辽北抚顺—清原地区太古宙基底地球化学组成特征及其地质意义

Geochemical Characteristics of Archaean Basement in the Fushun—Qingyuan Area, Northern Liaoning Province and Its Geological Significance

最近的研究表明 ,辽北抚顺、清原地区太古宙基底主体形成于新太古代 ,不存在广泛分布的中太古代地质体。表壳岩系以不同规模分布于 TTG花岗质岩石中 ,主要由斜长角闪岩、角闪变粒岩、黑云变粒岩及少量片麻岩组成。本文对该区新太古代表壳岩系和 TTG花岗质岩石进行了地球化学研究。小莱河斜长角闪岩具近于平坦型的稀土模式 [(L a/ Yb) n=1 .0 ],大离子亲石元素富集。存在 3种类型角闪变粒岩。类型 1角闪变粒岩具平坦型稀土模式 [(L a/ Yb) n=1 .1～ 2 .9],大离子亲石元素富集 ,其特征与共生的斜长角闪岩十分相似。类型 2角闪变粒岩常量元素组成与类型 1角闪变粒岩类似 ,但轻稀土富集 [(L a/ Yb) n=1 1 .4～ 2 0 .2 ],与之存在较明显区别。类型 3角闪变粒岩轻重稀土强烈分离 [(L a/ Yb) n=4 1 .8]。黑云变粒岩稀土模式和微量元素组成与类型 2角闪变粒岩相似 ,但显示出更为富集的组成特征。 TTG花岗质岩石构成辽北太古宙基底的主体。它们轻重稀土强烈分离 [(L a/ Yb) n=2 5 .8～1 94 .4 ],无明显负铕异常 (Eu/ Eu* =0 .75～ 1 .35 )。研究表明 ,辽北地区太古宙基底表壳岩系变质原岩主体为拉斑玄武岩、安山岩、英安岩及相应火山碎屑沉积岩 ,表壳岩系和 TTG花岗质岩石形成于板块汇聚的岛弧环境。

In terms of the recent zircon SHRIMP U-Pb geochronological research the basement in the Fushun—Qingyuan area, Liaoning Province was mainly formed at the end of Archaean, without wide distribution of Mesoarchaean material as previously thought. The supracrustal, mainly occurring as enclaves in different scales in TTG, is predominantly composed of amphibolite, hornblende fine-grained gneiss, biotite fine-grained gneiss and a few of schist. All the rocks analyzed show that Na 2O is higher in content than K 2O, only with an exception for some biotite fine-grained gneiss and gneiss of meta-sedimentary origin. The amphibolite is low in ΣREE (20.3×10 -6) and has flat REE pattern [(La/Yb) n=1.0], but with LIL enrichment. There also is amphibolite with fractionated REE pattern in the area. There are three kinds of hornblende fine-grained gneisss. Type 1 shows flat REE pattern [(La/Yb) n=1.1～2.9] and LIL enrichment, quite similar geochemically to the amphibolite with flat REE pattern. Type 2 hornblende fine-grained gneiss is similar in major element composition to the type 1, but different in being high in ΣREE (78.4×10 -6～133.0×10 -6) and having fractionated REE pattern [(La/Yb) n=11.4～20.2]. Type 3 hornblende fine-grained gneiss shows strong fractionated REE pattern [(La/Yb) n=41.8]. The biotite fine-grained gneiss shows more evolution characteristics, ΣREE and (La/Yb) n being 145.9×10 -6～165.6×10 -6 and 10.6～23.6 respectively. TTG granitoids, being the majority of the Archaean basement in the area, show strong fractionated REE pattern [(La/Yb) n=25.8～194.4]. Sm, Nd isotope compositions have also been analyzed for some kinds of rocks. According to geological and geochemical researches, some conclusions can be drawn as following: (1) Metamorphic protoliths of the supracrustal rocks are mainly composed of basalt, andesite, dacite and relevant volcano-sedimentary rocks, and some detrial sedimentary rocks with different maturities. (2) The supracrustal and TTG rocks are formed in an island-arc environment, meaning that the tectonic system similar to modern plate movement occurred at the end of Archaean. (3) The Archaean basement constitutes a typical arc-continent collision belt, being an important part of the Ji-Liao-Ji (Jilin-Liaoning-Hebei) huge orogenic belt of Neoarchaean in east of the North China Craton.