Characteristics And Geological Significance Of Silurian Intrusions In Nurgai Area, Inner Mongolia

Nurgai area is located in the southwestern margin of Alxa Block,Intrusive rocks are widely distributed in the block and magmatic activity is very intense.In this study,we present in geological observation records,lithofacies,zircon U-Pb age along with whole-rock element and Sr-Nd isotopic compositions for the Silurian instrusions in Nurgai area to constrain the magmatic types,magma evolution and source,timing and petrogenesis of the Silurian instrusions.There are five types of Silurian intrusions in Nurgai area:quartz diorite,tonalite,biotite granodiorite,porphyry granodiorite and monzonitic granite.The content of SiO₂ ranges from 56.42%to 74.76%and belongs to intermediate-acid rocks.Total alkali?Na₂O+K₂O?content ranges from 3.91%to 8.89%,K₂O/Na₂O from 0.56 to 2.20,Al₂O₃ from 13.22%to 18.88%,A/CNK from 0.92 to 1.29,AR from 1.49 to 3.53.In total,intrusions in Nurgai area are I-type granites with quasi-aluminous-weak aluminous and calcium-alkaline characteristics.LA-ICP-MS zircon U-Pb dating results yielded ages of quartz diorite of 423.8±5.7 Ma,424.4±4.6 Ma and 430.2±4.2 Ma,tonalite of 425.2±4.5 Ma,porphyry granodiorite of 433.4±4.8 Ma and 439.9±3.8 Ma and monzonitic granite of 434±13Ma and 437.7±2 Ma,respectively.In summary,the zircon U–Pb age data indicate that the instrusions in Nurgai area were formed at 440-424Ma,a Silurian period.From quartz diorite to monzonitic granite,the Rb/Sr ratio and DI value?differentiation index?and magma differentiation degree gradually increases,and the delta Eu value gradually decreases.The other parameter are(⁸⁷Sr/⁸⁶Sr)_i value of 0.705727-0.723430,?Nd?t?of-8.3-10.4,and the age of Nd isotope two-stage model(T_2DM)of1.8-2.0Ga.It has high Th,U,Pb content and negative Eu anomalies.Ti,Nb and Ta are depleted,which have the characteristics of island arc tectonic environment.It is a product of homologous magmatism and may originate from the relatively ancient Paleoproterozoic crustal source area.Related to the collision closure of the Paleo-Asian Ocean,the continuous southward subduction of the Paleo-Asian Ocean began or preceded the Silurian period,resulting in multi-stage magmatic activities.