Research On Precambrian Metamorphosed Plutonic Intrusions In The Northern Alxa, Inner Mongolia

Based on the study about the Zhunaohutu gneiss and Sunbuerwula gneiss, thispaper reaches the following conclusions.1.The previous studies suggest that the rock in the area of Zhunaohutu isgranodiorite, however, which factually is a gneiss series whose protolith is a graniteand granodiorite pluton. The main lithology of it are dark gray augen gneiss andreddish augen biotite gneiss.The geological body in the area of Sunbuerwula which is regarded as tectonicbelt and ductile shear zone by previous studies is a gneiss series of which the protolithis a granite in fact. The main lithology of Sunbuerwula gneiss are augen biotite gneissand augen gneissic granite.2.Both of the gneisses in the areas of Zhunaohutu and Sunbuerwula have thefollowing common features: The main elements show geochemical characteristics likethat:high－Si, low－calcium, metaluminous and high－K; the rock shows no iron－rich trend so they belong to sub－alkaline series; K2O(wt%)＞Na2O(wt%); σ＜3.3sothe rocks belong to calc－alkaline series; the rocks show the evolution characteristicof non－homologous magma; the features of trace elements in the rocks show relativeenrichment of La, Nd and Hf and relative depletion of Sr, Nb, P and Ti; Rocks showfeatures of higher total REE contents, higher fractionation of LREE and HREE andLREE enrichment.The differences between gneisses in the area of Zhunaohutu and Sunbuerwulainclude that REE features of Zhunaohutu gneiss is with both positive and negative Euabnormality. In contrast, there is only negative Eu abnormality in Sunbuerwula gneiss.The result of U－Pb isotopic dating by LA－ICP－MS method of Sunbuerwulagneiss is1433±8Ma while the result of Zhunaohutu gneiss is1510±11Ma.3. The protolith of Zhunaohutu gneiss formed in a continental magmatic arctectonic setting at a pro－collision and syncollisional active continental margin. Itsmagma source include partial melting of crustal material. The protolith of Sunbuerwula gneiss formed in a continental magmatic arctectonic setting at a pro－collision and syncollisional active continental margin. Itevolved from molten magma by non－homologous continental crust.4. The preliminary view is that the study area and the North China Plate shouldbelong to different tectonic units.