Magmatism And Crustal Evolution In The Eastern North Qinling Terrain

The North Qinling terrain, being an important part of Qinling orogenic belt, is characterized by diverse rocks, multiphase tectonic-magmatic-metamorphic events. This terrain is composed of not only basement rocks of the Qinling Group, but also numerous magmatic rocks in different ages of the Jinningian, Caledonian, and Hercynian periods. The Qinling Group is previously interpreted as the oldest Precambrian basement unit of the North Qinling Terrain. Therefore, North Qinling is a key location to reveal the relationship of collision and assemblage between the North and South China blocks, and also an ideal place to research formation and evolution of crustal terrains, genesis of granitoids and provenance of tectonic units.In order to better understand provenance and tectonic setting of the Qinling Group and to reveal magmatism and crustal evolution in the North Qinling terrain, a detailed study of major and trace elements and Rb-Sr and Sm-Nd isotopic composition of whole-rock samples, as well as U-Pb ages and Hf-O isotopic data of zircons from the same analytical spot by the SIMS and LA-MC-ICP MS techniques, was measured for different rocks collected from two major rock units of the Qinling Group (felsic gneiss and amphibolite) and granitoids (Jingningian, Caledonian and Hercynian periods) in eastern North Qinling. Rock associations and spatial-temporal distribution of the Jingningian and Caledonian-Hercynian magmatic rocks throughout North Qinling are investigated for petrogenesis, magma source, geochronological framework of different magmatic events. Further, by comparison with magmatism and crustal evolution of the adjacent areas, crustal growth and reworking in the North Qinling terrain are systematically summarized. Research results in this study not only place geochemical constraints on genesis of these rocks, but also provide insight into evolution of the basement and tectonic provenance of the Qinling orogenic belt, and further help to better understand geodynamic background during the formation and evolution of continental crust in orogenic processes.The Qinling Group consists predominantly of gneisses and amphibolites and minor marbles, and underwent multi-phase deformation and metamorphism. This geological unit records hence the formation and early crustal evolution of the North Qinling terrain. Felsic gneisses exhibit enrichment of LREEs and LILEs (Rb, Ba, Th, K, Pb), negative Eu-anomalies and depletion of HFSEs (Nb, Ta, P, Ti). The rocks have slightly elevated δ18O values (6.5～9.3‰) and initial εNd values of-4.6corresponding to two-stage Nd model age of1.99Ga. Amphibolites are also enriched in LILE and LREE and depleted in Nb and Ta, and have homogeneous δ18O values (5.0-6.0%o), but higher initiald values (2.8～3.3) and younger two-stage Nd model ages (1.29～1.24Ga) compared to the gneisses. These rocks were emplaced in a～960Ma volcanic arc environment rather than in a rift setting as previously suggested. A major metamorphic event took place in early Paleozoic. Based on the age spectrum of detrital zircons, the Qinling Group is interpreted as an independent geological unit, which was mainly derived mostly from1000～900Ma old granitoid rocks. The North Qinling terrain can be regarded as a remnant of the Grenville orogenic belt with an early Neoproterozoic evolution different from the North and South China blocks.Granitoids of Jingningian ages exposed both in the South and North Qinling terrains and record Precambrian crustal formation and evolution of the basements in the Qinling orogenic belt. However, Early Jingningian granitic intrusive are rare in South Qinling but numerous in North Qinling. In this study, the Fangzhuang and Dehe granitoid plutons emplaced into the Qinling Group in the North Qinling terrain are chosen as research object. Analytical results show that zircons of the Fangzhuang granite pluton formed in933.4±9.2Ma, zircons have δ18O values of8.3～11.9‰and whole-rock yields initial εNdd value of-6.0, Nd model age of2.09Ga (TDM2) and initial87Sr/86Sr ratio of0.7246, while zircons of the Dehe granite pluton crystallized in939.7±7.6Ma and948.1±8.9Ma and whole-rock has initial Nd values of-5.3～-4.8, corresponding to Nd model ages of2.05-2.00Ga (TDM2) and variable initial87Sr/86Sr ratios of0.7099～0.7246. Integrating with isotopic ages of fifteen Neoproterozoic granitoid plutons exposed within the North Qinling terrain previously reported, the Neoproterozoic magmatism in this terrain can be subdivided into two major stages resulted from980～870Ma compressional and～844Ma extensional tectonic settings, including granitoids of～940Ma syn-collisional,～880Ma post-collisional and～844Ma extensional origin. Geochemical and isotopic features of these granitoids indicate that they probably originated from partial melting mainly of crustal material similar to the Qinling Group, but contribution of mantle or juvenile material to the magmas became significant after～900Ma in the post-collisional processes. Neoproterozoic magmatic activities in the North Qinling terrain are broadly simultaneous with the formation of supercontinent Rodinia and hence, they record response of the basement in this terrain. The crustal evolution in North Qinling terrain prior to the Neoproterozoic was likely different from those in the North and South China blocks. Caledonian-Hercynian magmatic activities were extensively developed in the North Qinling terrain, but it is scarcely present in the adjacent terrains. In this study, nineteen