辽宁鞍山-本溪地区中太古代花岗质岩石的成因——地球化学及Nd同位素制约

The Origin of Mesoarchaean Granitic Rocks from Anshan Benxi Area:Constraints of Geochemistry and Nd Isotope

鞍山－本溪地区中太古代花岗质岩浆作用十分发育，形成了３．１Ｇａ立山细粒奥长花岗岩、３．０Ｇａ东鞍山片麻状花岗岩、３．０Ｇａ铁架山片麻状花岗岩和３．０Ｇａ弓长岭片麻状花岗岩等一系列花岗质岩石。岩石的εＮｄ（ｔ）分别为，立山奥长花岗岩（１０个样品）变化于－１．９６～４．２４之间，平均值为０．７６，东鞍山片麻状花岗岩（５个样品）变化于０．２６～２．７２之间，平均值为１．２６，铁架山片麻状花岗岩（５个样品）变化于－３．８５～－４．８３之间，平均值为－４．２８，弓长岭片麻状花岗岩（１个样品）为－２．５８。结合岩石的常量、微量和稀土元素特征，可以认为，立山奥长花岗岩和东鞍山片麻状花岗岩形成于地壳滞留时间不长的中酸性陆壳岩石的部分熔融，前者很可能有玄武质物质的参与。而铁架山片麻状花岗岩和弓长岭片麻状花岗岩则来自于成熟度很高的早期陆壳物质。由此可以推断，鞍山－本溪地区在中太古代以前陆壳基底就有了相当的规模，并具有很长的地壳演化历史。

This paper concerns the Anshan-Benxi area in the southwestern segment of the Liaoning Province. The area contains numerous occurrences of Mesoarchaean granitic rocks, including 3 1 Ga Lishan fine-grained trondhjemite, 3 0 Ga Donganshan gneissic granite, 3 0 Ga Tiejiashan gneissic granite and 3 0 Ga Gongchangling gneissic granite. According to neodymium isotopic analyses performed on whole rock powders, the Lishan trondhjemite(10 samples) and the Donganshan granite(5 samples) have the initial ε Nd ( t ) values varying between -1 96 and 4 24(with an average of 0 76) and between 0 26 and 2 72(with an average of 1 26) respectively. The Tiejiashan granite(5 samples) shows much more enriched signature, yielding the initial ε Nd (3 0 Ga) values from -3 85 to -4 83 and an average of -4 28. The Nd isotopic composition for the Gongchangling granite(1 sample) is somewhat similar to that for the Tiejiashan granite. Combined with element geochemical researches, some main conclusions can be obtained that first, the Lishan trondhjemite and Donganshan granite were formed by partial melting of different kinds of intermediate acid terrestrial rocks but noly with short crustal residence time, the source region of the Lishan trondhjemite might include some basaltic material. Second, the Tiejiashan granite and the Gongchangling granite were derived from more ancient continental crust with significantly higher maturity. In the Anshan Benxi area a relatively large scale continental basement with long evolutional history had already existed before the Mesoarchaean at the early stage of the Earth evolution.