鞍山地区铁架山花岗岩及表壳岩的锆石SHRIMP年代学和Hf同位素组成

SHRIMP geochronology and Hf isotope composition of zircons from the Tiejiashan granite and supracrustal rocks in the Anshan area,Liaoning province

辽宁鞍山中太古代铁架山花岗岩是华北克拉通时代最古老、分布范围最大的富钾质花岗岩。具相对高钾(4.77～5.75%)低钠(3.16～3.52%)、强烈负铕负钡异常(Eu/Eu*=0.40～0.51,Ba/Ba*=0.15～0.26)的组成特征,且高t_(DM)(Nd) (3.42～3.38Ga),低ε_(Nd)(t)(-3.61～-2.51)。花岗岩样品A9837和A0433岩浆锆石年龄分别为2992±10Ma和2983±10Ma。结合前人定年结果,可把铁架山花岗岩主体形成时代限制在2.96～2.99Ga之间。另一花岗岩样品A9825岩浆锆石年龄为2914±4Ma,可能代表了铁架山花岗岩形成后局部深熔作用的时代。首次在铁架山花岗岩中获得残余锆石年龄,其最大达3759Ma。2个花岗岩样品(A9837,A9825)岩浆锆石的t_(DM)(Hf)和ε_(Hf)(t)分别为3.48～3.32Ga和-7.85～-2.29.残余锆石的t_(DM)(Hf)和ε_(Hf)(t)分别为3.89～3.47Ga和-19.5～-6.2。这些资料为铁架山花岗岩形成于古老陆壳物质再循环提供了直接证据。存在于铁架山花岗岩中的表壳岩主要为变质沉积岩,其地球化学组成特征与铁架山花岗岩类似。3个变质沉积岩样品(A9819,A0435,A0436)的碎屑锆石年龄大都为～3.0Ga,其中2个样品(A0435,A0436)的碎屑锆石ε_(Hf)(t)和t_(DM)(Hf)分别为-9.93～-2.29和3672～3297Ma,与铁架山花岗岩中的岩浆锆石类似。表明这些变质沉积岩形成于铁架山花岗岩之后,而不是以前认为的那样为铁架山花岗岩中的包体。

Abstract Mesoarchaean Tiejiashan granite in the Anshan area is the largest and oldest K-rich granite intrusive in the North-China Craton. It is high in K2O （4.77～5.75% ） and low in Na2O （3.16～ 3.52% ） and shows obvious negative Eu and Ba anomalies （Eu/Eu＊=0.40～0.51, Ba/Ba＊ =0.15～0.26） , with tDM（Nd） and БNd（t） values being 3.42～3.38Ga and -3.61～ -2.51. Two granite samples A9837 and A0433 give magmatic zircon ages of 2992±10Ma and 2983±10Ma, respectively. Combined with the zircon age of 2.96Ga determined by Zhong （1984） and Song et al. （1996） , the formation time of the majority of the Tiejiashan granite can be restricted between 2.99 and 2.96Ga. The other granite sample （A9825） gives a magmatic zircon age of 2914±4Ma, which is considered to represent the time of anatexis of the Tiejiashan granite after its emplacement. All three samples contain inherited zircons, of which the oldest one is up to 3.76Ga. The magmatic/inberited zircons from samples A9837 and A9825 give the tDM （Hf） and БHT（t） of 3.48～ 3.32Ga/3.89～ 3.47Ga and -7.85～ - 2.29/- 19.5 ～ - 6.2, respectively. It is evident that the Tiejiashan granite was derived from ancient continental crust with high maturity, being consistent with the geochemical characteristics of the granite. The supracrustal rocks associated with the Tiejiashan granite are mainly metasedimentary rocks and show similar geochemical features to the Tiejiashan granite. Detrital zircons from three metasedimentary rock samples （ A9819, A0435, A0436 ） are around - 3.0Ga in ages （ Yin et al. , 2006）. The detrital zircons from two of the three samples （ A0435, A0436） give the tDM （Hf） and БHT（t） of 3.67 ～ 3.30Ga and - 9.93~ - 2.29, respectively. The detrital zircons are very similar in age and Hf isotope composition to the magmatic zircons from the Tiejiashan granite, indicating that the metasedimentary rocks were deposited later than the Tiejiashan granite and so they are not enclaves in the granite, as previously considered （ Wan et al. , 2002）.