Chronology And Geochemistry Of Changgouhe Diorite Pluton

The conjunction area of the Qinling-Qilian orogen,as the structural junction of the North Qinling orogenic belt and North Qilian orogenic belt,is the key area to study the structural junction between the North Qinling orogenic belt and the North Qilian orogenic belt,and is also the focus area of China's continental dynamics research.A large number of studies on the petrology,geochemistry and chronology of magmatic rocks have established a relatively complete chronology framework for the Northern Qinling orogenic belt and the Northern Qilian orogenic Belt.However,the eastern end of the Northern Qilian orogenic belt developed subducted magma at 455-440Ma,lacking the early stage of early Paleozoic records.The Changgouhe diorite pluton is located in the eastern end of the North Qilian orogenic belt with gneissic structural foliation,which has long been considered as an intrusive body formed in Precambrian.In this paper,the Changgouhe diorite pluton in the Guowan-Baijia section of Zhangjiachuan-Qingshui area is selected as the research object.Based on detailed field geological and petrographical,zircon U-Pb chronological,lithogeochemical and isotopic geochemical analysis,the petrogenesis,magma source and tectonic environment of Changgouhe diorite pluton are deeply recognized,and the early Paleozoic era in the eastern end of the North Qilian orogenic belt is discussed in the process of geological evolution.The following knowledge has been obtained:(1)Changgouhe diorite pluton in Guowan-Baijia section of Zhangjiachuan-Qingshui area is a magmatic complex from mafic rocks to intermediate-acid rocks.The rock assemblage includes gneissic diorite,plagioclase amphibolite,etc.,which is dominated by gneissic diorite,and the plagioclase amphibolite tectonic lens can be seen.Under the microscope,the amphiboles in plagioclase amphibolite are characterized by broken edges,subgranulization,and nucleo-mantle structure,which are the products of dynamic metamorphism,indicating that the rock mass may have undergone the transformation of ductile shear zone.The results of LA-ICP-MS zircon U-Pb dating in this study show two groups of magmatic zircon crystallization ages at 463.3±2.3Ma and 443.8±2.6Ma.Zircon trace element analyses show that these 463.3±2.3Ma and 443.8±2.6Ma share the similar trace element compositions,indicating that these two groups crystallized in a closed magmatic system.Zircon Hf isotope analysis shows that?_Hf(t)values of about 460Ma and about 440Ma zircons ranges from 6.36to 11.57,which is close to?_Hf(t)?75%depleted mantle evolution line,and has a relatively young two-stage model age,indicating that mantle-derived juvenile crustal magma was developed at about 460Ma and about 440Ma.t-Eu/Eu^*diagram shows that the magmatic zircons of about 460Ma have no Eu anomalies,similar to the REE patterns of mantle-derived zircons.The negative Eu anomaly of about 440Ma magmatic zircons indicates that the parent magma began to crystallize plagioclase,consistent with the process of the anatexis.Therefore,about 460Ma is the formation age of Changgouhe gneissic diorite,and about 440Ma is the crystallization age of anatexis magma of Changgouhe gneissic diorite.463Ma formation age of Changgouhe gneissic diorite limits that Changgouhe diorite pluton is early Paleozoic rock pluton.(2)The major element characteristics of Changgouhe diorite pluton show that it is medium potassium cal-alkaline medium-basic rock pluton.Al₂O₃content ranges from 15.22%to17.25%with an average of 16.48%(>16%),so it is high alumina basalt.In this paper,the correlation between Al₂O₃and MgO in Changgouhe high alumina basalts is not obvious and there is no Eu anomalies indicating that there is no fractional crystallization of plagioclase,which indicates delayed plagioclase nucleation or plagioclase cumulation.The negative correlation between the CaO/Al₂O₃and CaO/Na₂O ratios and the FeO^*/MgO ratios indicates the charactaristics of preferential crystallization of mafic minerals,which consistents with the delayed plagioclase nucleation.The lower Th/Yb and Th/Ce ratios and the large variable range of Ba/La and Ba/Nb ratios indicate that changgouhe high alumina basalt is formed by partial melting of subducted slab fluid metasomatism overlying mantle wedge.Therefore,the high aluminum cause of Changgouhe high alumina basalt is due to the delayed plagioclase nucleation under low crystallization pressure and water rich environment.The discrimination diagram of tectonic environment shows that changgouhe high alumina basalt is the product of the tectonic environment of intra-ocieanic arc of subduction zone.Changgouhe 463Ma high alumina basalt provides important geological evidence for early Paleozoic subduction in the eastern end of the North Qilian orogenic belt.(3)The tectonic position of Changgouhe diorite pluton is located in the northern side of Hongtubao back-arc basin.Therefore it is speculated that it is the product of hongtubao back-arc basin subduction,which indicates that hongtubao back-arc basin subduction started at least before 463Ma.