Petrogenesis And Tectonic Significance Of The Volcanic Rocks Of The Manketouebo Formation In The Keyouzhongqi Area,Central Great Xing'an Range

The Mesozoic volcanic rock belt in the Great Xing'an Range is an important carrier reflecting the tectonic influences in Northeast China,and has attracted the attention of many scholars and has been studied in detail.This paper takes the medium-acid volcanic rocks of the Manketouebo Formation in the Keyouzhongqi area of the central Great Xing'an Range as the research object.Through petrology,zircon U-Pb geochronology,Lu-Hf isotope and geochemical test analysis,combined with previous data,it is determined that The formation age of the volcanic rocks of the Manketouebo Formation in the study area was analyzed,and the petrogenesis and tectonic background of the volcanic rocks were analyzed.The volcanic rocks of the Manketouebo Formation in the study area are a set of acidic igneous rocks,mainly composed of rhyolite,rhyolite tuff,and rhyolite tuff with a small amount of dacite volcanic lava.The zircon U-Pb dating results of the rhyolite are162.4±1.5Ma and 160.2±1Ma,indicating that it was formed in the Late Jurassic.Lu-Hf isotopic analysis results show that the weighted average of ¹⁷⁶Lu/¹⁷⁷Hf zircon rhyolite is0.282984±0.000012(n=19),the?_Hf(t)value is positive(8.30?12.2),and the Hf two-stage model age(T_DM2)changes The range is 678-427Ma with a weighted mean age of 547Ma.Whole-rock geochemical analysis shows that the Si O₂ content of the Manketouebo Formation volcanic rocks from 61.52%to 82.28%,which is rich in silicon;Na₂O ranges from 0.13%to 5.64%;8.54%),and Al₂O₃varies between 9.38%and 16.76%.Most of the samples belong to the peraluminum high-potassium calc-alkaline series,and a small amount is the calc-alkaline series.The total amount of rare earths in the volcanic rocks is between?REE=56.8×10^–6?664×10^–6,with a large variation range and a strong negative Eu anomaly(?Eu=0.03?0.09,average 0.05).The trace elements are relatively enriched in large ion lithophile elements Rb,Th,K,and depleted in high field strength elements such as Nb and Ti.Basic compatible components Cr(2.0×10^–6?9.23×10^–6),Co(0.17×10^–6?2.27×10^–6),Ni(1.51×10^–6?5.40×10^–6)The content is low.The results of rock geochemistry and zircon isotope analysis show that the volcanic rocks of the Manketouebo Formation are characterized by high silicon,rich alkali,rich aluminum,and poor iron,calcium,and magnesium,rich in large ion lithophile elements,and depleted in high field strength elements.Typical features of crustal magma.Positive values of?_Hf(t)and T_MD2(678Ma?427Ma)indicate that the volcanic magma originated from the partial melting of young crustal material newly generated from the depleted mantle during the Neoproterozoic-Phanerozoic.Most of the volcanic rocks have strong negative Eu anomalies,indicating that amphibole exists in the residual minerals in the magma source area or has undergone the separation and crystallization of amphibole,and has the characteristics of high-fractionated I-type rhyolite.Based on the previous research results and the research data in this paper,it is believed that the volcanic rocks of the Manketouebo Formation in the study area were formed under the background of lithospheric extension after the Mongolia-Okhotsk Ocean closed orogeny.