Petrological Characteristics Of Mesozoic Granitoids In Zhengxiangbaiqi Banner Inner Mongolia

The area of Zhengxiangbaiqi in Inner Mongolia was affected by the subduction of the Pacific Ocean plate,developed a strong magmatism in the Mesozoic.The Jurassic-Cretaceous magmatic rocks were mainly developed,which consisted of deep intrusive rocks and ultra-shallow magmatic rocks.The granite rock mass,together with a large number of medium-acid volcanic rocks and volcanic clastic rocks,constitutes a mechanism for the formation of a caldera,reflecting the characteristics of the Mesozoic crustal evolution in this area.In this paper,the magmatic rock of Zhengxiangbaiqi is taken as the main research object.For a set of rocks with granite occurrence and lava structure characteristics,the understanding of the calcareous crater theme rock as ultra-shallow granite is proposed.LA-ICP-MS zircon is used.U-Pb dating,the ages of plaque-like granite,granite porphyry and monzonitic granites and syenite granites were(138.81±1.09,138.84±1.59)Ma,(135.76±1.08)Ma and(139.02±1.37),(136.38±1.04)Ma,with the intrusive rocks in the same period and the adjacent Zhenglanqi volcanic rocks,formed the magmatic activity zone of the Early Cretaceous period in the northern margin of the North China Sea;the elemental geochemistry of each granite was studied,the results showed that:Granite-like granites have higher Si O₂(62.4?77.8%),Al₂O₃(12.3?15.1%)and K₂O(3.23%?5.48%)content;trace element studies show that they have enriched Rb,Th,U and other large lithophiles Element and LREE,relative loss HREE and Ta,Nb,Ti high field strength elements,light and heavy rare earth elements are significantly different[(La/Yb)_N=5.51?18.32],with significant Eu negative anomaly(?Eu=0.03?0.30).Among them,ultra-shallow granite is a high-potassium-calcium-alkaline series-aluminous A type granite porphyry,while plaque-like granites and syenite granites are over-aluminous I type granites,and the whole is mixed with A-I type mixed granite Source feature.The Zhengxiangbaiqi rock mass has similar geochemical characteristics to the volcanic arc granite after collision,and the Sm/Nd ratio varies little(0.10?0.22),indicating that the magma source area is the lower crust.Rb content is high,Rb/Sr(1.21?12.56×10^-6),Th and U content are also high,high Th/U(5.72?7.44×10^-6),high field strength elements Nb,Ta content is low The Nb/Ta ratios are between(14.26?15.73×10^-6)and Nb/Ta,and the relative contents of Zr and Hf are relatively high,similar to the magmatic rocks formed by the re-melting of crustal materials caused by the intrusion of mantle source.The rock consolidation index SI(1.41?15.08),the La/Sm ratio(5.51?8.79)and the La/Yb ratio(11.17?31.82)indicate that the separation crystallization has been experienced.In addition,the average content of Mg O in biotite in the granitic rocks of Zhengxiangbaiqi rock is 10.03%,the average content of Fe O is 21.57%,the value of hornblende Mg^#is 0.48?0.59 and the ratio of Y/Nb,Ti/Yb and Ti/Zr.Both show the characteristics of the clam-shell mixed source.Therefore,the granitic rocks of the Zhengxiangbaiqi rock mass may be formed under the background of the westward subduction of the Pacific plate,and formed by the non-orogenic transformation in the plate after the orogenic formation and the extensional environment dominated by non-orogenic mountains in the plate.The basaltic magma that is melted undermines,causing remelting of the lower crust,and undergoing magma mixing and low-level separation and crystallization.The magma separation and crystallization base fractures cause the magma upwell to form a caldera mechanism.