Geochemistry And Petrogenesis Of Late Yanshanian Granites, Southern Manzhouli Area, Inner-Mongolia

Southern Manzhouli area is located in west of the famous Derbugan fracture belts，wherethe Paleo-Asian Ocean and Pacific Ocean, Mongolia-okhotsk Ocean strongly stacked,compound, belonging to the erguna massif, tectonic evolution complex. Predecessor has madenumerous researches about Formation time and geochemistry of granites in northern ergunamassif. But in marked contrast, chronology, geochemistry and isotope of granites in southernerguna massif, especially in Hf isotope research sharp shortage. The evidence of accurateisotopic chronology is insufficient. Due to chronology knowledge inadequate, the rockassociation of the Late Yanshanian granite is also not accurate. Meanwhile, lacking ofgeochemical data restricts restricted the deep understanding of its causes. What is more,Compared with agreement in the regional tectonic setting of the Early Yanshanian granite,Late Yanshanian granite formation is still controversial. Therefore, in this essay, I will givemore details to analyze the rock association and formation time of Late Yanshanian stageas well as lithogeochemical characteristics of rocks and the genesis through contentsincluding geology, petrography, chronology and geochemical characteristics of the granites inthis area.1. Rock association and the ageThe granites of Late Yanshanian stage in southern Manzhouli area is composed oforthoclase granite, alkali-feldspar granite, quartz syenite porphyry and granite-porphyry.The result of zircon U-Pb isotopic dating indicates that the ages of the granites are137~151Ma.2. Geochemical characteristics of the granitePetrogeochemistry characteristics of the granitic rocks show that orthoclase granite has77.21%~74.45%SiO2,12.33%~12.64%Al2O3,0.48%~0.67%Fe2O3, and comparatively rich potassium (Na2O/K2O=1.16~3.32). The total amount of rare earth elements is103.9~132.0ppm, with rich light rare earth elements and depleted heavy ones, and areenriched in LILE, such as Rb, K, etc., depleted in HFSE such as Nb, Ta, P, Ti, etc. and withobvious negative e Eu anomalies (δEu=0.24~0.38).Alkali-feldspar granite has71.68%~76.82%SiO2,12.41%~14.24%Al2O3,1.11%~1.83%Fe2O3,and comparatively rich potassium (Na2O/K2O=1.18~1.67). The total amount of rareearth elements is184.5-232.5ppm,with rich light rare earth elements and depleted heavy ones,and are enriched in LILE, such as Rb, K, Th, etc., depleted in HFSE such as Nb, Ta, P, Ti, etc.and with obvious negative δEu anomalies (δEu=0.34~0.44).Quartz syenite porphyry has66.45%~67.14%SiO2and comparatively rich potassium(Na2O/K2O=1.15~1.22). The total amount of rare earth elements is478.7~620.4ppm,with richlight rare earth elements and depleted heavy ones, and are enriched in LILE, such as Rb, K,Th,La,Ce, etc., depleted in HFSE such as Nb, Ta, P, Ti, etc. and with obvious negative δEuanomalies (δEu=0.34~0.44).As a conclusion, Major element and trace Element of the granites have some similarcharacteristics and change rules:rich silicon,low calcium and magnesium,and so on,butalkali-feldspar granite and quartz syenite porphyry in the alkali index(ALK), basicindex(NK/A), the total amount of rare earth elements,10000Ga/Al value, the contents ofZr+Nb+Ce+Y are higher than orthoclase granite. Identifying the genetic types of rock:orthoclase granite is I-type granite, alkali-feldspar granite and quartz syenite porphyry areA-type granite, both of them form Complex I-andA-type granites.3. Petrogenesis of the graniteZircon Hf analyses conducted by LA-MC-ICPMS show that these granites have Hf(t)values of0.7~9.5with Hf modal ages of0.6~1.1Ga, indicating that the crust in the Ergunblock was extracted from mantle during Meso-Neoproterozoic, But they may be derived fromdifferent positions of the lower crust, and lack Petrogenesis connection.Comprehensive research have shown that granites of Late Yashanian stage was mainlyinduced by lithospheric extension after closure of the Mongol-Okhotsk ocean,and mark theMongol-Okhotsk ocean was coming to an end.