鞍山中太古代铁架山花岗岩中表壳岩包体的地球化学特征及地质意义

GEOCHEMICAL CHARACTERISTICS OF SUPRACRUSTAL ENCLAVES IN MESOARCHEAN TIEJIASHAN GRANITE IN ANSHAN AREA AND ITS GEOLOGICAL SIGNIFICANCE

鞍山地区中太古代铁架山花岗岩中存在多种类型的表壳岩包体。本文对其中2类变泥砂质岩石进行了研究。一类包括云母石英片岩、黑云变粒岩、浅粒岩 ,它们高钾低钠 ,具有强的负铕异常 (Eu Eu =0 1 0～ 0 2 3 )和负钡异常 (Ba Ba =0 0 9～ 0 2 3 ) ,tDM 和εNd(3 0Ga)变化范围分别为 3 3 6～ 3 5 1Ga和 - 3 89～ - 1 5 9。岩石高的成熟度和长的地壳滞留时间表明鞍山地区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。它们在元素组成上与铁架山花岗岩十分相似 ,相互间存在物源上的成因联系。铁架山花岗岩的tDM和εNd(3 0Ga)变化范围分别为 3 3 9～ 3 70Ga和 - 4 99～ - 2 3 6 ,Sm、Nd同位素组成与变质泥砂质岩石存在一定差异 ,看来主要与前者形成过程中源区选择性部分熔融和稀土组成发生变化有关。另一类为黑云斜长片麻岩 ,组成上与第一类变泥砂质岩石存在较大区别 ,而与古太古代陈台沟花岗岩较为相似 ,相互间可能存在物源上的成因联系。它们对铁架山岩体部分轻、重稀土强烈分离花岗质岩石的形成可能起了一定作用。物源区的复杂性是铁架山花岗岩组成上存在较大变化的主要原因。

There is a lot of supracrustal enclaves in the Mesoarchean Tiejiashan granite in the Anshan area, Liaoning Province. Two kinds of meta argillo arenaceous rocks are studied in this paper. One of them, including mica quartz schist, biotite leptite and leuco leptite, is high in K 2O and low in Na 2O, with strongly negative Eu and Ba anomalies (Eu/Eu *=0 10～0 23, Ba/Ba *=0 09～0 23). Their t DM and ε Nd (3 0 Ga) range from 3 36 to 3 51 Ga and from -3.89 to -1.59, respectively. The high maturity and long crustal residence time indicate that in the Anshan area a relatively large scale continental basement with long evolutionary history has already existed before the Mesoarchean. These rocks are considered to constitute the main material of the source region from which the Tiejiashan granite is derived. However, the Tiejiashan granite shows slightly higher t DM ages (3.39～3.70 Ga) and lower ε Nd (3 0 Ga) (-4 99～-2 36) compared with the meta argillo arenaceous rocks. The Nd isotopic differences between them seem dominantly to be correlative to the selective partial melting of the material of the source region and LREE/HREE ratio varies in the forming process. The other kind of meta argillo arenaceous rock is biotite plagioclase gneiss, geochemically being somewhat different from the one mentioned above, but similar to the Paleoarchean Chentaigou granite in the Anshan area. They might play some important roles in the formation of some of the Tiejiashan granite with strongly LREE enriched chondrite normalized REE patterns. The complex compositions of the source region are doubtlessly one of the main reasons why there are largely composition variations in the Tiejiashan intrusion.