鞍山地区东鞍山花岗岩年代学、地球化学特征及成因研究

鞍山地区位于华北地台东北部辽宁省,区内保留有3.8~2.5Ga连续的地质记录。对位于鞍山市南侧的东鞍山花岗岩进行了SHRIMP锆石U-Pb同位素分析、白云母Ar-Ar同位素分析及岩石地球化学分析。SHRIMP测年结果为3004±7Ma,代表岩石的形成年龄,Ar-Ar测年结果为2545±16Ma,代表岩石受到构造热事件扰动的时间。岩体地化特征为富硅(Si O2=72.95%~75.37%,平均值为74.18%)、富碱(K2O+Na2O=7.05%~8.45%)、富铝(Al2O3=12.95%~15.44%),低钙(Ca O=0.13%~0.66%)。在稀土元素配分图上,曲线呈明显的右倾趋势,且有较明显的负Eu异常,重稀土分馏不明显。在微量元素洋中脊玄武岩标准化蛛网图上可以看出,东鞍山花岗岩强烈亏损Nb、P、Sr、Ti,富集大离子亲石元素Rb、K、Nd和高场强元素Th、U。岩石地球化学特征表明东鞍山花岗岩岩浆来源为壳源,残留相可能由石榴石+辉石+角闪石+斜长石组成。将鞍山-本溪地区3个3.0Ga花岗岩(东鞍山花岗岩岩体、铁架山二长花岗岩岩体以及弓长岭片麻状花岗岩)的地球化学数据进行对比,发现三者的地球化学存在较大差异,为三个独立的岩体。三个岩体最初可能发育在一个陆块之上,然后在25.5亿年左右分离开来,最后在25亿年左右再次拼贴到一起。

Chronology, petro-geochemistry and genesis of Donganshan granite in Anshan area

Anshan area is located in Liaoning Province, NE China, and is the place where consecutive geological records ranging from 3.8Ga to 2.5Ga are preserved. SHRIMP dating, muscovite Ar-Ar dating and geochemical data have been undertaken on the Donganshan granite. The results of SHRIMP dating and muscovite Ar-Ar dating indicate that Donganshan granite was formed in 3004±7Ma, and disturbed by structural thermal event in 2545±16Ma. Donganshan granite is high in SiO₂, alkaloid, Al₂O₃ and low in CaO. The chondrite-normalized REE diagram shows right-dipping linear trends and slightly negative Eu anomalies. The NMORB-normalized trace element diagram shows negative anomalies of Nb, P, Sr and Ti, and positive anomalies of Rb, K, Nd, Th and U. The geochemical features indicate that Donganshan granite is derived from a magma source with crustal rock composed of garnet+pyroxene+amphibole+plagioclase as a residual phase. According to the result of comparison on petrologic geochemistry of three 3.0Ga granitic plutons(Donganshan granite, Tiejiashan adamellite and Gongchangling gneissic granite) in Anshan-Benxi area, those plutons have different petrologic geochemical features and are relatively independent. Initially they were located in the same block, then separated at 2.55Ga and collaged together at 2.50Ga.