Isotopic Chronological Restriction Of Granulite Facies Metamorphism In Daqingshan Area, Inner Mongolia

Granulite is one kind of high grade metamorphites. As one kind of Archean high metamorphites, the lithology of Granulite series exist extensively in Daqingshan Mountain ,Inner Mongolia Autonomous Region.. As one part of the oldest lithology series of Archean high metamorphites in North China Craton,and beening effected by perplexing deformation and metamorphism , Daqingshan Mountain laying in Yinshan Intra-plate Mountain which was builded in Mesozoic, have been researched by domestic and overseas sophists for a few decades. Although a great many achievements have been made, there are some puzzles and arguments on high grade metamorphites production in craton of Northern China worry those geologists.,including the others high grade metamorphitic immingle lithologies. The most disputative problem is about the chronology of Granulite series in Daqingshan Mountain. Although many different data of age and methods of partitioning the age to several stages have been made by different researchers using different testing means, these data are no much convictive. So , researches on granulite series in the area could resolve many puzzles and argrutive problems.This article is based on this thought, through studying the granulite series rocks of Daqingshan area systematically,including geochemistry, chronology , the times of metamorphosing studying,we can get several aspects as the following. 1. The metamorphic periodsMake certain that the granulite series rocks of Xinghe Group in the area have been undergone three metamorphic periods: the early granulite faces metamorphism,mid-granulite faces metamorphism and anaphase superimpose metamorphism.Basic granulite veins have been undergone three times transformation because three periods of veins intrusion.2. Granulite protolith RestorationWith studying the petrochemistry and geochemistry of the granulite in the area and charnockitic gneis systematically,especially,studying from rock trace element and lanthanon in detail and getting the protolith restorated , we can make certain that the protoliths formation of Xinghe group granulite in the area is composed of basic rock ,neutral rock and acid volcanic rock and some deposit dross.This kind of lava formed from the surrounding of active continent edge belongs to calc-alkaline series volcanic rock. Charnockitic gneiss is formed by granulite remelting,so they two are similar chemic elements and succession.The petrochemistry characteristic and geochemistry characteristic of granulite of Xinghe Group this area are similar to that of which in Jining and in northwest of Hebei Province, they both have the same formation.3. Granulite series isotopic agesAfter dating the U-Pb isotopic ages of the single zircon in the granulite series by SHRIMP and analyzing the former aging data and the reliability ageing data,we can draw conclusion that the formatic age of protoliths is 2516±15Ma～2506±9Ma which belongs to pre-Archean to mid-Archean,the metamorphosing age of granulite is 2478±14Ma～2472±14Ma which belongs to mid-Archean to new-Archean,the metamorphosing age of charnockitic gneiss is 2496±22 Ma which may be older than the true age because of the metamorphosing ages of charnockitic gneiss being much older than the formatic ages. The reasons of this discrepancy: it may be lost Pb when the charnockitic gneiss is metamorphosing,or through the later metamorphosing process maybe the mantle of zircon thansform other new substance after forming gneissose zircon.