The Geochronological,Geochemical Researches And Tectonic Implications Of Middle Triassic Monzonitic Granites In The Naomuhunni Area,Southern Sonid Zuoqi

Sonid Zuoqi area of Inner Mongolia autonomous region belongs to the northern orogenic belt of XMOB,and it belongs to the eastern part of the central Asian orogenic belt.Magmatic activity is very active in the area.Between the Mesoproterozoic and the Mesozoic,there are a lot of different types of granites.And there have been a lot of detailed studies about the granitic magmatic activities in the Sonid Zuoqi area,and there are different understandings on its formation age,petrogenesis and tectonic setting.The study of Muscovite granites(or di-micaceous adamellite)in the study area is relatively less,and there is a lack of accurate information about chronology and rock geochemical.In addition,although many scholars still have different perspectives on the magma source and rock types of the granites that containing muscovite.Because of its typical geochemical characteristics and special indicative meaning of tectonic environment,it has attracted the attention of many scholars.Therefore,the petrology,chronology and whole rock geochemistry of Muscovite monzonitic granite and Two-mica monzonitic granite in the study area were studied in detail.It is determined that the formation age of these two types of monzogranites are middle Triassic,which were probably derived from the Neoproterozoic accreted crustal materials.The tectonic environment of the middle Triassic in the study area is discussed through the correlation study of intrusive rocks in the same area and other areas of the same tectonic unit,which provides a fine constraint for the study of the complex tectonic evolution history of the Xing-Meng Intracontinent Orogenic Belt.By observing the image of zircon cathode glow of Muscovite monzogranite and Two-mica Monzogranite were found in the Naomuhunni area,it can be seen from the the image of zircon cathode glow of the zircons that the zircons have clear growth belts.At the same time,Th/U ratio of both is higher(0.05-2.34,0.31-2.26).Suggesting a magmatic origin.Dating results showed the formation ages of these granites were 239.7±2.1Ma and240.2±1.9Ma,respectively,and it is showed that the two mozogranites are products of magmatic activity during the Middle Triassic.The geochemical analysis results showed that the two types of monzogranite were rich in LREE,deficient in HREE,and significantly differentiated between LREE and HREE((La/Yb)_N=2.43-24.69).Enrichment of LILEs(Rb,K),loss of HFSEs(U,Nb,Ta,P,Ti),has the remarkable Eu negative anomaly(?Eu=0.27-0.95,<0.1),and their P₂O₅ content is between 0.001%to 0.07%,the two types of monzonite granites have obvious Si O₂-P₂O₅negative correlation,indicates that the two types of adamellite granites are I-type granites.In addition,two types of adamellite granites have low content of Mg O(0.03%-0.06%),TFe₂O₃(0.59%-0.90%),Ca O(0.58%-1.35%),Ti O₂(0.04%-0.09%)and P₂O₅(0.001%-0.07%),The negative correlation of Si O₂-P₂O₅ is obvious.Combined with the high differentiation index(DI=92.60-94.04),it fully indicates that the two types of monzonite granites belong to the highly differentiated i-type granites.In addition,deficit the HREE,A high index of La/Yb and high index of Y,Yb content,indicate that the source area is garnet residues,suggested that two kinds of granite formed in continental crust thickening environment,combined with positive?Hf(t)(5.24-13.68)and younger age of two-phase model(T_DM2=553-673 Ma),and in the granite tectonic environment discrimination diagrams are fall into two classes of adamellite with the result of the collision tectonic area,show their original magma originated in the thickening of the neoproterozoic new gave birth to the partial melting of crust.In conclusion,the formation of two types of monzonite granites in the Naomuhunni area was under the same collision tectonic The two types of monzonite granites in the Naomuhunni area were formed in the synclastic tectonic environment during the intraplate orogenic stage.In conclusion,the two types of monzonite granites in the Naomuhunni area were formed the newly formed and thick lower crust.The cause of the two types of monzonite granites is related to the crustal thickening caused by the collision-regional compression of the introntinental orogenic belt in late Permian to early Triassic.This paper provides important evidence for the tectonic evolution process from early Paleozoic subduction-collision to late Paleozoic to early Mesozoic extension-thickening of continental crust.