| As granite is the main component of the continental crust,the study of its geochemical characteristics,magma source area characteristics and geodynamic characteristics is of great significance for studying the formation history,evolution process and tectonic environment of the continental crust.The study area is located in Sonid Left Banner,Xilingol League,Inner Mongolia,and the tectonic location is north of the Erenhot-Hegenshan tectonic hybrid zone.The intrusive rocks in the study area are generally distributed in a north-east direction,consistent with the trend of the Mongolia-Okhotsk suture zone.Late Carboniferous,Late Jurassic,and Early Cretaceous are the three peak periods of magmatic activity in the region.To determine the diagenetic period of the Mesozoic intrusive rock in the area,to clarify the its geochemical characteristics,and to expound on its relationship with the Mongolia-Okhotsk Ocean Basin,this paper selects the monzonitic granite and biotite monzonitic granite located in the northern part of the area as the research object,in combination with field survey,geological mapping,geochemical analysis,isotope dating and structural geology studying,this paper explored the geochemical characteristics and geological significance of these two kinds of intrusive rocks and got the following results:(1)Monzonitic granite is mainly distributed in Xinshenghuo Production Team Subdivision,showing a near-east-to-west distribution.It turns to a north-east distribution which is similar to the overall structural trend of the region,when going eastward entering Darai Obo Subdivision.And the monzonitic granite which covers an area of 302.11km~2 as the largest intrusion in the area,exposes as large band-shaped rock foudation.The structure and mineral composition of the intrusive mass show facies variation,there are medium-fine grained porphyritic granitic texture,medium-fine grained speckled granitic texture,medium-fine grained granitic texture,and fine grained granitic texture.Diorite xenoliths and dark enclaves can be found at the edge of the intrusive mass.Joints and vein rocks are relatively developed in the intrusive mass.Most of the vein rocks have a same strike with regional structure,which have a northeast strike.The characteristics above show that Xinshenghuo Production Team monzonitic granite is a dome shaped rock base which has an occurrence which is obviously controlled by regional main structure.(2)Biotite monzonitic granite is mainly distributed in Xinshenghuo Production Team Subdivision,and it extends eastward into the Darai Obo Subdivision and northward into the territory of Mongolia.In the study area,the biotite monzonitic granite which covers an area of 83.47km~2 is the southern edge of the extensional dome which has a long axis in the northeast direction.The structure and mineral composition of the intrusive mass show facies variation,there are medium-fine grained porphyritic granitic texture,medium-fine grained granitic texture,and fine grained granitic texture.The intrusive mass has a series of horizontal joints,due to the later weathering,a series of layered stacked stone columns and piers were formed.The faults developed with the emplacement of the intrusive mass,and filled with granite-pegmatite vein which is the exclusive dyke of the time.Ductile shear zone is found at the southwest part of the intrusive mass,which shows that the emplacement of the intrusive mass is significantly controlled by fracture structure.Combined with the age background,it is concluded that the intrusive mass is a granite dome formed by active emplacement in the regional extensional environment.(3)The LA-MC-ICP-MS zircon U-Pb isotope dating shows that the age of the monzonitic granite obtained by Xinshenghuo Production Team is 162.3±1.4Ma~159.8±1.2Ma,which belongs to the Late Jurassic.Therefore,the monzonitic granite should be formed in the Early Yanshanian,rather than the Late Variscan determined by the predecessors.The age of the biotite monzonitic granite obtained by Baoderchulu is 133.3±2.8Ma,which belongs to the Early Cretaceous.Therefore,the biotite monzonitic granite should be formed in the Late Yanshanian.(4)The two kinds of intrusive rocks both have the characteristics of high silicon,high alkali,poor calcium and poor magnesium.The A/CNK values are all in the range of 1.0~1.1,which is weakly peraluminous high-K calc-alkaline rock.In terms of trace elements,it is rich in Rb,Th,and K,and depleted in Ba,Sr P,Ti and Eu.The REE distribution curve shows a right-sloping”searull”type with LREE enrichment and negative Eu anomaly.The rock from the Early Cretaceous has less REE than the rock from the Late Jurassic,and has less obvious negative Eu anomaly.(5)In combination with its geochemical characteristics and the lack of accessory minerals,the two kinds of intrusive rocks both should be highly fractionated I-type granite.Zircon Hf isotope tests show that the?Hf(t)values range from+9.3~+13.3and+10.1~+13.5,the crustal mode age ranges from 677Ma~380Ma and 604Ma~353Ma.It is inferred that the magma came from the melting of crustal material,which is newly accreted from depleted mantle during Neoproterozoic to Phanerozoic.And the crustal material is the product of the post-orogenic extensional environment due to the closing of Mongolia-Okhotsk Ocean Basin. |
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