The Formation And Geodynamics Signification Of Garnet Granites In Daqingshan Area, Inner Mongolia

The research of anatexis and anatexis gneiss have already become one of the hottest points in the studies of the high-metamorphic rock. Through these researches geologists can discuss the infrastructure fuction of the continent crust, and these consequences provide a basis for the understanding of dynamics of continental lithosphere and ecolutional of tectonic of the high-metamorphic rock area. The existing Geological Survey indicate that large-scale anatexises are observed in Precambrian crystalline basement and the root of the large orogenic belt, so in-depth studies of the mechanism of anatexis, is very important for exploration the cause of the crustal derived Granitic Magmas and the early Crustal evolution. Currently, the garnet granites fromed by anatexis is mainly distributed in high-grade metamorphic terranes, but to this kind of garnet granites in high-grade metamorphic terranes research do not distinct. Daqingshan region in Inner Mongolia locates the northern margin of the North China craton, in which explorates a great deal of khondalite series and granulite series , and the garnet granites also develops among them. At present, the formation and characteristics of the garnet granites are not yet finally settled, and many problems still exist.Owing to this, this text uses the garnet granites of the Daqingshan region in Inner Mongolia as a research object, by detailed field geological surveying,by chronology, mineralogy, petrology and geochemistry and the analysis of electron microprobe to the granite, making sure the garnet granites of the Daqingshan region of formation ages, origin type and the correlation with surrounding rock, and with structure of method to discuss the formation and geodynamics signification of Garnet granites.1.granate granites of formation agesMake use of a SHRIMP analysis method to the U-Pb isotope analysis of zircons on garnet granites of the Daqingshan region, The results indicated that the granate granites formation after sillimanite garnet biotite gneiss, not directly formed by magmatic activities, but formed by anatexis of the garnet biotite gneiss, the age is 2388±20 Ma,can make sure its formation ages is terminal new- Archaean.2.The relationship of the garnet granites with the surrounding rock and the enclosureThe garnet granites are strictly chontroled by strata, but do not have unregulation gushing intrusive contact relationships and chilled margins with garnet biotite gneiss, this obviously indicate both of them are transitional, and its boundary misty. We can see a great deal of garnet biotite gneiss with dark mineral in garnet granites, Both of them have no obvious interface, indicate they are transitional. The garnet granites and it inner part of enclosure or relict have the relation of growth and decay on the mineral composition.3.P-T conditions of garnet granitesThrough electron probe microanalysis analysis, the garnet granites formed at peak temperature 701.2⁷48.5℃, when granites magmatism is over, the close temperature down to 662.3⁶90℃, but the change of the press is unprominence. The garnet granites is clockwise PT track, and lie in garnet biotite gneiss PT track, showing the relation of them formation.4.The geochemistry characteristics of garnet granitesThe chemistry composition characteristics of garnet granites:A/CNK (numerator ratio)=1.2¹.59, they all bigger then 1.1, belong to the strongly peraluminous granitoids. The Na₂O content is usually <3.52%, correspondly the sample with SiO2 appears that K₂O>Na₂O>1, widespread contain garnet in the rock Indicate that the garnet granites are peraluminous "S" type granitoids with crust anatexis type characteristics in Daqingshan area, its formation depends on the partial melting of pure crustal materials. The microelement geochemistry characteristic(enrichment LILE(K, Rb, Ba), poor HSFE(Nb, Ti, P)) of garnet granites in this area suggested that its source rock is meta-pelite rock of crustal materials. But the distribute characteristic of garnet granites and garnet biotite gneiss are very similitude, indicate both of them with succession, explaining the relative relations of them. The content differerce of lithophile and transition elements tallies the partial melting. REE distribution patterns of garnet granites change prominence. rich LREE and radiation HREE, mainly expressing to two kinds of forms:Low K₂O and high Na₂O, CaO with high REE (∑REE=233.20-841.97 ppms), and rich LREE, having an obvious negative Eu abnormality (Eu*/Eu=0.32-0.98), reflecting rich garnet, monazite and apatite in the garnet granites.; High K₂O low Na₂O, CaO with low REE(∑REE=87.47- 208.96 ppms), having an obvious positive Eu abnormality (Eu*/Eu=1.30-2.83), suggest that a great deal of plagioclase riched when process of melting. REE distribution patterns of garnet granites different from garnet biotite gneiss, this means geochemistry composition especially REE readijust in the anatexis process.5.The origin type of garnet granitesThe garnet granites plot out syn-collision and post-collision uplift granitoids by tectonic environment in Daqingshan area, inner Mongolia. Thatsuggest the development evolution of garnet granites with the character of collision and orogenics, which represents the lithology combination at the tectonic turning period.The garnet granites of this area formed at granulite facies by anatexis, which has closed contact with regional metamorphism. The garnet granites formation is not intrusived and crystallized by typical magma, but formed by in situ anatexis of sillimanite garnet biotite gneiss and garnet biotite gneiss The metamorphic mineral combined character in this area also confirmed exist anatexis and migmatization and the exist scope more widely.6.The relation of garnet granites and Sub-horizontal Shearing Structures Sub-horizontal Shearing Structures and anatexis are based on easch other and The interaction between resolution. The rock growthed at Sub-horizontal Shearing Structures with higher melting; and in the anatexis gneiss with at higher melting develop Sub-horizontal Shearing Structures. Sub-horizontal Shearing Structures and anatexis take place at the same time.