The Preliminary Study Of The Genesis And Tectonic Significance Of Early Paleozoic Granitoids In The Eastern Section Of The North Qilian

Qilian orogenic belt,which locates in the western part of Qinling-Qilian-Kunlun Central Orogenic System,and adjacents to Qaidam Block in the south and Alxa massif in the north,and the tectonic position is very important,is an ideal place to study the plate tectonics and the process of crust-mantle movement.The article will discuss the mineralogy,petrology,geochemistry and petrogenesis,tectonic significance of the four typically granites on the basis of the plate tectonics theory,structure-magmatism and detailed field geological survey,in order to provide a significant scientific basis for the research of the evolution of plate tectonics in the North Qilian orogenic belt.The results are as follow:Firstly,Lianhua Mountain acid intrusive rock body is mainly composed of fine-grained diorite,tonalite rocks,granodiorite rocks,formed in the Ordovician;Maozang Temple acid intrusive rock body is mainly composed of granodiorite,monzogranite rocks, formed in the Ordovician(435±5.8Ma);Zhigou acid intrusive rock body is chiefly composed of diorite,quartz diorite rocks,Which formed in the Silurian Period(423±2.8Ma);North Daban acid intrusive rock body is consisted of fine-grained granite rock, crude medium-grained granite,porphyritic coarse-grained rocks composed of granite, formed in the Late Silurian(414±4Ma).Secondly,Lianhua Mountain,Maozang Temple and Zhigou rock bodies have the characteristic of sodium-rich and potassium-depleted,mostly belong to quasi-weakly peraluminous,calc-alkaline series,A/CNK＜1.1,and Na₂O-K₂O diagram shows all are "I"-type granites,strongly enriched in large ion lithophile elements and light rare earth elements,as well as slightly enriched HFSE.North Daban rock body have the characteristic of potassium-rich and sodium-depleted,belongs to peraluminous rocks,in the Na₂O-K₂O diagram which are plotted in "S"-type granite region.Rocks are enriched in large ion lithophile elements,slightly enriched HFSE,with the rare earth distribution patterns enriched in LREE. Thirdly,in the Th-Ni diagram,the trend of the map projection of Lianhua Mountain, Maozang Temple and Zhigou rock bodies nearly parallels to the X-axis,which suggests that are the result of fractional crystallization,while that of North Daban rock body parallels to the Y-axis,indicating that it formed by partial melting.Four rock bodies have low TiO₂,P₂O₅ content and higher K₂O content.In the diagrams of K₂O/P₂O₅-SiO₂, K₂O/TiO₂-SiO₂,Ti/Yb-Zr/Nb and La/Nb,there are significantly covariant laws between them.The range of(La/Sm)_N ratios is wide and all imply that the rock body is strongly mixed by the earth's crust.Fourthly,Yb-Ta,Y-Nb,(Y+Nb)-Rb,(Yb+Ta)-Nb and Hf-Rb-Ta diagrams indicate that Lianhua Mountain,Maozang Temple and Zhigou rock bodies formed in the volcanic arc environment,and are the product of Caledonian oceanic subduction.However,North Daban rock body formed in syn-collision environment,is a product of arc-continental collision when the North Qilian Mountains basin(ocean basin and back-arc basin) tends to closure.Finally,it is thought that Lianhua Mountain,Maozang Temple and Zhigou rock bodies may formed with two-stage magmatism.In the active continental margin arc background, the expansion of back-arc mantle caused partial melting of magma,which generated magmatic underplating,while continuing underplating makes tha early underplating material partial melting,forming granite.Whereas,North Daban rock body is mainly from the earth's crust.