SHRIMP geochronology and Hf isotope composition of zircons from the Tiejiashan granite and supracrustal rocks in the Anshan area,Liaoning province
SHRIMP geochronology and Hf isotope composition of zircons from the Tiejiashan granite and supracrustal rocks in the Anshan area,Liaoning province
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作者：
Wan Yu
鞍山地区中太古代铁家山花岗岩是华北地区最大、最古老的富钾花岗岩侵入体（4.77~5.75%），Na_2O含量低（3.16~3.52%），具明显的Eu、Ba负异常（Eu/Eu ~*=0.40~ 0.51，Ba/Ba ~*=0.15~0.26），tDM（Nd）和deNd（t）值分别为3.42~3.38Ga和-3.61~-2.51。两个花岗岩样品A9837和A0433的岩浆锆石年龄分别为2992± 10 Ma和2983± 10 Ma，结合钟（1984）和宋（1984）等人的锆石年龄2.96Ga。（1996），大部分铁家山花岗岩的形成时间可限定在2.99 ~ 2.96Ga之间，另一个花岗岩样品（A9825）的岩浆锆石年龄为2914± 4 Ma，代表了铁家山花岗岩侵位后的深熔时间。A9837和A9825样品的岩浆/继承锆石的tDM（Hf）和deHf（t）分别为3.48~3.32Ga/3.89~3.47Ga和-7.85~-2.29/-19.5~-6.2，结果表明，铁家山花岗岩是高成熟度的古陆壳源花岗岩，与花岗岩的地球化学特征相一致。与铁家山花岗岩共生的表壳岩主要为变质沉积岩，其地球化学特征与铁家山花岗岩相似。3个变质沉积岩样品中的碎屑锆石与铁家山花岗岩的地球化学特征基本一致。（A9819，A0435，A0436）的年龄约为3.0Ga（Yin et al.，2006).三个样本中的两个样本中的碎屑锆石（A0435，A0436）的tDM（Hf）和deHf（t）分别为3.67~3.30Ga和-9.93~-2.29。表明变质沉积岩的沉积晚于铁甲山花岗岩，因此它们不是花岗岩中的包体，如先前所认为的（Wan等人，2002年）。
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 K_2O(4.77～5.75%)and low in Na_2O(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 t_(DM)(Nd)ande_(Nd)(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/inherited zircons from samples A9837 and A9825 give the t_(DM)(Hf)ande_(Hf)(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 t_(DM)(Hf)ande_(Hf)(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).