samples from ten intermediate-acidic intrusions and four samples from the intermediate-basic Fushui complex in the North Qinling terrain were measured for zircon U-Pb ages using SIMS and LA-ICP MS techniques. Integrating with fifty-three zircon isotopic ages previously reported in North Qinling, a geochronologic framework can be established for spatial and temporal distribution of the Caledonian-Hercynian magmatism in this terrain. It can be subdivided into three major phases of granitoids:-487Ma mainly distributed in eastern part of the study area,～450Ma occurring throughout the study area, and～417Ma scattered only in middle part of the study area.Stage one (-487Ma):I-type granitoids in geochemical composition, represented by the Banshanping, Xizhuanghe and Machihexiang plutons, are similar to adakites related to subducted oceanic-crust. They have initial87Sr/86Sr ratios of0.7026～0.7052, high initial εNd values of0.19-3.17and young two-stage Nd model ages of0.92-1.21Ga, and δ18O values between7.03and7.60%o. Coexisting with the island-arc related Fushui complex in the terrain, two types of magmatism together confirm an Early Paleozoic subduction event, which points to this I-type granitoids may be derived from partial melting of subducted oceanic crust. S-type granitoids, particularly the Piaochi and Manziying plutons, have high SiO2and K2O contents and low TiO2and MgO contents and define a high-K calc-alkaline magma trend. They have initial87Sr/86Sr ratios of0.7035～0.7274, lower initial εNd values of-7.98～-10.05and two-stage Nd model ages of1.89～2.06Ga, and zircon δ18O values of7.88～8.65‰, lower than whole-rock δ18O values of10.9-11.7%o, likely indicating S-type granites resulted from melting of crustal material induced by the subduction.Stage two (-450Ma):I-type granitoids (-460Ma) represented by the Huichizi pluton is analogous to thickened lower crust derived adakitic rocks, which have initial87Sr/86Sr ratios of0.70463～0.70612, initial εNd values of-0.51～1.54and two-stage Nd model ages of1.08～1.25Ga and two-stage Hf model ages of0.61～1.70Ga, and whole-rock δ18O values between7.06and9.08%o. This type of plutons does not contain intermediate-basic magmatic enclaves but many inherited zircons, implying that granites might be products of partial melting of thickened juvenile lower crust, whose source is the same as I-type granites from Stage one and related to Paleozoic Sangdan oceanic crust subduction-converge-collision, resulting in juvenile lower crust shortened, superimposed and thickened. I-S-type granites (-450Ma), represented by the Wuduoshan, Kuanping, Zaoyuan, Huangbaicha and Taoping plutons, have some characteristics of both collision-related and subduction-related granitoids. They overall display similar feature and more enriched in Nd and O isotopic composition compared to I-type granite (-460Ma), indicating contribution of mantle-derived material is relatively less than the older granites. I-type granitoids (-440Ma), represented by the Wuguan and Xiaowangjian plutons distributed along the Shangdan suture zone, might be the product of partial melting of pre-existing crustal component in subduction zone. S-type granites represented by the Anjiping and Tuqiaogang plutons display a high-K calc-alkaline magma trend as the stage one, which have an initial87Sr/86Sr ratios of0.71025～0.71263, lower initiald values of-7.64～-8.20and two-stage Nd model age of1.81～1.88Ga, and zircon δ18O values of9.60～13.30‰, indicating feature of compression and collision origin.Stage three (-417Ma):I-type granitoids, represented by the Zhangjiazhuang pluton, display a low-K (Na2O/K2O=3.30～28.83), peraluminous tholeiite magma trend, and have lower initial87Sr/86Sr ratios of0.70331～0.70471, high initial εNd values of3.19-5.07, corresponding to two-stage model Nd age of0.74-0.89Ga, implying that another juvenile crustal growth may be occurred in late Neoproterozoic. Geochemical and isotopic features indicate that they probably originated from the partial melting of the underplating juvenile basic magma or magma that had already been emplaced into orogenic root during the early stage, mixing with small amounts of ancient crustal material. S-type granites, represented by the Danfeng pluton, have high SiO2and K2O contents and low MgO content. Their geochemical characteristics point to products of re-melting of continental crust.Overall, I-type granitoids of three stages originated from the sources closely related to basic volcanic rocks in the South and North Qinling terrains but irrelevant to the basement of North China. However, S-type granites were mainly derived from crustal material similar with metamorphic rocks of the Qinling and Kuanping Groups, without contribution of the basement of North China block.Crustal formation in the eastern North Qinling is characterized by episodic growth that mostly took place in Proterozoic through reworking of ancient crustal material at～2300Ma and2000Ma, crustal growth and recycling of ancient crust at-1600Ma,1000～630Ma and～500Ma, respectively. By comparison with the adjacent terrains, it can be observed that the North Qinling terrain can be interpreted as a separate geological entity with a special evolution different from the South and North China blocks before Proterozoic. Crustal growth in the southern margin of the North China block took place in Archean (～2700Ma), the South Qinling terrain mainly in Neoproterozoic and the northern margin of the Yangtze block in Archean and Mesoproterozoic, while the North Qinling terrain began massive growth after Mesoproterozoic.