Early Mesozoic Granitic Magmatism And Its Metallogenic Significance In The Moerdaoga,Inner Mongolia

Moerdaoga is located in the Ergun block situtated at the southern part of Mongolian-Okhotsk suture line in the Xingmeng orogenic belt.Its tectonic evolution has been affected by the superposition of the three major tectonic domains: Paleo-Asian Ocean,Mongolia-Okhotsk and the Pacific Ocean,which has provided a favorable geological background for polymetallic mineralization.Based on more geological study and improvement of analysis in recent years,new point view of the evolution of the Early Mesozoic magmatism has been proposed.The author collected samples of the Early Mesozoic intrusive rock in the area.Detailed zircon U-Pb isotope dating and petrochemical analysis were carried out to investigate the stage and tectonic setting of the Early Mesozoic magmatism in the study area,and to illustrate the collision time of the Mongolian-Okhotsk ocean in the area.Granitic intrusive rocks caused by the magmatic activity of the Ergun block focused on Late Proterozoic,Paleozoic and Mesozoic,and the Late Proterozoic and Early Mesozoic magmatic rocks were the main period of northeastern part of this area.Early Mesozoic magmatic rocks(240-190Ma)in the northern part of Moerdaoga are widely distributed,and magmatism can be divided into three phases: 250-230 Ma,220-210 Ma,210-195 Ma.The 250-230 Ma intrusions include granite and intermediate rock and mafic rocks,is feature by intermediate rock and basic rock such as gabbro and diorite.The major minerals of the intermediate and mafic rocks are plagioclase,hornblende,and minor minerals are quartz,biotite.It is low in Si,rich in Fe,Mg,Al is also high.K,Na are less.Intrusions of 220-210 Ma is mainly intermediate rock and acidic rock stocks such as granodiorite,granodiorite.These intrusive rocks usually occurred as NE-trending,their major minerals are quartz and plagioclase,minor minerals are biotite and amphibole.It is riched in K,Na,low in Fe,Mg.210-195 Ma is mainly composed of acidic rock such as biotite granite.The main minerals are quartz,potassium feldspar,plagioclase.The minor mineral is biotite.It is also the largest intrusion in the Early Mesozoic.It is higher in Si;K and Na are also higher;Fe and Mg are lower.The closure time of the Mongolia-Okhotsk ocean has been debated for a long time because of lack of reliable evidence such as dating,structure,and rock combination.Based on factors such as magmatic rock combination,petrogenesis and geochemical analysis of rock in research area,the study suggests that the 250-230 Ma magmatism activity is likely to represent the southward subduction of the Mongolian-Okhotsk ocean,and then Siberian plate collided with the Ergun block after 220 Ma.The 215 Ma volcanic rock at the bottom of the Hailar basin is likely to represent the start of the collision.Magma activity reached a peak.period at 210-195 Ma.Along with the magmatism of the Early Mesozoic,the corresponding copper-molybdenum polymetallic deposits were formed in the study area.This study shows that there are two phases of copper-molybdenum polymetallic mineralization associated with early Mesozoic magmatism,which are the copper-molybdenum polymetallic mineralization associated with the subduction of Mongolian-Okhotsk ocean in the Early-Middle Triassic and the copper-molybdenum polymetallic mineralization associated with the collision of Mongolian-Okhotsk ocean in later Triassic-Jurassic.These deposit types are a major direction for future polymetallic prospecting in the region.The major direction for polymetallic prospecting in the region are the porphyry copper-molybdenum deposits related to the subduction,the porphyry copper-molybdenum deposits,medium-high temperature hydrothermal tungsten-molybdenum deposits,and skarn-type deposits related to the collision.