively, with Th/U ratios of 0. 7 - 44. 9. ^206Pb/^238 U apparent ages of hydrothermal zircons are negatively correlated with the contents of U, radiogenic and common Pb. Because of the U and Th concentrations of the magmatic zircons are rather low, the α-decay doses （3.65 × 10^14 - 2.04 × 10^15 /mg） are much lower than the first percolation point （3.5 × 10^15 /mg）, thus, lead loss resulted from diffusion could be safely neglected. The U-Pb disturbance of the zircons is most likely due to the intensive sericitization coupled with mylonitization, the hydrothermal fluids altered the zircons along the rims and fractures of the distorted zircon crystals and formed the chemically inhomogeneous hydrothermal zircons through a series complicated mechanisms such as dissolution-reprecipitation. The significance of the U-Pb age （the lower intercept age on discordia U-Pb plot） of the hydrothermal zircons is uncertain. We argue that interpretations of hydrothermal zircon data as the age of hydrothermal events or hydrothermal ore-forming processes are questionable and cautions must be taken.